path: root/gfx/wr/webrender/src/picture.rs

/* This Source Code Form is subject to the terms of the Mozilla Public
 * License, v. 2.0. If a copy of the MPL was not distributed with this
 * file, You can obtain one at http://mozilla.org/MPL/2.0/. */

//! A picture represents a dynamically rendered image.
//!
//! # Overview
//!
//! Pictures consists of:
//!
//! - A number of primitives that are drawn onto the picture.
//! - A composite operation describing how to composite this
//!   picture into its parent.
//! - A configuration describing how to draw the primitives on
//!   this picture (e.g. in screen space or local space).
//!
//! The tree of pictures are generated during scene building.
//!
//! Depending on their composite operations pictures can be rendered into
//! intermediate targets or folded into their parent picture.
//!
//! ## Picture caching
//!
//! Pictures can be cached to reduce the amount of rasterization happening per
//! frame.
//!
//! When picture caching is enabled, the scene is cut into a small number of slices,
//! typically:
//!
//! - content slice
//! - UI slice
//! - background UI slice which is hidden by the other two slices most of the time.
//!
//! Each of these slice is made up of fixed-size large tiles of 2048x512 pixels
//! (or 128x128 for the UI slice).
//!
//! Tiles can be either cached rasterized content into a texture or "clear tiles"
//! that contain only a solid color rectangle rendered directly during the composite
//! pass.
//!
//! ## Invalidation
//!
//! Each tile keeps track of the elements that affect it, which can be:
//!
//! - primitives
//! - clips
//! - image keys
//! - opacity bindings
//! - transforms
//!
//! These dependency lists are built each frame and compared to the previous frame to
//! see if the tile changed.
//!
//! The tile's primitive dependency information is organized in a quadtree, each node
//! storing an index buffer of tile primitive dependencies.
//!
//! The union of the invalidated leaves of each quadtree produces a per-tile dirty rect
//! which defines the scissor rect used when replaying the tile's drawing commands and
//! can be used for partial present.
//!
//! ## Display List shape
//!
//! WR will first look for an iframe item in the root stacking context to apply
//! picture caching to. If that's not found, it will apply to the entire root
//! stacking context of the display list. Apart from that, the format of the
//! display list is not important to picture caching. Each time a new scroll root
//! is encountered, a new picture cache slice will be created. If the display
//! list contains more than some arbitrary number of slices (currently 8), the
//! content will all be squashed into a single slice, in order to save GPU memory
//! and compositing performance.
//!
//! ## Compositor Surfaces
//!
//! Sometimes, a primitive would prefer to exist as a native compositor surface.
//! This allows a large and/or regularly changing primitive (such as a video, or
//! webgl canvas) to be updated each frame without invalidating the content of
//! tiles, and can provide a significant performance win and battery saving.
//!
//! Since drawing a primitive as a compositor surface alters the ordering of
//! primitives in a tile, we use 'overlay tiles' to ensure correctness. If a
//! tile has a compositor surface, _and_ that tile has primitives that overlap
//! the compositor surface rect, the tile switches to be drawn in alpha mode.
//!
//! We rely on only promoting compositor surfaces that are opaque primitives.
//! With this assumption, the tile(s) that intersect the compositor surface get
//! a 'cutout' in the rectangle where the compositor surface exists (not the
//! entire tile), allowing that tile to be drawn as an alpha tile after the
//! compositor surface.
//!
//! Tiles are only drawn in overlay mode if there is content that exists on top
//! of the compositor surface. Otherwise, we can draw the tiles in the normal fast
//! path before the compositor surface is drawn. Use of the per-tile valid and
//! dirty rects ensure that we do a minimal amount of per-pixel work here to
//! blend the overlay tile (this is not always optimal right now, but will be
//! improved as a follow up).

use api::{MixBlendMode, PremultipliedColorF, FilterPrimitiveKind};
use api::{PropertyBinding, PropertyBindingId, FilterPrimitive};
use api::{DebugFlags, ImageKey, ColorF, ColorU, PrimitiveFlags};
use api::{ImageRendering, ColorDepth, YuvColorSpace, YuvFormat};
use api::units::*;
use crate::box_shadow::BLUR_SAMPLE_SCALE;
use crate::clip::{ClipStore, ClipChainInstance, ClipChainId, ClipInstance};
use crate::spatial_tree::{ROOT_SPATIAL_NODE_INDEX,
    SpatialTree, CoordinateSpaceMapping, SpatialNodeIndex, VisibleFace
};
use crate::composite::{CompositorKind, CompositeState, NativeSurfaceId, NativeTileId};
use crate::composite::{ExternalSurfaceDescriptor, ExternalSurfaceDependency};
use crate::debug_colors;
use euclid::{vec2, vec3, Point2D, Scale, Size2D, Vector2D, Vector3D, Rect, Transform3D, SideOffsets2D};
use euclid::approxeq::ApproxEq;
use crate::filterdata::SFilterData;
use crate::intern::ItemUid;
use crate::internal_types::{FastHashMap, FastHashSet, PlaneSplitter, Filter, PlaneSplitAnchor, TextureSource};
use crate::frame_builder::{FrameBuildingContext, FrameBuildingState, PictureState, PictureContext};
use crate::gpu_cache::{GpuCache, GpuCacheAddress, GpuCacheHandle};
use crate::gpu_types::{UvRectKind, ZBufferId};
use plane_split::{Clipper, Polygon, Splitter};
use crate::prim_store::{PrimitiveTemplateKind, PictureIndex, PrimitiveInstance, PrimitiveInstanceKind};
use crate::prim_store::{ColorBindingStorage, ColorBindingIndex, PrimitiveScratchBuffer};
use crate::print_tree::{PrintTree, PrintTreePrinter};
use crate::render_backend::{DataStores, FrameId};
use crate::render_task_graph::RenderTaskId;
use crate::render_target::RenderTargetKind;
use crate::render_task::{BlurTask, RenderTask, RenderTaskLocation, BlurTaskCache};
use crate::render_task::{StaticRenderTaskSurface, RenderTaskKind};
use crate::resource_cache::{ResourceCache, ImageGeneration};
use crate::space::{SpaceMapper, SpaceSnapper};
use crate::scene::SceneProperties;
use smallvec::SmallVec;
use std::{mem, u8, marker, u32};
use std::sync::atomic::{AtomicUsize, Ordering};
use std::collections::hash_map::Entry;
use std::ops::Range;
use crate::texture_cache::TextureCacheHandle;
use crate::util::{MaxRect, VecHelper, MatrixHelpers, Recycler, raster_rect_to_device_pixels, ScaleOffset};
use crate::filterdata::{FilterDataHandle};
use crate::tile_cache::{SliceDebugInfo, TileDebugInfo, DirtyTileDebugInfo};
use crate::visibility::{PrimitiveVisibilityMask, PrimitiveVisibilityFlags, FrameVisibilityContext};
use crate::visibility::{VisibilityState, FrameVisibilityState};
#[cfg(any(feature = "capture", feature = "replay"))]
use ron;
#[cfg(feature = "capture")]
use crate::scene_builder_thread::InternerUpdates;
#[cfg(any(feature = "capture", feature = "replay"))]
use crate::intern::{Internable, UpdateList};
#[cfg(any(feature = "capture", feature = "replay"))]
use crate::clip::ClipIntern;
#[cfg(any(feature = "capture", feature = "replay"))]
use crate::filterdata::FilterDataIntern;
#[cfg(any(feature = "capture", feature = "replay"))]
use api::PrimitiveKeyKind;
#[cfg(any(feature = "capture", feature = "replay"))]
use crate::prim_store::backdrop::Backdrop;
#[cfg(any(feature = "capture", feature = "replay"))]
use crate::prim_store::borders::{ImageBorder, NormalBorderPrim};
#[cfg(any(feature = "capture", feature = "replay"))]
use crate::prim_store::gradient::{LinearGradient, RadialGradient, ConicGradient};
#[cfg(any(feature = "capture", feature = "replay"))]
use crate::prim_store::image::{Image, YuvImage};
#[cfg(any(feature = "capture", feature = "replay"))]
use crate::prim_store::line_dec::LineDecoration;
#[cfg(any(feature = "capture", feature = "replay"))]
use crate::prim_store::picture::Picture;
#[cfg(any(feature = "capture", feature = "replay"))]
use crate::prim_store::text_run::TextRun;

#[cfg(feature = "capture")]
use std::fs::File;
#[cfg(feature = "capture")]
use std::io::prelude::*;
#[cfg(feature = "capture")]
use std::path::PathBuf;
use crate::scene_building::{SliceFlags};

#[cfg(feature = "replay")]
// used by tileview so don't use an internal_types FastHashMap
use std::collections::HashMap;

// Maximum blur radius for blur filter (different than box-shadow blur).
// Taken from FilterNodeSoftware.cpp in Gecko.
pub const MAX_BLUR_RADIUS: f32 = 100.;

/// Specify whether a surface allows subpixel AA text rendering.
#[derive(Debug, Clone, PartialEq)]
pub enum SubpixelMode {
    /// This surface allows subpixel AA text
    Allow,
    /// Subpixel AA text cannot be drawn on this surface
    Deny,
    /// Subpixel AA can be drawn on this surface, if not intersecting
    /// with the excluded regions, and inside the allowed rect.
    Conditional {
        allowed_rect: PictureRect,
        excluded_rects: Vec<PictureRect>,
    },
}

/// A comparable transform matrix, that compares with epsilon checks.
#[derive(Debug, Clone)]
struct MatrixKey {
    m: [f32; 16],
}

impl PartialEq for MatrixKey {
    fn eq(&self, other: &Self) -> bool {
        const EPSILON: f32 = 0.001;

        // TODO(gw): It's possible that we may need to adjust the epsilon
        //           to be tighter on most of the matrix, except the
        //           translation parts?
        for (i, j) in self.m.iter().zip(other.m.iter()) {
            if !i.approx_eq_eps(j, &EPSILON) {
                return false;
            }
        }

        true
    }
}

/// A comparable / hashable version of a coordinate space mapping. Used to determine
/// if a transform dependency for a tile has changed.
#[derive(Debug, PartialEq, Clone)]
enum TransformKey {
    Local,
    ScaleOffset {
        scale_x: f32,
        scale_y: f32,
        offset_x: f32,
        offset_y: f32,
    },
    Transform {
        m: MatrixKey,
    }
}

impl<Src, Dst> From<CoordinateSpaceMapping<Src, Dst>> for TransformKey {
    fn from(transform: CoordinateSpaceMapping<Src, Dst>) -> TransformKey {
        match transform {
            CoordinateSpaceMapping::Local => {
                TransformKey::Local
            }
            CoordinateSpaceMapping::ScaleOffset(ref scale_offset) => {
                TransformKey::ScaleOffset {
                    scale_x: scale_offset.scale.x,
                    scale_y: scale_offset.scale.y,
                    offset_x: scale_offset.offset.x,
                    offset_y: scale_offset.offset.y,
                }
            }
            CoordinateSpaceMapping::Transform(ref m) => {
                TransformKey::Transform {
                    m: MatrixKey {
                        m: m.to_array(),
                    },
                }
            }
        }
    }
}

/// Information about a picture that is pushed / popped on the
/// PictureUpdateState during picture traversal pass.
struct PictureInfo {
    /// The spatial node for this picture.
    _spatial_node_index: SpatialNodeIndex,
}

/// Unit for tile coordinates.
#[derive(Hash, Clone, Copy, Debug, Eq, PartialEq, Ord, PartialOrd)]
pub struct TileCoordinate;

// Geometry types for tile coordinates.
pub type TileOffset = Point2D<i32, TileCoordinate>;
pub type TileSize = Size2D<i32, TileCoordinate>;
pub type TileRect = Rect<i32, TileCoordinate>;

/// The maximum number of compositor surfaces that are allowed per picture cache. This
/// is an arbitrary number that should be enough for common cases, but low enough to
/// prevent performance and memory usage drastically degrading in pathological cases.
const MAX_COMPOSITOR_SURFACES: usize = 4;

/// The size in device pixels of a normal cached tile.
pub const TILE_SIZE_DEFAULT: DeviceIntSize = DeviceIntSize {
    width: 1024,
    height: 512,
    _unit: marker::PhantomData,
};

/// The size in device pixels of a tile for horizontal scroll bars
pub const TILE_SIZE_SCROLLBAR_HORIZONTAL: DeviceIntSize = DeviceIntSize {
    width: 1024,
    height: 32,
    _unit: marker::PhantomData,
};

/// The size in device pixels of a tile for vertical scroll bars
pub const TILE_SIZE_SCROLLBAR_VERTICAL: DeviceIntSize = DeviceIntSize {
    width: 32,
    height: 1024,
    _unit: marker::PhantomData,
};

/// The maximum size per axis of a surface,
///  in WorldPixel coordinates.
const MAX_SURFACE_SIZE: f32 = 4096.0;
/// Maximum size of a compositor surface.
const MAX_COMPOSITOR_SURFACES_SIZE: f32 = 8192.0;

/// The maximum number of sub-dependencies (e.g. clips, transforms) we can handle
/// per-primitive. If a primitive has more than this, it will invalidate every frame.
const MAX_PRIM_SUB_DEPS: usize = u8::MAX as usize;

/// Used to get unique tile IDs, even when the tile cache is
/// destroyed between display lists / scenes.
static NEXT_TILE_ID: AtomicUsize = AtomicUsize::new(0);

fn clamp(value: i32, low: i32, high: i32) -> i32 {
    value.max(low).min(high)
}

fn clampf(value: f32, low: f32, high: f32) -> f32 {
    value.max(low).min(high)
}

/// Clamps the blur radius depending on scale factors.
fn clamp_blur_radius(blur_radius: f32, scale_factors: (f32, f32)) -> f32 {
    // Clamping must occur after scale factors are applied, but scale factors are not applied
    // until later on. To clamp the blur radius, we first apply the scale factors and then clamp
    // and finally revert the scale factors.

    // TODO: the clamping should be done on a per-axis basis, but WR currently only supports
    // having a single value for both x and y blur.
    let largest_scale_factor = f32::max(scale_factors.0, scale_factors.1);
    let scaled_blur_radius = blur_radius * largest_scale_factor;

    if scaled_blur_radius > MAX_BLUR_RADIUS {
        MAX_BLUR_RADIUS / largest_scale_factor
    } else {
        // Return the original blur radius to avoid any rounding errors
        blur_radius
    }
}

/// An index into the prims array in a TileDescriptor.
#[derive(Debug, Copy, Clone, PartialEq, Eq, PartialOrd, Ord, Hash)]
#[cfg_attr(feature = "capture", derive(Serialize))]
#[cfg_attr(feature = "replay", derive(Deserialize))]
pub struct PrimitiveDependencyIndex(pub u32);

/// Information about the state of a binding.
#[derive(Debug)]
pub struct BindingInfo<T> {
    /// The current value retrieved from dynamic scene properties.
    value: T,
    /// True if it was changed (or is new) since the last frame build.
    changed: bool,
}

/// Information stored in a tile descriptor for a binding.
#[derive(Debug, PartialEq, Clone, Copy)]
#[cfg_attr(feature = "capture", derive(Serialize))]
#[cfg_attr(feature = "replay", derive(Deserialize))]
pub enum Binding<T> {
    Value(T),
    Binding(PropertyBindingId),
}

impl<T> From<PropertyBinding<T>> for Binding<T> {
    fn from(binding: PropertyBinding<T>) -> Binding<T> {
        match binding {
            PropertyBinding::Binding(key, _) => Binding::Binding(key.id),
            PropertyBinding::Value(value) => Binding::Value(value),
        }
    }
}

pub type OpacityBinding = Binding<f32>;
pub type OpacityBindingInfo = BindingInfo<f32>;

pub type ColorBinding = Binding<ColorU>;
pub type ColorBindingInfo = BindingInfo<ColorU>;

/// A dependency for a transform is defined by the spatial node index + frame it was used
#[derive(Copy, Clone, Debug, Eq, PartialEq, Hash)]
#[cfg_attr(feature = "capture", derive(Serialize))]
#[cfg_attr(feature = "replay", derive(Deserialize))]
pub struct SpatialNodeKey {
    spatial_node_index: SpatialNodeIndex,
    frame_id: FrameId,
}

/// A helper for comparing spatial nodes between frames. The comparisons
/// are done by-value, so that if the shape of the spatial node tree
/// changes, invalidations aren't done simply due to the spatial node
/// index changing between display lists.
struct SpatialNodeComparer {
    /// The root spatial node index of the tile cache
    ref_spatial_node_index: SpatialNodeIndex,
    /// Maintains a map of currently active transform keys
    spatial_nodes: FastHashMap<SpatialNodeKey, TransformKey>,
    /// A cache of recent comparisons between prev and current spatial nodes
    compare_cache: FastHashMap<(SpatialNodeKey, SpatialNodeKey), bool>,
    /// A set of frames that we need to retain spatial node entries for
    referenced_frames: FastHashSet<FrameId>,
}

impl SpatialNodeComparer {
    /// Construct a new comparer
    fn new() -> Self {
        SpatialNodeComparer {
            ref_spatial_node_index: ROOT_SPATIAL_NODE_INDEX,
            spatial_nodes: FastHashMap::default(),
            compare_cache: FastHashMap::default(),
            referenced_frames: FastHashSet::default(),
        }
    }

    /// Advance to the next frame
    fn next_frame(
        &mut self,
        ref_spatial_node_index: SpatialNodeIndex,
    ) {
        // Drop any node information for unreferenced frames, to ensure that the
        // hashmap doesn't grow indefinitely!
        let referenced_frames = &self.referenced_frames;
        self.spatial_nodes.retain(|key, _| {
            referenced_frames.contains(&key.frame_id)
        });

        // Update the root spatial node for this comparer
        self.ref_spatial_node_index = ref_spatial_node_index;
        self.compare_cache.clear();
        self.referenced_frames.clear();
    }

    /// Register a transform that is used, and build the transform key for it if new.
    fn register_used_transform(
        &mut self,
        spatial_node_index: SpatialNodeIndex,
        frame_id: FrameId,
        spatial_tree: &SpatialTree,
    ) {
        let key = SpatialNodeKey {
            spatial_node_index,
            frame_id,
        };

        if let Entry::Vacant(entry) = self.spatial_nodes.entry(key) {
            entry.insert(
                get_transform_key(
                    spatial_node_index,
                    self.ref_spatial_node_index,
                    spatial_tree,
                )
            );
        }
    }

    /// Return true if the transforms for two given spatial nodes are considered equivalent
    fn are_transforms_equivalent(
        &mut self,
        prev_spatial_node_key: &SpatialNodeKey,
        curr_spatial_node_key: &SpatialNodeKey,
    ) -> bool {
        let key = (*prev_spatial_node_key, *curr_spatial_node_key);
        let spatial_nodes = &self.spatial_nodes;

        *self.compare_cache
            .entry(key)
            .or_insert_with(|| {
                let prev = &spatial_nodes[&prev_spatial_node_key];
                let curr = &spatial_nodes[&curr_spatial_node_key];
                curr == prev
            })
    }

    /// Ensure that the comparer won't GC any nodes for a given frame id
    fn retain_for_frame(&mut self, frame_id: FrameId) {
        self.referenced_frames.insert(frame_id);
    }
}

// Immutable context passed to picture cache tiles during pre_update
struct TilePreUpdateContext {
    /// Maps from picture cache coords -> world space coords.
    pic_to_world_mapper: SpaceMapper<PicturePixel, WorldPixel>,

    /// The fractional position of the picture cache, which may
    /// require invalidation of all tiles.
    fract_offset: PictureVector2D,
    device_fract_offset: DeviceVector2D,

    /// The optional background color of the picture cache instance
    background_color: Option<ColorF>,

    /// The visible part of the screen in world coords.
    global_screen_world_rect: WorldRect,

    /// Current size of tiles in picture units.
    tile_size: PictureSize,

    /// The current frame id for this picture cache
    frame_id: FrameId,
}

// Immutable context passed to picture cache tiles during post_update
struct TilePostUpdateContext<'a> {
    /// Maps from picture cache coords -> world space coords.
    pic_to_world_mapper: SpaceMapper<PicturePixel, WorldPixel>,

    /// Global scale factor from world -> device pixels.
    global_device_pixel_scale: DevicePixelScale,

    /// The local clip rect (in picture space) of the entire picture cache
    local_clip_rect: PictureRect,

    /// The calculated backdrop information for this cache instance.
    backdrop: BackdropInfo,

    /// Information about opacity bindings from the picture cache.
    opacity_bindings: &'a FastHashMap<PropertyBindingId, OpacityBindingInfo>,

    /// Information about color bindings from the picture cache.
    color_bindings: &'a FastHashMap<PropertyBindingId, ColorBindingInfo>,

    /// Current size in device pixels of tiles for this cache
    current_tile_size: DeviceIntSize,

    /// The local rect of the overall picture cache
    local_rect: PictureRect,

    /// A list of the external surfaces that are present on this slice
    external_surfaces: &'a [ExternalSurfaceDescriptor],

    /// Pre-allocated z-id to assign to opaque tiles during post_update. We
    /// use a different z-id for opaque/alpha tiles, so that compositor
    /// surfaces (such as videos) can have a z-id between these values,
    /// which allows compositor surfaces to occlude opaque tiles, but not
    /// alpha tiles.
    z_id_opaque: ZBufferId,

    /// Pre-allocated z-id to assign to alpha tiles during post_update
    z_id_alpha: ZBufferId,

    /// If true, the scale factor of the root transform for this picture
    /// cache changed, so we need to invalidate the tile and re-render.
    invalidate_all: bool,
}

// Mutable state passed to picture cache tiles during post_update
struct TilePostUpdateState<'a> {
    /// Allow access to the texture cache for requesting tiles
    resource_cache: &'a mut ResourceCache,

    /// Current configuration and setup for compositing all the picture cache tiles in renderer.
    composite_state: &'a mut CompositeState,

    /// A cache of comparison results to avoid re-computation during invalidation.
    compare_cache: &'a mut FastHashMap<PrimitiveComparisonKey, PrimitiveCompareResult>,

    /// Information about transform node differences from last frame.
    spatial_node_comparer: &'a mut SpatialNodeComparer,
}

/// Information about the dependencies of a single primitive instance.
struct PrimitiveDependencyInfo {
    /// Unique content identifier of the primitive.
    prim_uid: ItemUid,

    /// The (conservative) clipped area in picture space this primitive occupies.
    prim_clip_box: PictureBox2D,

    /// Image keys this primitive depends on.
    images: SmallVec<[ImageDependency; 8]>,

    /// Opacity bindings this primitive depends on.
    opacity_bindings: SmallVec<[OpacityBinding; 4]>,

    /// Color binding this primitive depends on.
    color_binding: Option<ColorBinding>,

    /// Clips that this primitive depends on.
    clips: SmallVec<[ItemUid; 8]>,

    /// Spatial nodes references by the clip dependencies of this primitive.
    spatial_nodes: SmallVec<[SpatialNodeIndex; 4]>,

    /// If true, this primitive has been promoted to be a compositor surface.
    is_compositor_surface: bool,
}

impl PrimitiveDependencyInfo {
    /// Construct dependency info for a new primitive.
    fn new(
        prim_uid: ItemUid,
        prim_clip_box: PictureBox2D,
    ) -> Self {
        PrimitiveDependencyInfo {
            prim_uid,
            images: SmallVec::new(),
            opacity_bindings: SmallVec::new(),
            color_binding: None,
            prim_clip_box,
            clips: SmallVec::new(),
            spatial_nodes: SmallVec::new(),
            is_compositor_surface: false,
        }
    }
}

/// A stable ID for a given tile, to help debugging. These are also used
/// as unique identifiers for tile surfaces when using a native compositor.
#[derive(Debug, Copy, Clone, PartialEq, PartialOrd, Ord, Eq)]
#[cfg_attr(feature = "capture", derive(Serialize))]
#[cfg_attr(feature = "replay", derive(Deserialize))]
pub struct TileId(pub usize);

/// A descriptor for the kind of texture that a picture cache tile will
/// be drawn into.
#[derive(Debug)]
pub enum SurfaceTextureDescriptor {
    /// When using the WR compositor, the tile is drawn into an entry
    /// in the WR texture cache.
    TextureCache {
        handle: TextureCacheHandle
    },
    /// When using an OS compositor, the tile is drawn into a native
    /// surface identified by arbitrary id.
    Native {
        /// The arbitrary id of this tile.
        id: Option<NativeTileId>,
    },
}

/// This is the same as a `SurfaceTextureDescriptor` but has been resolved
/// into a texture cache handle (if appropriate) that can be used by the
/// batching and compositing code in the renderer.
#[derive(Clone, Debug, Eq, PartialEq, Hash)]
#[cfg_attr(feature = "capture", derive(Serialize))]
#[cfg_attr(feature = "replay", derive(Deserialize))]
pub enum ResolvedSurfaceTexture {
    TextureCache {
        /// The texture ID to draw to.
        texture: TextureSource,
        /// Slice index in the texture array to draw to.
        layer: i32,
    },
    Native {
        /// The arbitrary id of this tile.
        id: NativeTileId,
        /// The size of the tile in device pixels.
        size: DeviceIntSize,
    }
}

impl SurfaceTextureDescriptor {
    /// Create a resolved surface texture for this descriptor
    pub fn resolve(
        &self,
        resource_cache: &ResourceCache,
        size: DeviceIntSize,
    ) -> ResolvedSurfaceTexture {
        match self {
            SurfaceTextureDescriptor::TextureCache { handle } => {
                let cache_item = resource_cache.texture_cache.get(handle);

                ResolvedSurfaceTexture::TextureCache {
                    texture: cache_item.texture_id,
                    layer: cache_item.texture_layer,
                }
            }
            SurfaceTextureDescriptor::Native { id } => {
                ResolvedSurfaceTexture::Native {
                    id: id.expect("bug: native surface not allocated"),
                    size,
                }
            }
        }
    }
}

/// The backing surface for this tile.
#[derive(Debug)]
pub enum TileSurface {
    Texture {
        /// Descriptor for the surface that this tile draws into.
        descriptor: SurfaceTextureDescriptor,
        /// Bitfield specifying the dirty region(s) that are relevant to this tile.
        visibility_mask: PrimitiveVisibilityMask,
    },
    Color {
        color: ColorF,
    },
    Clear,
}

impl TileSurface {
    fn kind(&self) -> &'static str {
        match *self {
            TileSurface::Color { .. } => "Color",
            TileSurface::Texture { .. } => "Texture",
            TileSurface::Clear => "Clear",
        }
    }
}

/// Optional extra information returned by is_same when
/// logging is enabled.
#[derive(Debug, Copy, Clone, PartialEq)]
#[cfg_attr(feature = "capture", derive(Serialize))]
#[cfg_attr(feature = "replay", derive(Deserialize))]
pub enum CompareHelperResult<T> {
    /// Primitives match
    Equal,
    /// Counts differ
    Count {
        prev_count: u8,
        curr_count: u8,
    },
    /// Sentinel
    Sentinel,
    /// Two items are not equal
    NotEqual {
        prev: T,
        curr: T,
    },
    /// User callback returned true on item
    PredicateTrue {
        curr: T
    },
}

/// The result of a primitive dependency comparison. Size is a u8
/// since this is a hot path in the code, and keeping the data small
/// is a performance win.
#[derive(Debug, Copy, Clone, PartialEq)]
#[cfg_attr(feature = "capture", derive(Serialize))]
#[cfg_attr(feature = "replay", derive(Deserialize))]
#[repr(u8)]
pub enum PrimitiveCompareResult {
    /// Primitives match
    Equal,
    /// Something in the PrimitiveDescriptor was different
    Descriptor,
    /// The clip node content or spatial node changed
    Clip,
    /// The value of the transform changed
    Transform,
    /// An image dependency was dirty
    Image,
    /// The value of an opacity binding changed
    OpacityBinding,
    /// The value of a color binding changed
    ColorBinding,
}

/// A more detailed version of PrimitiveCompareResult used when
/// debug logging is enabled.
#[derive(Debug, Copy, Clone, PartialEq)]
#[cfg_attr(feature = "capture", derive(Serialize))]
#[cfg_attr(feature = "replay", derive(Deserialize))]
pub enum PrimitiveCompareResultDetail {
    /// Primitives match
    Equal,
    /// Something in the PrimitiveDescriptor was different
    Descriptor {
        old: PrimitiveDescriptor,
        new: PrimitiveDescriptor,
    },
    /// The clip node content or spatial node changed
    Clip {
        detail: CompareHelperResult<ItemUid>,
    },
    /// The value of the transform changed
    Transform {
        detail: CompareHelperResult<SpatialNodeKey>,
    },
    /// An image dependency was dirty
    Image {
        detail: CompareHelperResult<ImageDependency>,
    },
    /// The value of an opacity binding changed
    OpacityBinding {
        detail: CompareHelperResult<OpacityBinding>,
    },
    /// The value of a color binding changed
    ColorBinding {
        detail: CompareHelperResult<ColorBinding>,
    },
}

/// Debugging information about why a tile was invalidated
#[derive(Debug,Clone)]
#[cfg_attr(feature = "capture", derive(Serialize))]
#[cfg_attr(feature = "replay", derive(Deserialize))]
pub enum InvalidationReason {
    /// The fractional offset changed
    FractionalOffset {
        old: DeviceVector2D,
        new: DeviceVector2D,
    },
    /// The background color changed
    BackgroundColor {
        old: Option<ColorF>,
        new: Option<ColorF>,
    },
    /// The opaque state of the backing native surface changed
    SurfaceOpacityChanged{
        became_opaque: bool
    },
    /// There was no backing texture (evicted or never rendered)
    NoTexture,
    /// There was no backing native surface (never rendered, or recreated)
    NoSurface,
    /// The primitive count in the dependency list was different
    PrimCount {
        old: Option<Vec<ItemUid>>,
        new: Option<Vec<ItemUid>>,
    },
    /// The content of one of the primitives was different
    Content {
        /// What changed in the primitive that was different
        prim_compare_result: PrimitiveCompareResult,
        prim_compare_result_detail: Option<PrimitiveCompareResultDetail>,
    },
    // The compositor type changed
    CompositorKindChanged,
    // The valid region of the tile changed
    ValidRectChanged,
    // The overall scale of the picture cache changed
    ScaleChanged,
}

/// A minimal subset of Tile for debug capturing
#[cfg_attr(feature = "capture", derive(Serialize))]
#[cfg_attr(feature = "replay", derive(Deserialize))]
pub struct TileSerializer {
    pub rect: PictureRect,
    pub current_descriptor: TileDescriptor,
    pub device_fract_offset: DeviceVector2D,
    pub id: TileId,
    pub root: TileNode,
    pub background_color: Option<ColorF>,
    pub invalidation_reason: Option<InvalidationReason>
}

/// A minimal subset of TileCacheInstance for debug capturing
#[cfg_attr(feature = "capture", derive(Serialize))]
#[cfg_attr(feature = "replay", derive(Deserialize))]
pub struct TileCacheInstanceSerializer {
    pub slice: usize,
    pub tiles: FastHashMap<TileOffset, TileSerializer>,
    pub background_color: Option<ColorF>,
    pub fract_offset: PictureVector2D,
}

