path: root/servo/components/style/values/generics/transform.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 https://mozilla.org/MPL/2.0/. */

//! Generic types for CSS values that are related to transformations.

use crate::values::computed::length::Length as ComputedLength;
use crate::values::computed::length::LengthPercentage as ComputedLengthPercentage;
use crate::values::specified::angle::Angle as SpecifiedAngle;
use crate::values::specified::length::Length as SpecifiedLength;
use crate::values::specified::length::LengthPercentage as SpecifiedLengthPercentage;
use crate::values::{computed, CSSFloat};
use crate::{Zero, ZeroNoPercent};
use euclid;
use euclid::default::{Rect, Transform3D};
use std::fmt::{self, Write};
use style_traits::{CssWriter, ToCss};

/// A generic 2D transformation matrix.
#[allow(missing_docs)]
#[derive(
    Clone,
    Copy,
    Debug,
    Deserialize,
    MallocSizeOf,
    PartialEq,
    Serialize,
    SpecifiedValueInfo,
    ToComputedValue,
    ToCss,
    ToResolvedValue,
    ToShmem,
)]
#[css(comma, function = "matrix")]
#[repr(C)]
pub struct GenericMatrix<T> {
    pub a: T,
    pub b: T,
    pub c: T,
    pub d: T,
    pub e: T,
    pub f: T,
}

pub use self::GenericMatrix as Matrix;

#[allow(missing_docs)]
#[cfg_attr(rustfmt, rustfmt_skip)]
#[derive(
    Clone,
    Copy,
    Debug,
    Deserialize,
    MallocSizeOf,
    PartialEq,
    Serialize,
    SpecifiedValueInfo,
    ToComputedValue,
    ToCss,
    ToResolvedValue,
    ToShmem,
)]
#[css(comma, function = "matrix3d")]
#[repr(C)]
pub struct GenericMatrix3D<T> {
    pub m11: T, pub m12: T, pub m13: T, pub m14: T,
    pub m21: T, pub m22: T, pub m23: T, pub m24: T,
    pub m31: T, pub m32: T, pub m33: T, pub m34: T,
    pub m41: T, pub m42: T, pub m43: T, pub m44: T,
}

pub use self::GenericMatrix3D as Matrix3D;

#[cfg_attr(rustfmt, rustfmt_skip)]
impl<T: Into<f64>> From<Matrix<T>> for Transform3D<f64> {
    #[inline]
    fn from(m: Matrix<T>) -> Self {
        Transform3D::new(
            m.a.into(), m.b.into(), 0.0, 0.0,
            m.c.into(), m.d.into(), 0.0, 0.0,
            0.0,        0.0,        1.0, 0.0,
            m.e.into(), m.f.into(), 0.0, 1.0,
        )
    }
}

#[cfg_attr(rustfmt, rustfmt_skip)]
impl<T: Into<f64>> From<Matrix3D<T>> for Transform3D<f64> {
    #[inline]
    fn from(m: Matrix3D<T>) -> Self {
        Transform3D::new(
            m.m11.into(), m.m12.into(), m.m13.into(), m.m14.into(),
            m.m21.into(), m.m22.into(), m.m23.into(), m.m24.into(),
            m.m31.into(), m.m32.into(), m.m33.into(), m.m34.into(),
            m.m41.into(), m.m42.into(), m.m43.into(), m.m44.into(),
        )
    }
}

/// A generic transform origin.
#[derive(
    Animate,
    Clone,
    ComputeSquaredDistance,
    Copy,
    Debug,
    MallocSizeOf,
    PartialEq,
    SpecifiedValueInfo,
    ToAnimatedZero,
    ToComputedValue,
    ToCss,
    ToResolvedValue,
    ToShmem,
)]
#[repr(C)]
pub struct GenericTransformOrigin<H, V, Depth> {
    /// The horizontal origin.
    pub horizontal: H,
    /// The vertical origin.
    pub vertical: V,
    /// The depth.
    pub depth: Depth,
}

pub use self::GenericTransformOrigin as TransformOrigin;

impl<H, V, D> TransformOrigin<H, V, D> {
    /// Returns a new transform origin.
    pub fn new(horizontal: H, vertical: V, depth: D) -> Self {
        Self {
            horizontal,
            vertical,
            depth,
        }
    }
}

fn is_same<N: PartialEq>(x: &N, y: &N) -> bool {
    x == y
}

/// A value for the `perspective()` transform function, which is either a
/// non-negative `<length>` or `none`.
#[derive(
    Clone,
    Debug,
    Deserialize,
    MallocSizeOf,
    PartialEq,
    Serialize,
    SpecifiedValueInfo,
    ToComputedValue,
    ToCss,
    ToResolvedValue,
    ToShmem,
)]
#[repr(C, u8)]
pub enum GenericPerspectiveFunction<L> {
    /// `none`
    None,
    /// A `<length>`.
    Length(L),
}

impl<L> GenericPerspectiveFunction<L> {
    /// Returns `f32::INFINITY` or the result of a function on the length value.
    pub fn infinity_or(&self, f: impl FnOnce(&L) -> f32) -> f32 {
        match *self {
            Self::None => f32::INFINITY,
            Self::Length(ref l) => f(l),
        }
    }
}

pub use self::GenericPerspectiveFunction as PerspectiveFunction;

#[derive(
    Clone,
    Debug,
    Deserialize,
    MallocSizeOf,
    PartialEq,
    Serialize,
    SpecifiedValueInfo,
    ToComputedValue,
    ToCss,
    ToResolvedValue,
    ToShmem,
)]
#[repr(C, u8)]
/// A single operation in the list of a `transform` value
pub enum GenericTransformOperation<Angle, Number, Length, Integer, LengthPercentage>
where
    Angle: Zero,
    LengthPercentage: Zero + ZeroNoPercent,
    Number: PartialEq,
{
    /// Represents a 2D 2x3 matrix.
    Matrix(GenericMatrix<Number>),
    /// Represents a 3D 4x4 matrix.
    Matrix3D(GenericMatrix3D<Number>),
    /// A 2D skew.
    ///
    /// If the second angle is not provided it is assumed zero.
    ///
    /// Syntax can be skew(angle) or skew(angle, angle)
    #[css(comma, function)]
    Skew(Angle, #[css(skip_if = "Zero::is_zero")] Angle),
    /// skewX(angle)
    #[css(function = "skewX")]
    SkewX(Angle),
    /// skewY(angle)
    #[css(function = "skewY")]
    SkewY(Angle),
    /// translate(x, y) or translate(x)
    #[css(comma, function)]
    Translate(
        LengthPercentage,
        #[css(skip_if = "ZeroNoPercent::is_zero_no_percent")] LengthPercentage,
    ),
    /// translateX(x)
    #[css(function = "translateX")]
    TranslateX(LengthPercentage),
    /// translateY(y)
    #[css(function = "translateY")]
    TranslateY(LengthPercentage),
    /// translateZ(z)
    #[css(function = "translateZ")]
    TranslateZ(Length),
    /// translate3d(x, y, z)
    #[css(comma, function = "translate3d")]
    Translate3D(LengthPercentage, LengthPercentage, Length),
    /// A 2D scaling factor.
    ///
    /// Syntax can be scale(factor) or scale(factor, factor)
    #[css(comma, function)]
    Scale(Number, #[css(contextual_skip_if = "is_same")