/// Information about a cached tile.
pub struct Tile {
    /// The grid position of this tile within the picture cache
    pub tile_offset: TileOffset,
    /// The current world rect of this tile.
    pub world_tile_rect: WorldRect,
    /// The current local rect of this tile.
    pub local_tile_rect: PictureRect,
    /// Same as local_tile_rect, but in min/max form as an optimization
    pub local_tile_box: PictureBox2D,
    /// The picture space dirty rect for this tile.
    local_dirty_rect: PictureRect,
    /// The device space dirty rect for this tile.
    /// TODO(gw): We have multiple dirty rects available due to the quadtree above. In future,
    ///           expose these as multiple dirty rects, which will help in some cases.
    pub device_dirty_rect: DeviceRect,
    /// Device space rect that contains valid pixels region of this tile.
    pub device_valid_rect: DeviceRect,
    /// Uniquely describes the content of this tile, in a way that can be
    /// (reasonably) efficiently hashed and compared.
    pub current_descriptor: TileDescriptor,
    /// The content descriptor for this tile from the previous frame.
    pub prev_descriptor: TileDescriptor,
    /// Handle to the backing surface for this tile.
    pub surface: Option<TileSurface>,
    /// If true, this tile is marked valid, and the existing texture
    /// cache handle can be used. Tiles are invalidated during the
    /// build_dirty_regions method.
    pub is_valid: bool,
    /// If true, this tile intersects with the currently visible screen
    /// rect, and will be drawn.
    pub is_visible: bool,
    /// The current fractional offset of the cache transform root. If this changes,
    /// all tiles need to be invalidated and redrawn, since snapping differences are
    /// likely to occur.
    device_fract_offset: DeviceVector2D,
    /// The tile id is stable between display lists and / or frames,
    /// if the tile is retained. Useful for debugging tile evictions.
    pub id: TileId,
    /// If true, the tile was determined to be opaque, which means blending
    /// can be disabled when drawing it.
    pub is_opaque: bool,
    /// Root node of the quadtree dirty rect tracker.
    root: TileNode,
    /// The last rendered background color on this tile.
    background_color: Option<ColorF>,
    /// The first reason the tile was invalidated this frame.
    invalidation_reason: Option<InvalidationReason>,
    /// If true, this tile has one or more compositor surfaces affecting it.
    pub has_compositor_surface: bool,
    /// The local space valid rect for any primitives found prior to the first compositor
    /// surface that affects this tile.
    bg_local_valid_rect: PictureBox2D,
    /// The local space valid rect for any primitives found after the first compositor
    /// surface that affects this tile.
    fg_local_valid_rect: PictureBox2D,
    /// z-buffer id for this tile, which is one of z_id_opaque or z_id_alpha, depending on tile opacity
    pub z_id: ZBufferId,
    /// The last frame this tile had its dependencies updated (dependency updating is
    /// skipped if a tile is off-screen).
    pub last_updated_frame_id: FrameId,
}

impl Tile {
    /// Construct a new, invalid tile.
    fn new(tile_offset: TileOffset) -> Self {
        let id = TileId(NEXT_TILE_ID.fetch_add(1, Ordering::Relaxed));

        Tile {
            tile_offset,
            local_tile_rect: PictureRect::zero(),
            local_tile_box: PictureBox2D::zero(),
            world_tile_rect: WorldRect::zero(),
            device_valid_rect: DeviceRect::zero(),
            local_dirty_rect: PictureRect::zero(),
            device_dirty_rect: DeviceRect::zero(),
            surface: None,
            current_descriptor: TileDescriptor::new(),
            prev_descriptor: TileDescriptor::new(),
            is_valid: false,
            is_visible: false,
            device_fract_offset: DeviceVector2D::zero(),
            id,
            is_opaque: false,
            root: TileNode::new_leaf(Vec::new()),
            background_color: None,
            invalidation_reason: None,
            has_compositor_surface: false,
            bg_local_valid_rect: PictureBox2D::zero(),
            fg_local_valid_rect: PictureBox2D::zero(),
            z_id: ZBufferId::invalid(),
            last_updated_frame_id: FrameId::INVALID,
        }
    }

    /// Print debug information about this tile to a tree printer.
    fn print(&self, pt: &mut dyn PrintTreePrinter) {
        pt.new_level(format!("Tile {:?}", self.id));
        pt.add_item(format!("local_tile_rect: {:?}", self.local_tile_rect));
        pt.add_item(format!("device_fract_offset: {:?}", self.device_fract_offset));
        pt.add_item(format!("background_color: {:?}", self.background_color));
        pt.add_item(format!("invalidation_reason: {:?}", self.invalidation_reason));
        self.current_descriptor.print(pt);
        pt.end_level();
    }

    /// Check if the content of the previous and current tile descriptors match
    fn update_dirty_rects(
        &mut self,
        ctx: &TilePostUpdateContext,
        state: &mut TilePostUpdateState,
        invalidation_reason: &mut Option<InvalidationReason>,
        frame_context: &FrameVisibilityContext,
    ) -> PictureRect {
        let mut prim_comparer = PrimitiveComparer::new(
            &self.prev_descriptor,
            &self.current_descriptor,
            state.resource_cache,
            state.spatial_node_comparer,
            ctx.opacity_bindings,
            ctx.color_bindings,
        );

        let mut dirty_rect = PictureBox2D::zero();
        self.root.update_dirty_rects(
            &self.prev_descriptor.prims,
            &self.current_descriptor.prims,
            &mut prim_comparer,
            &mut dirty_rect,
            state.compare_cache,
            invalidation_reason,
            frame_context,
        );

        dirty_rect.to_rect()
    }

    /// Invalidate a tile based on change in content. This
    /// must be called even if the tile is not currently
    /// visible on screen. We might be able to improve this
    /// later by changing how ComparableVec is used.
    fn update_content_validity(
        &mut self,
        ctx: &TilePostUpdateContext,
        state: &mut TilePostUpdateState,
        frame_context: &FrameVisibilityContext,
    ) {
        // Check if the contents of the primitives, clips, and
        // other dependencies are the same.
        state.compare_cache.clear();
        let mut invalidation_reason = None;
        let dirty_rect = self.update_dirty_rects(
            ctx,
            state,
            &mut invalidation_reason,
            frame_context,
        );
        if !dirty_rect.is_empty() {
            self.invalidate(
                Some(dirty_rect),
                invalidation_reason.expect("bug: no invalidation_reason"),
            );
        }
        if ctx.invalidate_all {
            self.invalidate(None, InvalidationReason::ScaleChanged);
        }
        // TODO(gw): We can avoid invalidating the whole tile in some cases here,
        //           but it should be a fairly rare invalidation case.
        if self.current_descriptor.local_valid_rect != self.prev_descriptor.local_valid_rect {
            self.invalidate(None, InvalidationReason::ValidRectChanged);
            state.composite_state.dirty_rects_are_valid = false;
        }
    }

    /// Invalidate this tile. If `invalidation_rect` is None, the entire
    /// tile is invalidated.
    fn invalidate(
        &mut self,
        invalidation_rect: Option<PictureRect>,
        reason: InvalidationReason,
    ) {
        self.is_valid = false;

        match invalidation_rect {
            Some(rect) => {
                self.local_dirty_rect = self.local_dirty_rect.union(&rect);
            }
            None => {
                self.local_dirty_rect = self.local_tile_rect;
            }
        }

        if self.invalidation_reason.is_none() {
            self.invalidation_reason = Some(reason);
        }
    }

    /// Called during pre_update of a tile cache instance. Allows the
    /// tile to setup state before primitive dependency calculations.
    fn pre_update(
        &mut self,
        ctx: &TilePreUpdateContext,
    ) {
        // Ensure each tile is offset by the appropriate amount from the
        // origin, such that the content origin will be a whole number and
        // the snapping will be consistent.
        self.local_tile_rect = PictureRect::new(
            PicturePoint::new(
                self.tile_offset.x as f32 * ctx.tile_size.width + ctx.fract_offset.x,
                self.tile_offset.y as f32 * ctx.tile_size.height + ctx.fract_offset.y,
            ),
            ctx.tile_size,
        );
        self.local_tile_box = PictureBox2D::new(
            self.local_tile_rect.origin,
            self.local_tile_rect.bottom_right(),
        );
        self.bg_local_valid_rect = PictureBox2D::zero();
        self.fg_local_valid_rect = PictureBox2D::zero();
        self.invalidation_reason  = None;
        self.has_compositor_surface = false;

        self.world_tile_rect = ctx.pic_to_world_mapper
            .map(&self.local_tile_rect)
            .expect("bug: map local tile rect");

        // Check if this tile is currently on screen.
        self.is_visible = self.world_tile_rect.intersects(&ctx.global_screen_world_rect);

        // If the tile isn't visible, early exit, skipping the normal set up to
        // validate dependencies. Instead, we will only compare the current tile
        // dependencies the next time it comes into view.
        if !self.is_visible {
            return;
        }

        // We may need to rerender if glyph subpixel positions have changed. Note
        // that we update the tile fract offset itself after we have completed
        // invalidation. This allows for other whole tile invalidation cases to
        // update the fract offset appropriately.
        let fract_delta = self.device_fract_offset - ctx.device_fract_offset;
        let fract_changed = fract_delta.x.abs() > 0.01 || fract_delta.y.abs() > 0.01;
        if fract_changed {
            self.invalidate(None, InvalidationReason::FractionalOffset {
                                    old: self.device_fract_offset,
                                    new: ctx.device_fract_offset });
        }

        if ctx.background_color != self.background_color {
            self.invalidate(None, InvalidationReason::BackgroundColor {
                                    old: self.background_color,
                                    new: ctx.background_color });
            self.background_color = ctx.background_color;
        }

        // Clear any dependencies so that when we rebuild them we
        // can compare if the tile has the same content.
        mem::swap(
            &mut self.current_descriptor,
            &mut self.prev_descriptor,
        );
        self.current_descriptor.clear();
        self.root.clear(self.local_tile_rect.to_box2d());

        // Since this tile is determined to be visible, it will get updated
        // dependencies, so update the frame id we are storing dependencies for.
        self.last_updated_frame_id = ctx.frame_id;
    }

    /// Add dependencies for a given primitive to this tile.
    fn add_prim_dependency(
        &mut self,
        info: &PrimitiveDependencyInfo,
    ) {
        // If this tile isn't currently visible, we don't want to update the dependencies
        // for this tile, as an optimization, since it won't be drawn anyway.
        if !self.is_visible {
            return;
        }

        // If this primitive is a compositor surface, any tile it affects must be
        // drawn as an overlay tile.
        if info.is_compositor_surface {
            self.has_compositor_surface = true;
        } else {
            // Incorporate the bounding rect of the primitive in the local valid rect
            // for this tile. This is used to minimize the size of the scissor rect
            // during rasterization and the draw rect during composition of partial tiles.

            // Once we have encountered 1+ compositor surfaces affecting this tile, include
            // this bounding rect in the foreground. Otherwise, include in the background rect.
            // This allows us to determine if we found any primitives that are on top of the
            // compositor surface(s) for this tile. If so, we need to draw the tile with alpha
            // blending as an overlay.
            if self.has_compositor_surface {
                self.fg_local_valid_rect = self.fg_local_valid_rect.union(&info.prim_clip_box);
            } else {
                self.bg_local_valid_rect = self.bg_local_valid_rect.union(&info.prim_clip_box);
            }
        }

        // Include any image keys this tile depends on.
        self.current_descriptor.images.extend_from_slice(&info.images);

        // Include any opacity bindings this primitive depends on.
        self.current_descriptor.opacity_bindings.extend_from_slice(&info.opacity_bindings);

        // Include any clip nodes that this primitive depends on.
        self.current_descriptor.clips.extend_from_slice(&info.clips);

        // Include any transforms that this primitive depends on.
        for spatial_node_index in &info.spatial_nodes {
            self.current_descriptor.transforms.push(
                SpatialNodeKey {
                    spatial_node_index: *spatial_node_index,
                    frame_id: self.last_updated_frame_id,
                }
            );
        }

        // Include any color bindings this primitive depends on.
        if info.color_binding.is_some() {
            self.current_descriptor.color_bindings.insert(
                self.current_descriptor.color_bindings.len(), info.color_binding.unwrap());
        }

        // TODO(gw): The prim_clip_rect can be impacted by the clip rect of the display port,
        //           which can cause invalidations when a new display list with changed
        //           display port is received. To work around this, clamp the prim clip rect
        //           to the tile boundaries - if the clip hasn't affected the tile, then the
        //           changed clip can't affect the content of the primitive on this tile.
        //           In future, we could consider supplying the display port clip from Gecko
        //           in a different way (e.g. as a scroll frame clip) which still provides
        //           the desired clip for checkerboarding, but doesn't require this extra
        //           work below.

        // TODO(gw): This is a hot part of the code - we could probably optimize further by:
        //           - Using min/max instead of clamps below (if we guarantee the rects are well formed)

        let tile_p0 = self.local_tile_box.min;
        let tile_p1 = self.local_tile_box.max;

        let prim_clip_box = PictureBox2D::new(
            PicturePoint::new(
                clampf(info.prim_clip_box.min.x, tile_p0.x, tile_p1.x),
                clampf(info.prim_clip_box.min.y, tile_p0.y, tile_p1.y),
            ),
            PicturePoint::new(
                clampf(info.prim_clip_box.max.x, tile_p0.x, tile_p1.x),
                clampf(info.prim_clip_box.max.y, tile_p0.y, tile_p1.y),
            ),
        );

        // Update the tile descriptor, used for tile comparison during scene swaps.
        let prim_index = PrimitiveDependencyIndex(self.current_descriptor.prims.len() as u32);

        // We know that the casts below will never overflow because the array lengths are
        // truncated to MAX_PRIM_SUB_DEPS during update_prim_dependencies.
        debug_assert!(info.spatial_nodes.len() <= MAX_PRIM_SUB_DEPS);
        debug_assert!(info.clips.len() <= MAX_PRIM_SUB_DEPS);
        debug_assert!(info.images.len() <= MAX_PRIM_SUB_DEPS);
        debug_assert!(info.opacity_bindings.len() <= MAX_PRIM_SUB_DEPS);

        self.current_descriptor.prims.push(PrimitiveDescriptor {
            prim_uid: info.prim_uid,
            prim_clip_box,
            transform_dep_count: info.spatial_nodes.len()  as u8,
            clip_dep_count: info.clips.len() as u8,
            image_dep_count: info.images.len() as u8,
            opacity_binding_dep_count: info.opacity_bindings.len() as u8,
            color_binding_dep_count: if info.color_binding.is_some() { 1 } else { 0 } as u8,
        });

        // Add this primitive to the dirty rect quadtree.
        self.root.add_prim(prim_index, &info.prim_clip_box);
    }

    /// Called during tile cache instance post_update. Allows invalidation and dirty
    /// rect calculation after primitive dependencies have been updated.
    fn post_update(
        &mut self,
        ctx: &TilePostUpdateContext,
        state: &mut TilePostUpdateState,
        frame_context: &FrameVisibilityContext,
    ) -> bool {
        // Register the frame id of this tile with the spatial node comparer, to ensure
        // that it doesn't GC any spatial nodes from the comparer that are referenced
        // by this tile. Must be done before we early exit below, so that we retain
        // spatial node info even for tiles that are currently not visible.
        state.spatial_node_comparer.retain_for_frame(self.last_updated_frame_id);

        // If tile is not visible, just early out from here - we don't update dependencies
        // so don't want to invalidate, merge, split etc. The tile won't need to be drawn
        // (and thus updated / invalidated) until it is on screen again.
        if !self.is_visible {
            return false;
        }

        // Calculate the overall valid rect for this tile, including both the foreground
        // and background local valid rects.
        self.current_descriptor.local_valid_rect =
            self.bg_local_valid_rect
                .union(&self.fg_local_valid_rect)
                .to_rect();

        // TODO(gw): In theory, the local tile rect should always have an
        //           intersection with the overall picture rect. In practice,
        //           due to some accuracy issues with how fract_offset (and
        //           fp accuracy) are used in the calling method, this isn't
        //           always true. In this case, it's safe to set the local
        //           valid rect to zero, which means it will be clipped out
        //           and not affect the scene. In future, we should fix the
        //           accuracy issue above, so that this assumption holds, but
        //           it shouldn't have any noticeable effect on performance
        //           or memory usage (textures should never get allocated).
        self.current_descriptor.local_valid_rect = self.local_tile_rect
            .intersection(&ctx.local_rect)
            .and_then(|r| r.intersection(&self.current_descriptor.local_valid_rect))
            .unwrap_or_else(PictureRect::zero);

        // The device_valid_rect is referenced during `update_content_validity` so it
        // must be updated here first.
        let world_valid_rect = ctx.pic_to_world_mapper
            .map(&self.current_descriptor.local_valid_rect)
            .expect("bug: map local valid rect");

        // The device rect is guaranteed to be aligned on a device pixel - the round
        // is just to deal with float accuracy. However, the valid rect is not
        // always aligned to a device pixel. To handle this, round out to get all
        // required pixels, and intersect with the tile device rect.
        let device_rect = (self.world_tile_rect * ctx.global_device_pixel_scale).round();
        self.device_valid_rect = (world_valid_rect * ctx.global_device_pixel_scale)
            .round_out()
            .intersection(&device_rect)
            .unwrap_or_else(DeviceRect::zero);

        // Invalidate the tile based on the content changing.
        self.update_content_validity(ctx, state, frame_context);

        // If there are no primitives there is no need to draw or cache it.
        if self.current_descriptor.prims.is_empty() {
            // If there is a native compositor surface allocated for this (now empty) tile
            // it must be freed here, otherwise the stale tile with previous contents will
            // be composited. If the tile subsequently gets new primitives added to it, the
            // surface will be re-allocated when it's added to the composite draw list.
            if let Some(TileSurface::Texture { descriptor: SurfaceTextureDescriptor::Native { mut id, .. }, .. }) = self.surface.take() {
                if let Some(id) = id.take() {
                    state.resource_cache.destroy_compositor_tile(id);
                }
            }

            self.is_visible = false;
            return false;
        }

        // Check if this tile can be considered opaque. Opacity state must be updated only
        // after all early out checks have been performed. Otherwise, we might miss updating
        // the native surface next time this tile becomes visible.
        let clipped_rect = self.current_descriptor.local_valid_rect
            .intersection(&ctx.local_clip_rect)
            .unwrap_or_else(PictureRect::zero);
        let mut is_opaque = ctx.backdrop.opaque_rect.contains_rect(&clipped_rect);

        if self.has_compositor_surface {
            // If we found primitive(s) that are ordered _after_ the first compositor
            // surface, _and_ intersect with any compositor surface, then we will need
            // to draw this tile with alpha blending, as an overlay to the compositor surface.
            let fg_world_valid_rect = ctx.pic_to_world_mapper
                .map(&self.fg_local_valid_rect.to_rect())
                .expect("bug: map fg local valid rect");
            let fg_device_valid_rect = fg_world_valid_rect * ctx.global_device_pixel_scale;

            for surface in ctx.external_surfaces {
                if surface.device_rect.intersects(&fg_device_valid_rect) {
                    is_opaque = false;
                    break;
                }
            }
        }

        // Set the correct z_id for this tile based on opacity
        if is_opaque {
            self.z_id = ctx.z_id_opaque;
        } else {
            self.z_id = ctx.z_id_alpha;
        }

        if is_opaque != self.is_opaque {
            // If opacity changed, the native compositor surface and all tiles get invalidated.
            // (this does nothing if not using native compositor mode).
            // TODO(gw): This property probably changes very rarely, so it is OK to invalidate
            //           everything in this case. If it turns out that this isn't true, we could
            //           consider other options, such as per-tile opacity (natively supported
            //           on CoreAnimation, and supported if backed by non-virtual surfaces in
            //           DirectComposition).
            if let Some(TileSurface::Texture { descriptor: SurfaceTextureDescriptor::Native { ref mut id, .. }, .. }) = self.surface {
                if let Some(id) = id.take() {
                    state.resource_cache.destroy_compositor_tile(id);
                }
            }

            // Invalidate the entire tile to force a redraw.
            self.invalidate(None, InvalidationReason::SurfaceOpacityChanged { became_opaque: is_opaque });
            self.is_opaque = is_opaque;
        }

        // Check if the selected composite mode supports dirty rect updates. For Draw composite
        // mode, we can always update the content with smaller dirty rects, unless there is a
        // driver bug to workaround. For native composite mode, we can only use dirty rects if
        // the compositor supports partial surface updates.
        let (supports_dirty_rects, supports_simple_prims) = match state.composite_state.compositor_kind {
            CompositorKind::Draw { .. } => {
                (frame_context.config.gpu_supports_render_target_partial_update, true)
            }
            CompositorKind::Native { max_update_rects, .. } => {
                (max_update_rects > 0, false)
            }
        };

        // TODO(gw): Consider using smaller tiles and/or tile splits for
        //           native compositors that don't support dirty rects.
        if supports_dirty_rects {
            // Only allow splitting for normal content sized tiles
            if ctx.current_tile_size == state.resource_cache.texture_cache.default_picture_tile_size() {
                let max_split_level = 3;

                // Consider splitting / merging dirty regions
                self.root.maybe_merge_or_split(
                    0,
                    &self.current_descriptor.prims,
                    max_split_level,
                );
            }
        }

        // The dirty rect will be set correctly by now. If the underlying platform
        // doesn't support partial updates, and this tile isn't valid, force the dirty
        // rect to be the size of the entire tile.
        if !self.is_valid && !supports_dirty_rects {
            self.local_dirty_rect = self.local_tile_rect;
        }

        // See if this tile is a simple color, in which case we can just draw
        // it as a rect, and avoid allocating a texture surface and drawing it.
        // TODO(gw): Initial native compositor interface doesn't support simple
        //           color tiles. We can definitely support this in DC, so this
        //           should be added as a follow up.
        let is_simple_prim =
            ctx.backdrop.kind.is_some() &&
            self.current_descriptor.prims.len() == 1 &&
            self.is_opaque &&
            supports_simple_prims;

        // Set up the backing surface for this tile.
        let surface = if is_simple_prim {
            // If we determine the tile can be represented by a color, set the
            // surface unconditionally (this will drop any previously used
            // texture cache backing surface).
            match ctx.backdrop.kind {
                Some(BackdropKind::Color { color }) => {
                    TileSurface::Color {
                        color,
                    }
                }
                Some(BackdropKind::Clear) => {
                    TileSurface::Clear
                }
                None => {
                    // This should be prevented by the is_simple_prim check above.
                    unreachable!();
                }
            }
        } else {
            // If this tile will be backed by a surface, we want to retain
            // the texture handle from the previous frame, if possible. If
            // the tile was previously a color, or not set, then just set
            // up a new texture cache handle.
            match self.surface.take() {
                Some(TileSurface::Texture { descriptor, visibility_mask }) => {
                    // Reuse the existing descriptor and vis mask
                    TileSurface::Texture {
                        descriptor,
                        visibility_mask,
                    }
                }
                Some(TileSurface::Color { .. }) | Some(TileSurface::Clear) | None => {
                    // This is the case where we are constructing a tile surface that
                    // involves drawing to a texture. Create the correct surface
                    // descriptor depending on the compositing mode that will read
                    // the output.
                    let descriptor = match state.composite_state.compositor_kind {
                        CompositorKind::Draw { .. } => {
                            // For a texture cache entry, create an invalid handle that
                            // will be allocated when update_picture_cache is called.
                            SurfaceTextureDescriptor::TextureCache {
                                handle: TextureCacheHandle::invalid(),
                            }
                        }
                        CompositorKind::Native { .. } => {
                            // Create a native surface surface descriptor, but don't allocate
                            // a surface yet. The surface is allocated *after* occlusion
                            // culling occurs, so that only visible tiles allocate GPU memory.
                            SurfaceTextureDescriptor::Native {
                                id: None,
                            }
                        }
                    };

                    TileSurface::Texture {
                        descriptor,
                        visibility_mask: PrimitiveVisibilityMask::empty(),
                    }
                }
            }
        };

        // Store the current surface backing info for use during batching.
        self.surface = Some(surface);

        true
    }
}

/// Defines a key that uniquely identifies a primitive instance.
#[derive(Debug, Copy, Clone)]
#[cfg_attr(feature = "capture", derive(Serialize))]
#[cfg_attr(feature = "replay", derive(Deserialize))]
pub struct PrimitiveDescriptor {
    /// Uniquely identifies the content of the primitive template.
    pub prim_uid: ItemUid,
    /// The clip rect for this primitive. Included here in
    /// dependencies since there is no entry in the clip chain
    /// dependencies for the local clip rect.
    pub prim_clip_box: PictureBox2D,
    /// The number of extra dependencies that this primitive has.
    transform_dep_count: u8,
    image_dep_count: u8,
    opacity_binding_dep_count: u8,
    clip_dep_count: u8,
    color_binding_dep_count: u8,
}

impl PartialEq for PrimitiveDescriptor {
    fn eq(&self, other: &Self) -> bool {
        const EPSILON: f32 = 0.001;

        if self.prim_uid != other.prim_uid {
            return false;
        }

        if !self.prim_clip_box.min.x.approx_eq_eps(&other.prim_clip_box.min.x, &EPSILON) {
            return false;
        }
        if !self.prim_clip_box.min.y.approx_eq_eps(&other.prim_clip_box.min.y, &EPSILON) {
            return false;
        }
        if !self.prim_clip_box.max.x.approx_eq_eps(&other.prim_clip_box.max.x, &EPSILON) {
            return false;
        }
        if !self.prim_clip_box.max.y.approx_eq_eps(&other.prim_clip_box.max.y, &EPSILON) {
            return false;
        }

        true
    }
}

/// A small helper to compare two arrays of primitive dependencies.
struct CompareHelper<'a, T> where T: Copy {
    offset_curr: usize,
    offset_prev: usize,
    curr_items: &'a [T],
    prev_items: &'a [T],
}

impl<'a, T> CompareHelper<'a, T> where T: Copy + PartialEq {
    /// Construct a new compare helper for a current / previous set of dependency information.
    fn new(
        prev_items: &'a [T],
        curr_items: &'a [T],
    ) -> Self {
        CompareHelper {
            offset_curr: 0,
            offset_prev: 0,
            curr_items,
            prev_items,
        }
    }

    /// Reset the current position in the dependency array to the start
    fn reset(&mut self) {
        self.offset_prev = 0;
        self.offset_curr = 0;
    }

    /// Test if two sections of the dependency arrays are the same, by checking both
    /// item equality, and a user closure to see if the content of the item changed.
    fn is_same<F>(
        &self,
        prev_count: u8,
        curr_count: u8,
        mut f: F,
        opt_detail: Option<&mut CompareHelperResult<T>>,
    ) -> bool where F: FnMut(&T, &T) -> bool {
        // If the number of items is different, trivial reject.
        if prev_count != curr_count {
            if let Some(detail) = opt_detail { *detail = CompareHelperResult::Count{ prev_count, curr_count }; }
            return false;
        }
        // If both counts are 0, then no need to check these dependencies.
        if curr_count == 0 {
            if let Some(detail) = opt_detail { *detail = CompareHelperResult::Equal; }
            return true;
        }
        // If both counts are u8::MAX, this is a sentinel that we can't compare these
        // deps, so just trivial reject.
        if curr_count as usize == MAX_PRIM_SUB_DEPS {
            if let Some(detail) = opt_detail { *detail = CompareHelperResult::Sentinel; }
            return false;
        }

        let end_prev = self.offset_prev + prev_count as usize;
        let end_curr = self.offset_curr + curr_count as usize;

        let curr_items = &self.curr_items[self.offset_curr .. end_curr];
        let prev_items = &self.prev_items[self.offset_prev .. end_prev];

        for (curr, prev) in curr_items.iter().zip(prev_items.iter()) {
            if !f(prev, curr) {
                if let Some(detail) = opt_detail { *detail = CompareHelperResult::PredicateTrue{ curr: *curr }; }
                return false;
            }
        }

        if let Some(detail) = opt_detail { *detail = CompareHelperResult::Equal; }
        true
    }

    // Advance the prev dependency array by a given amount
    fn advance_prev(&mut self, count: u8) {
        self.offset_prev += count as usize;
    }

    // Advance the current dependency array by a given amount
    fn advance_curr(&mut self, count: u8) {
        self.offset_curr  += count as usize;
    }
}

/// Uniquely describes the content of this tile, in a way that can be
/// (reasonably) efficiently hashed and compared.
#[cfg_attr(any(feature="capture",feature="replay"), derive(Clone))]
#[cfg_attr(feature = "capture", derive(Serialize))]
#[cfg_attr(feature = "replay", derive(Deserialize))]
pub struct TileDescriptor {
    /// List of primitive instance unique identifiers. The uid is guaranteed
    /// to uniquely describe the content of the primitive template, while
    /// the other parameters describe the clip chain and instance params.
    pub prims: Vec<PrimitiveDescriptor>,

    /// List of clip node descriptors.
    clips: Vec<ItemUid>,

    /// List of image keys that this tile depends on.
    images: Vec<ImageDependency>,

    /// The set of opacity bindings that this tile depends on.
    // TODO(gw): Ugh, get rid of all opacity binding support!
    opacity_bindings: Vec<OpacityBinding>,

    /// List of the effects of transforms that we care about
    /// tracking for this tile.
    transforms: Vec<SpatialNodeKey>,

    /// Picture space rect that contains valid pixels region of this tile.
    local_valid_rect: PictureRect,

    /// List of the effects of color that we care about
    /// tracking for this tile.
    color_bindings: Vec<ColorBinding>,
}

impl TileDescriptor {
    fn new() -> Self {
        TileDescriptor {
            prims: Vec::new(),
            clips: Vec::new(),
            opacity_bindings: Vec::new(),
            images: Vec::new(),
            transforms: Vec::new(),
            local_valid_rect: PictureRect::zero(),
            color_bindings: Vec::new(),
        }
    }

    /// Print debug information about this tile descriptor to a tree printer.
    fn print(&self, pt: &mut dyn PrintTreePrinter) {
        pt.new_level("current_descriptor".to_string());

        pt.new_level("prims".to_string());
        for prim in &self.prims {
            pt.new_level(format!("prim uid={}", prim.prim_uid.get_uid()));
            pt.add_item(format!("clip: p0={},{} p1={},{}",
                prim.prim_clip_box.min.x,
                prim.prim_clip_box.min.y,
                prim.prim_clip_box.max.x,
                prim.prim_clip_box.max.y,
            ));
            pt.add_item(format!("deps: t={} i={} o={} c={} color={}",
                prim.transform_dep_count,
                prim.image_dep_count,
                prim.opacity_binding_dep_count,
                prim.clip_dep_count,
                prim.color_binding_dep_count,
            ));
            pt.end_level();
        }
        pt.end_level();

        if !self.clips.is_empty() {
            pt.new_level("clips".to_string());
            for clip in &self.clips {
                pt.new_level(format!("clip uid={}", clip.get_uid()));
                pt.end_level();
            }
            pt.end_level();
        }

        if !self.images.is_empty() {
            pt.new_level("images".to_string());
            for info in &self.images {
                pt.new_level(format!("key={:?}", info.key));
                pt.add_item(format!("generation={:?}", info.generation));
                pt.end_level();
            }
            pt.end_level();
        }

        if !self.opacity_bindings.is_empty() {
            pt.new_level("opacity_bindings".to_string());
            for opacity_binding in &self.opacity_bindings {
                pt.new_level(format!("binding={:?}", opacity_binding));
                pt.end_level();
            }
            pt.end_level();
        }

        if !self.transforms.is_empty() {
            pt.new_level("transforms".to_string());
            for transform in &self.transforms {
                pt.new_level(format!("spatial_node={:?}", transform));
                pt.end_level();
            }
            pt.end_level();
        }

        if !self.color_bindings.is_empty() {
            pt.new_level("color_bindings".to_string());
            for color_binding in &self.color_bindings {
                pt.new_level(format!("binding={:?}", color_binding));
                pt.end_level();
            }
            pt.end_level();
        }

        pt.end_level();
    }

    /// Clear the dependency information for a tile, when the dependencies
    /// are being rebuilt.
    fn clear(&mut self) {
        self.prims.clear();
        self.clips.clear();
        self.opacity_bindings.clear();
        self.images.clear();
        self.transforms.clear();
        self.local_valid_rect = PictureRect::zero();
        self.color_bindings.clear();
    }
}

/// Stores both the world and devices rects for a single dirty rect.
#[derive(Debug, Clone)]
pub struct DirtyRegionRect {
    /// Bitfield for picture render tasks that draw this dirty region.
    pub visibility_mask: PrimitiveVisibilityMask,
    /// The dirty region in space of the picture cache
    pub rect_in_pic_space: PictureRect,
}

/// Represents the dirty region of a tile cache picture.
#[derive(Debug, Clone)]
pub struct DirtyRegion {
    /// The individual dirty rects of this region.
    pub dirty_rects: Vec<DirtyRegionRect>,

    /// The overall dirty rect, a combination of dirty_rects
    pub combined: WorldRect,

    /// Spatial node of the picture cache this region represents
    spatial_node_index: SpatialNodeIndex,
}

impl DirtyRegion {
    /// Construct a new dirty region tracker.
    pub fn new(
        spatial_node_index: SpatialNodeIndex,
    ) -> Self {
        DirtyRegion {
            dirty_rects: Vec::with_capacity(PrimitiveVisibilityMask::MAX_DIRTY_REGIONS),
            combined: WorldRect::zero(),
            spatial_node_index,
        }
    }

    /// Reset the dirty regions back to empty
    pub fn reset(
        &mut self,
        spatial_node_index: SpatialNodeIndex,
    ) {
        self.dirty_rects.clear();
        self.combined = WorldRect::zero();
        self.spatial_node_index = spatial_node_index;
    }

    /// Add a dirty region to the tracker. Returns the visibility mask that corresponds to
    /// this region in the tracker.
    pub fn add_dirty_region(
        &mut self,
        rect_in_pic_space: PictureRect,
        spatial_tree: &SpatialTree,
    ) -> PrimitiveVisibilityMask {
        let map_pic_to_world = SpaceMapper::new_with_target(
            ROOT_SPATIAL_NODE_INDEX,
            self.spatial_node_index,
            WorldRect::max_rect(),
            spatial_tree,
        );

        let world_rect = map_pic_to_world
            .map(&rect_in_pic_space)
            .expect("bug");

        // Include this in the overall dirty rect
        self.combined = self.combined.union(&world_rect);

        let dirty_region_index = self.dirty_rects.len();
        let mut visibility_mask = PrimitiveVisibilityMask::empty();

        if dirty_region_index < PrimitiveVisibilityMask::MAX_DIRTY_REGIONS {
            visibility_mask.set_visible(dirty_region_index);

            self.dirty_rects.push(DirtyRegionRect {
                visibility_mask,
                rect_in_pic_space,
            });
        } else {
            // If we run out of dirty regions, then force the last dirty region to
            // be a union of any remaining regions. This is an inefficiency, in that
            // we'll add items to batches later on that are redundant / outside this
            // tile, but it's really rare except in pathological cases (even on a
            // 4k screen, the typical dirty region count is < 16).
            visibility_mask.set_visible(PrimitiveVisibilityMask::MAX_DIRTY_REGIONS - 1);

            let combined_region = self.dirty_rects.last_mut().unwrap();
            combined_region.rect_in_pic_space = combined_region.rect_in_pic_space.union(&rect_in_pic_space);
        }

        visibility_mask
    }

    // TODO(gw): This returns a heap allocated object. Perhaps we can simplify this
    //           logic? Although - it's only used very rarely so it may not be an issue.
    pub fn inflate(
        &self,
        inflate_amount: f32,
        spatial_tree: &SpatialTree,
    ) -> DirtyRegion {
        let map_pic_to_world = SpaceMapper::new_with_target(
            ROOT_SPATIAL_NODE_INDEX,
            self.spatial_node_index,
            WorldRect::max_rect(),
            spatial_tree,
        );

        let mut dirty_rects = Vec::with_capacity(self.dirty_rects.len());
        let mut combined = WorldRect::zero();

        for rect in &self.dirty_rects {
            let rect_in_pic_space = rect.rect_in_pic_space.inflate(inflate_amount, inflate_amount);

            let world_rect = map_pic_to_world
                .map(&rect_in_pic_space)
                .expect("bug");

            combined = combined.union(&world_rect);
            dirty_rects.push(DirtyRegionRect {
                visibility_mask: rect.visibility_mask,
                rect_in_pic_space,
            });
        }

        DirtyRegion {
            dirty_rects,
            combined,
            spatial_node_index: self.spatial_node_index,
        }
    }
}

#[derive(Debug, Copy, Clone)]
pub enum BackdropKind {
    Color {
        color: ColorF,
    },
    Clear,
}

/// Stores information about the calculated opaque backdrop of this slice.
#[derive(Debug, Copy, Clone)]
pub struct BackdropInfo {
    /// The picture space rectangle that is known to be opaque. This is used
    /// to determine where subpixel AA can be used, and where alpha blending
    /// can be disabled.
    pub opaque_rect: PictureRect,
    /// Kind of the backdrop
    pub kind: Option<BackdropKind>,
}

impl BackdropInfo {
    fn empty() -> Self {
        BackdropInfo {
            opaque_rect: PictureRect::zero(),
            kind: None,
        }
    }
}

#[derive(Clone)]
pub struct TileCacheLoggerSlice {
    pub serialized_slice: String,
    pub local_to_world_transform: Transform3D<f32, PicturePixel, WorldPixel>,
}

#[cfg(any(feature = "capture", feature = "replay"))]
macro_rules! declare_tile_cache_logger_updatelists {
    ( $( $name:ident : $ty:ty, )+ ) => {
        #[cfg_attr(feature = "capture", derive(Serialize))]
        #[cfg_attr(feature = "replay", derive(Deserialize))]
        struct TileCacheLoggerUpdateListsSerializer {
            pub ron_string: Vec<String>,
        }

        pub struct TileCacheLoggerUpdateLists {
            $(
                /// Generate storage, one per interner.
                /// the tuple is a workaround to avoid the need for multiple
                /// fields that start with $name (macro concatenation).
                /// the string is .ron serialized updatelist at capture time;
                /// the updates is the list of DataStore updates (avoid UpdateList
                /// due to Default() requirements on the Keys) reconstructed at
                /// load time.
                pub $name: (Vec<String>, Vec<UpdateList<<$ty as Internable>::Key>>),
            )+
        }

        impl TileCacheLoggerUpdateLists {
            pub fn new() -> Self {
                TileCacheLoggerUpdateLists {
                    $(
                        $name : ( Vec::new(), Vec::new() ),
                    )+
                }
            }

            /// serialize all interners in updates to .ron
            #[cfg(feature = "capture")]
            fn serialize_updates(
                &mut self,
                updates: &InternerUpdates
            ) {
                $(
                    self.$name.0.push(ron::ser::to_string_pretty(&updates.$name, Default::default()).unwrap());
                )+
            }

            fn is_empty(&self) -> bool {
                $(
                    if !self.$name.0.is_empty() { return false; }
                )+
                true
            }

            #[cfg(feature = "capture")]
            fn to_ron(&self) -> String {
                let mut serializer =
                    TileCacheLoggerUpdateListsSerializer { ron_string: Vec::new() };
                $(
                    serializer.ron_string.push(
                        ron::ser::to_string_pretty(&self.$name.0, Default::default()).unwrap());
                )+
                ron::ser::to_string_pretty(&serializer, Default::default()).unwrap()
            }

            #[cfg(feature = "replay")]
            pub fn from_ron(&mut self, text: &str) {
                let serializer : TileCacheLoggerUpdateListsSerializer =
                    match ron::de::from_str(&text) {
                        Ok(data) => { data }
                        Err(e) => {
                            println!("ERROR: failed to deserialize updatelist: {:?}\n{:?}", &text, e);
                            return;
                        }
                    };
                let mut index = 0;
                $(
                    let ron_lists : Vec<String> = ron::de::from_str(&serializer.ron_string[index]).unwrap();
                    self.$name.1 = ron_lists.iter()
                                            .map( |list| ron::de::from_str(&list).unwrap() )
                                            .collect();
                    index = index + 1;
                )+
                // error: value assigned to `index` is never read
                let _ = index;
            }

            /// helper method to add a stringified version of all interned keys into
            /// a lookup table based on ItemUid.  Use strings as a form of type erasure
            /// so all UpdateLists can go into a single map.
            /// Then during analysis, when we see an invalidation reason due to
            /// "ItemUid such and such was added to the tile primitive list", the lookup
            /// allows mapping that back into something readable.
            #[cfg(feature = "replay")]
            pub fn insert_in_lookup(
                        &mut self,
                        itemuid_to_string: &mut HashMap<ItemUid, String>)
            {
                $(
                    {
                        for list in &self.$name.1 {
                            for insertion in &list.insertions {
                                itemuid_to_string.insert(
                                    insertion.uid,
                                    format!("{:?}", insertion.value));
                            }
                        }
                    }
                )+
            }
        }
    }
}

#[cfg(any(feature = "capture", feature = "replay"))]
crate::enumerate_interners!(declare_tile_cache_logger_updatelists);

#[cfg(not(any(feature = "capture", feature = "replay")))]
pub struct TileCacheLoggerUpdateLists {
}

#[cfg(not(any(feature = "capture", feature = "replay")))]
impl TileCacheLoggerUpdateLists {
    pub fn new() -> Self { TileCacheLoggerUpdateLists {} }
    fn is_empty(&self) -> bool { true }
}

/// Log tile cache activity for one single frame.
/// Also stores the commands sent to the interning data_stores
/// so we can see which items were created or destroyed this frame,
/// and correlate that with tile invalidation activity.
pub struct TileCacheLoggerFrame {
    /// slices in the frame, one per take_context call
    pub slices: Vec<TileCacheLoggerSlice>,
    /// interning activity
    pub update_lists: TileCacheLoggerUpdateLists
}

impl TileCacheLoggerFrame {
    pub fn new() -> Self {
        TileCacheLoggerFrame {
            slices: Vec::new(),
            update_lists: TileCacheLoggerUpdateLists::new()
        }
    }

    pub fn is_empty(&self) -> bool {
        self.slices.is_empty() && self.update_lists.is_empty()
    }
}

/// Log tile cache activity whenever anything happens in take_context.
pub struct TileCacheLogger {
    /// next write pointer
    pub write_index : usize,
    /// ron serialization of tile caches;
    pub frames: Vec<TileCacheLoggerFrame>
}

impl TileCacheLogger {
    pub fn new(
        num_frames: usize
    ) -> Self {
        let mut frames = Vec::with_capacity(num_frames);
        for _i in 0..num_frames { // no Clone so no resize
            frames.push(TileCacheLoggerFrame::new());
        }
        TileCacheLogger {
            write_index: 0,
            frames
        }
    }

    pub fn is_enabled(&self) -> bool {
        !self.frames.is_empty()
    }

    #[cfg(feature = "capture")]
    pub fn add(
            &mut self,
            serialized_slice: String,
            local_to_world_transform: Transform3D<f32, PicturePixel, WorldPixel>
    ) {
        if !self.is_enabled() {
            return;
        }
        self.frames[self.write_index].slices.push(
            TileCacheLoggerSlice {
                serialized_slice,
                local_to_world_transform });
    }

    #[cfg(feature = "capture")]
    pub fn serialize_updates(&mut self, updates: &InternerUpdates) {
        if !self.is_enabled() {
            return;
        }
        self.frames[self.write_index].update_lists.serialize_updates(updates);
    }

    /// see if anything was written in this frame, and if so,
    /// advance the write index in a circular way and clear the
    /// recorded string.
    pub fn advance(&mut self) {
        if !self.is_enabled() || self.frames[self.write_index].is_empty() {
            return;
        }
        self.write_index = self.write_index + 1;
        if self.write_index >= self.frames.len() {
            self.write_index = 0;
        }
        self.frames[self.write_index] = TileCacheLoggerFrame::new();
    }

    #[cfg(feature = "capture")]
    pub fn save_capture(
        &self, root: &PathBuf
    ) {
        if !self.is_enabled() {
            return;
        }
        use std::fs;

        info!("saving tile cache log");
        let path_tile_cache = root.join("tile_cache");
        if !path_tile_cache.is_dir() {
            fs::create_dir(&path_tile_cache).unwrap();
        }

        let mut files_written = 0;
        for ix in 0..self.frames.len() {
            // ...and start with write_index, since that's the oldest entry
            // that we're about to overwrite. However when we get to
            // save_capture, we've add()ed entries but haven't advance()d yet,
            // so the actual oldest entry is write_index + 1
            let index = (self.write_index + 1 + ix) % self.frames.len();
            if self.frames[index].is_empty() {
                continue;
            }

            let filename = path_tile_cache.join(format!("frame{:05}.ron", files_written));
            let mut output = File::create(filename).unwrap();
            output.write_all(b"// slice data\n").unwrap();
            output.write_all(b"[\n").unwrap();
            for item in &self.frames[index].slices {
                output.write_all(b"( transform:\n").unwrap();
                let transform =
                    ron::ser::to_string_pretty(
                        &item.local_to_world_transform, Default::default()).unwrap();
                output.write_all(transform.as_bytes()).unwrap();
                output.write_all(b",\n tile_cache:\n").unwrap();
                output.write_all(item.serialized_slice.as_bytes()).unwrap();
                output.write_all(b"\n),\n").unwrap();
            }
            output.write_all(b"]\n\n").unwrap();

            output.write_all(b"// @@@ chunk @@@\n\n").unwrap();

            output.write_all(b"// interning data\n").unwrap();
            output.write_all(self.frames[index].update_lists.to_ron().as_bytes()).unwrap();

            files_written = files_written + 1;
        }
    }
}

/// Represents the native surfaces created for a picture cache, if using
/// a native compositor. An opaque and alpha surface is always created,
/// but tiles are added to a surface based on current opacity. If the
/// calculated opacity of a tile changes, the tile is invalidated and
/// attached to a different native surface. This means that we don't
/// need to invalidate the entire surface if only some tiles are changing
/// opacity. It also means we can take advantage of opaque tiles on cache
/// slices where only some of the tiles are opaque. There is an assumption
/// that creating a native surface is cheap, and only when a tile is added
/// to a surface is there a significant cost. This assumption holds true
/// for the current native compositor implementations on Windows and Mac.
pub struct NativeSurface {
    /// Native surface for opaque tiles
    pub opaque: NativeSurfaceId,
    /// Native surface for alpha tiles
    pub alpha: NativeSurfaceId,
}

/// Hash key for an external native compositor surface
#[derive(PartialEq, Eq, Hash)]
pub struct ExternalNativeSurfaceKey {
    /// The YUV/RGB image keys that are used to draw this surface.
    pub image_keys: [ImageKey; 3],
    /// The current device size of the surface.
    pub size: DeviceIntSize,
    /// True if this is an 'external' compositor surface created via
    /// Compositor::create_external_surface.
    pub is_external_surface: bool,
}

/// Information about a native compositor surface cached between frames.
pub struct ExternalNativeSurface {
    /// If true, the surface was used this frame. Used for a simple form
    /// of GC to remove old surfaces.
    pub used_this_frame: bool,
    /// The native compositor surface handle
    pub native_surface_id: NativeSurfaceId,
    /// List of image keys, and current image generations, that are drawn in this surface.
    /// The image generations are used to check if the compositor surface is dirty and
    /// needs to be updated.
    pub image_dependencies: [ImageDependency; 3],
}

/// The key that identifies a tile cache instance. For now, it's simple the index of
/// the slice as it was created during scene building.
#[derive(Debug, Copy, Clone, PartialEq, Eq, Hash)]
#[cfg_attr(feature = "capture", derive(Serialize))]
#[cfg_attr(feature = "replay", derive(Deserialize))]
pub struct SliceId(usize);

impl SliceId {
    pub fn new(index: usize) -> Self {
        SliceId(index)
    }
}

/// Information that is required to reuse or create a new tile cache. Created
/// during scene building and passed to the render backend / frame builder.
pub struct TileCacheParams {
    // Index of the slice (also effectively the key of the tile cache, though we use SliceId where that matters)
    pub slice: usize,
    // Flags describing content of this cache (e.g. scrollbars)
    pub slice_flags: SliceFlags,
    // The anchoring spatial node / scroll root
    pub spatial_node_index: SpatialNodeIndex,
    // Optional background color of this tilecache. If present, can be used as an optimization
    // to enable opaque blending and/or subpixel AA in more places.
    pub background_color: Option<ColorF>,
    // List of clips shared by all prims that are promoted to this tile cache
    pub shared_clips: Vec<ClipInstance>,
    // The clip chain handle representing `shared_clips`
    pub shared_clip_chain: ClipChainId,
    // Virtual surface sizes are always square, so this represents both the width and height
    pub virtual_surface_size: i32,
}

/// Represents a cache of tiles that make up a picture primitives.
pub struct TileCacheInstance {
    /// Index of the tile cache / slice for this frame builder. It's determined
    /// by the setup_picture_caching method during flattening, which splits the
    /// picture tree into multiple slices. It's used as a simple input to the tile
    /// keys. It does mean we invalidate tiles if a new layer gets inserted / removed
    /// between display lists - this seems very unlikely to occur on most pages, but
    /// can be revisited if we ever notice that.
    pub slice: usize,
    /// Propagated information about the slice
    pub slice_flags: SliceFlags,
    /// The currently selected tile size to use for this cache
    pub current_tile_size: DeviceIntSize,
    /// The positioning node for this tile cache.
    pub spatial_node_index: SpatialNodeIndex,
    /// Hash of tiles present in this picture.
    pub tiles: FastHashMap<TileOffset, Box<Tile>>,
    /// A helper struct to map local rects into surface coords.
    map_local_to_surface: SpaceMapper<LayoutPixel, PicturePixel>,
    /// A helper struct to map child picture rects into picture cache surface coords.
    map_child_pic_to_surface: SpaceMapper<PicturePixel, PicturePixel>,
    /// List of opacity bindings, with some extra information
    /// about whether they changed since last frame.
    opacity_bindings: FastHashMap<PropertyBindingId, OpacityBindingInfo>,
    /// Switch back and forth between old and new bindings hashmaps to avoid re-allocating.
    old_opacity_bindings: FastHashMap<PropertyBindingId, OpacityBindingInfo>,
    /// A helper to compare transforms between previous and current frame.
    spatial_node_comparer: SpatialNodeComparer,
    /// List of color bindings, with some extra information
    /// about whether they changed since last frame.
    color_bindings: FastHashMap<PropertyBindingId, ColorBindingInfo>,
    /// Switch back and forth between old and new bindings hashmaps to avoid re-allocating.
    old_color_bindings: FastHashMap<PropertyBindingId, ColorBindingInfo>,
    /// The current dirty region tracker for this picture.
    pub dirty_region: DirtyRegion,
    /// Current size of tiles in picture units.
    tile_size: PictureSize,
    /// Tile coords of the currently allocated grid.
    tile_rect: TileRect,
    /// Pre-calculated versions of the tile_rect above, used to speed up the
    /// calculations in get_tile_coords_for_rect.
    tile_bounds_p0: TileOffset,
    tile_bounds_p1: TileOffset,
    /// Local rect (unclipped) of the picture this cache covers.
    pub local_rect: PictureRect,
    /// The local clip rect, from the shared clips of this picture.
    pub local_clip_rect: PictureRect,
    /// The surface index that this tile cache will be drawn into.
    surface_index: SurfaceIndex,
    /// The background color from the renderer. If this is set opaque, we know it's
    /// fine to clear the tiles to this and allow subpixel text on the first slice.
    pub background_color: Option<ColorF>,
    /// Information about the calculated backdrop content of this cache.
    pub backdrop: BackdropInfo,
    /// The allowed subpixel mode for this surface, which depends on the detected
    /// opacity of the background.
    pub subpixel_mode: SubpixelMode,
    /// A list of clip handles that exist on every (top-level) primitive in this picture.
    /// It's often the case that these are root / fixed position clips. By handling them
    /// here, we can avoid applying them to the items, which reduces work, but more importantly
    /// reduces invalidations.
    pub shared_clips: Vec<ClipInstance>,
    /// The clip chain that represents the shared_clips above. Used to build the local
    /// clip rect for this tile cache.
    shared_clip_chain: ClipChainId,
    /// The current transform of the picture cache root spatial node
    root_transform: ScaleOffset,
    /// The number of frames until this cache next evaluates what tile size to use.
    /// If a picture rect size is regularly changing just around a size threshold,
    /// we don't want to constantly invalidate and reallocate different tile size
    /// configuration each frame.
    frames_until_size_eval: usize,
    /// The current fractional offset of the cached picture
    fract_offset: PictureVector2D,
    /// The current device fractional offset of the cached picture
    device_fract_offset: DeviceVector2D,
    /// For DirectComposition, virtual surfaces don't support negative coordinates. However,
    /// picture cache tile coordinates can be negative. To handle this, we apply an offset
    /// to each tile in DirectComposition. We want to change this as little as possible,
    /// to avoid invalidating tiles. However, if we have a picture cache tile coordinate
    /// which is outside the virtual surface bounds, we must change this to allow
    /// correct remapping of the coordinates passed to BeginDraw in DC.
    virtual_offset: DeviceIntPoint,
    /// keep around the hash map used as compare_cache to avoid reallocating it each
    /// frame.
    compare_cache: FastHashMap<PrimitiveComparisonKey, PrimitiveCompareResult>,
    /// The allocated compositor surfaces for this picture cache. May be None if
    /// not using native compositor, or if the surface was destroyed and needs
    /// to be reallocated next time this surface contains valid tiles.
    pub native_surface: Option<NativeSurface>,
    /// The current device position of this cache. Used to set the compositor
    /// offset of the surface when building the visual tree.
    pub device_position: DevicePoint,
    /// The currently considered tile size override. Used to check if we should
    /// re-evaluate tile size, even if the frame timer hasn't expired.
    tile_size_override: Option<DeviceIntSize>,
    /// List of external surfaces that have been promoted from primitives
    /// in this tile cache.
    pub external_surfaces: Vec<ExternalSurfaceDescriptor>,
    /// z-buffer ID assigned to opaque tiles in this slice
    pub z_id_opaque: ZBufferId,
    /// A cache of compositor surfaces that are retained between frames
    pub external_native_surface_cache: FastHashMap<ExternalNativeSurfaceKey, ExternalNativeSurface>,
    /// Current frame ID of this tile cache instance. Used for book-keeping / garbage collecting
    frame_id: FrameId,
}

enum SurfacePromotionResult {
    Failed,
    Success {
        flip_y: bool,
    }
}

impl TileCacheInstance {
    pub fn new(params: TileCacheParams) -> Self {
        TileCacheInstance {
            slice: params.slice,
            slice_flags: params.slice_flags,
            spatial_node_index: params.spatial_node_index,
            tiles: FastHashMap::default(),
            map_local_to_surface: SpaceMapper::new(
                ROOT_SPATIAL_NODE_INDEX,
                PictureRect::zero(),
            ),
            map_child_pic_to_surface: SpaceMapper::new(
                ROOT_SPATIAL_NODE_INDEX,
                PictureRect::zero(),
            ),
            opacity_bindings: FastHashMap::default(),
            old_opacity_bindings: FastHashMap::default(),
            spatial_node_comparer: SpatialNodeComparer::new(),
            color_bindings: FastHashMap::default(),
            old_color_bindings: FastHashMap::default(),
            dirty_region: DirtyRegion::new(params.spatial_node_index),
            tile_size: PictureSize::zero(),
            tile_rect: TileRect::zero(),
            tile_bounds_p0: TileOffset::zero(),
            tile_bounds_p1: TileOffset::zero(),
            local_rect: PictureRect::zero(),
            local_clip_rect: PictureRect::zero(),
            surface_index: SurfaceIndex(0),
            background_color: params.background_color,
            backdrop: BackdropInfo::empty(),
            subpixel_mode: SubpixelMode::Allow,
            root_transform: ScaleOffset::identity(),
            shared_clips: params.shared_clips,
            shared_clip_chain: params.shared_clip_chain,
            current_tile_size: DeviceIntSize::zero(),
            frames_until_size_eval: 0,
            fract_offset: PictureVector2D::zero(),
            device_fract_offset: DeviceVector2D::zero(),
            // Default to centering the virtual offset in the middle of the DC virtual surface
            virtual_offset: DeviceIntPoint::new(
                params.virtual_surface_size / 2,
                params.virtual_surface_size / 2,
            ),
            compare_cache: FastHashMap::default(),
            native_surface: None,
            device_position: DevicePoint::zero(),
            tile_size_override: None,
            external_surfaces: Vec::new(),
            z_id_opaque: ZBufferId::invalid(),
            external_native_surface_cache: FastHashMap::default(),
            frame_id: FrameId::INVALID,
        }
    }

    /// Reset this tile cache with the updated parameters from a new scene
    /// that has arrived. This allows the tile cache to be retained across
    /// new scenes.
    pub fn prepare_for_new_scene(
        &mut self,
        params: TileCacheParams,
    ) {
        // We should only receive updated state for matching slice key
        assert_eq!(self.slice, params.slice);

        // Store the parameters from the scene builder for this slice. Other
        // params in the tile cache are retained and reused, or are always
        // updated during pre/post_update.
        self.slice_flags = params.slice_flags;
        self.spatial_node_index = params.spatial_node_index;
        self.background_color = params.background_color;
        self.shared_clips = params.shared_clips;
        self.shared_clip_chain = params.shared_clip_chain;

        // Since the slice flags may have changed, ensure we re-evaluate the
        // appropriate tile size for this cache next update.
        self.frames_until_size_eval = 0;
    }

    /// Destroy any manually managed resources before this picture cache is
    /// destroyed, such as native compositor surfaces.
    pub fn destroy(
        self,
        resource_cache: &mut ResourceCache,
    ) {
        if let Some(native_surface) = self.native_surface {
            resource_cache.destroy_compositor_surface(native_surface.opaque);
            resource_cache.destroy_compositor_surface(native_surface.alpha);
        }

        for (_, external_surface) in self.external_native_surface_cache {
            resource_cache.destroy_compositor_surface(external_surface.native_surface_id)
        }
    }

    /// Returns true if this tile cache is considered opaque.
    pub fn is_opaque(&self) -> bool {
        // If known opaque due to background clear color and being the first slice.
        // The background_color will only be Some(..) if this is the first slice.
        match self.background_color {
            Some(color) => color.a >= 1.0,
            None => false
        }
    }

    /// Get the tile coordinates for a given rectangle.
    fn get_tile_coords_for_rect(
        &self,
        rect: &PictureRect,
    ) -> (TileOffset, TileOffset) {
        // Get the tile coordinates in the picture space.
        let mut p0 = TileOffset::new(
            (rect.origin.x / self.tile_size.width).floor() as i32,
            (rect.origin.y / self.tile_size.height).floor() as i32,
        );

        let mut p1 = TileOffset::new(
            ((rect.origin.x + rect.size.width) / self.tile_size.width).ceil() as i32,
            ((rect.origin.y + rect.size.height) / self.tile_size.height).ceil() as i32,
        );

        // Clamp the tile coordinates here to avoid looping over irrelevant tiles later on.
        p0.x = clamp(p0.x, self.tile_bounds_p0.x, self.tile_bounds_p1.x);
        p0.y = clamp(p0.y, self.tile_bounds_p0.y, self.tile_bounds_p1.y);
        p1.x = clamp(p1.x, self.tile_bounds_p0.x, self.tile_bounds_p1.x);
        p1.y = clamp(p1.y, self.tile_bounds_p0.y, self.tile_bounds_p1.y);

        (p0, p1)
    }

    /// Update transforms, opacity, color bindings and tile rects.
    pub fn pre_update(
        &mut self,
        pic_rect: PictureRect,
        surface_index: SurfaceIndex,
        frame_context: &FrameVisibilityContext,
        frame_state: &mut FrameVisibilityState,
    ) -> WorldRect {
        self.external_surfaces.clear();
        self.surface_index = surface_index;
        self.local_rect = pic_rect;
        self.local_clip_rect = PictureRect::max_rect();

        // Opaque surfaces get the first z_id. Compositor surfaces then get
        // allocated a z_id each. After all compositor surfaces are added,
        // then we allocate a z_id for alpha tiles.
        self.z_id_opaque = frame_state.composite_state.z_generator.next();

        // Reset the opaque rect + subpixel mode, as they are calculated
        // during the prim dependency checks.
        self.backdrop = BackdropInfo::empty();

        self.map_local_to_surface = SpaceMapper::new(
            self.spatial_node_index,
            pic_rect,
        );
        self.map_child_pic_to_surface = SpaceMapper::new(
            self.spatial_node_index,
            pic_rect,
        );

        let pic_to_world_mapper = SpaceMapper::new_with_target(
            ROOT_SPATIAL_NODE_INDEX,
            self.spatial_node_index,
            frame_context.global_screen_world_rect,
            frame_context.spatial_tree,
        );

        // If there is a valid set of shared clips, build a clip chain instance for this,
        // which will provide a local clip rect. This is useful for establishing things
        // like whether the backdrop rect supplied by Gecko can be considered opaque.
        if self.shared_clip_chain != ClipChainId::NONE {
            let shared_clips = &mut frame_state.scratch.picture.clip_chain_ids;
            shared_clips.clear();

            let mut current_clip_chain_id = self.shared_clip_chain;
            while current_clip_chain_id != ClipChainId::NONE {
                shared_clips.push(current_clip_chain_id);
                let clip_chain_node = &frame_state.clip_store.clip_chain_nodes[current_clip_chain_id.0 as usize];
                current_clip_chain_id = clip_chain_node.parent_clip_chain_id;
            }

            frame_state.clip_store.set_active_clips(
                LayoutRect::max_rect(),
                self.spatial_node_index,
                self.map_local_to_surface.ref_spatial_node_index,
                &shared_clips,
                frame_context.spatial_tree,
                &mut frame_state.data_stores.clip,
            );

            let clip_chain_instance = frame_state.clip_store.build_clip_chain_instance(
                pic_rect.cast_unit(),
                &self.map_local_to_surface,
                &pic_to_world_mapper,
                frame_context.spatial_tree,
                frame_state.gpu_cache,
                frame_state.resource_cache,
                frame_context.global_device_pixel_scale,
                &frame_context.global_screen_world_rect,
                &mut frame_state.data_stores.clip,
                true,
                false,
            );

            // Ensure that if the entire picture cache is clipped out, the local
            // clip rect is zero. This makes sure we don't register any occluders
            // that are actually off-screen.
            self.local_clip_rect = clip_chain_instance.map_or(PictureRect::zero(), |clip_chain_instance| {
                clip_chain_instance.pic_clip_rect
            });
        }

        // Advance the current frame ID counter for this picture cache (must be done
        // after any retained prev state is taken above).
        self.frame_id.advance();

        // Notify the spatial node comparer that a new frame has started, and the
        // current reference spatial node for this tile cache.
        self.spatial_node_comparer.next_frame(self.spatial_node_index);

        // At the start of the frame, step through each current compositor surface
        // and mark it as unused. Later, this is used to free old compositor surfaces.
        // TODO(gw): In future, we might make this more sophisticated - for example,
        //           retaining them for >1 frame if unused, or retaining them in some
        //           kind of pool to reduce future allocations.
        for external_native_surface in self.external_native_surface_cache.values_mut() {
            external_native_surface.used_this_frame = false;
        }

        // Only evaluate what tile size to use fairly infrequently, so that we don't end
        // up constantly invalidating and reallocating tiles if the picture rect size is
        // changing near a threshold value.
        if self.frames_until_size_eval == 0 ||
           self.tile_size_override != frame_context.config.tile_size_override {

            // Work out what size tile is appropriate for this picture cache.
            let desired_tile_size = match frame_context.config.tile_size_override {
                Some(tile_size_override) => {
                    tile_size_override
                }
                None => {
                    if self.slice_flags.contains(SliceFlags::IS_SCROLLBAR) {
                        if pic_rect.size.width <= pic_rect.size.height {
                            TILE_SIZE_SCROLLBAR_VERTICAL
                        } else {
                            TILE_SIZE_SCROLLBAR_HORIZONTAL
                        }
                    } else {
                        frame_state.resource_cache.texture_cache.default_picture_tile_size()
                    }
                }
            };

            // If the desired tile size has changed, then invalidate and drop any
            // existing tiles.
            if desired_tile_size != self.current_tile_size {
                // Destroy any native surfaces on the tiles that will be dropped due
                // to resizing.
                if let Some(native_surface) = self.native_surface.take() {
                    frame_state.resource_cache.destroy_compositor_surface(native_surface.opaque);
                    frame_state.resource_cache.destroy_compositor_surface(native_surface.alpha);
                }
                self.tiles.clear();
                self.tile_rect = TileRect::zero();
                self.current_tile_size = desired_tile_size;
            }

            // Reset counter until next evaluating the desired tile size. This is an
            // arbitrary value.
            self.frames_until_size_eval = 120;
            self.tile_size_override = frame_context.config.tile_size_override;
        }

        // Map an arbitrary point in picture space to world space, to work out
        // what the fractional translation is that's applied by this scroll root.
        // TODO(gw): I'm not 100% sure this is right. At least, in future, we should
        //           make a specific API for this, and/or enforce that the picture
        //           cache transform only includes scale and/or translation (we
        //           already ensure it doesn't have perspective).
        let world_origin = pic_to_world_mapper
            .map(&PictureRect::new(PicturePoint::zero(), PictureSize::new(1.0, 1.0)))
            .expect("bug: unable to map origin to world space")
            .origin;

        // Get the desired integer device coordinate
        let device_origin = world_origin * frame_context.global_device_pixel_scale;
        let desired_device_origin = device_origin.round();
        self.device_position = desired_device_origin;
        self.device_fract_offset = desired_device_origin - device_origin;

        // Unmap from device space to world space rect
        let ref_world_rect = WorldRect::new(
            desired_device_origin / frame_context.global_device_pixel_scale,
            WorldSize::new(1.0, 1.0),
        );

        // Unmap from world space to picture space; this should be the fractional offset
        // required in picture space to align in device space
        self.fract_offset = pic_to_world_mapper
            .unmap(&ref_world_rect)
            .expect("bug: unable to unmap ref world rect")
            .origin
            .to_vector();

        // Do a hacky diff of opacity binding values from the last frame. This is
        // used later on during tile invalidation tests.
        let current_properties = frame_context.scene_properties.float_properties();
        mem::swap(&mut self.opacity_bindings, &mut self.old_opacity_bindings);

        self.opacity_bindings.clear();
        for (id, value) in current_properties {
            let changed = match self.old_opacity_bindings.get(id) {
                Some(old_property) => !old_property.value.approx_eq(value),
                None => true,
            };
            self.opacity_bindings.insert(*id, OpacityBindingInfo {
                value: *value,
                changed,
            });
        }

        // Do a hacky diff of color binding values from the last frame. This is
        // used later on during tile invalidation tests.
        let current_properties = frame_context.scene_properties.color_properties();
        mem::swap(&mut self.color_bindings, &mut self.old_color_bindings);

        self.color_bindings.clear();
        for (id, value) in current_properties {
            let changed = match self.old_color_bindings.get(id) {
                Some(old_property) => old_property.value != (*value).into(),
                None => true,
            };
            self.color_bindings.insert(*id, ColorBindingInfo {
                value: (*value).into(),
                changed,
            });
        }

        let world_tile_size = WorldSize::new(
            self.current_tile_size.width as f32 / frame_context.global_device_pixel_scale.0,
            self.current_tile_size.height as f32 / frame_context.global_device_pixel_scale.0,
        );

        // We know that this is an exact rectangle, since we (for now) only support tile
        // caches where the scroll root is in the root coordinate system.
        let local_tile_rect = pic_to_world_mapper
            .unmap(&WorldRect::new(WorldPoint::zero(), world_tile_size))
            .expect("bug: unable to get local tile rect");

        self.tile_size = local_tile_rect.size;

        let screen_rect_in_pic_space = pic_to_world_mapper
            .unmap(&frame_context.global_screen_world_rect)
            .expect("unable to unmap screen rect");

        // Inflate the needed rect a bit, so that we retain tiles that we have drawn
        // but have just recently gone off-screen. This means that we avoid re-drawing
        // tiles if the user is scrolling up and down small amounts, at the cost of
        // a bit of extra texture memory.
        let desired_rect_in_pic_space = screen_rect_in_pic_space
            .inflate(0.0, 1.0 * self.tile_size.height);

        let needed_rect_in_pic_space = desired_rect_in_pic_space
            .intersection(&pic_rect)
            .unwrap_or_else(PictureRect::zero);

        let p0 = needed_rect_in_pic_space.origin;
        let p1 = needed_rect_in_pic_space.bottom_right();

        let x0 = (p0.x / local_tile_rect.size.width).floor() as i32;
        let x1 = (p1.x / local_tile_rect.size.width).ceil() as i32;

        let y0 = (p0.y / local_tile_rect.size.height).floor() as i32;
        let y1 = (p1.y / local_tile_rect.size.height).ceil() as i32;

        let x_tiles = x1 - x0;
        let y_tiles = y1 - y0;
        let new_tile_rect = TileRect::new(
            TileOffset::new(x0, y0),
            TileSize::new(x_tiles, y_tiles),
        );

        // Determine whether the current bounds of the tile grid will exceed the
        // bounds of the DC virtual surface, taking into account the current
        // virtual offset. If so, we need to invalidate all tiles, and set up
        // a new virtual offset, centered around the current tile grid.

        if let CompositorKind::Native { virtual_surface_size, .. } = frame_context.config.compositor_kind {
            // We only need to invalidate in this case if the underlying platform
            // uses virtual surfaces.
            if virtual_surface_size > 0 {
                // Get the extremities of the tile grid after virtual offset is applied
                let tx0 = self.virtual_offset.x + x0 * self.current_tile_size.width;
                let ty0 = self.virtual_offset.y + y0 * self.current_tile_size.height;
                let tx1 = self.virtual_offset.x + (x1+1) * self.current_tile_size.width;
                let ty1 = self.virtual_offset.y + (y1+1) * self.current_tile_size.height;

                let need_new_virtual_offset = tx0 < 0 ||
                                              ty0 < 0 ||
                                              tx1 >= virtual_surface_size ||
                                              ty1 >= virtual_surface_size;

                if need_new_virtual_offset {
                    // Calculate a new virtual offset, centered around the middle of the
                    // current tile grid. This means we won't need to invalidate and get
                    // a new offset for a long time!
                    self.virtual_offset = DeviceIntPoint::new(
                        (virtual_surface_size/2) - ((x0 + x1) / 2) * self.current_tile_size.width,
                        (virtual_surface_size/2) - ((y0 + y1) / 2) * self.current_tile_size.height,
                    );

                    // Invalidate all native tile surfaces. They will be re-allocated next time
                    // they are scheduled to be rasterized.
                    for tile in self.tiles.values_mut() {
                        if let Some(TileSurface::Texture { descriptor: SurfaceTextureDescriptor::Native { ref mut id, .. }, .. }) = tile.surface {
                            if let Some(id) = id.take() {
                                frame_state.resource_cache.destroy_compositor_tile(id);
                                tile.surface = None;
                                // Invalidate the entire tile to force a redraw.
                                // TODO(gw): Add a new invalidation reason for virtual offset changing
                                tile.invalidate(None, InvalidationReason::CompositorKindChanged);
                            }
                        }
                    }

                    // Destroy the native virtual surfaces. They will be re-allocated next time a tile
                    // that references them is scheduled to draw.
                    if let Some(native_surface) = self.native_surface.take() {
                        frame_state.resource_cache.destroy_compositor_surface(native_surface.opaque);
                        frame_state.resource_cache.destroy_compositor_surface(native_surface.alpha);
                    }
                }
            }
        }

        // Rebuild the tile grid if the picture cache rect has changed.
        if new_tile_rect != self.tile_rect {
            let mut old_tiles = mem::replace(&mut self.tiles, FastHashMap::default());
            self.tiles.reserve(new_tile_rect.size.area() as usize);

            for y in y0 .. y1 {
                for x in x0 .. x1 {
                    let key = TileOffset::new(x, y);
                    let tile = old_tiles
                        .remove(&key)
                        .unwrap_or_else(|| {
                            Box::new(Tile::new(key))
                        });
                    self.tiles.insert(key, tile);
                }
            }

            // When old tiles that remain after the loop, dirty rects are not valid.
            if !old_tiles.is_empty() {
                frame_state.composite_state.dirty_rects_are_valid = false;
            }

            // Any old tiles that remain after the loop above are going to be dropped. For
            // simple composite mode, the texture cache handle will expire and be collected
            // by the texture cache. For native compositor mode, we need to explicitly
            // invoke a callback to the client to destroy that surface.
            frame_state.composite_state.destroy_native_tiles(
                old_tiles.values_mut(),
                frame_state.resource_cache,
            );
        }

        // This is duplicated information from tile_rect, but cached here to avoid
        // redundant calculations during get_tile_coords_for_rect
        self.tile_bounds_p0 = TileOffset::new(x0, y0);
        self.tile_bounds_p1 = TileOffset::new(x1, y1);
        self.tile_rect = new_tile_rect;

        let mut world_culling_rect = WorldRect::zero();

        let ctx = TilePreUpdateContext {
            pic_to_world_mapper,
            fract_offset: self.fract_offset,
            device_fract_offset: self.device_fract_offset,
            background_color: self.background_color,
            global_screen_world_rect: frame_context.global_screen_world_rect,
            tile_size: self.tile_size,
            frame_id: self.frame_id,
        };

        // Pre-update each tile
        for tile in self.tiles.values_mut() {
            tile.pre_update(&ctx);

            // Only include the tiles that are currently in view into the world culling
            // rect. This is a very important optimization for a couple of reasons:
            // (1) Primitives that intersect with tiles in the grid that are not currently
            //     visible can be skipped from primitive preparation, clip chain building
            //     and tile dependency updates.
            // (2) When we need to allocate an off-screen surface for a child picture (for
            //     example a CSS filter) we clip the size of the GPU surface to the world
            //     culling rect below (to ensure we draw enough of it to be sampled by any
            //     tiles that reference it). Making the world culling rect only affected
            //     by visible tiles (rather than the entire virtual tile display port) can
            //     result in allocating _much_ smaller GPU surfaces for cases where the
            //     true off-screen surface size is very large.
            if tile.is_visible {
                world_culling_rect = world_culling_rect.union(&tile.world_tile_rect);
            }
        }

        // If compositor mode is changed, need to drop all incompatible tiles.
        match frame_context.config.compositor_kind {
            CompositorKind::Draw { .. } => {
                for tile in self.tiles.values_mut() {
                    if let Some(TileSurface::Texture { descriptor: SurfaceTextureDescriptor::Native { ref mut id, .. }, .. }) = tile.surface {
                        if let Some(id) = id.take() {
                            frame_state.resource_cache.destroy_compositor_tile(id);
                        }
                        tile.surface = None;
                        // Invalidate the entire tile to force a redraw.
                        tile.invalidate(None, InvalidationReason::CompositorKindChanged);
                    }
                }

                if let Some(native_surface) = self.native_surface.take() {
                    frame_state.resource_cache.destroy_compositor_surface(native_surface.opaque);
                    frame_state.resource_cache.destroy_compositor_surface(native_surface.alpha);
                }

                for (_, external_surface) in self.external_native_surface_cache.drain() {
                    frame_state.resource_cache.destroy_compositor_surface(external_surface.native_surface_id)
                }
            }
            CompositorKind::Native { .. } => {
                // This could hit even when compositor mode is not changed,
                // then we need to check if there are incompatible tiles.
                for tile in self.tiles.values_mut() {
                    if let Some(TileSurface::Texture { descriptor: SurfaceTextureDescriptor::TextureCache { .. }, .. }) = tile.surface {
                        tile.surface = None;
                        // Invalidate the entire tile to force a redraw.
                        tile.invalidate(None, InvalidationReason::CompositorKindChanged);
                    }
                }
            }
        }

        world_culling_rect
    }

    fn can_promote_to_surface(
        &mut self,
        flags: PrimitiveFlags,
        prim_clip_chain: &ClipChainInstance,
        prim_spatial_node_index: SpatialNodeIndex,
        on_picture_surface: bool,
        frame_context: &FrameVisibilityContext,
    ) -> SurfacePromotionResult {
        // Check if this primitive _wants_ to be promoted to a compositor surface.
        if !flags.contains(PrimitiveFlags::PREFER_COMPOSITOR_SURFACE) {
            return SurfacePromotionResult::Failed;
        }

        // For now, only support a small (arbitrary) number of compositor surfaces.
        if self.external_surfaces.len() == MAX_COMPOSITOR_SURFACES {
            return SurfacePromotionResult::Failed;
        }

        // If a complex clip is being applied to this primitive, it can't be
        // promoted directly to a compositor surface (we might be able to
        // do this in limited cases in future, some native compositors do
        // support rounded rect clips, for example)
        if prim_clip_chain.needs_mask {
            return SurfacePromotionResult::Failed;
        }

        // If not on the same surface as the picture cache, it has some kind of
        // complex effect (such as a filter, mix-blend-mode or 3d transform).
        if !on_picture_surface {
            return SurfacePromotionResult::Failed;
        }

        let mapper : SpaceMapper<PicturePixel, WorldPixel> = SpaceMapper::new_with_target(
            ROOT_SPATIAL_NODE_INDEX,
            prim_spatial_node_index,
            frame_context.global_screen_world_rect,
            &frame_context.spatial_tree);
        let transform = mapper.get_transform();
        if !transform.is_2d_scale_translation() {
            return SurfacePromotionResult::Failed;
        }
        if transform.m11 < 0.0 {
            return SurfacePromotionResult::Failed;
        }

        SurfacePromotionResult::Success {
            flip_y: transform.m22 < 0.0,
        }
    }

    fn setup_compositor_surfaces_yuv(
        &mut self,
        prim_info: &mut PrimitiveDependencyInfo,
        flags: PrimitiveFlags,
        prim_rect: PictureRect,
        local_prim_rect: LayoutRect,
        prim_spatial_node_index: SpatialNodeIndex,
        frame_context: &FrameVisibilityContext,
        image_dependencies: &[ImageDependency;3],
        api_keys: &[ImageKey; 3],
        resource_cache: &mut ResourceCache,
        composite_state: &mut CompositeState,
        image_rendering: ImageRendering,
        color_depth: ColorDepth,
        color_space: YuvColorSpace,
        format: YuvFormat,
    ) -> bool {
        self.setup_compositor_surfaces_impl(
            prim_info,
            flags,
            prim_rect,
            local_prim_rect,
            prim_spatial_node_index,
            frame_context,
            ExternalSurfaceDependency::Yuv {
                image_dependencies: *image_dependencies,
                color_space,
                format,
                rescale: color_depth.rescaling_factor(),
            },
            api_keys,
            resource_cache,
            composite_state,
            image_rendering,
        )
    }

    fn setup_compositor_surfaces_rgb(
        &mut self,
        prim_info: &mut PrimitiveDependencyInfo,
        flags: PrimitiveFlags,
        prim_rect: PictureRect,
        local_prim_rect: LayoutRect,
        prim_spatial_node_index: SpatialNodeIndex,
        frame_context: &FrameVisibilityContext,
        image_dependency: ImageDependency,
        api_key: ImageKey,
        resource_cache: &mut ResourceCache,
        composite_state: &mut CompositeState,
        image_rendering: ImageRendering,
        flip_y: bool,
    ) -> bool {
        let mut api_keys = [ImageKey::DUMMY; 3];
        api_keys[0] = api_key;
        self.setup_compositor_surfaces_impl(
            prim_info,
            flags,
            prim_rect,
            local_prim_rect,
            prim_spatial_node_index,
            frame_context,
            ExternalSurfaceDependency::Rgb {
                image_dependency,
                flip_y,
            },
            &api_keys,
            resource_cache,
            composite_state,
            image_rendering,
        )
    }

    // returns false if composition is not available for this surface,
    // and the non-compositor path should be used to draw it instead.
    fn setup_compositor_surfaces_impl(
        &mut self,
        prim_info: &mut PrimitiveDependencyInfo,
        flags: PrimitiveFlags,
        prim_rect: PictureRect,
        local_prim_rect: LayoutRect,
        prim_spatial_node_index: SpatialNodeIndex,
        frame_context: &FrameVisibilityContext,
        dependency: ExternalSurfaceDependency,
        api_keys: &[ImageKey; 3],
        resource_cache: &mut ResourceCache,
        composite_state: &mut CompositeState,
        image_rendering: ImageRendering,
    ) -> bool {
        prim_info.is_compositor_surface = true;

        let pic_to_world_mapper = SpaceMapper::new_with_target(
            ROOT_SPATIAL_NODE_INDEX,
            self.spatial_node_index,
            frame_context.global_screen_world_rect,
            frame_context.spatial_tree,
        );

        let world_clip_rect = pic_to_world_mapper
            .map(&prim_info.prim_clip_box.to_rect())
            .expect("bug: unable to map clip to world space");

        let is_visible = world_clip_rect.intersects(&frame_context.global_screen_world_rect);
        if !is_visible {
            return true;
        }

        let world_rect = pic_to_world_mapper
            .map(&prim_rect)
            .expect("bug: unable to map the primitive to world space");
        let device_rect = (world_rect * frame_context.global_device_pixel_scale).round();

        // TODO(gw): Is there any case where if the primitive ends up on a fractional
        //           boundary we want to _skip_ promoting to a compositor surface and
        //           draw it as part of the content?
        let (surface_rect, transform) = match composite_state.compositor_kind {
            CompositorKind::Draw { .. } => {
                (device_rect, Transform3D::identity())
            }
            CompositorKind::Native { .. } => {
                // If we have a Native Compositor, then we can support doing the transformation
                // as part of compositing. Use the local prim rect for the external surface, and
                // compute the full local to device transform to provide to the compositor.
                let surface_to_world_mapper : SpaceMapper<PicturePixel, WorldPixel> = SpaceMapper::new_with_target(
                    ROOT_SPATIAL_NODE_INDEX,
                    prim_spatial_node_index,
                    frame_context.global_screen_world_rect,
                    frame_context.spatial_tree,
                );
                let prim_origin = Vector3D::new(local_prim_rect.origin.x, local_prim_rect.origin.y, 0.0);
                let world_to_device_scale = Transform3D::from_scale(frame_context.global_device_pixel_scale);
                let transform = surface_to_world_mapper.get_transform().pre_translate(prim_origin).then(&world_to_device_scale);

                (local_prim_rect.cast_unit(), transform)
            }
        };

        let clip_rect = (world_clip_rect * frame_context.global_device_pixel_scale).round();

        if surface_rect.size.width >= MAX_COMPOSITOR_SURFACES_SIZE ||
           surface_rect.size.height >= MAX_COMPOSITOR_SURFACES_SIZE {
               return false;
        }

        // If this primitive is an external image, and supports being used
        // directly by a native compositor, then lookup the external image id
        // so we can pass that through.
        let external_image_id = if flags.contains(PrimitiveFlags::SUPPORTS_EXTERNAL_COMPOSITOR_SURFACE) {
            resource_cache.get_image_properties(api_keys[0])
                .and_then(|properties| properties.external_image)
                .and_then(|image| Some(image.id))
        } else {
            None
        };

        // When using native compositing, we need to find an existing native surface
        // handle to use, or allocate a new one. For existing native surfaces, we can
        // also determine whether this needs to be updated, depending on whether the
        // image generation(s) of the planes have changed since last composite.
        let (native_surface_id, update_params) = match composite_state.compositor_kind {
            CompositorKind::Draw { .. } => {
                (None, None)
            }
            CompositorKind::Native { .. } => {
                let native_surface_size = surface_rect.size.round().to_i32();

                let key = ExternalNativeSurfaceKey {
                    image_keys: *api_keys,
                    size: native_surface_size,
                    is_external_surface: external_image_id.is_some(),
                };

                let native_surface = self.external_native_surface_cache
                    .entry(key)
                    .or_insert_with(|| {
                        // No existing surface, so allocate a new compositor surface.
                        let native_surface_id = match external_image_id {
                            Some(_external_image) => {
                                // If we have a suitable external image, then create an external
                                // surface to attach to.
                                resource_cache.create_compositor_external_surface(true)
                            }
                            None => {
                                // Otherwise create a normal compositor surface and a single
                                // compositor tile that covers the entire surface.
                                let native_surface_id =
                                resource_cache.create_compositor_surface(
                                    DeviceIntPoint::zero(),
                                    native_surface_size,
                                    true,
                                );

                                let tile_id = NativeTileId {
                                    surface_id: native_surface_id,
                                    x: 0,
                                    y: 0,
                                };
                                resource_cache.create_compositor_tile(tile_id);

                                native_surface_id
                            }
                        };

                        ExternalNativeSurface {
                            used_this_frame: true,
                            native_surface_id,
                            image_dependencies: [ImageDependency::INVALID; 3],
                        }
                    });

                // Mark that the surface is referenced this frame so that the
                // backing native surface handle isn't freed.
                native_surface.used_this_frame = true;

                let update_params = match external_image_id {
                    Some(external_image) => {
                        // If this is an external image surface, then there's no update
                        // to be done. Just attach the current external image to the surface
                        // and we're done.
                        resource_cache.attach_compositor_external_image(
                            native_surface.native_surface_id,
                            external_image,
                        );
                        None
                    }
                    None => {
                        // If the image dependencies match, there is no need to update
                        // the backing native surface.
                        match dependency {
                            ExternalSurfaceDependency::Yuv{ image_dependencies, .. } => {
                                if image_dependencies == native_surface.image_dependencies {
                                    None
                                } else {
                                    Some(native_surface_size)
                                }
                            },
                            ExternalSurfaceDependency::Rgb{ image_dependency, .. } => {
                                if image_dependency == native_surface.image_dependencies[0] {
                                    None
                                } else {
                                    Some(native_surface_size)
                                }
                            },
                        }
                    }
                };

                (Some(native_surface.native_surface_id), update_params)
            }
        };

        // Each compositor surface allocates a unique z-id
        self.external_surfaces.push(ExternalSurfaceDescriptor {
            local_rect: prim_info.prim_clip_box.to_rect(),
            local_clip_rect: prim_info.prim_clip_box.to_rect(),
            dependency,
            image_rendering,
            device_rect,
            surface_rect,
            clip_rect,
            transform: transform.cast_unit(),
            z_id: composite_state.z_generator.next(),
            native_surface_id,
            update_params,
        });

        true
    }

    /// Update the dependencies for each tile for a given primitive instance.
    pub fn update_prim_dependencies(
        &mut self,
        prim_instance: &mut PrimitiveInstance,
        prim_spatial_node_index: SpatialNodeIndex,
        local_prim_rect: LayoutRect,
        frame_context: &FrameVisibilityContext,
        data_stores: &DataStores,
        clip_store: &ClipStore,
        pictures: &[PicturePrimitive],
        resource_cache: &mut ResourceCache,
        color_bindings: &ColorBindingStorage,
        surface_stack: &[SurfaceIndex],
        composite_state: &mut CompositeState,
    ) {
        // This primitive exists on the last element on the current surface stack.
        profile_scope!("update_prim_dependencies");
        let prim_surface_index = *surface_stack.last().unwrap();
        let prim_clip_chain = &prim_instance.vis.clip_chain;

        self.map_local_to_surface.set_target_spatial_node(
            prim_spatial_node_index,
            frame_context.spatial_tree,
        );

        // Map the primitive local rect into picture space.
        let prim_rect = match self.map_local_to_surface.map(&local_prim_rect) {
            Some(rect) => rect,
            None => return,
        };

        // If the rect is invalid, no need to create dependencies.
        if prim_rect.size.is_empty() {
            return;
        }

        // If the primitive is directly drawn onto this picture cache surface, then
        // the pic_clip_rect is in the same space. If not, we need to map it from
        // the surface space into the picture cache space.
        let on_picture_surface = prim_surface_index == self.surface_index;
        let pic_clip_rect = if on_picture_surface {
            prim_clip_chain.pic_clip_rect
        } else {
            // We want to get the rect in the tile cache surface space that this primitive
            // occupies, in order to enable correct invalidation regions. Each surface
            // that exists in the chain between this primitive and the tile cache surface
            // may have an arbitrary inflation factor (for example, in the case of a series
            // of nested blur elements). To account for this, step through the current
            // surface stack, mapping the primitive rect into each surface space, including
            // the inflation factor from each intermediate surface.
            let mut current_pic_clip_rect = prim_clip_chain.pic_clip_rect;
            let mut current_spatial_node_index = frame_context
                .surfaces[prim_surface_index.0]
                .surface_spatial_node_index;

            for surface_index in surface_stack.iter().rev() {
                let surface = &frame_context.surfaces[surface_index.0];

                let map_local_to_surface = SpaceMapper::new_with_target(
                    surface.surface_spatial_node_index,
                    current_spatial_node_index,
                    surface.rect,
                    frame_context.spatial_tree,
                );

                // Map the rect into the parent surface, and inflate if this surface requires
                // it. If the rect can't be mapping (e.g. due to an invalid transform) then
                // just bail out from the dependencies and cull this primitive.
                current_pic_clip_rect = match map_local_to_surface.map(&current_pic_clip_rect) {
                    Some(rect) => {
                        rect.inflate(surface.inflation_factor, surface.inflation_factor)
                    }
                    None => {
                        return;
                    }
                };

                current_spatial_node_index = surface.surface_spatial_node_index;
            }

            current_pic_clip_rect
        };

        // Get the tile coordinates in the picture space.
        let (p0, p1) = self.get_tile_coords_for_rect(&pic_clip_rect);

        // If the primitive is outside the tiling rects, it's known to not
        // be visible.
        if p0.x == p1.x || p0.y == p1.y {
            return;
        }

        // Build the list of resources that this primitive has dependencies on.
        let mut prim_info = PrimitiveDependencyInfo::new(
            prim_instance.uid(),
            pic_clip_rect.to_box2d(),
        );

        // Include the prim spatial node, if differs relative to cache root.
        if prim_spatial_node_index != self.spatial_node_index {
            prim_info.spatial_nodes.push(prim_spatial_node_index);
        }

        // If there was a clip chain, add any clip dependencies to the list for this tile.
        let clip_instances = &clip_store
            .clip_node_instances[prim_clip_chain.clips_range.to_range()];
        for clip_instance in clip_instances {
            prim_info.clips.push(clip_instance.handle.uid());

            // If the clip has the same spatial node, the relative transform
            // will always be the same, so there's no need to depend on it.
            if clip_instance.spatial_node_index != self.spatial_node_index
                && !prim_info.spatial_nodes.contains(&clip_instance.spatial_node_index) {
                prim_info.spatial_nodes.push(clip_instance.spatial_node_index);
            }
        }

        // Certain primitives may select themselves to be a backdrop candidate, which is
        // then applied below.
        let mut backdrop_candidate = None;


        // For pictures, we don't (yet) know the valid clip rect, so we can't correctly
        // use it to calculate the local bounding rect for the tiles. If we include them
        // then we may calculate a bounding rect that is too large, since it won't include
        // the clip bounds of the picture. Excluding them from the bounding rect here
        // fixes any correctness issues (the clips themselves are considered when we
        // consider the bounds of the primitives that are *children* of the picture),
        // however it does potentially result in some un-necessary invalidations of a
        // tile (in cases where the picture local rect affects the tile, but the clip
        // rect eventually means it doesn't affect that tile).
        // TODO(gw): Get picture clips earlier (during the initial picture traversal
        //           pass) so that we can calculate these correctly.
        match prim_instance.kind {
            PrimitiveInstanceKind::Picture { pic_index,.. } => {
                // Pictures can depend on animated opacity bindings.
                let pic = &pictures[pic_index.0];
                if let Some(PictureCompositeMode::Filter(Filter::Opacity(binding, _))) = pic.requested_composite_mode {
                    prim_info.opacity_bindings.push(binding.into());
                }
            }
            PrimitiveInstanceKind::Rectangle { data_handle, color_binding_index, .. } => {
                // Rectangles can only form a backdrop candidate if they are known opaque.
                // TODO(gw): We could resolve the opacity binding here, but the common
                //           case for background rects is that they don't have animated opacity.
                let color = match data_stores.prim[data_handle].kind {
                    PrimitiveTemplateKind::Rectangle { color, .. } => {
                        frame_context.scene_properties.resolve_color(&color)
                    }
                    _ => unreachable!(),
                };
                if color.a >= 1.0 {
                    backdrop_candidate = Some(BackdropInfo {
                        opaque_rect: pic_clip_rect,
                        kind: Some(BackdropKind::Color { color }),
                    });
                }

                if color_binding_index != ColorBindingIndex::INVALID {
                    prim_info.color_binding = Some(color_bindings[color_binding_index].into());
                }
            }
            PrimitiveInstanceKind::Image { data_handle, ref mut is_compositor_surface, .. } => {
                let image_key = &data_stores.image[data_handle];
                let image_data = &image_key.kind;

                let mut promote_to_surface = false;
                let mut promote_with_flip_y = false;
                match self.can_promote_to_surface(image_key.common.flags,
                                                  prim_clip_chain,
                                                  prim_spatial_node_index,
                                                  on_picture_surface,
                                                  frame_context) {
                    SurfacePromotionResult::Failed => {
                    }
                    SurfacePromotionResult::Success{flip_y} => {
                        promote_to_surface = true;
                        promote_with_flip_y = flip_y;
                    }
                }

                if let Some(image_properties) = resource_cache.get_image_properties(image_data.key) {
                    // For an image to be a possible opaque backdrop, it must:
                    // - Have a valid, opaque image descriptor
                    // - Not use tiling (since they can fail to draw)
                    // - Not having any spacing / padding
                    if image_properties.descriptor.is_opaque() &&
                       image_properties.tiling.is_none() &&
                       image_data.tile_spacing == LayoutSize::zero() {
                        backdrop_candidate = Some(BackdropInfo {
                            opaque_rect: pic_clip_rect,
                            kind: None,
                        });
                    }
                }

                if promote_to_surface {
                    promote_to_surface = self.setup_compositor_surfaces_rgb(
                        &mut prim_info,
                        image_key.common.flags,
                        prim_rect,
                        local_prim_rect,
                        prim_spatial_node_index,
                        frame_context,
                        ImageDependency {
                            key: image_data.key,
                            generation: resource_cache.get_image_generation(image_data.key),
                        },
                        image_data.key,
                        resource_cache,
                        composite_state,
                        image_data.image_rendering,
                        promote_with_flip_y,
                    );
                }

                if !promote_to_surface {
                    prim_info.images.push(ImageDependency {
                        key: image_data.key,
                        generation: resource_cache.get_image_generation(image_data.key),
                    });
                }

                *is_compositor_surface = promote_to_surface;
            }
            PrimitiveInstanceKind::YuvImage { data_handle, ref mut is_compositor_surface, .. } => {
                let prim_data = &data_stores.yuv_image[data_handle];
                let mut promote_to_surface = match self.can_promote_to_surface(
                                            prim_data.common.flags,
                                            prim_clip_chain,
                                            prim_spatial_node_index,
                                            on_picture_surface,
                                            frame_context) {
                    SurfacePromotionResult::Failed => false,
                    SurfacePromotionResult::Success{flip_y} => !flip_y,
                };

                // TODO(gw): When we support RGBA images for external surfaces, we also
                //           need to check if opaque (YUV images are implicitly opaque).

                // If this primitive is being promoted to a surface, construct an external
                // surface descriptor for use later during batching and compositing. We only
                // add the image keys for this primitive as a dependency if this is _not_
                // a promoted surface, since we don't want the tiles to invalidate when the
                // video content changes, if it's a compositor surface!
                if promote_to_surface {
                    // Build dependency for each YUV plane, with current image generation for
                    // later detection of when the composited surface has changed.
                    let mut image_dependencies = [ImageDependency::INVALID; 3];
                    for (key, dep) in prim_data.kind.yuv_key.iter().cloned().zip(image_dependencies.iter_mut()) {
                        *dep = ImageDependency {
                            key,
                            generation: resource_cache.get_image_generation(key),
                        }
                    }

                    promote_to_surface = self.setup_compositor_surfaces_yuv(
                        &mut prim_info,
                        prim_data.common.flags,
                        prim_rect,
                        local_prim_rect,
                        prim_spatial_node_index,
                        frame_context,
                        &image_dependencies,
                        &prim_data.kind.yuv_key,
                        resource_cache,
                        composite_state,
                        prim_data.kind.image_rendering,
                        prim_data.kind.color_depth,
                        prim_data.kind.color_space,
                        prim_data.kind.format,
                    );
                }

                if !promote_to_surface {
                    prim_info.images.extend(
                        prim_data.kind.yuv_key.iter().map(|key| {
                            ImageDependency {
                                key: *key,
                                generation: resource_cache.get_image_generation(*key),
                            }
                        })
                    );
                }

                // Store on the YUV primitive instance whether this is a promoted surface.
                // This is used by the batching code to determine whether to draw the
                // image to the content tiles, or just a transparent z-write.
                *is_compositor_surface = promote_to_surface;

            }
            PrimitiveInstanceKind::ImageBorder { data_handle, .. } => {
                let border_data = &data_stores.image_border[data_handle].kind;
                prim_info.images.push(ImageDependency {
                    key: border_data.request.key,
                    generation: resource_cache.get_image_generation(border_data.request.key),
                });
            }
            PrimitiveInstanceKind::Clear { .. } => {
                backdrop_candidate = Some(BackdropInfo {
                    opaque_rect: pic_clip_rect,
                    kind: Some(BackdropKind::Clear),
                });
            }
            PrimitiveInstanceKind::LinearGradient { data_handle, .. } => {
                let gradient_data = &data_stores.linear_grad[data_handle];
                if gradient_data.stops_opacity.is_opaque
                    && gradient_data.tile_spacing == LayoutSize::zero()
                {
                    backdrop_candidate = Some(BackdropInfo {
                        opaque_rect: pic_clip_rect,
                        kind: None,
                    });
                }
            }
            PrimitiveInstanceKind::ConicGradient { data_handle, .. } => {
                let gradient_data = &data_stores.conic_grad[data_handle];
                if gradient_data.stops_opacity.is_opaque
                    && gradient_data.tile_spacing == LayoutSize::zero()
                {
                    backdrop_candidate = Some(BackdropInfo {
                        opaque_rect: pic_clip_rect,
                        kind: None,
                    });
                }
            }
            PrimitiveInstanceKind::RadialGradient { data_handle, .. } => {
                let gradient_data = &data_stores.radial_grad[data_handle];
                if gradient_data.stops_opacity.is_opaque
                    && gradient_data.tile_spacing == LayoutSize::zero()
                {
                    backdrop_candidate = Some(BackdropInfo {
                        opaque_rect: pic_clip_rect,
                        kind: None,
                    });
                }
            }
            PrimitiveInstanceKind::LineDecoration { .. } |
            PrimitiveInstanceKind::NormalBorder { .. } |
            PrimitiveInstanceKind::TextRun { .. } |
            PrimitiveInstanceKind::Backdrop { .. } => {
                // These don't contribute dependencies
            }
        };

        // If this primitive considers itself a backdrop candidate, apply further
        // checks to see if it matches all conditions to be a backdrop.
        let mut vis_flags = PrimitiveVisibilityFlags::empty();

        if let Some(backdrop_candidate) = backdrop_candidate {
            let is_suitable_backdrop = match backdrop_candidate.kind {
                Some(BackdropKind::Clear) => {
                    // Clear prims are special - they always end up in their own slice,
                    // and always set the backdrop. In future, we hope to completely
                    // remove clear prims, since they don't integrate with the compositing
                    // system cleanly.
                    true
                }
                Some(BackdropKind::Color { .. }) | None => {
                    // Check a number of conditions to see if we can consider this
                    // primitive as an opaque backdrop rect. Several of these are conservative
                    // checks and could be relaxed in future. However, these checks
                    // are quick and capture the common cases of background rects and images.
                    // Specifically, we currently require:
                    //  - The primitive is on the main picture cache surface.
                    //  - Same coord system as picture cache (ensures rects are axis-aligned).
                    //  - No clip masks exist.
                    let same_coord_system = {
                        let prim_spatial_node = &frame_context.spatial_tree
                            .spatial_nodes[prim_spatial_node_index.0 as usize];
                        let surface_spatial_node = &frame_context.spatial_tree
                            .spatial_nodes[self.spatial_node_index.0 as usize];

                        prim_spatial_node.coordinate_system_id == surface_spatial_node.coordinate_system_id
                    };

                    same_coord_system && on_picture_surface
                }
            };

            if is_suitable_backdrop
                && self.external_surfaces.is_empty()
                && !prim_clip_chain.needs_mask {

                if backdrop_candidate.opaque_rect.contains_rect(&self.backdrop.opaque_rect) {
                    self.backdrop.opaque_rect = backdrop_candidate.opaque_rect;
                }

                if let Some(kind) = backdrop_candidate.kind {
                    if backdrop_candidate.opaque_rect.contains_rect(&self.local_rect) {
                        // If we have a color backdrop, mark the visibility flags
                        // of the primitive so it is skipped during batching (and
                        // also clears any previous primitives).
                        if let BackdropKind::Color { .. } = kind {
                            vis_flags |= PrimitiveVisibilityFlags::IS_BACKDROP;
                        }

                        self.backdrop.kind = Some(kind);
                    }
                }
            }
        }

        // Record any new spatial nodes in the used list.
        for spatial_node_index in &prim_info.spatial_nodes {
            self.spatial_node_comparer.register_used_transform(
                *spatial_node_index,
                self.frame_id,
                frame_context.spatial_tree,
            );
        }

        // Truncate the lengths of dependency arrays to the max size we can handle.
        // Any arrays this size or longer will invalidate every frame.
        prim_info.clips.truncate(MAX_PRIM_SUB_DEPS);
        prim_info.opacity_bindings.truncate(MAX_PRIM_SUB_DEPS);
        prim_info.spatial_nodes.truncate(MAX_PRIM_SUB_DEPS);
        prim_info.images.truncate(MAX_PRIM_SUB_DEPS);

        // Normalize the tile coordinates before adding to tile dependencies.
        // For each affected tile, mark any of the primitive dependencies.
        for y in p0.y .. p1.y {
            for x in p0.x .. p1.x {
                // TODO(gw): Convert to 2d array temporarily to avoid hash lookups per-tile?
                let key = TileOffset::new(x, y);
                let tile = self.tiles.get_mut(&key).expect("bug: no tile");

                tile.add_prim_dependency(&prim_info);
            }
        }

        prim_instance.vis.flags = vis_flags;
        prim_instance.vis.state = VisibilityState::Coarse {
            rect_in_pic_space: pic_clip_rect,
        };
    }

    /// Print debug information about this picture cache to a tree printer.
    fn print(&self) {
        // TODO(gw): This initial implementation is very basic - just printing
        //           the picture cache state to stdout. In future, we can
        //           make this dump each frame to a file, and produce a report
        //           stating which frames had invalidations. This will allow
        //           diff'ing the invalidation states in a visual tool.
        let mut pt = PrintTree::new("Picture Cache");

        pt.new_level(format!("Slice {:?}", self.slice));

        pt.add_item(format!("fract_offset: {:?}", self.fract_offset));
        pt.add_item(format!("background_color: {:?}", self.background_color));

        for y in self.tile_bounds_p0.y .. self.tile_bounds_p1.y {
            for x in self.tile_bounds_p0.x .. self.tile_bounds_p1.x {
                let key = TileOffset::new(x, y);
                let tile = &self.tiles[&key];
                tile.print(&mut pt);
            }
        }

        pt.end_level();
    }

    fn calculate_subpixel_mode(&self) -> SubpixelMode {
        // If the overall tile cache is known opaque, subpixel AA is allowed everywhere
        if self.is_opaque() {
            return SubpixelMode::Allow;
        }

        // If we didn't find any valid opaque backdrop, no subpixel AA allowed
        if self.backdrop.opaque_rect.is_empty() {
            return SubpixelMode::Deny;
        }

        // If the opaque backdrop rect covers the entire tile cache surface,
        // we can allow subpixel AA anywhere, skipping the per-text-run tests
        // later on during primitive preparation.
        if self.backdrop.opaque_rect.contains_rect(&self.local_rect) {
            return SubpixelMode::Allow;
        }

        // If none of the simple cases above match, we need to build a list
        // of excluded rects (compositor surfaces) and a valid inclusion rect
        // (known opaque area) where we can support subpixel AA.
        // TODO(gw): In future, it may make sense to have > 1 inclusion rect,
        //           but this handles the common cases.
        // TODO(gw): If a text run gets animated such that it's moving in a way that is
        //           sometimes intersecting with the video rect, this can result in subpixel
        //           AA flicking on/off for that text run. It's probably very rare, but
        //           something we should handle in future.

        let excluded_rects = self.external_surfaces
            .iter()
            .map(|s| {
                s.local_rect
            })
            .collect();

        SubpixelMode::Conditional {
            allowed_rect: self.backdrop.opaque_rect,
            excluded_rects,
        }
    }

    /// Apply any updates after prim dependency updates. This applies
    /// any late tile invalidations, and sets up the dirty rect and
    /// set of tile blits.
    pub fn post_update(
        &mut self,
        frame_context: &FrameVisibilityContext,
        frame_state: &mut FrameVisibilityState,
    ) {
        self.dirty_region.reset(self.spatial_node_index);
        self.subpixel_mode = self.calculate_subpixel_mode();

        let map_pic_to_world = SpaceMapper::new_with_target(
            ROOT_SPATIAL_NODE_INDEX,
            self.spatial_node_index,
            frame_context.global_screen_world_rect,
            frame_context.spatial_tree,
        );

        // Register the opaque region of this tile cache as an occluder, which
        // is used later in the frame to occlude other tiles.
        if !self.backdrop.opaque_rect.is_empty() {
            let backdrop_rect = self.backdrop.opaque_rect
                .intersection(&self.local_rect)
                .and_then(|r| {
                    r.intersection(&self.local_clip_rect)
                });

            if let Some(backdrop_rect) = backdrop_rect {
                let world_backdrop_rect = map_pic_to_world
                    .map(&backdrop_rect)
                    .expect("bug: unable to map backdrop to world space");

                // Since we register the entire backdrop rect, use the opaque z-id for the
                // picture cache slice.
                frame_state.composite_state.register_occluder(
                    self.z_id_opaque,
                    world_backdrop_rect,
                );
            }
        }

        // Register any external compositor surfaces as potential occluders. This
        // is especially useful when viewing video in full-screen mode, as it is
        // able to occlude every background tile (avoiding allocation, rasterizion
        // and compositing).
        for external_surface in &self.external_surfaces {
            let local_surface_rect = external_surface.local_rect
                .intersection(&external_surface.local_clip_rect)
                .and_then(|r| {
                    r.intersection(&self.local_clip_rect)
                });

            if let Some(local_surface_rect) = local_surface_rect {
                let world_surface_rect = map_pic_to_world
                    .map(&local_surface_rect)
                    .expect("bug: unable to map external surface to world space");

                frame_state.composite_state.register_occluder(
                    external_surface.z_id,
                    world_surface_rect,
                );
            }
        }

        // A simple GC of the native external surface cache, to remove and free any
        // surfaces that were not referenced during the update_prim_dependencies pass.
        self.external_native_surface_cache.retain(|_, surface| {
            if !surface.used_this_frame {
                frame_state.resource_cache.destroy_compositor_surface(surface.native_surface_id);
            }

            surface.used_this_frame
        });

        // Detect if the picture cache was scrolled or scaled. In this case,
        // the device space dirty rects aren't applicable (until we properly
        // integrate with OS compositors that can handle scrolling slices).
        let root_transform = frame_context
            .spatial_tree
            .get_relative_transform(
                self.spatial_node_index,
                ROOT_SPATIAL_NODE_INDEX,
            );
        let root_transform = match root_transform {
            CoordinateSpaceMapping::Local => ScaleOffset::identity(),
            CoordinateSpaceMapping::ScaleOffset(scale_offset) => scale_offset,
            CoordinateSpaceMapping::Transform(..) => panic!("bug: picture caches don't support complex transforms"),
        };
        const EPSILON: f32 = 0.001;
        let root_translation_changed =
            !root_transform.offset.x.approx_eq_eps(&self.root_transform.offset.x, &EPSILON) ||
            !root_transform.offset.y.approx_eq_eps(&self.root_transform.offset.y, &EPSILON);
        let root_scale_changed =
            !root_transform.scale.x.approx_eq_eps(&self.root_transform.scale.x, &EPSILON) ||
            !root_transform.scale.y.approx_eq_eps(&self.root_transform.scale.y, &EPSILON);

        if root_translation_changed || root_scale_changed || frame_context.config.force_invalidation {
            self.root_transform = root_transform;
            frame_state.composite_state.dirty_rects_are_valid = false;
        }

        let pic_to_world_mapper = SpaceMapper::new_with_target(
            ROOT_SPATIAL_NODE_INDEX,
            self.spatial_node_index,
            frame_context.global_screen_world_rect,
            frame_context.spatial_tree,
        );

        // All compositor surfaces have allocated a z_id, so reserve a z_id for alpha tiles.
        let z_id_alpha = frame_state.composite_state.z_generator.next();

        let ctx = TilePostUpdateContext {
            pic_to_world_mapper,
            global_device_pixel_scale: frame_context.global_device_pixel_scale,
            local_clip_rect: self.local_clip_rect,
            backdrop: self.backdrop,
            opacity_bindings: &self.opacity_bindings,
            color_bindings: &self.color_bindings,
            current_tile_size: self.current_tile_size,
            local_rect: self.local_rect,
            external_surfaces: &self.external_surfaces,
            z_id_opaque: self.z_id_opaque,
            z_id_alpha,
            invalidate_all: root_scale_changed || frame_context.config.force_invalidation,
        };

        let mut state = TilePostUpdateState {
            resource_cache: frame_state.resource_cache,
            composite_state: frame_state.composite_state,
            compare_cache: &mut self.compare_cache,
            spatial_node_comparer: &mut self.spatial_node_comparer,
        };

        // Step through each tile and invalidate if the dependencies have changed. Determine
        // the current opacity setting and whether it's changed.
        for tile in self.tiles.values_mut() {
            tile.post_update(&ctx, &mut state, frame_context);
        }
    }
}

pub struct PictureScratchBuffer {
    surface_stack: Vec<SurfaceIndex>,
    picture_stack: Vec<PictureInfo>,
    clip_chain_ids: Vec<ClipChainId>,
}

impl Default for PictureScratchBuffer {
    fn default() -> Self {
        PictureScratchBuffer {
            surface_stack: Vec::new(),
            picture_stack: Vec::new(),
            clip_chain_ids: Vec::new(),
        }
    }
}

impl PictureScratchBuffer {
    pub fn begin_frame(&mut self) {
        self.surface_stack.clear();
        self.picture_stack.clear();
        self.clip_chain_ids.clear();
    }

    pub fn recycle(&mut self, recycler: &mut Recycler) {
        recycler.recycle_vec(&mut self.surface_stack);
        recycler.recycle_vec(&mut self.picture_stack);
    }
 }

/// Maintains a stack of picture and surface information, that
/// is used during the initial picture traversal.
pub struct PictureUpdateState<'a> {
    surfaces: &'a mut Vec<SurfaceInfo>,
    surface_stack: Vec<SurfaceIndex>,
    picture_stack: Vec<PictureInfo>,
}

impl<'a> PictureUpdateState<'a> {
    pub fn update_all(
        buffers: &mut PictureScratchBuffer,
        surfaces: &'a mut Vec<SurfaceInfo>,
        pic_index: PictureIndex,
        picture_primitives: &mut [PicturePrimitive],
        frame_context: &FrameBuildingContext,
        gpu_cache: &mut GpuCache,
        clip_store: &ClipStore,
        data_stores: &mut DataStores,
    ) {
        profile_scope!("UpdatePictures");
        profile_marker!("UpdatePictures");

        let mut state = PictureUpdateState {
            surfaces,
            surface_stack: buffers.surface_stack.take().cleared(),
            picture_stack: buffers.picture_stack.take().cleared(),
        };

        state.surface_stack.push(SurfaceIndex(0));

        state.update(
            pic_index,
            picture_primitives,
            frame_context,
            gpu_cache,
            clip_store,
            data_stores,
        );

        buffers.surface_stack = state.surface_stack.take();
        buffers.picture_stack = state.picture_stack.take();
    }

    /// Return the current surface
    fn current_surface(&self) -> &SurfaceInfo {
        &self.surfaces[self.surface_stack.last().unwrap().0]
    }

    /// Return the current surface (mutable)
    fn current_surface_mut(&mut self) -> &mut SurfaceInfo {
        &mut self.surfaces[self.surface_stack.last().unwrap().0]
    }

    /// Push a new surface onto the update stack.
    fn push_surface(
        &mut self,
        surface: SurfaceInfo,
    ) -> SurfaceIndex {
        let surface_index = SurfaceIndex(self.surfaces.len());
        self.surfaces.push(surface);
        self.surface_stack.push(surface_index);
        surface_index
    }

    /// Pop a surface on the way up the picture traversal
    fn pop_surface(&mut self) -> SurfaceIndex{
        self.surface_stack.pop().unwrap()
    }

    /// Push information about a picture on the update stack
    fn push_picture(
        &mut self,
        info: PictureInfo,
    ) {
        self.picture_stack.push(info);
    }

    /// Pop the picture info off, on the way up the picture traversal
    fn pop_picture(
        &mut self,
    ) -> PictureInfo {
        self.picture_stack.pop().unwrap()
    }

    /// Update a picture, determining surface configuration,
    /// rasterization roots, and (in future) whether there
    /// are cached surfaces that can be used by this picture.
    fn update(
        &mut self,
        pic_index: PictureIndex,
        picture_primitives: &mut [PicturePrimitive],
        frame_context: &FrameBuildingContext,
        gpu_cache: &mut GpuCache,
        clip_store: &ClipStore,
        data_stores: &mut DataStores,
    ) {
        if let Some(prim_list) = picture_primitives[pic_index.0].pre_update(
            self,
            frame_context,
        ) {
            for cluster in &prim_list.clusters {
                if !cluster.flags.contains(ClusterFlags::IS_PICTURE) {
                    continue;
                }
                for prim_instance in &prim_list.prim_instances[cluster.prim_range()] {
                    let child_pic_index = match prim_instance.kind {
                        PrimitiveInstanceKind::Picture { pic_index, .. } => pic_index,
                        _ => unreachable!(),
                    };

                    self.update(
                        child_pic_index,
                        picture_primitives,
                        frame_context,
                        gpu_cache,
                        clip_store,
                        data_stores,
                    );
                }
            }

            picture_primitives[pic_index.0].post_update(
                prim_list,
                self,
                frame_context,
                data_stores,
            );
        }
    }
}

#[derive(Debug, Copy, Clone, PartialEq)]
#[cfg_attr(feature = "capture", derive(Serialize))]
pub struct SurfaceIndex(pub usize);

pub const ROOT_SURFACE_INDEX: SurfaceIndex = SurfaceIndex(0);

/// Describes the render task configuration for a picture surface.
#[derive(Debug)]
pub enum SurfaceRenderTasks {
    /// The common type of surface is a single render task
    Simple(RenderTaskId),
    /// Some surfaces draw their content, and then have further tasks applied
    /// to that input (such as blur passes for shadows). These tasks have a root
    /// (the output of the surface), and a port (for attaching child task dependencies
    /// to the content).
    Chained { root_task_id: RenderTaskId, port_task_id: RenderTaskId },
    /// Picture caches are a single surface consisting of multiple render
    /// tasks, one per tile with dirty content.
    Tiled(Vec<RenderTaskId>),
}

/// Information about an offscreen surface. For now,
/// it contains information about the size and coordinate
/// system of the surface. In the future, it will contain
/// information about the contents of the surface, which
/// will allow surfaces to be cached / retained between
/// frames and display lists.
#[derive(Debug)]
pub struct SurfaceInfo {
    /// A local rect defining the size of this surface, in the
    /// coordinate system of the surface itself.
    pub rect: PictureRect,
    /// Part of the surface that we know to be opaque.
    pub opaque_rect: PictureRect,
    /// Helper structs for mapping local rects in different
    /// coordinate systems into the surface coordinates.
    pub map_local_to_surface: SpaceMapper<LayoutPixel, PicturePixel>,
    /// Defines the positioning node for the surface itself,
    /// and the rasterization root for this surface.
    pub raster_spatial_node_index: SpatialNodeIndex,
    pub surface_spatial_node_index: SpatialNodeIndex,
    /// This is set when the render task is created.
    pub render_tasks: Option<SurfaceRenderTasks>,
    /// How much the local surface rect should be inflated (for blur radii).
    pub inflation_factor: f32,
    /// The device pixel ratio specific to this surface.
    pub device_pixel_scale: DevicePixelScale,
    /// The scale factors of the surface to raster transform.
    pub scale_factors: (f32, f32),
}

impl SurfaceInfo {
    pub fn new(
        surface_spatial_node_index: SpatialNodeIndex,
        raster_spatial_node_index: SpatialNodeIndex,
        inflation_factor: f32,
        world_rect: WorldRect,
        spatial_tree: &SpatialTree,
        device_pixel_scale: DevicePixelScale,
        scale_factors: (f32, f32),
    ) -> Self {
        let map_surface_to_world = SpaceMapper::new_with_target(
            ROOT_SPATIAL_NODE_INDEX,
            surface_spatial_node_index,
            world_rect,
            spatial_tree,
        );

        let pic_bounds = map_surface_to_world
            .unmap(&map_surface_to_world.bounds)
            .unwrap_or_else(PictureRect::max_rect);

        let map_local_to_surface = SpaceMapper::new(
            surface_spatial_node_index,
            pic_bounds,
        );

        SurfaceInfo {
            rect: PictureRect::zero(),
            opaque_rect: PictureRect::zero(),
            map_local_to_surface,
            render_tasks: None,
            raster_spatial_node_index,
            surface_spatial_node_index,
            inflation_factor,
            device_pixel_scale,
            scale_factors,
        }
    }
}

#[derive(Debug)]
#[cfg_attr(feature = "capture", derive(Serialize))]
pub struct RasterConfig {
    /// How this picture should be composited into
    /// the parent surface.
    pub composite_mode: PictureCompositeMode,
    /// Index to the surface descriptor for this
    /// picture.
    pub surface_index: SurfaceIndex,
    /// Whether this picture establishes a rasterization root.
    pub establishes_raster_root: bool,
    /// Scaling factor applied to fit within MAX_SURFACE_SIZE when
    /// establishing a raster root.
    /// Most code doesn't need to know about it, since it is folded
    /// into device_pixel_scale when the rendertask is set up.
    /// However e.g. text rasterization uses it to ensure consistent
    /// on-screen font size.
    pub root_scaling_factor: f32,
    /// The world rect of this picture clipped to the current culling
    /// rect. This is used for determining the size of the render
    /// target rect for this surface, and calculating raster scale
    /// factors.
    pub clipped_bounding_rect: WorldRect,
}

bitflags! {
    /// A set of flags describing why a picture may need a backing surface.
    #[cfg_attr(feature = "capture", derive(Serialize))]
    pub struct BlitReason: u32 {
        /// Mix-blend-mode on a child that requires isolation.
        const ISOLATE = 1;
        /// Clip node that _might_ require a surface.
        const CLIP = 2;
        /// Preserve-3D requires a surface for plane-splitting.
        const PRESERVE3D = 4;
        /// A backdrop that is reused which requires a surface.
        const BACKDROP = 8;
    }
}

/// Specifies how this Picture should be composited
/// onto the target it belongs to.
#[allow(dead_code)]
#[derive(Debug, Clone)]
#[cfg_attr(feature = "capture", derive(Serialize))]
pub enum PictureCompositeMode {
    /// Apply CSS mix-blend-mode effect.
    MixBlend(MixBlendMode),
    /// Apply a CSS filter (except component transfer).
    Filter(Filter),
    /// Apply a component transfer filter.
    ComponentTransferFilter(FilterDataHandle),
    /// Draw to intermediate surface, copy straight across. This
    /// is used for CSS isolation, and plane splitting.
    Blit(BlitReason),
    /// Used to cache a picture as a series of tiles.
    TileCache {
        slice_id: SliceId,
    },
    /// Apply an SVG filter
    SvgFilter(Vec<FilterPrimitive>, Vec<SFilterData>),
}

impl PictureCompositeMode {
    pub fn inflate_picture_rect(&self, picture_rect: PictureRect, scale_factors: (f32, f32)) -> PictureRect {
        let mut result_rect = picture_rect;
        match self {
            PictureCompositeMode::Filter(filter) => match filter {
                Filter::Blur(width, height) => {
                    let width_factor = clamp_blur_radius(*width, scale_factors).ceil() * BLUR_SAMPLE_SCALE;
                    let height_factor = clamp_blur_radius(*height, scale_factors).ceil() * BLUR_SAMPLE_SCALE;
                    result_rect = picture_rect.inflate(width_factor, height_factor);
                },
                Filter::DropShadows(shadows) => {
                    let mut max_inflation: f32 = 0.0;
                    for shadow in shadows {
                        max_inflation = max_inflation.max(shadow.blur_radius);
                    }
                    max_inflation = clamp_blur_radius(max_inflation, scale_factors).ceil() * BLUR_SAMPLE_SCALE;
                    result_rect = picture_rect.inflate(max_inflation, max_inflation);
                },
                _ => {}
            }
            PictureCompositeMode::SvgFilter(primitives, _) => {
                let mut output_rects = Vec::with_capacity(primitives.len());
                for (cur_index, primitive) in primitives.iter().enumerate() {
                    let output_rect = match primitive.kind {
                        FilterPrimitiveKind::Blur(ref primitive) => {
                            let input = primitive.input.to_index(cur_index).map(|index| output_rects[index]).unwrap_or(picture_rect);
                            let width_factor = primitive.width.round() * BLUR_SAMPLE_SCALE;
                            let height_factor = primitive.height.round() * BLUR_SAMPLE_SCALE;
                            input.inflate(width_factor, height_factor)
                        }
                        FilterPrimitiveKind::DropShadow(ref primitive) => {
                            let inflation_factor = primitive.shadow.blur_radius.ceil() * BLUR_SAMPLE_SCALE;
                            let input = primitive.input.to_index(cur_index).map(|index| output_rects[index]).unwrap_or(picture_rect);
                            let shadow_rect = input.inflate(inflation_factor, inflation_factor);
                            input.union(&shadow_rect.translate(primitive.shadow.offset * Scale::new(1.0)))
                        }
                        FilterPrimitiveKind::Blend(ref primitive) => {
                            primitive.input1.to_index(cur_index).map(|index| output_rects[index]).unwrap_or(picture_rect)
                                .union(&primitive.input2.to_index(cur_index).map(|index| output_rects[index]).unwrap_or(picture_rect))
                        }
                        FilterPrimitiveKind::Composite(ref primitive) => {
                            primitive.input1.to_index(cur_index).map(|index| output_rects[index]).unwrap_or(picture_rect)
                                .union(&primitive.input2.to_index(cur_index).map(|index| output_rects[index]).unwrap_or(picture_rect))
                        }
                        FilterPrimitiveKind::Identity(ref primitive) =>
                            primitive.input.to_index(cur_index).map(|index| output_rects[index]).unwrap_or(picture_rect),
                        FilterPrimitiveKind::Opacity(ref primitive) =>
                            primitive.input.to_index(cur_index).map(|index| output_rects[index]).unwrap_or(picture_rect),
                        FilterPrimitiveKind::ColorMatrix(ref primitive) =>
                            primitive.input.to_index(cur_index).map(|index| output_rects[index]).unwrap_or(picture_rect),
                        FilterPrimitiveKind::ComponentTransfer(ref primitive) =>
                            primitive.input.to_index(cur_index).map(|index| output_rects[index]).unwrap_or(picture_rect),
                        FilterPrimitiveKind::Offset(ref primitive) => {
                            let input_rect = primitive.input.to_index(cur_index).map(|index| output_rects[index]).unwrap_or(picture_rect);
                            input_rect.translate(primitive.offset * Scale::new(1.0))
                        },

                        FilterPrimitiveKind::Flood(..) => picture_rect,
                    };
                    output_rects.push(output_rect);
                    result_rect = result_rect.union(&output_rect);
                }
            }
            _ => {},
        }
        result_rect
    }
}

/// Enum value describing the place of a picture in a 3D context.
#[derive(Clone, Debug)]
#[cfg_attr(feature = "capture", derive(Serialize))]
pub enum Picture3DContext<C> {
    /// The picture is not a part of 3D context sub-hierarchy.
    Out,
    /// The picture is a part of 3D context.
    In {
        /// Additional data per child for the case of this a root of 3D hierarchy.
        root_data: Option<Vec<C>>,
        /// The spatial node index of an "ancestor" element, i.e. one
        /// that establishes the transformed element's containing block.
        ///
        /// See CSS spec draft for more details:
        /// https://drafts.csswg.org/css-transforms-2/#accumulated-3d-transformation-matrix-computation
        ancestor_index: SpatialNodeIndex,
    },
}

/// Information about a preserve-3D hierarchy child that has been plane-split
/// and ordered according to the view direction.
#[derive(Clone, Debug)]
#[cfg_attr(feature = "capture", derive(Serialize))]
pub struct OrderedPictureChild {
    pub anchor: PlaneSplitAnchor,
    pub spatial_node_index: SpatialNodeIndex,
    pub gpu_address: GpuCacheAddress,
}

bitflags! {
    /// A set of flags describing why a picture may need a backing surface.
    #[cfg_attr(feature = "capture", derive(Serialize))]
    pub struct ClusterFlags: u32 {
        /// This cluster is a picture
        const IS_PICTURE = 1;
        /// Whether this cluster is visible when the position node is a backface.
        const IS_BACKFACE_VISIBLE = 2;
        /// This flag is set during the first pass picture traversal, depending on whether
        /// the cluster is visible or not. It's read during the second pass when primitives
        /// consult their owning clusters to see if the primitive itself is visible.
        const IS_VISIBLE = 4;
        /// Is a backdrop-filter cluster that requires special handling during post_update.
        const IS_BACKDROP_FILTER = 8;
    }
}

/// Descriptor for a cluster of primitives. For now, this is quite basic but will be
/// extended to handle more spatial clustering of primitives.
#[cfg_attr(feature = "capture", derive(Serialize))]
pub struct PrimitiveCluster {
    /// The positioning node for this cluster.
    pub spatial_node_index: SpatialNodeIndex,
    /// The bounding rect of the cluster, in the local space of the spatial node.
    /// This is used to quickly determine the overall bounding rect for a picture
    /// during the first picture traversal, which is needed for local scale
    /// determination, and render task size calculations.
    bounding_rect: LayoutRect,
    /// a part of the cluster that we know to be opaque if any. Does not always
    /// describe the entire opaque region, but all content within that rect must
    /// be opaque.
    pub opaque_rect: LayoutRect,
    /// The range of primitive instance indices associated with this cluster.
    pub prim_range: Range<usize>,
    /// Various flags / state for this cluster.
    pub flags: ClusterFlags,
}

impl PrimitiveCluster {
    /// Construct a new primitive cluster for a given positioning node.
    fn new(
        spatial_node_index: SpatialNodeIndex,
        flags: ClusterFlags,
        first_instance_index: usize,
    ) -> Self {
        PrimitiveCluster {
            bounding_rect: LayoutRect::zero(),
            opaque_rect: LayoutRect::zero(),
            spatial_node_index,
            flags,
            prim_range: first_instance_index..first_instance_index
        }
    }

    /// Return true if this cluster is compatible with the given params
    pub fn is_compatible(
        &self,
        spatial_node_index: SpatialNodeIndex,
        flags: ClusterFlags,
    ) -> bool {
        self.flags == flags && self.spatial_node_index == spatial_node_index
    }

    pub fn prim_range(&self) -> Range<usize> {
        self.prim_range.clone()
    }

    /// Add a primitive instance to this cluster, at the start or end
    fn add_instance(
        &mut self,
        culling_rect: &LayoutRect,
        instance_index: usize,
    ) {
        debug_assert_eq!(instance_index, self.prim_range.end);
        self.bounding_rect = self.bounding_rect.union(culling_rect);
        self.prim_range.end += 1;
    }
}

/// A list of primitive instances that are added to a picture
/// This ensures we can keep a list of primitives that
/// are pictures, for a fast initial traversal of the picture
/// tree without walking the instance list.
#[cfg_attr(feature = "capture", derive(Serialize))]
pub struct PrimitiveList {
    /// List of primitives grouped into clusters.
    pub clusters: Vec<PrimitiveCluster>,
    pub prim_instances: Vec<PrimitiveInstance>,
}

impl PrimitiveList {
    /// Construct an empty primitive list. This is
    /// just used during the take_context / restore_context
    /// borrow check dance, which will be removed as the
    /// picture traversal pass is completed.
    pub fn empty() -> Self {
        PrimitiveList {
            clusters: Vec::new(),
            prim_instances: Vec::new(),
        }
    }

    /// Add a primitive instance to the end of the list
    pub fn add_prim(
        &mut self,
        prim_instance: PrimitiveInstance,
        prim_rect: LayoutRect,
        spatial_node_index: SpatialNodeIndex,
        prim_flags: PrimitiveFlags,
    ) {
        let mut flags = ClusterFlags::empty();

        // Pictures are always put into a new cluster, to make it faster to
        // iterate all pictures in a given primitive list.
        match prim_instance.kind {
            PrimitiveInstanceKind::Picture { .. } => {
                flags.insert(ClusterFlags::IS_PICTURE);
            }
            PrimitiveInstanceKind::Backdrop { .. } => {
                flags.insert(ClusterFlags::IS_BACKDROP_FILTER);
            }
            _ => {}
        }

        if prim_flags.contains(PrimitiveFlags::IS_BACKFACE_VISIBLE) {
            flags.insert(ClusterFlags::IS_BACKFACE_VISIBLE);
        }

        let culling_rect = prim_instance.clip_set.local_clip_rect
            .intersection(&prim_rect)
            .unwrap_or_else(LayoutRect::zero);

        // Primitive lengths aren't evenly distributed among primitive lists:
        // We often have a large amount of single primitive lists, a
        // few below 20~30 primitives, and even fewer lists (maybe a couple)
        // in the multiple hundreds with nothing in between.
        // We can see in profiles that reallocating vectors while pushing
        // primitives is taking a large amount of the total scene build time,
        // so we take advantage of what we know about the length distributions
        // to go for an adapted vector growth pattern that avoids over-allocating
        // for the many small allocations while avoiding a lot of reallocation by
        // quickly converging to the common sizes.
        // Rust's default vector growth strategy (when pushing elements one by one)
        // is to double the capacity every time.
        let prims_len = self.prim_instances.len();
        if prims_len == self.prim_instances.capacity() {
            let next_alloc = match prims_len {
                1 ..= 31 => 32 - prims_len,
                32 ..= 256 => 512 - prims_len,
                _ => prims_len * 2,
            };

            self.prim_instances.reserve(next_alloc);
        }

        let instance_index = prims_len;
        self.prim_instances.push(prim_instance);

        if let Some(cluster) = self.clusters.last_mut() {
            if cluster.is_compatible(spatial_node_index, flags) {
                cluster.add_instance(&culling_rect, instance_index);
                return;
            }
        }

        // Same idea with clusters, using a different distribution.
        let clusters_len = self.clusters.len();
        if clusters_len == self.clusters.capacity() {
            let next_alloc = match clusters_len {
                1 ..= 15 => 16 - clusters_len,
                16 ..= 127 => 128 - clusters_len,
                _ => clusters_len * 2,
            };

            self.clusters.reserve(next_alloc);
        }

        let mut cluster = PrimitiveCluster::new(
            spatial_node_index,
            flags,
            instance_index,
        );

        cluster.add_instance(&culling_rect, instance_index);
        self.clusters.push(cluster);
    }

    /// Returns true if there are no clusters (and thus primitives)
    pub fn is_empty(&self) -> bool {
        self.clusters.is_empty()
    }
}

/// Defines configuration options for a given picture primitive.
#[cfg_attr(feature = "capture", derive(Serialize))]
pub struct PictureOptions {
    /// If true, WR should inflate the bounding rect of primitives when
    /// using a filter effect that requires inflation.
    pub inflate_if_required: bool,
}

impl Default for PictureOptions {
    fn default() -> Self {
        PictureOptions {
            inflate_if_required: true,
        }
    }
}

#[cfg_attr(feature = "capture", derive(Serialize))]
pub struct PicturePrimitive {
    /// List of primitives, and associated info for this picture.
    pub prim_list: PrimitiveList,

    #[cfg_attr(feature = "capture", serde(skip))]
    pub state: Option<PictureState>,

    /// If true, apply the local clip rect to primitive drawn
    /// in this picture.
    pub apply_local_clip_rect: bool,
    /// If false and transform ends up showing the back of the picture,
    /// it will be considered invisible.
    pub is_backface_visible: bool,

    // If a mix-blend-mode, contains the render task for
    // the readback of the framebuffer that we use to sample
    // from in the mix-blend-mode shader.
    // For drop-shadow filter, this will store the original
    // picture task which would be rendered on screen after
    // blur pass.
    pub secondary_render_task_id: Option<RenderTaskId>,
    /// How this picture should be composited.
    /// If None, don't composite - just draw directly on parent surface.
    pub requested_composite_mode: Option<PictureCompositeMode>,

    pub raster_config: Option<RasterConfig>,
    pub context_3d: Picture3DContext<OrderedPictureChild>,

    // Optional cache handles for storing extra data
    // in the GPU cache, depending on the type of
    // picture.
    pub extra_gpu_data_handles: SmallVec<[GpuCacheHandle; 1]>,

    /// The spatial node index of this picture when it is
    /// composited into the parent picture.
    pub spatial_node_index: SpatialNodeIndex,

    /// The conservative local rect of this picture. It is
    /// built dynamically during the first picture traversal.
    /// It is composed of already snapped primitives.
    pub estimated_local_rect: LayoutRect,

    /// The local rect of this picture. It is built
    /// dynamically during the frame visibility update. It
    /// differs from the estimated_local_rect because it
    /// will not contain culled primitives, takes into
    /// account surface inflation and the whole clip chain.
    /// It is frequently the same, but may be quite
    /// different depending on how much was culled.
    pub precise_local_rect: LayoutRect,

    /// Store the state of the previous precise local rect
    /// for this picture. We need this in order to know when
    /// to invalidate segments / drop-shadow gpu cache handles.
    pub prev_precise_local_rect: LayoutRect,

    /// If false, this picture needs to (re)build segments
    /// if it supports segment rendering. This can occur
    /// if the local rect of the picture changes due to
    /// transform animation and/or scrolling.
    pub segments_are_valid: bool,

    /// The config options for this picture.
    pub options: PictureOptions,

    /// Set to true if we know for sure the picture is fully opaque.
    pub is_opaque: bool,
}

impl PicturePrimitive {
    pub fn print<T: PrintTreePrinter>(
        &self,
        pictures: &[Self],
        self_index: PictureIndex,
        pt: &mut T,
    ) {
        pt.new_level(format!("{:?}", self_index));
        pt.add_item(format!("cluster_count: {:?}", self.prim_list.clusters.len()));
        pt.add_item(format!("estimated_local_rect: {:?}", self.estimated_local_rect));
        pt.add_item(format!("precise_local_rect: {:?}", self.precise_local_rect));
        pt.add_item(format!("spatial_node_index: {:?}", self.spatial_node_index));
        pt.add_item(format!("raster_config: {:?}", self.raster_config));
        pt.add_item(format!("requested_composite_mode: {:?}", self.requested_composite_mode));

        for cluster in &self.prim_list.clusters {
            if !cluster.flags.contains(ClusterFlags::IS_PICTURE) {
                continue;
            }
            for instance in &self.prim_list.prim_instances[cluster.prim_range()] {
                let index = match instance.kind {
                    PrimitiveInstanceKind::Picture { pic_index, .. } => pic_index,
                    _ => unreachable!(),
                };
                pictures[index.0].print(pictures, index, pt);
            }
        }

        pt.end_level();
    }

    /// Returns true if this picture supports segmented rendering.
    pub fn can_use_segments(&self) -> bool {
        match self.raster_config {
            // TODO(gw): Support brush segment rendering for filter and mix-blend
            //           shaders. It's possible this already works, but I'm just
            //           applying this optimization to Blit mode for now.
            Some(RasterConfig { composite_mode: PictureCompositeMode::MixBlend(..), .. }) |
            Some(RasterConfig { composite_mode: PictureCompositeMode::Filter(..), .. }) |
            Some(RasterConfig { composite_mode: PictureCompositeMode::ComponentTransferFilter(..), .. }) |
            Some(RasterConfig { composite_mode: PictureCompositeMode::TileCache { .. }, .. }) |
            Some(RasterConfig { composite_mode: PictureCompositeMode::SvgFilter(..), .. }) |
            None => {
                false
            }
            Some(RasterConfig { composite_mode: PictureCompositeMode::Blit(reason), ..}) => {
                reason == BlitReason::CLIP
            }
        }
    }

    fn resolve_scene_properties(&mut self, properties: &SceneProperties) -> bool {
        match self.requested_composite_mode {
            Some(PictureCompositeMode::Filter(ref mut filter)) => {
                match *filter {
                    Filter::Opacity(ref binding, ref mut value) => {
                        *value = properties.resolve_float(binding);
                    }
                    _ => {}
                }

                filter.is_visible()
            }
            _ => true,
        }
    }

    pub fn is_visible(&self) -> bool {
        match self.requested_composite_mode {
            Some(PictureCompositeMode::Filter(ref filter)) => {
                filter.is_visible()
            }
            _ => true,
        }
    }

    // TODO(gw): We have the PictureOptions struct available. We
    //           should move some of the parameter list in this
    //           method to be part of the PictureOptions, and
    //           avoid adding new parameters here.
    pub fn new_image(
        requested_composite_mode: Option<PictureCompositeMode>,
        context_3d: Picture3DContext<OrderedPictureChild>,
        apply_local_clip_rect: bool,
        flags: PrimitiveFlags,
        prim_list: PrimitiveList,
        spatial_node_index: SpatialNodeIndex,
        options: PictureOptions,
    ) -> Self {
        PicturePrimitive {
            prim_list,
            state: None,
            secondary_render_task_id: None,
            requested_composite_mode,
            raster_config: None,
            context_3d,
            extra_gpu_data_handles: SmallVec::new(),
            apply_local_clip_rect,
            is_backface_visible: flags.contains(PrimitiveFlags::IS_BACKFACE_VISIBLE),
            spatial_node_index,
            estimated_local_rect: LayoutRect::zero(),
            precise_local_rect: LayoutRect::zero(),
            prev_precise_local_rect: LayoutRect::zero(),
            options,
            segments_are_valid: false,
            is_opaque: false,
        }
    }

    pub fn take_context(
        &mut self,
        pic_index: PictureIndex,
        surface_spatial_node_index: SpatialNodeIndex,
        raster_spatial_node_index: SpatialNodeIndex,
        parent_surface_index: SurfaceIndex,
        parent_subpixel_mode: &SubpixelMode,
        frame_state: &mut FrameBuildingState,
        frame_context: &FrameBuildingContext,
        scratch: &mut PrimitiveScratchBuffer,
        tile_cache_logger: &mut TileCacheLogger,
        tile_caches: &mut FastHashMap<SliceId, Box<TileCacheInstance>>,
    ) -> Option<(PictureContext, PictureState, PrimitiveList)> {
        if !self.is_visible() {
            return None;
        }

        profile_scope!("take_context");

        // Extract the raster and surface spatial nodes from the raster
        // config, if this picture establishes a surface. Otherwise just
        // pass in the spatial node indices from the parent context.
        let (raster_spatial_node_index, surface_spatial_node_index, surface_index, inflation_factor) = match self.raster_config {
            Some(ref raster_config) => {
                let surface = &frame_state.surfaces[raster_config.surface_index.0];

                (
                    surface.raster_spatial_node_index,
                    self.spatial_node_index,
                    raster_config.surface_index,
                    surface.inflation_factor,
                )
            }
            None => {
                (
                    raster_spatial_node_index,
                    surface_spatial_node_index,
                    parent_surface_index,
                    0.0,
                )
            }
        };

        let map_pic_to_world = SpaceMapper::new_with_target(
            ROOT_SPATIAL_NODE_INDEX,
            surface_spatial_node_index,
            frame_context.global_screen_world_rect,
            frame_context.spatial_tree,
        );

        let pic_bounds = map_pic_to_world.unmap(&map_pic_to_world.bounds)
                                         .unwrap_or_else(PictureRect::max_rect);

        let map_local_to_pic = SpaceMapper::new(
            surface_spatial_node_index,
            pic_bounds,
        );

        let (map_raster_to_world, map_pic_to_raster) = create_raster_mappers(
            surface_spatial_node_index,
            raster_spatial_node_index,
            frame_context.global_screen_world_rect,
            frame_context.spatial_tree,
        );

        let plane_splitter = match self.context_3d {
            Picture3DContext::Out => {
                None
            }
            Picture3DContext::In { root_data: Some(_), .. } => {
                Some(PlaneSplitter::new())
            }
            Picture3DContext::In { root_data: None, .. } => {
                None
            }
        };

        match self.raster_config {
            Some(RasterConfig { surface_index, composite_mode: PictureCompositeMode::TileCache { slice_id }, .. }) => {
                let tile_cache = tile_caches.get_mut(&slice_id).unwrap();
                let mut debug_info = SliceDebugInfo::new();
                let mut surface_tasks = Vec::with_capacity(tile_cache.tiles.len());
                let device_pixel_scale = frame_state
                    .surfaces[surface_index.0]
                    .device_pixel_scale;

                // Get the overall world space rect of the picture cache. Used to clip
                // the tile rects below for occlusion testing to the relevant area.
                let world_clip_rect = map_pic_to_world
                    .map(&tile_cache.local_clip_rect)
                    .expect("bug: unable to map clip rect");
                let device_clip_rect = (world_clip_rect * frame_context.global_device_pixel_scale).round();

                for tile in tile_cache.tiles.values_mut() {

                    if tile.is_visible {
                        // Get the world space rect that this tile will actually occupy on screem
                        let device_draw_rect = device_clip_rect.intersection(&tile.device_valid_rect);

                        // If that draw rect is occluded by some set of tiles in front of it,
                        // then mark it as not visible and skip drawing. When it's not occluded
                        // it will fail this test, and get rasterized by the render task setup
                        // code below.
                        match device_draw_rect {
                            Some(device_draw_rect) => {
                                // Only check for occlusion on visible tiles that are fixed position.
                                if tile_cache.spatial_node_index == ROOT_SPATIAL_NODE_INDEX &&
                                   frame_state.composite_state.occluders.is_tile_occluded(tile.z_id, device_draw_rect) {
                                    // If this tile has an allocated native surface, free it, since it's completely
                                    // occluded. We will need to re-allocate this surface if it becomes visible,
                                    // but that's likely to be rare (e.g. when there is no content display list
                                    // for a frame or two during a tab switch).
                                    let surface = tile.surface.as_mut().expect("no tile surface set!");

                                    if let TileSurface::Texture { descriptor: SurfaceTextureDescriptor::Native { id, .. }, .. } = surface {
                                        if let Some(id) = id.take() {
                                            frame_state.resource_cache.destroy_compositor_tile(id);
                                        }
                                    }

                                    tile.is_visible = false;

                                    if frame_context.fb_config.testing {
                                        debug_info.tiles.insert(
                                            tile.tile_offset,
                                            TileDebugInfo::Occluded,
                                        );
                                    }

                                    continue;
                                }
                            }
                            None => {
                                tile.is_visible = false;
                            }
                        }
                    }

                    // If we get here, we want to ensure that the surface remains valid in the texture
                    // cache, _even if_ it's not visible due to clipping or being scrolled off-screen.
                    // This ensures that we retain valid tiles that are off-screen, but still in the
                    // display port of this tile cache instance.
                    if let Some(TileSurface::Texture { descriptor, .. }) = tile.surface.as_ref() {
                        if let SurfaceTextureDescriptor::TextureCache { ref handle, .. } = descriptor {
                            frame_state.resource_cache.texture_cache.request(
                                handle,
                                frame_state.gpu_cache,
                            );
                        }
                    }

                    // If the tile has been found to be off-screen / clipped, skip any further processing.
                    if !tile.is_visible {
                        if frame_context.fb_config.testing {
                            debug_info.tiles.insert(
                                tile.tile_offset,
                                TileDebugInfo::Culled,
                            );
                        }

                        continue;
                    }

                    if frame_context.debug_flags.contains(DebugFlags::PICTURE_CACHING_DBG) {
                        tile.root.draw_debug_rects(
                            &map_pic_to_world,
                            tile.is_opaque,
                            tile.current_descriptor.local_valid_rect,
                            scratch,
                            frame_context.global_device_pixel_scale,
                        );

                        let label_offset = DeviceVector2D::new(20.0, 30.0);
                        let tile_device_rect = tile.world_tile_rect * frame_context.global_device_pixel_scale;
                        if tile_device_rect.size.height >= label_offset.y {
                            let surface = tile.surface.as_ref().expect("no tile surface set!");

                            scratch.push_debug_string(
                                tile_device_rect.origin + label_offset,
                                debug_colors::RED,
                                format!("{:?}: s={} is_opaque={} surface={}",
                                        tile.id,
                                        tile_cache.slice,
                                        tile.is_opaque,
                                        surface.kind(),
                                ),
                            );
                        }
                    }

                    if let TileSurface::Texture { descriptor, .. } = tile.surface.as_mut().unwrap() {
                        match descriptor {
                            SurfaceTextureDescriptor::TextureCache { ref handle, .. } => {
                                // Invalidate if the backing texture was evicted.
                                if frame_state.resource_cache.texture_cache.is_allocated(handle) {
                                    // Request the backing texture so it won't get evicted this frame.
                                    // We specifically want to mark the tile texture as used, even
                                    // if it's detected not visible below and skipped. This is because
                                    // we maintain the set of tiles we care about based on visibility
                                    // during pre_update. If a tile still exists after that, we are
                                    // assuming that it's either visible or we want to retain it for
                                    // a while in case it gets scrolled back onto screen soon.
                                    // TODO(gw): Consider switching to manual eviction policy?
                                    frame_state.resource_cache.texture_cache.request(handle, frame_state.gpu_cache);
                                } else {
                                    // If the texture was evicted on a previous frame, we need to assume
                                    // that the entire tile rect is dirty.
                                    tile.invalidate(None, InvalidationReason::NoTexture);
                                }
                            }
                            SurfaceTextureDescriptor::Native { id, .. } => {
                                if id.is_none() {
                                    // There is no current surface allocation, so ensure the entire tile is invalidated
                                    tile.invalidate(None, InvalidationReason::NoSurface);
                                }
                            }
                        }
                    }

                    // Ensure that the dirty rect doesn't extend outside the local valid rect.
                    tile.local_dirty_rect = tile.local_dirty_rect
                        .intersection(&tile.current_descriptor.local_valid_rect)
                        .unwrap_or_else(PictureRect::zero);

                    // Update the world/device dirty rect
                    let world_dirty_rect = map_pic_to_world.map(&tile.local_dirty_rect).expect("bug");

                    let device_rect = (tile.world_tile_rect * frame_context.global_device_pixel_scale).round();
                    tile.device_dirty_rect = (world_dirty_rect * frame_context.global_device_pixel_scale)
                        .round_out()
                        .intersection(&device_rect)
                        .unwrap_or_else(DeviceRect::zero);

                    if tile.is_valid {
                        if frame_context.fb_config.testing {
                            debug_info.tiles.insert(
                                tile.tile_offset,
                                TileDebugInfo::Valid,
                            );
                        }

                        continue;
                    }

                    // Get the visibility mask bit(s) for this tile from the dirty region tracker. This must be done
                    // outside the if statement below, so that we include in the dirty region tiles that are handled
                    // by a background color only (no surface allocation).
                    let tile_vis_mask = tile_cache.dirty_region.add_dirty_region(
                        tile.local_dirty_rect,
                        frame_context.spatial_tree,
                    );

                    // Ensure that this texture is allocated.
                    if let TileSurface::Texture { ref mut descriptor, ref mut visibility_mask } = tile.surface.as_mut().unwrap() {
                        *visibility_mask = tile_vis_mask;

                        match descriptor {
                            SurfaceTextureDescriptor::TextureCache { ref mut handle } => {
                                if !frame_state.resource_cache.texture_cache.is_allocated(handle) {
                                    frame_state.resource_cache.texture_cache.update_picture_cache(
                                        tile_cache.current_tile_size,
                                        handle,
                                        frame_state.gpu_cache,
                                    );
                                }
                            }
                            SurfaceTextureDescriptor::Native { id } => {
                                if id.is_none() {
                                    // Allocate a native surface id if we're in native compositing mode,
                                    // and we don't have a surface yet (due to first frame, or destruction
                                    // due to tile size changing etc).
                                    if tile_cache.native_surface.is_none() {
                                        let opaque = frame_state
                                            .resource_cache
                                            .create_compositor_surface(
                                                tile_cache.virtual_offset,
                                                tile_cache.current_tile_size,
                                                true,
                                            );

                                        let alpha = frame_state
                                            .resource_cache
                                            .create_compositor_surface(
                                                tile_cache.virtual_offset,
                                                tile_cache.current_tile_size,
                                                false,
                                            );

                                        tile_cache.native_surface = Some(NativeSurface {
                                            opaque,
                                            alpha,
                                        });
                                    }

                                    // Create the tile identifier and allocate it.
                                    let surface_id = if tile.is_opaque {
                                        tile_cache.native_surface.as_ref().unwrap().opaque
                                    } else {
                                        tile_cache.native_surface.as_ref().unwrap().alpha
                                    };

                                    let tile_id = NativeTileId {
                                        surface_id,
                                        x: tile.tile_offset.x,
                                        y: tile.tile_offset.y,
                                    };

                                    frame_state.resource_cache.create_compositor_tile(tile_id);

                                    *id = Some(tile_id);
                                }
                            }
                        }

                        let content_origin_f = tile.world_tile_rect.origin * device_pixel_scale;
                        let content_origin = content_origin_f.round();
                        debug_assert!((content_origin_f.x - content_origin.x).abs() < 0.01);
                        debug_assert!((content_origin_f.y - content_origin.y).abs() < 0.01);

                        let surface = descriptor.resolve(
                            frame_state.resource_cache,
                            tile_cache.current_tile_size,
                        );

                        let scissor_rect = tile.device_dirty_rect
                            .translate(-device_rect.origin.to_vector())
                            .round()
                            .to_i32();

                        let valid_rect = tile.device_valid_rect
                            .translate(-device_rect.origin.to_vector())
                            .round()
                            .to_i32();

                        let task_size = tile_cache.current_tile_size;

                        let render_task_id = frame_state.rg_builder.add().init(
                            RenderTask::new(
                                RenderTaskLocation::Static {
                                    surface: StaticRenderTaskSurface::PictureCache {
                                        surface,
                                    },
                                    rect: task_size.into(),
                                },
                                RenderTaskKind::new_picture(
                                    task_size,
                                    tile_cache.current_tile_size.to_f32(),
                                    pic_index,
                                    content_origin,
                                    UvRectKind::Rect,
                                    surface_spatial_node_index,
                                    device_pixel_scale,
                                    *visibility_mask,
                                    Some(scissor_rect),
                                    Some(valid_rect),
                                )
                            ),
                        );

                        surface_tasks.push(render_task_id);
                    }

                    if frame_context.fb_config.testing {
                        debug_info.tiles.insert(
                            tile.tile_offset,
                            TileDebugInfo::Dirty(DirtyTileDebugInfo {
                                local_valid_rect: tile.current_descriptor.local_valid_rect,
                                local_dirty_rect: tile.local_dirty_rect,
                            }),
                        );
                    }

                    // If the entire tile valid region is dirty, we can update the fract offset
                    // at which the tile was rendered.
                    if tile.device_dirty_rect.contains_rect(&tile.device_valid_rect) {
                        tile.device_fract_offset = tile_cache.device_fract_offset;
                    }

                    // Now that the tile is valid, reset the dirty rect.
                    tile.local_dirty_rect = PictureRect::zero();
                    tile.is_valid = true;
                }

                // If invalidation debugging is enabled, dump the picture cache state to a tree printer.
                if frame_context.debug_flags.contains(DebugFlags::INVALIDATION_DBG) {
                    tile_cache.print();
                }

                // If testing mode is enabled, write some information about the current state
                // of this picture cache (made available in RenderResults).
                if frame_context.fb_config.testing {
                    frame_state.composite_state
                        .picture_cache_debug
                        .slices
                        .insert(
                            tile_cache.slice,
                            debug_info,
                        );
                }

                frame_state.init_surface_tiled(
                    surface_index,
                    surface_tasks,
                );
            }
            Some(ref mut raster_config) => {
                let pic_rect = self.precise_local_rect.cast_unit();

                let mut device_pixel_scale = frame_state
                    .surfaces[raster_config.surface_index.0]
                    .device_pixel_scale;

                let scale_factors = frame_state
                    .surfaces[raster_config.surface_index.0]
                    .scale_factors;

                // If the primitive has a filter that can sample with an offset, the clip rect has
                // to take it into account.
                let clip_inflation = match raster_config.composite_mode {
                    PictureCompositeMode::Filter(Filter::DropShadows(ref shadows)) => {
                        let mut max_offset = vec2(0.0, 0.0);
                        let mut min_offset = vec2(0.0, 0.0);
                        for shadow in shadows {
                            let offset = layout_vector_as_picture_vector(shadow.offset);
                            max_offset = max_offset.max(offset);
                            min_offset = min_offset.min(offset);
                        }

                        // Get the shadow offsets in world space.
                        let raster_min = map_pic_to_raster.map_vector(min_offset);
                        let raster_max = map_pic_to_raster.map_vector(max_offset);
                        let world_min = map_raster_to_world.map_vector(raster_min);
                        let world_max = map_raster_to_world.map_vector(raster_max);

                        // Grow the clip in the opposite direction of the shadow's offset.
                        SideOffsets2D::from_vectors_outer(
                            -world_max.max(vec2(0.0, 0.0)),
                            -world_min.min(vec2(0.0, 0.0)),
                        )
                    }
                    _ => SideOffsets2D::zero(),
                };

                let (mut clipped, mut unclipped) = match get_raster_rects(
                    pic_rect,
                    &map_pic_to_raster,
                    &map_raster_to_world,
                    raster_config.clipped_bounding_rect.outer_rect(clip_inflation),
                    device_pixel_scale,
                ) {
                    Some(info) => info,
                    None => {
                        return None
                    }
                };
                let transform = map_pic_to_raster.get_transform();

                /// If the picture (raster_config) establishes a raster root,
                /// its requested resolution won't be clipped by the parent or
                /// viewport; so we need to make sure the requested resolution is
                /// "reasonable", ie. <= MAX_SURFACE_SIZE.  If not, scale the
                /// picture down until it fits that limit.  This results in a new
                /// device_rect, a new unclipped rect, and a new device_pixel_scale.
                ///
                /// Since the adjusted device_pixel_scale is passed into the
                /// RenderTask (and then the shader via RenderTaskData) this mostly
                /// works transparently, reusing existing support for variable DPI
                /// support.  The on-the-fly scaling can be seen as on-the-fly,
                /// per-task DPI adjustment.  Logical pixels are unaffected.
                ///
                /// The scaling factor is returned to the caller; blur radius,
                /// font size, etc. need to be scaled accordingly.
                fn adjust_scale_for_max_surface_size(
                    raster_config: &RasterConfig,
                    max_target_size: i32,
                    pic_rect: PictureRect,
                    map_pic_to_raster: &SpaceMapper<PicturePixel, RasterPixel>,
                    map_raster_to_world: &SpaceMapper<RasterPixel, WorldPixel>,
                    clipped_prim_bounding_rect: WorldRect,
                    device_pixel_scale : &mut DevicePixelScale,
                    device_rect: &mut DeviceRect,
                    unclipped: &mut DeviceRect) -> Option<f32>
                {
                    let limit = if raster_config.establishes_raster_root {
                        MAX_SURFACE_SIZE
                    } else {
                        max_target_size as f32
                    };
                    if device_rect.size.width > limit || device_rect.size.height > limit {
                        // round_out will grow by 1 integer pixel if origin is on a
                        // fractional position, so keep that margin for error with -1:
                        let scale = (limit as f32 - 1.0) /
                                    (f32::max(device_rect.size.width, device_rect.size.height));
                        *device_pixel_scale = *device_pixel_scale * Scale::new(scale);
                        let new_device_rect = device_rect.to_f32() * Scale::new(scale);
                        *device_rect = new_device_rect.round_out();

                        *unclipped = match get_raster_rects(
                            pic_rect,
                            &map_pic_to_raster,
                            &map_raster_to_world,
                            clipped_prim_bounding_rect,
                            *device_pixel_scale
                        ) {
                            Some(info) => info.1,
                            None => {
                                return None
                            }
                        };
                        Some(scale)
                    }
                    else
                    {
                        None
                    }
                }

                match raster_config.composite_mode {
                    PictureCompositeMode::TileCache { .. } => {
                        unreachable!("handled above");
                    }
                    PictureCompositeMode::Filter(Filter::Blur(width, height)) => {
                        let width_std_deviation = clamp_blur_radius(width, scale_factors) * device_pixel_scale.0;
                        let height_std_deviation = clamp_blur_radius(height, scale_factors) * device_pixel_scale.0;
                        let mut blur_std_deviation = DeviceSize::new(
                            width_std_deviation * scale_factors.0,
                            height_std_deviation * scale_factors.1
                        );
                        let mut device_rect = if self.options.inflate_if_required {
                            let inflation_factor = frame_state.surfaces[raster_config.surface_index.0].inflation_factor;
                            let inflation_factor = inflation_factor * device_pixel_scale.0;

                            // The clipped field is the part of the picture that is visible
                            // on screen. The unclipped field is the screen-space rect of
                            // the complete picture, if no screen / clip-chain was applied
                            // (this includes the extra space for blur region). To ensure
                            // that we draw a large enough part of the picture to get correct
                            // blur results, inflate that clipped area by the blur range, and
                            // then intersect with the total screen rect, to minimize the
                            // allocation size.
                            clipped
                                .inflate(inflation_factor * scale_factors.0, inflation_factor * scale_factors.1)
                                .intersection(&unclipped)
                                .unwrap()
                        } else {
                            clipped
                        };

                        let mut original_size = device_rect.size;

                        // Adjust the size to avoid introducing sampling errors during the down-scaling passes.
                        // what would be even better is to rasterize the picture at the down-scaled size
                        // directly.
                        device_rect.size = BlurTask::adjusted_blur_source_size(
                            device_rect.size,
                            blur_std_deviation,
                        );

                        if let Some(scale) = adjust_scale_for_max_surface_size(
                            raster_config, frame_context.fb_config.max_target_size,
                            pic_rect, &map_pic_to_raster, &map_raster_to_world,
                            raster_config.clipped_bounding_rect,
                            &mut device_pixel_scale, &mut device_rect, &mut unclipped,
                        ) {
                            blur_std_deviation = blur_std_deviation * scale;
                            original_size = original_size.to_f32() * scale;
                            raster_config.root_scaling_factor = scale;
                        }

                        let uv_rect_kind = calculate_uv_rect_kind(
                            &pic_rect,
                            &transform,
                            &device_rect,
                            device_pixel_scale,
                        );

                        let task_size = device_rect.size.to_i32();

                        let picture_task_id = frame_state.rg_builder.add().init(
                            RenderTask::new_dynamic(
                                task_size,
                                RenderTaskKind::new_picture(
                                    task_size,
                                    unclipped.size,
                                    pic_index,
                                    device_rect.origin,
                                    uv_rect_kind,
                                    surface_spatial_node_index,
                                    device_pixel_scale,
                                    PrimitiveVisibilityMask::all(),
                                    None,
                                    None,
                                )
                            )
                        );

                        let blur_render_task_id = RenderTask::new_blur(
                            blur_std_deviation,
                            picture_task_id,
                            frame_state.rg_builder,
                            RenderTargetKind::Color,
                            None,
                            original_size.to_i32(),
                        );

                        frame_state.init_surface_chain(
                            raster_config.surface_index,
                            blur_render_task_id,
                            picture_task_id,
                            parent_surface_index,
                        );
                    }
                    PictureCompositeMode::Filter(Filter::DropShadows(ref shadows)) => {
                        let mut max_std_deviation = 0.0;
                        for shadow in shadows {
                            max_std_deviation = f32::max(max_std_deviation, shadow.blur_radius);
                        }
                        max_std_deviation = clamp_blur_radius(max_std_deviation, scale_factors) * device_pixel_scale.0;
                        let max_blur_range = max_std_deviation * BLUR_SAMPLE_SCALE;

                        // We cast clipped to f32 instead of casting unclipped to i32
                        // because unclipped can overflow an i32.
                        let mut device_rect = clipped
                                .inflate(max_blur_range * scale_factors.0, max_blur_range * scale_factors.1)
                                .intersection(&unclipped)
                                .unwrap();

                        device_rect.size = BlurTask::adjusted_blur_source_size(
                            device_rect.size,
                            DeviceSize::new(
                                max_std_deviation * scale_factors.0,
                                max_std_deviation * scale_factors.1
                            ),
                        );

                        if let Some(scale) = adjust_scale_for_max_surface_size(
                            raster_config, frame_context.fb_config.max_target_size,
                            pic_rect, &map_pic_to_raster, &map_raster_to_world,
                            raster_config.clipped_bounding_rect,
                            &mut device_pixel_scale, &mut device_rect, &mut unclipped,
                        ) {
                            // std_dev adjusts automatically from using device_pixel_scale
                            raster_config.root_scaling_factor = scale;
                        }

                        let uv_rect_kind = calculate_uv_rect_kind(
                            &pic_rect,
                            &transform,
                            &device_rect,
                            device_pixel_scale,
                        );

                        let task_size = device_rect.size.to_i32();

                        let picture_task_id = frame_state.rg_builder.add().init(
                            RenderTask::new_dynamic(
                                task_size,
                                RenderTaskKind::new_picture(
                                    task_size,
                                    unclipped.size,
                                    pic_index,
                                    device_rect.origin,
                                    uv_rect_kind,
                                    surface_spatial_node_index,
                                    device_pixel_scale,
                                    PrimitiveVisibilityMask::all(),
                                    None,
                                    None,
                                ),
                            )
                        );

                        // Add this content picture as a dependency of the parent surface, to
                        // ensure it isn't free'd after the shadow uses it as an input.
                        frame_state.add_child_render_task(
                            parent_surface_index,
                            picture_task_id,
                        );

                        self.secondary_render_task_id = Some(picture_task_id);

                        let mut blur_tasks = BlurTaskCache::default();

                        self.extra_gpu_data_handles.resize(shadows.len(), GpuCacheHandle::new());

                        let mut blur_render_task_id = picture_task_id;
                        for shadow in shadows {
                            let blur_radius = clamp_blur_radius(shadow.blur_radius, scale_factors) * device_pixel_scale.0;
                            blur_render_task_id = RenderTask::new_blur(
                                DeviceSize::new(
                                    blur_radius * scale_factors.0,
                                    blur_radius * scale_factors.1,
                                ),
                                picture_task_id,
                                frame_state.rg_builder,
                                RenderTargetKind::Color,
                                Some(&mut blur_tasks),
                                device_rect.size.to_i32(),
                            );
                        }

                        // TODO(nical) the second one should to be the blur's task id but we have several blurs now
                        frame_state.init_surface_chain(
                            raster_config.surface_index,
                            blur_render_task_id,
                            picture_task_id,
                            parent_surface_index,
                        );
                    }
                    PictureCompositeMode::MixBlend(..) if !frame_context.fb_config.gpu_supports_advanced_blend => {
                        if let Some(scale) = adjust_scale_for_max_surface_size(
                            raster_config, frame_context.fb_config.max_target_size,
                            pic_rect, &map_pic_to_raster, &map_raster_to_world,
                            raster_config.clipped_bounding_rect,
                            &mut device_pixel_scale, &mut clipped, &mut unclipped,
                        ) {
                            raster_config.root_scaling_factor = scale;
                        }

                        let uv_rect_kind = calculate_uv_rect_kind(
                            &pic_rect,
                            &transform,
                            &clipped,
                            device_pixel_scale,
                        );

                        let readback_task_id = frame_state.rg_builder.add().init(
                            RenderTask::new_dynamic(
                                clipped.size.to_i32(),
                                RenderTaskKind::new_readback(),
                            )
                        );

                        frame_state.add_child_render_task(
                            parent_surface_index,
                            readback_task_id,
                        );

                        self.secondary_render_task_id = Some(readback_task_id);

                        let task_size = clipped.size.to_i32();

                        let render_task_id = frame_state.rg_builder.add().init(
                            RenderTask::new_dynamic(
                                task_size,
                                RenderTaskKind::new_picture(
                                    task_size,
                                    unclipped.size,
                                    pic_index,
                                    clipped.origin,
                                    uv_rect_kind,
                                    surface_spatial_node_index,
                                    device_pixel_scale,
                                    PrimitiveVisibilityMask::all(),
                                    None,
                                    None,
                                )
                            )
                        );

                        frame_state.init_surface(
                            raster_config.surface_index,
                            render_task_id,
                            parent_surface_index,
                        );
                    }
                    PictureCompositeMode::Filter(..) => {

                        if let Some(scale) = adjust_scale_for_max_surface_size(
                            raster_config, frame_context.fb_config.max_target_size,
                            pic_rect, &map_pic_to_raster, &map_raster_to_world,
                            raster_config.clipped_bounding_rect,
                            &mut device_pixel_scale, &mut clipped, &mut unclipped,
                        ) {
                            raster_config.root_scaling_factor = scale;
                        }

                        let uv_rect_kind = calculate_uv_rect_kind(
                            &pic_rect,
                            &transform,
                            &clipped,
                            device_pixel_scale,
                        );

                        let task_size = clipped.size.to_i32();

                        let render_task_id = frame_state.rg_builder.add().init(
                            RenderTask::new_dynamic(
                                task_size,
                                RenderTaskKind::new_picture(
                                    task_size,
                                    unclipped.size,
                                    pic_index,
                                    clipped.origin,
                                    uv_rect_kind,
                                    surface_spatial_node_index,
                                    device_pixel_scale,
                                    PrimitiveVisibilityMask::all(),
                                    None,
                                    None,
                                )
                            )
                        );

                        frame_state.init_surface(
                            raster_config.surface_index,
                            render_task_id,
                            parent_surface_index,
                        );
                    }
                    PictureCompositeMode::ComponentTransferFilter(..) => {
                        if let Some(scale) = adjust_scale_for_max_surface_size(
                            raster_config, frame_context.fb_config.max_target_size,
                            pic_rect, &map_pic_to_raster, &map_raster_to_world,
                            raster_config.clipped_bounding_rect,
                            &mut device_pixel_scale, &mut clipped, &mut unclipped,
                        ) {
                            raster_config.root_scaling_factor = scale;
                        }

                        let uv_rect_kind = calculate_uv_rect_kind(
                            &pic_rect,
                            &transform,
                            &clipped,
                            device_pixel_scale,
                        );

                        let task_size = clipped.size.to_i32();

                        let render_task_id = frame_state.rg_builder.add().init(
                            RenderTask::new_dynamic(
                                task_size,
                                RenderTaskKind::new_picture(
                                    task_size,
                                    unclipped.size,
                                    pic_index,
                                    clipped.origin,
                                    uv_rect_kind,
                                    surface_spatial_node_index,
                                    device_pixel_scale,
                                    PrimitiveVisibilityMask::all(),
                                    None,
                                    None,
                                )
                            )
                        );

                        frame_state.init_surface(
                            raster_config.surface_index,
                            render_task_id,
                            parent_surface_index,
                        );
                    }
                    PictureCompositeMode::MixBlend(..) |
                    PictureCompositeMode::Blit(_) => {
                        if let Some(scale) = adjust_scale_for_max_surface_size(
                            raster_config, frame_context.fb_config.max_target_size,
                            pic_rect, &map_pic_to_raster, &map_raster_to_world,
                            raster_config.clipped_bounding_rect,
                            &mut device_pixel_scale, &mut clipped, &mut unclipped,
                        ) {
                            raster_config.root_scaling_factor = scale;
                        }

                        let uv_rect_kind = calculate_uv_rect_kind(
                            &pic_rect,
                            &transform,
                            &clipped,
                            device_pixel_scale,
                        );

                        let task_size = clipped.size.to_i32();

                        let render_task_id = frame_state.rg_builder.add().init(
                            RenderTask::new_dynamic(
                                task_size,
                                RenderTaskKind::new_picture(
                                    task_size,
                                    unclipped.size,
                                    pic_index,
                                    clipped.origin,
                                    uv_rect_kind,
                                    surface_spatial_node_index,
                                    device_pixel_scale,
                                    PrimitiveVisibilityMask::all(),
                                    None,
                                    None,
                                )
                            )
                        );

                        frame_state.init_surface(
                            raster_config.surface_index,
                            render_task_id,
                            parent_surface_index,
                        );
                    }
                    PictureCompositeMode::SvgFilter(ref primitives, ref filter_datas) => {

                        if let Some(scale) = adjust_scale_for_max_surface_size(
                            raster_config, frame_context.fb_config.max_target_size,
                            pic_rect, &map_pic_to_raster, &map_raster_to_world,
                            raster_config.clipped_bounding_rect,
                            &mut device_pixel_scale, &mut clipped, &mut unclipped,
                        ) {
                            raster_config.root_scaling_factor = scale;
                        }

                        let uv_rect_kind = calculate_uv_rect_kind(
                            &pic_rect,
                            &transform,
                            &clipped,
                            device_pixel_scale,
                        );

                        let task_size = clipped.size.to_i32();

                        let picture_task_id = frame_state.rg_builder.add().init(
                            RenderTask::new_dynamic(
                                task_size,
                                RenderTaskKind::new_picture(
                                    task_size,
                                    unclipped.size,
                                    pic_index,
                                    clipped.origin,
                                    uv_rect_kind,
                                    surface_spatial_node_index,
                                    device_pixel_scale,
                                    PrimitiveVisibilityMask::all(),
                                    None,
                                    None,
                                )
                            )
                        );

                        let filter_task_id = RenderTask::new_svg_filter(
                            primitives,
                            filter_datas,
                            frame_state.rg_builder,
                            clipped.size.to_i32(),
                            uv_rect_kind,
                            picture_task_id,
                            device_pixel_scale,
                        );

                        frame_state.init_surface_chain(
                            raster_config.surface_index,
                            filter_task_id,
                            picture_task_id,
                            parent_surface_index,
                        );
                    }
                }
            }
            None => {}
        };

        #[cfg(feature = "capture")]
        {
            if frame_context.debug_flags.contains(DebugFlags::TILE_CACHE_LOGGING_DBG) {
                if let Some(PictureCompositeMode::TileCache { slice_id }) = self.requested_composite_mode {
                    if let Some(ref tile_cache) = tile_caches.get(&slice_id) {
                        // extract just the fields that we're interested in
                        let mut tile_cache_tiny = TileCacheInstanceSerializer {
                            slice: tile_cache.slice,
                            tiles: FastHashMap::default(),
                            background_color: tile_cache.background_color,
                            fract_offset: tile_cache.fract_offset
                        };
                        for (key, tile) in &tile_cache.tiles {
                            tile_cache_tiny.tiles.insert(*key, TileSerializer {
                                rect: tile.local_tile_rect,
                                current_descriptor: tile.current_descriptor.clone(),
                                device_fract_offset: tile.device_fract_offset,
                                id: tile.id,
                                root: tile.root.clone(),
                                background_color: tile.background_color,
                                invalidation_reason: tile.invalidation_reason.clone()
                            });
                        }
                        let text = ron::ser::to_string_pretty(&tile_cache_tiny, Default::default()).unwrap();
                        tile_cache_logger.add(text, map_pic_to_world.get_transform());
                    }
                }
            }
        }
        #[cfg(not(feature = "capture"))]
        {
            let _tile_cache_logger = tile_cache_logger;   // unused variable fix
        }

        let state = PictureState {
            //TODO: check for MAX_CACHE_SIZE here?
            map_local_to_pic,
            map_pic_to_world,
            map_pic_to_raster,
            map_raster_to_world,
            plane_splitter,
        };

        let mut dirty_region_count = 0;

        // If this is a picture cache, push the dirty region to ensure any
        // child primitives are culled and clipped to the dirty rect(s).
        if let Some(RasterConfig { composite_mode: PictureCompositeMode::TileCache { slice_id }, .. }) = self.raster_config {
            let dirty_region = tile_caches[&slice_id].dirty_region.clone();
            frame_state.push_dirty_region(dirty_region);
            dirty_region_count += 1;
        }

        if inflation_factor > 0.0 {
            let inflated_region = frame_state.current_dirty_region().inflate(
                inflation_factor,
                frame_context.spatial_tree,
            );
            frame_state.push_dirty_region(inflated_region);
            dirty_region_count += 1;
        }

        // Disallow subpixel AA if an intermediate surface is needed.
        // TODO(lsalzman): allow overriding parent if intermediate surface is opaque
        let (is_passthrough, subpixel_mode) = match self.raster_config {
            Some(RasterConfig { ref composite_mode, .. }) => {
                let subpixel_mode = match composite_mode {
                    PictureCompositeMode::TileCache { slice_id } => {
                        tile_caches[&slice_id].subpixel_mode.clone()
                    }
                    PictureCompositeMode::Blit(..) |
                    PictureCompositeMode::ComponentTransferFilter(..) |
                    PictureCompositeMode::Filter(..) |
                    PictureCompositeMode::MixBlend(..) |
                    PictureCompositeMode::SvgFilter(..) => {
                        // TODO(gw): We can take advantage of the same logic that
                        //           exists in the opaque rect detection for tile
                        //           caches, to allow subpixel text on other surfaces
                        //           that can be detected as opaque.
                        SubpixelMode::Deny
                    }
                };

                (false, subpixel_mode)
            }
            None => {
                (true, SubpixelMode::Allow)
            }
        };

        // Still disable subpixel AA if parent forbids it
        let subpixel_mode = match (parent_subpixel_mode, subpixel_mode) {
            (SubpixelMode::Allow, SubpixelMode::Allow) => {
                // Both parent and this surface unconditionally allow subpixel AA
                SubpixelMode::Allow
            }
            (SubpixelMode::Allow, SubpixelMode::Conditional { allowed_rect, excluded_rects }) => {
                // Parent allows, but we are conditional subpixel AA
                SubpixelMode::Conditional {
                    allowed_rect,
                    excluded_rects,
                }
            }
            (SubpixelMode::Conditional { allowed_rect, excluded_rects }, SubpixelMode::Allow) => {
                // Propagate conditional subpixel mode to child pictures that allow subpixel AA
                SubpixelMode::Conditional {
                    allowed_rect: *allowed_rect,
                    excluded_rects: excluded_rects.clone(),
                }
            }
            (SubpixelMode::Conditional { .. }, SubpixelMode::Conditional { ..}) => {
                unreachable!("bug: only top level picture caches have conditional subpixel");
            }
            (SubpixelMode::Deny, _) | (_, SubpixelMode::Deny) => {
                // Either parent or this surface explicitly deny subpixel, these take precedence
                SubpixelMode::Deny
            }
        };

        let context = PictureContext {
            pic_index,
            apply_local_clip_rect: self.apply_local_clip_rect,
            is_passthrough,
            raster_spatial_node_index,
            surface_spatial_node_index,
            surface_index,
            dirty_region_count,
            subpixel_mode,
        };

        let prim_list = mem::replace(&mut self.prim_list, PrimitiveList::empty());

        Some((context, state, prim_list))
    }

    pub fn restore_context(
        &mut self,
        prim_list: PrimitiveList,
        context: PictureContext,
        state: PictureState,
        frame_state: &mut FrameBuildingState,
    ) {
        // Pop any dirty regions this picture set
        for _ in 0 .. context.dirty_region_count {
            frame_state.pop_dirty_region();
        }

        self.prim_list = prim_list;
        self.state = Some(state);
    }

    pub fn take_state(&mut self) -> PictureState {
        self.state.take().expect("bug: no state present!")
    }

    /// Add a primitive instance to the plane splitter. The function would generate
    /// an appropriate polygon, clip it against the frustum, and register with the
    /// given plane splitter.
    pub fn add_split_plane(
        splitter: &mut PlaneSplitter,
        spatial_tree: &SpatialTree,
        prim_spatial_node_index: SpatialNodeIndex,
        original_local_rect: LayoutRect,
        combined_local_clip_rect: &LayoutRect,
        world_rect: WorldRect,
        plane_split_anchor: PlaneSplitAnchor,
    ) -> bool {
        let transform = spatial_tree
            .get_world_transform(prim_spatial_node_index);
        let matrix = transform.clone().into_transform().cast();

        // Apply the local clip rect here, before splitting. This is
        // because the local clip rect can't be applied in the vertex
        // shader for split composites, since we are drawing polygons
        // rather that rectangles. The interpolation still works correctly
        // since we determine the UVs by doing a bilerp with a factor
        // from the original local rect.
        let local_rect = match original_local_rect
            .intersection(combined_local_clip_rect)
        {
            Some(rect) => rect.cast(),
            None => return false,
        };
        let world_rect = world_rect.cast();

        match transform {
            CoordinateSpaceMapping::Local => {
                let polygon = Polygon::from_rect(
                    local_rect * Scale::new(1.0),
                    plane_split_anchor,
                );
                splitter.add(polygon);
            }
            CoordinateSpaceMapping::ScaleOffset(scale_offset) if scale_offset.scale == Vector2D::new(1.0, 1.0) => {
                let inv_matrix = scale_offset.inverse().to_transform().cast();
                let polygon = Polygon::from_transformed_rect_with_inverse(
                    local_rect,
                    &matrix,
                    &inv_matrix,
                    plane_split_anchor,
                ).unwrap();
                splitter.add(polygon);
            }
            CoordinateSpaceMapping::ScaleOffset(_) |
            CoordinateSpaceMapping::Transform(_) => {
                let mut clipper = Clipper::new();
                let results = clipper.clip_transformed(
                    Polygon::from_rect(
                        local_rect,
                        plane_split_anchor,
                    ),
                    &matrix,
                    Some(world_rect),
                );
                if let Ok(results) = results {
                    for poly in results {
                        splitter.add(poly);
                    }
                }
            }
        }

        true
    }

    pub fn resolve_split_planes(
        &mut self,
        splitter: &mut PlaneSplitter,
        gpu_cache: &mut GpuCache,
        spatial_tree: &SpatialTree,
    ) {
        let ordered = match self.context_3d {
            Picture3DContext::In { root_data: Some(ref mut list), .. } => list,
            _ => panic!("Expected to find 3D context root"),
        };
        ordered.clear();

        // Process the accumulated split planes and order them for rendering.
        // Z axis is directed at the screen, `sort` is ascending, and we need back-to-front order.
        let sorted = splitter.sort(vec3(0.0, 0.0, 1.0));
        ordered.reserve(sorted.len());
        for poly in sorted {
            let cluster = &self.prim_list.clusters[poly.anchor.cluster_index];
            let spatial_node_index = cluster.spatial_node_index;
            let transform = match spatial_tree
                .get_world_transform(spatial_node_index)
                .inverse()
            {
                Some(transform) => transform.into_transform(),
                // logging this would be a bit too verbose
                None => continue,
            };

            let local_points = [
                transform.transform_point3d(poly.points[0].cast()),
                transform.transform_point3d(poly.points[1].cast()),
                transform.transform_point3d(poly.points[2].cast()),
                transform.transform_point3d(poly.points[3].cast()),
            ];

            // If any of the points are un-transformable, just drop this
            // plane from drawing.
            if local_points.iter().any(|p| p.is_none()) {
                continue;
            }

            let p0 = local_points[0].unwrap();
            let p1 = local_points[1].unwrap();
            let p2 = local_points[2].unwrap();
            let p3 = local_points[3].unwrap();
            let gpu_blocks = [
                [p0.x, p0.y, p1.x, p1.y].into(),
                [p2.x, p2.y, p3.x, p3.y].into(),
            ];
            let gpu_handle = gpu_cache.push_per_frame_blocks(&gpu_blocks);
            let gpu_address = gpu_cache.get_address(&gpu_handle);

            ordered.push(OrderedPictureChild {
                anchor: poly.anchor,
                spatial_node_index,
                gpu_address,
            });
        }
    }

    /// Called during initial picture traversal, before we know the
    /// bounding rect of children. It is possible to determine the
    /// surface / raster config now though.
    fn pre_update(
        &mut self,
        state: &mut PictureUpdateState,
        frame_context: &FrameBuildingContext,
    ) -> Option<PrimitiveList> {
        // Reset raster config in case we early out below.
        self.raster_config = None;

        // Resolve animation properties, and early out if the filter
        // properties make this picture invisible.
        if !self.resolve_scene_properties(frame_context.scene_properties) {
            return None;
        }

        // For out-of-preserve-3d pictures, the backface visibility is determined by
        // the local transform only.
        // Note: we aren't taking the transform relativce to the parent picture,
        // since picture tree can be more dense than the corresponding spatial tree.
        if !self.is_backface_visible {
            if let Picture3DContext::Out = self.context_3d {
                match frame_context.spatial_tree.get_local_visible_face(self.spatial_node_index) {
                    VisibleFace::Front => {}
                    VisibleFace::Back => return None,
                }
            }
        }

        // Push information about this pic on stack for children to read.
        state.push_picture(PictureInfo {
            _spatial_node_index: self.spatial_node_index,
        });

        // See if this picture actually needs a surface for compositing.
        // TODO(gw): FPC: Remove the actual / requested composite mode distinction.
        let actual_composite_mode = self.requested_composite_mode.clone();

        if let Some(composite_mode) = actual_composite_mode {
            // Retrieve the positioning node information for the parent surface.
            let parent_raster_node_index = state.current_surface().raster_spatial_node_index;
            let parent_device_pixel_scale = state.current_surface().device_pixel_scale;
            let surface_spatial_node_index = self.spatial_node_index;

            // Filters must be applied before transforms, to do this, we can mark this picture as establishing a raster root.
            let has_svg_filter = if let PictureCompositeMode::SvgFilter(..) = composite_mode {
                true
            } else {
                false
            };

            let surface_to_parent_transform = frame_context.spatial_tree
                .get_relative_transform(surface_spatial_node_index, parent_raster_node_index);

            // Check if there is perspective or if an SVG filter is applied, and thus whether a new
            // rasterization root should be established.
            let establishes_raster_root = has_svg_filter || surface_to_parent_transform.is_perspective();

            let (raster_spatial_node_index, device_pixel_scale) = if establishes_raster_root {
                // If a raster root is established, this surface should be scaled based on the scale factors of the surface raster to parent raster transform.
                // This scaling helps ensure that the content in this surface does not become blurry or pixelated when composited in the parent surface.
                let scale_factors = surface_to_parent_transform.scale_factors();

                // Pick the largest scale factor of the transform for the scaling factor.
                // Currently, we ensure that the scaling factor is >= 1.0 as a smaller scale factor can result in blurry output.
                let scaling_factor = scale_factors.0.max(scale_factors.1).max(1.0);

                let device_pixel_scale = parent_device_pixel_scale * Scale::new(scaling_factor);
                (surface_spatial_node_index, device_pixel_scale)
            } else {
                (parent_raster_node_index, parent_device_pixel_scale)
            };

            let scale_factors = frame_context
                    .spatial_tree
                    .get_relative_transform(surface_spatial_node_index, raster_spatial_node_index)
                    .scale_factors();

            // This inflation factor is to be applied to all primitives within the surface.
            // Only inflate if the caller hasn't already inflated the bounding rects for this filter.
            let mut inflation_factor = 0.0;
            if self.options.inflate_if_required {
                match composite_mode {
                    PictureCompositeMode::Filter(Filter::Blur(width, height)) => {
                        let blur_radius = f32::max(clamp_blur_radius(width, scale_factors), clamp_blur_radius(height, scale_factors));
                        // The amount of extra space needed for primitives inside
                        // this picture to ensure the visibility check is correct.
                        inflation_factor = blur_radius * BLUR_SAMPLE_SCALE;
                    }
                    PictureCompositeMode::SvgFilter(ref primitives, _) => {
                        let mut max = 0.0;
                        for primitive in primitives {
                            if let FilterPrimitiveKind::Blur(ref blur) = primitive.kind {
                                max = f32::max(max, blur.width);
                                max = f32::max(max, blur.height);
                            }
                        }
                        inflation_factor = clamp_blur_radius(max, scale_factors) * BLUR_SAMPLE_SCALE;
                    }
                    PictureCompositeMode::Filter(Filter::DropShadows(ref shadows)) => {
                        // TODO(gw): This is incorrect, since we don't consider the drop shadow
                        //           offset. However, fixing that is a larger task, so this is
                        //           an improvement on the current case (this at least works where
                        //           the offset of the drop-shadow is ~0, which is often true).

                        // Can't use max_by_key here since f32 isn't Ord
                        let mut max_blur_radius: f32 = 0.0;
                        for shadow in shadows {
                            max_blur_radius = max_blur_radius.max(shadow.blur_radius);
                        }

                        inflation_factor = clamp_blur_radius(max_blur_radius, scale_factors) * BLUR_SAMPLE_SCALE;
                    }
                    _ => {}
                }
            }

            let surface = SurfaceInfo::new(
                surface_spatial_node_index,
                raster_spatial_node_index,
                inflation_factor,
                frame_context.global_screen_world_rect,
                &frame_context.spatial_tree,
                device_pixel_scale,
                scale_factors,
            );

            self.raster_config = Some(RasterConfig {
                composite_mode,
                establishes_raster_root,
                surface_index: state.push_surface(surface),
                root_scaling_factor: 1.0,
                clipped_bounding_rect: WorldRect::zero(),
            });
        }

        Some(mem::replace(&mut self.prim_list, PrimitiveList::empty()))
    }

    /// Called after updating child pictures during the initial
    /// picture traversal.
    fn post_update(
        &mut self,
        prim_list: PrimitiveList,
        state: &mut PictureUpdateState,
        frame_context: &FrameBuildingContext,
        data_stores: &mut DataStores,
    ) {
        // Restore the pictures list used during recursion.
        self.prim_list = prim_list;

        // Pop the state information about this picture.
        state.pop_picture();

        let surface = state.current_surface_mut();

        for cluster in &mut self.prim_list.clusters {
            cluster.flags.remove(ClusterFlags::IS_VISIBLE);

            // Skip the cluster if backface culled.
            if !cluster.flags.contains(ClusterFlags::IS_BACKFACE_VISIBLE) {
                // For in-preserve-3d primitives and pictures, the backface visibility is
                // evaluated relative to the containing block.
                if let Picture3DContext::In { ancestor_index, .. } = self.context_3d {
                    let mut face = VisibleFace::Front;
                    frame_context.spatial_tree.get_relative_transform_with_face(
                        cluster.spatial_node_index,
                        ancestor_index,
                        Some(&mut face),
                    );
                    if face == VisibleFace::Back {
                        continue
                    }
                }
            }

            // No point including this cluster if it can't be transformed
            let spatial_node = &frame_context
                .spatial_tree
                .spatial_nodes[cluster.spatial_node_index.0 as usize];
            if !spatial_node.invertible {
                continue;
            }

            // Update any primitives/cluster bounding rects that can only be done
            // with information available during frame building.
            if cluster.flags.contains(ClusterFlags::IS_BACKDROP_FILTER) {
                let backdrop_to_world_mapper = SpaceMapper::new_with_target(
                    ROOT_SPATIAL_NODE_INDEX,
                    cluster.spatial_node_index,
                    LayoutRect::max_rect(),
                    frame_context.spatial_tree,
                );

                for prim_instance in &mut self.prim_list.prim_instances[cluster.prim_range()] {
                    match prim_instance.kind {
                        PrimitiveInstanceKind::Backdrop { data_handle, .. } => {
                            // The actual size and clip rect of this primitive are determined by computing the bounding
                            // box of the projected rect of the backdrop-filter element onto the backdrop.
                            let prim_data = &mut data_stores.backdrop[data_handle];
                            let spatial_node_index = prim_data.kind.spatial_node_index;

                            // We cannot use the relative transform between the backdrop and the element because
                            // that doesn't take into account any projection transforms that both spatial nodes are children of.
                            // Instead, we first project from the element to the world space and get a flattened 2D bounding rect
                            // in the screen space, we then map this rect from the world space to the backdrop space to get the
                            // proper bounding box where the backdrop-filter needs to be processed.

                            let prim_to_world_mapper = SpaceMapper::new_with_target(
                                ROOT_SPATIAL_NODE_INDEX,
                                spatial_node_index,
                                LayoutRect::max_rect(),
                                frame_context.spatial_tree,
                            );

                            // First map to the screen and get a flattened rect
                            let prim_rect = prim_to_world_mapper.map(&prim_data.kind.border_rect).unwrap_or_else(LayoutRect::zero);
                            // Backwards project the flattened rect onto the backdrop
                            let prim_rect = backdrop_to_world_mapper.unmap(&prim_rect).unwrap_or_else(LayoutRect::zero);

                            // TODO(aosmond): Is this safe? Updating the primitive size during
                            // frame building is usually problematic since scene building will cache
                            // the primitive information in the GPU already.
                            prim_data.common.prim_rect = prim_rect;
                            prim_instance.clip_set.local_clip_rect = prim_rect;

                            // Update the cluster bounding rect now that we have the backdrop rect.
                            cluster.bounding_rect = cluster.bounding_rect.union(&prim_rect);
                        }
                        _ => {
                            panic!("BUG: unexpected deferred primitive kind for cluster updates");
                        }
                    }
                }
            }

            // Map the cluster bounding rect into the space of the surface, and
            // include it in the surface bounding rect. 
            surface.map_local_to_surface.set_target_spatial_node(
                cluster.spatial_node_index,
                frame_context.spatial_tree,
            );

            // Mark the cluster visible, since it passed the invertible and
            // backface checks.
            cluster.flags.insert(ClusterFlags::IS_VISIBLE);
            if let Some(cluster_rect) = surface.map_local_to_surface.map(&cluster.bounding_rect) {
                surface.rect = surface.rect.union(&cluster_rect);
            }
        }

        // If this picture establishes a surface, then map the surface bounding
        // rect into the parent surface coordinate space, and propagate that up
        // to the parent.
        if let Some(ref mut raster_config) = self.raster_config {
            let surface = state.current_surface_mut();
            // Inflate the local bounding rect if required by the filter effect.
            // This inflaction factor is to be applied to the surface itself.
            if self.options.inflate_if_required {
                surface.rect = raster_config.composite_mode.inflate_picture_rect(surface.rect, surface.scale_factors);

                // The picture's local rect is calculated as the union of the
                // snapped primitive rects, which should result in a snapped
                // local rect, unless it was inflated. This is also done during
                // update visibility when calculating the picture's precise
                // local rect.
                let snap_surface_to_raster = SpaceSnapper::new_with_target(
                    surface.raster_spatial_node_index,
                    self.spatial_node_index,
                    surface.device_pixel_scale,
                    frame_context.spatial_tree,
                );

                surface.rect = snap_surface_to_raster.snap_rect(&surface.rect);
            }

            let mut surface_rect = surface.rect * Scale::new(1.0);

            // Pop this surface from the stack
            let surface_index = state.pop_surface();
            debug_assert_eq!(surface_index, raster_config.surface_index);

            // Set the estimated and precise local rects. The precise local rect
            // may be changed again during frame visibility.
            self.estimated_local_rect = surface_rect;
            self.precise_local_rect = surface_rect;

            // Drop shadows draw both a content and shadow rect, so need to expand the local
            // rect of any surfaces to be composited in parent surfaces correctly.
            match raster_config.composite_mode {
                PictureCompositeMode::Filter(Filter::DropShadows(ref shadows)) => {
                    for shadow in shadows {
                        let shadow_rect = self.estimated_local_rect.translate(shadow.offset);
                        surface_rect = surface_rect.union(&shadow_rect);
                    }
                }
                _ => {}
            }

            // Propagate up to parent surface, now that we know this surface's static rect
            let parent_surface = state.current_surface_mut();
            parent_surface.map_local_to_surface.set_target_spatial_node(
                self.spatial_node_index,
                frame_context.spatial_tree,
            );
            if let Some(parent_surface_rect) = parent_surface
                .map_local_to_surface
                .map(&surface_rect)
            {
                parent_surface.rect = parent_surface.rect.union(&parent_surface_rect);
            }
        }
    }

    pub fn prepare_for_render(
        &mut self,
        frame_context: &FrameBuildingContext,
        frame_state: &mut FrameBuildingState,
        data_stores: &mut DataStores,
    ) -> bool {
        let mut pic_state_for_children = self.take_state();

        if let Some(ref mut splitter) = pic_state_for_children.plane_splitter {
            self.resolve_split_planes(
                splitter,
                &mut frame_state.gpu_cache,
                &frame_context.spatial_tree,
            );
        }

        let raster_config = match self.raster_config {
            Some(ref mut raster_config) => raster_config,
            None => {
                return true
            }
        };

        // TODO(gw): Almost all of the Picture types below use extra_gpu_cache_data
        //           to store the same type of data. The exception is the filter
        //           with a ColorMatrix, which stores the color matrix here. It's
        //           probably worth tidying this code up to be a bit more consistent.
        //           Perhaps store the color matrix after the common data, even though
        //           it's not used by that shader.

        match raster_config.composite_mode {
            PictureCompositeMode::TileCache { .. } => {}
            PictureCompositeMode::Filter(Filter::Blur(..)) => {}
            PictureCompositeMode::Filter(Filter::DropShadows(ref shadows)) => {
                self.extra_gpu_data_handles.resize(shadows.len(), GpuCacheHandle::new());
                for (shadow, extra_handle) in shadows.iter().zip(self.extra_gpu_data_handles.iter_mut()) {
                    if let Some(mut request) = frame_state.gpu_cache.request(extra_handle) {
                        // Basic brush primitive header is (see end of prepare_prim_for_render_inner in prim_store.rs)
                        //  [brush specific data]
                        //  [segment_rect, segment data]
                        let shadow_rect = self.precise_local_rect.translate(shadow.offset);

                        // ImageBrush colors
                        request.push(shadow.color.premultiplied());
                        request.push(PremultipliedColorF::WHITE);
                        request.push([
                            self.precise_local_rect.size.width,
                            self.precise_local_rect.size.height,
                            0.0,
                            0.0,
                        ]);

                        // segment rect / extra data
                        request.push(shadow_rect);
                        request.push([0.0, 0.0, 0.0, 0.0]);
                    }
                }
            }
            PictureCompositeMode::MixBlend(..) if !frame_context.fb_config.gpu_supports_advanced_blend => {}
            PictureCompositeMode::Filter(ref filter) => {
                match *filter {
                    Filter::ColorMatrix(ref m) => {
                        if self.extra_gpu_data_handles.is_empty() {
                            self.extra_gpu_data_handles.push(GpuCacheHandle::new());
                        }
                        if let Some(mut request) = frame_state.gpu_cache.request(&mut self.extra_gpu_data_handles[0]) {
                            for i in 0..5 {
                                request.push([m[i*4], m[i*4+1], m[i*4+2], m[i*4+3]]);
                            }
                        }
                    }
                    Filter::Flood(ref color) => {
                        if self.extra_gpu_data_handles.is_empty() {
                            self.extra_gpu_data_handles.push(GpuCacheHandle::new());
                        }
                        if let Some(mut request) = frame_state.gpu_cache.request(&mut self.extra_gpu_data_handles[0]) {
                            request.push(color.to_array());
                        }
                    }
                    _ => {}
                }
            }
            PictureCompositeMode::ComponentTransferFilter(handle) => {
                let filter_data = &mut data_stores.filter_data[handle];
                filter_data.update(frame_state);
            }
            PictureCompositeMode::MixBlend(..) |
            PictureCompositeMode::Blit(_) |
            PictureCompositeMode::SvgFilter(..) => {}
        }

        true
    }
}

// Calculate a single homogeneous screen-space UV for a picture.
fn calculate_screen_uv(
    local_pos: &PicturePoint,
    transform: &PictureToRasterTransform,
    rendered_rect: &DeviceRect,
    device_pixel_scale: DevicePixelScale,
) -> DeviceHomogeneousVector {
    let raster_pos = transform.transform_point2d_homogeneous(*local_pos);

    DeviceHomogeneousVector::new(
        (raster_pos.x * device_pixel_scale.0 - rendered_rect.origin.x * raster_pos.w) / rendered_rect.size.width,
        (raster_pos.y * device_pixel_scale.0 - rendered_rect.origin.y * raster_pos.w) / rendered_rect.size.height,
        0.0,
        raster_pos.w,
    )
}

// Calculate a UV rect within an image based on the screen space
// vertex positions of a picture.
fn calculate_uv_rect_kind(
    pic_rect: &PictureRect,
    transform: &PictureToRasterTransform,
    rendered_rect: &DeviceRect,
    device_pixel_scale: DevicePixelScale,
) -> UvRectKind {
    let top_left = calculate_screen_uv(
        &pic_rect.origin,
        transform,
        &rendered_rect,
        device_pixel_scale,
    );

    let top_right = calculate_screen_uv(
        &pic_rect.top_right(),
        transform,
        &rendered_rect,
        device_pixel_scale,
    );

    let bottom_left = calculate_screen_uv(
        &pic_rect.bottom_left(),
        transform,
        &rendered_rect,
        device_pixel_scale,
    );

    let bottom_right = calculate_screen_uv(
        &pic_rect.bottom_right(),
        transform,
        &rendered_rect,
        device_pixel_scale,
    );

    UvRectKind::Quad {
        top_left,
        top_right,
        bottom_left,
        bottom_right,
    }
}

fn create_raster_mappers(
    surface_spatial_node_index: SpatialNodeIndex,
    raster_spatial_node_index: SpatialNodeIndex,
    world_rect: WorldRect,
    spatial_tree: &SpatialTree,
) -> (SpaceMapper<RasterPixel, WorldPixel>, SpaceMapper<PicturePixel, RasterPixel>) {
    let map_raster_to_world = SpaceMapper::new_with_target(
        ROOT_SPATIAL_NODE_INDEX,
        raster_spatial_node_index,
        world_rect,
        spatial_tree,
    );

    let raster_bounds = map_raster_to_world.unmap(&world_rect)
                                           .unwrap_or_else(RasterRect::max_rect);

    let map_pic_to_raster = SpaceMapper::new_with_target(
        raster_spatial_node_index,
        surface_spatial_node_index,
        raster_bounds,
        spatial_tree,
    );

    (map_raster_to_world, map_pic_to_raster)
}

fn get_transform_key(
    spatial_node_index: SpatialNodeIndex,
    cache_spatial_node_index: SpatialNodeIndex,
    spatial_tree: &SpatialTree,
) -> TransformKey {
    // Note: this is the only place where we don't know beforehand if the tile-affecting
    // spatial node is below or above the current picture.
    let transform = if cache_spatial_node_index >= spatial_node_index {
        spatial_tree
            .get_relative_transform(
                cache_spatial_node_index,
                spatial_node_index,
            )
    } else {
        spatial_tree
            .get_relative_transform(
                spatial_node_index,
                cache_spatial_node_index,
            )
    };
    transform.into()
}

/// A key for storing primitive comparison results during tile dependency tests.
#[derive(Debug, Copy, Clone, Eq, Hash, PartialEq)]
struct PrimitiveComparisonKey {
    prev_index: PrimitiveDependencyIndex,
    curr_index: PrimitiveDependencyIndex,
}

/// Information stored an image dependency
#[derive(Debug, Copy, Clone, PartialEq)]
#[cfg_attr(feature = "capture", derive(Serialize))]
#[cfg_attr(feature = "replay", derive(Deserialize))]
pub struct ImageDependency {
    pub key: ImageKey,
    pub generation: ImageGeneration,
}

impl ImageDependency {
    pub const INVALID: ImageDependency = ImageDependency {
        key: ImageKey::DUMMY,
        generation: ImageGeneration::INVALID,
    };
}

/// A helper struct to compare a primitive and all its sub-dependencies.
struct PrimitiveComparer<'a> {
    clip_comparer: CompareHelper<'a, ItemUid>,
    transform_comparer: CompareHelper<'a, SpatialNodeKey>,
    image_comparer: CompareHelper<'a, ImageDependency>,
    opacity_comparer: CompareHelper<'a, OpacityBinding>,
    color_comparer: CompareHelper<'a, ColorBinding>,
    resource_cache: &'a ResourceCache,
    spatial_node_comparer: &'a mut SpatialNodeComparer,
    opacity_bindings: &'a FastHashMap<PropertyBindingId, OpacityBindingInfo>,
    color_bindings: &'a FastHashMap<PropertyBindingId, ColorBindingInfo>,
}

impl<'a> PrimitiveComparer<'a> {
    fn new(
        prev: &'a TileDescriptor,
        curr: &'a TileDescriptor,
        resource_cache: &'a ResourceCache,
        spatial_node_comparer: &'a mut SpatialNodeComparer,
        opacity_bindings: &'a FastHashMap<PropertyBindingId, OpacityBindingInfo>,
        color_bindings: &'a FastHashMap<PropertyBindingId, ColorBindingInfo>,
    ) -> Self {
        let clip_comparer = CompareHelper::new(
            &prev.clips,
            &curr.clips,
        );

        let transform_comparer = CompareHelper::new(
            &prev.transforms,
            &curr.transforms,
        );

        let image_comparer = CompareHelper::new(
            &prev.images,
            &curr.images,
        );

        let opacity_comparer = CompareHelper::new(
            &prev.opacity_bindings,
            &curr.opacity_bindings,
        );

        let color_comparer = CompareHelper::new(
            &prev.color_bindings,
            &curr.color_bindings,
        );

        PrimitiveComparer {
            clip_comparer,
            transform_comparer,
            image_comparer,
            opacity_comparer,
            color_comparer,
            resource_cache,
            spatial_node_comparer,
            opacity_bindings,
            color_bindings,
        }
    }

    fn reset(&mut self) {
        self.clip_comparer.reset();
        self.transform_comparer.reset();
        self.image_comparer.reset();
        self.opacity_comparer.reset();
        self.color_comparer.reset();
    }

    fn advance_prev(&mut self, prim: &PrimitiveDescriptor) {
        self.clip_comparer.advance_prev(prim.clip_dep_count);
        self.transform_comparer.advance_prev(prim.transform_dep_count);
        self.image_comparer.advance_prev(prim.image_dep_count);
        self.opacity_comparer.advance_prev(prim.opacity_binding_dep_count);
        self.color_comparer.advance_prev(prim.color_binding_dep_count);
    }

    fn advance_curr(&mut self, prim: &PrimitiveDescriptor) {
        self.clip_comparer.advance_curr(prim.clip_dep_count);
        self.transform_comparer.advance_curr(prim.transform_dep_count);
        self.image_comparer.advance_curr(prim.image_dep_count);
        self.opacity_comparer.advance_curr(prim.opacity_binding_dep_count);
        self.color_comparer.advance_curr(prim.color_binding_dep_count);
    }

    /// Check if two primitive descriptors are the same.
    fn compare_prim(
        &mut self,
        prev: &PrimitiveDescriptor,
        curr: &PrimitiveDescriptor,
        opt_detail: Option<&mut PrimitiveCompareResultDetail>,
    ) -> PrimitiveCompareResult {
        let resource_cache = self.resource_cache;
        let spatial_node_comparer = &mut self.spatial_node_comparer;
        let opacity_bindings = self.opacity_bindings;
        let color_bindings = self.color_bindings;

        // Check equality of the PrimitiveDescriptor
        if prev != curr {
            if let Some(detail) = opt_detail {
                *detail = PrimitiveCompareResultDetail::Descriptor{ old: *prev, new: *curr };
            }
            return PrimitiveCompareResult::Descriptor;
        }

        // Check if any of the clips  this prim has are different.
        let mut clip_result = CompareHelperResult::Equal;
        if !self.clip_comparer.is_same(
            prev.clip_dep_count,
            curr.clip_dep_count,
            |prev, curr| {
                prev == curr
            },
            if opt_detail.is_some() { Some(&mut clip_result) } else { None }
        ) {
            if let Some(detail) = opt_detail { *detail = PrimitiveCompareResultDetail::Clip{ detail: clip_result }; }
            return PrimitiveCompareResult::Clip;
        }

        // Check if any of the transforms  this prim has are different.
        let mut transform_result = CompareHelperResult::Equal;
        if !self.transform_comparer.is_same(
            prev.transform_dep_count,
            curr.transform_dep_count,
            |prev, curr| {
                spatial_node_comparer.are_transforms_equivalent(prev, curr)
            },
            if opt_detail.is_some() { Some(&mut transform_result) } else { None },
        ) {
            if let Some(detail) = opt_detail {
                *detail = PrimitiveCompareResultDetail::Transform{ detail: transform_result };
            }
            return PrimitiveCompareResult::Transform;
        }

        // Check if any of the images this prim has are different.
        let mut image_result = CompareHelperResult::Equal;
        if !self.image_comparer.is_same(
            prev.image_dep_count,
            curr.image_dep_count,
            |prev, curr| {
                prev == curr &&
                resource_cache.get_image_generation(curr.key) == curr.generation
            },
            if opt_detail.is_some() { Some(&mut image_result) } else { None },
        ) {
            if let Some(detail) = opt_detail {
                *detail = PrimitiveCompareResultDetail::Image{ detail: image_result };
            }
            return PrimitiveCompareResult::Image;
        }

        // Check if any of the opacity bindings this prim has are different.
        let mut bind_result = CompareHelperResult::Equal;
        if !self.opacity_comparer.is_same(
            prev.opacity_binding_dep_count,
            curr.opacity_binding_dep_count,
            |prev, curr| {
                if prev != curr {
                    return false;
                }

                if let OpacityBinding::Binding(id) = curr {
                    if opacity_bindings
                        .get(id)
                        .map_or(true, |info| info.changed) {
                        return false;
                    }
                }

                true
            },
            if opt_detail.is_some() { Some(&mut bind_result) } else { None },
        ) {
            if let Some(detail) = opt_detail {
                *detail = PrimitiveCompareResultDetail::OpacityBinding{ detail: bind_result };
            }
            return PrimitiveCompareResult::OpacityBinding;
        }

        // Check if any of the color bindings this prim has are different.
        let mut bind_result = CompareHelperResult::Equal;
        if !self.color_comparer.is_same(
            prev.color_binding_dep_count,
            curr.color_binding_dep_count,
            |prev, curr| {
                if prev != curr {
                    return false;
                }

                if let ColorBinding::Binding(id) = curr {
                    if color_bindings
                        .get(id)
                        .map_or(true, |info| info.changed) {
                        return false;
                    }
                }

                true
            },
            if opt_detail.is_some() { Some(&mut bind_result) } else { None },
        ) {
            if let Some(detail) = opt_detail {
                *detail = PrimitiveCompareResultDetail::ColorBinding{ detail: bind_result };
            }
            return PrimitiveCompareResult::ColorBinding;
        }

        PrimitiveCompareResult::Equal
    }
}

/// Details for a node in a quadtree that tracks dirty rects for a tile.
#[cfg_attr(any(feature="capture",feature="replay"), derive(Clone))]
#[cfg_attr(feature = "capture", derive(Serialize))]
#[cfg_attr(feature = "replay", derive(Deserialize))]
pub enum TileNodeKind {
    Leaf {
        /// The index buffer of primitives that affected this tile previous frame
        #[cfg_attr(any(feature = "capture", feature = "replay"), serde(skip))]
        prev_indices: Vec<PrimitiveDependencyIndex>,
        /// The index buffer of primitives that affect this tile on this frame
        #[cfg_attr(any(feature = "capture", feature = "replay"), serde(skip))]
        curr_indices: Vec<PrimitiveDependencyIndex>,
        /// A bitset of which of the last 64 frames have been dirty for this leaf.
        #[cfg_attr(any(feature = "capture", feature = "replay"), serde(skip))]
        dirty_tracker: u64,
        /// The number of frames since this node split or merged.
        #[cfg_attr(any(feature = "capture", feature = "replay"), serde(skip))]
        frames_since_modified: usize,
    },
    Node {
        /// The four children of this node
        children: Vec<TileNode>,
    },
}

/// The kind of modification that a tile wants to do
#[derive(Copy, Clone, PartialEq, Debug)]
enum TileModification {
    Split,
    Merge,
}

/// A node in the dirty rect tracking quadtree.
#[cfg_attr(any(feature="capture",feature="replay"), derive(Clone))]
#[cfg_attr(feature = "capture", derive(Serialize))]
#[cfg_attr(feature = "replay", derive(Deserialize))]
pub struct TileNode {
    /// Leaf or internal node
    pub kind: TileNodeKind,
    /// Rect of this node in the same space as the tile cache picture
    pub rect: PictureBox2D,
}

impl TileNode {
    /// Construct a new leaf node, with the given primitive dependency index buffer
    fn new_leaf(curr_indices: Vec<PrimitiveDependencyIndex>) -> Self {
        TileNode {
            kind: TileNodeKind::Leaf {
                prev_indices: Vec::new(),
                curr_indices,
                dirty_tracker: 0,
                frames_since_modified: 0,
            },
            rect: PictureBox2D::zero(),
        }
    }

    /// Draw debug information about this tile node
    fn draw_debug_rects(
        &self,
        pic_to_world_mapper: &SpaceMapper<PicturePixel, WorldPixel>,
        is_opaque: bool,
        local_valid_rect: PictureRect,
        scratch: &mut PrimitiveScratchBuffer,
        global_device_pixel_scale: DevicePixelScale,
    ) {
        match self.kind {
            TileNodeKind::Leaf { dirty_tracker, .. } => {
                let color = if (dirty_tracker & 1) != 0 {
                    debug_colors::RED
                } else if is_opaque {
                    debug_colors::GREEN
                } else {
                    debug_colors::YELLOW
                };

                if let Some(local_rect) = local_valid_rect.intersection(&self.rect.to_rect()) {
                    let world_rect = pic_to_world_mapper
                        .map(&local_rect)
                        .unwrap();
                    let device_rect = world_rect * global_device_pixel_scale;

                    let outer_color = color.scale_alpha(0.3);
                    let inner_color = outer_color.scale_alpha(0.5);
                    scratch.push_debug_rect(
                        device_rect.inflate(-3.0, -3.0),
                        outer_color,
                        inner_color
                    );
                }
            }
            TileNodeKind::Node { ref children, .. } => {
                for child in children.iter() {
                    child.draw_debug_rects(
                        pic_to_world_mapper,
                        is_opaque,
                        local_valid_rect,
                        scratch,
                        global_device_pixel_scale,
                    );
                }
            }
        }
    }

    /// Calculate the four child rects for a given node
    fn get_child_rects(
        rect: &PictureBox2D,
        result: &mut [PictureBox2D; 4],
    ) {
        let p0 = rect.min;
        let p1 = rect.max;
        let pc = p0 + rect.size() * 0.5;

        *result = [
            PictureBox2D::new(
                p0,
                pc,
            ),
            PictureBox2D::new(
                PicturePoint::new(pc.x, p0.y),
                PicturePoint::new(p1.x, pc.y),
            ),
            PictureBox2D::new(
                PicturePoint::new(p0.x, pc.y),
                PicturePoint::new(pc.x, p1.y),
            ),
            PictureBox2D::new(
                pc,
                p1,
            ),
        ];
    }

    /// Called during pre_update, to clear the current dependencies
    fn clear(
        &mut self,
        rect: PictureBox2D,
    ) {
        self.rect = rect;

        match self.kind {
            TileNodeKind::Leaf { ref mut prev_indices, ref mut curr_indices, ref mut dirty_tracker, ref mut frames_since_modified } => {
                // Swap current dependencies to be the previous frame
                mem::swap(prev_indices, curr_indices);
                curr_indices.clear();
                // Note that another frame has passed in the dirty bit trackers
                *dirty_tracker = *dirty_tracker << 1;
                *frames_since_modified += 1;
            }
            TileNodeKind::Node { ref mut children, .. } => {
                let mut child_rects = [PictureBox2D::zero(); 4];
                TileNode::get_child_rects(&rect, &mut child_rects);
                assert_eq!(child_rects.len(), children.len());

                for (child, rect) in children.iter_mut().zip(child_rects.iter()) {
                    child.clear(*rect);
                }
            }
        }
    }

    /// Add a primitive dependency to this node
    fn add_prim(
        &mut self,
        index: PrimitiveDependencyIndex,
        prim_rect: &PictureBox2D,
    ) {
        match self.kind {
            TileNodeKind::Leaf { ref mut curr_indices, .. } => {
                curr_indices.push(index);
            }
            TileNodeKind::Node { ref mut children, .. } => {
                for child in children.iter_mut() {
                    if child.rect.intersects(prim_rect) {
                        child.add_prim(index, prim_rect);
                    }
                }
            }
        }
    }

    /// Apply a merge or split operation to this tile, if desired
    fn maybe_merge_or_split(
        &mut self,
        level: i32,
        curr_prims: &[PrimitiveDescriptor],
        max_split_levels: i32,
    ) {
        // Determine if this tile wants to split or merge
        let mut tile_mod = None;

        fn get_dirty_frames(
            dirty_tracker: u64,
            frames_since_modified: usize,
        ) -> Option<u32> {
            // Only consider splitting or merging at least 64 frames since we last changed
            if frames_since_modified > 64 {
                // Each bit in the tracker is a frame that was recently invalidated
                Some(dirty_tracker.count_ones())
            } else {
                None
            }
        }

        match self.kind {
            TileNodeKind::Leaf { dirty_tracker, frames_since_modified, .. } => {
                // Only consider splitting if the tree isn't too deep.
                if level < max_split_levels {
                    if let Some(dirty_frames) = get_dirty_frames(dirty_tracker, frames_since_modified) {
                        // If the tile has invalidated > 50% of the recent number of frames, split.
                        if dirty_frames > 32 {
                            tile_mod = Some(TileModification::Split);
                        }
                    }
                }
            }
            TileNodeKind::Node { ref children, .. } => {
                // There's two conditions that cause a node to merge its children:
                // (1) If _all_ the child nodes are constantly invalidating, then we are wasting
                //     CPU time tracking dependencies for each child, so merge them.
                // (2) If _none_ of the child nodes are recently invalid, then the page content
                //     has probably changed, and we no longer need to track fine grained dependencies here.

                let mut static_count = 0;
                let mut changing_count = 0;

                for child in children {
                    // Only consider merging nodes at the edge of the tree.
                    if let TileNodeKind::Leaf { dirty_tracker, frames_since_modified, .. } = child.kind {
                        if let Some(dirty_frames) = get_dirty_frames(dirty_tracker, frames_since_modified) {
                            if dirty_frames == 0 {
                                // Hasn't been invalidated for some time
                                static_count += 1;
                            } else if dirty_frames == 64 {
                                // Is constantly being invalidated
                                changing_count += 1;
                            }
                        }
                    }

                    // Only merge if all the child tiles are in agreement. Otherwise, we have some
                    // that are invalidating / static, and it's worthwhile tracking dependencies for
                    // them individually.
                    if static_count == 4 || changing_count == 4 {
                        tile_mod = Some(TileModification::Merge);
                    }
                }
            }
        }

        match tile_mod {
            Some(TileModification::Split) => {
                // To split a node, take the current dependency index buffer for this node, and
                // split it into child index buffers.
                let curr_indices = match self.kind {
                    TileNodeKind::Node { .. } => {
                        unreachable!("bug - only leaves can split");
                    }
                    TileNodeKind::Leaf { ref mut curr_indices, .. } => {
                        curr_indices.take()
                    }
                };

                let mut child_rects = [PictureBox2D::zero(); 4];
                TileNode::get_child_rects(&self.rect, &mut child_rects);

                let mut child_indices = [
                    Vec::new(),
                    Vec::new(),
                    Vec::new(),
                    Vec::new(),
                ];

                // Step through the index buffer, and add primitives to each of the children
                // that they intersect.
                for index in curr_indices {
                    let prim = &curr_prims[index.0 as usize];
                    for (child_rect, indices) in child_rects.iter().zip(child_indices.iter_mut()) {
                        if prim.prim_clip_box.intersects(child_rect) {
                            indices.push(index);
                        }
                    }
                }

                // Create the child nodes and switch from leaf -> node.
                let children = child_indices
                    .iter_mut()
                    .map(|i| TileNode::new_leaf(mem::replace(i, Vec::new())))
                    .collect();

                self.kind = TileNodeKind::Node {
                    children,
                };
            }
            Some(TileModification::Merge) => {
                // Construct a merged index buffer by collecting the dependency index buffers
                // from each child, and merging them into a de-duplicated index buffer.
                let merged_indices = match self.kind {
                    TileNodeKind::Node { ref mut children, .. } => {
                        let mut merged_indices = Vec::new();

                        for child in children.iter() {
                            let child_indices = match child.kind {
                                TileNodeKind::Leaf { ref curr_indices, .. } => {
                                    curr_indices
                                }
                                TileNodeKind::Node { .. } => {
                                    unreachable!("bug: child is not a leaf");
                                }
                            };
                            merged_indices.extend_from_slice(child_indices);
                        }

                        merged_indices.sort();
                        merged_indices.dedup();

                        merged_indices
                    }
                    TileNodeKind::Leaf { .. } => {
                        unreachable!("bug - trying to merge a leaf");
                    }
                };

                // Switch from a node to a leaf, with the combined index buffer
                self.kind = TileNodeKind::Leaf {
                    prev_indices: Vec::new(),
                    curr_indices: merged_indices,
                    dirty_tracker: 0,
                    frames_since_modified: 0,
                };
            }
            None => {
                // If this node didn't merge / split, then recurse into children
                // to see if they want to split / merge.
                if let TileNodeKind::Node { ref mut children, .. } = self.kind {
                    for child in children.iter_mut() {
                        child.maybe_merge_or_split(
                            level+1,
                            curr_prims,
                            max_split_levels,
                        );
                    }
                }
            }
        }
    }

    /// Update the dirty state of this node, building the overall dirty rect
    fn update_dirty_rects(
        &mut self,
        prev_prims: &[PrimitiveDescriptor],
        curr_prims: &[PrimitiveDescriptor],
        prim_comparer: &mut PrimitiveComparer,
        dirty_rect: &mut PictureBox2D,
        compare_cache: &mut FastHashMap<PrimitiveComparisonKey, PrimitiveCompareResult>,
        invalidation_reason: &mut Option<InvalidationReason>,
        frame_context: &FrameVisibilityContext,
    ) {
        match self.kind {
            TileNodeKind::Node { ref mut children, .. } => {
                for child in children.iter_mut() {
                    child.update_dirty_rects(
                        prev_prims,
                        curr_prims,
                        prim_comparer,
                        dirty_rect,
                        compare_cache,
                        invalidation_reason,
                        frame_context,
                    );
                }
            }
            TileNodeKind::Leaf { ref prev_indices, ref curr_indices, ref mut dirty_tracker, .. } => {
                // If the index buffers are of different length, they must be different
                if prev_indices.len() == curr_indices.len() {
                    let mut prev_i0 = 0;
                    let mut prev_i1 = 0;
                    prim_comparer.reset();

                    // Walk each index buffer, comparing primitives
                    for (prev_index, curr_index) in prev_indices.iter().zip(curr_indices.iter()) {
                        let i0 = prev_index.0 as usize;
                        let i1 = curr_index.0 as usize;

                        // Advance the dependency arrays for each primitive (this handles
                        // prims that may be skipped by these index buffers).
                        for i in prev_i0 .. i0 {
                            prim_comparer.advance_prev(&prev_prims[i]);
                        }
                        for i in prev_i1 .. i1 {
                            prim_comparer.advance_curr(&curr_prims[i]);
                        }

                        // Compare the primitives, caching the result in a hash map
                        // to save comparisons in other tree nodes.
                        let key = PrimitiveComparisonKey {
                            prev_index: *prev_index,
                            curr_index: *curr_index,
                        };

                        #[cfg(any(feature = "capture", feature = "replay"))]
                        let mut compare_detail = PrimitiveCompareResultDetail::Equal;
                        #[cfg(any(feature = "capture", feature = "replay"))]
                        let prim_compare_result_detail =
                            if frame_context.debug_flags.contains(DebugFlags::TILE_CACHE_LOGGING_DBG) {
                                Some(&mut compare_detail)
                            } else {
                                None
                            };

                        #[cfg(not(any(feature = "capture", feature = "replay")))]
                        let compare_detail = PrimitiveCompareResultDetail::Equal;
                        #[cfg(not(any(feature = "capture", feature = "replay")))]
                        let prim_compare_result_detail = None;

                        let prim_compare_result = *compare_cache
                            .entry(key)
                            .or_insert_with(|| {
                                let prev = &prev_prims[i0];
                                let curr = &curr_prims[i1];
                                prim_comparer.compare_prim(prev, curr, prim_compare_result_detail)
                            });

                        // If not the same, mark this node as dirty and update the dirty rect
                        if prim_compare_result != PrimitiveCompareResult::Equal {
                            if invalidation_reason.is_none() {
                                *invalidation_reason = Some(InvalidationReason::Content {
                                    prim_compare_result,
                                    prim_compare_result_detail: Some(compare_detail)
                                });
                            }
                            *dirty_rect = self.rect.union(dirty_rect);
                            *dirty_tracker = *dirty_tracker | 1;
                            break;
                        }

                        prev_i0 = i0;
                        prev_i1 = i1;
                    }
                } else {
                    if invalidation_reason.is_none() {
                        // if and only if tile logging is enabled, do the expensive step of
                        // converting indices back to ItemUids and allocating old and new vectors
                        // to store them in.
                        #[cfg(any(feature = "capture", feature = "replay"))]
                        {
                            if frame_context.debug_flags.contains(DebugFlags::TILE_CACHE_LOGGING_DBG) {
                                let old = prev_indices.iter().map( |i| prev_prims[i.0 as usize].prim_uid ).collect();
                                let new = curr_indices.iter().map( |i| curr_prims[i.0 as usize].prim_uid ).collect();
                                *invalidation_reason = Some(InvalidationReason::PrimCount {
                                                                old: Some(old),
                                                                new: Some(new) });
                            } else {
                                *invalidation_reason = Some(InvalidationReason::PrimCount {
                                                                old: None,
                                                                new: None });
                            }
                        }
                        #[cfg(not(any(feature = "capture", feature = "replay")))]
                        {
                            *invalidation_reason = Some(InvalidationReason::PrimCount {
                                                                old: None,
                                                                new: None });
                        }
                    }
                    *dirty_rect = self.rect.union(dirty_rect);
                    *dirty_tracker = *dirty_tracker | 1;
                }
            }
        }
    }
}

impl CompositeState {
    // A helper function to destroy all native surfaces for a given list of tiles
    pub fn destroy_native_tiles<'a, I: Iterator<Item = &'a mut Box<Tile>>>(
        &mut self,
        tiles_iter: I,
        resource_cache: &mut ResourceCache,
    ) {
        // Any old tiles that remain after the loop above are going to be dropped. For
        // simple composite mode, the texture cache handle will expire and be collected
        // by the texture cache. For native compositor mode, we need to explicitly
        // invoke a callback to the client to destroy that surface.
        if let CompositorKind::Native { .. } = self.compositor_kind {
            for tile in tiles_iter {
                // Only destroy native surfaces that have been allocated. It's
                // possible for display port tiles to be created that never
                // come on screen, and thus never get a native surface allocated.
                if let Some(TileSurface::Texture { descriptor: SurfaceTextureDescriptor::Native { ref mut id, .. }, .. }) = tile.surface {
                    if let Some(id) = id.take() {
                        resource_cache.destroy_compositor_tile(id);
                    }
                }
            }
        }
    }
}

pub fn get_raster_rects(
    pic_rect: PictureRect,
    map_to_raster: &SpaceMapper<PicturePixel, RasterPixel>,
    map_to_world: &SpaceMapper<RasterPixel, WorldPixel>,
    prim_bounding_rect: WorldRect,
    device_pixel_scale: DevicePixelScale,
) -> Option<(DeviceRect, DeviceRect)> {
    let unclipped_raster_rect = map_to_raster.map(&pic_rect)?;

    let unclipped = raster_rect_to_device_pixels(
        unclipped_raster_rect,
        device_pixel_scale,
    );

    let unclipped_world_rect = map_to_world.map(&unclipped_raster_rect)?;
    let clipped_world_rect = unclipped_world_rect.intersection(&prim_bounding_rect)?;

    // We don't have to be able to do the back-projection from world into raster.
    // Rendering only cares one way, so if that fails, we fall back to the full rect.
    let clipped_raster_rect = match map_to_world.unmap(&clipped_world_rect) {
        Some(rect) => rect.intersection(&unclipped_raster_rect)?,
        None => return Some((unclipped, unclipped)),
    };

    let clipped = raster_rect_to_device_pixels(
        clipped_raster_rect,
        device_pixel_scale,
    );

    // Ensure that we won't try to allocate a zero-sized clip render task.
    if clipped.is_empty() {
        return None;
    }

    Some((clipped, unclipped))
}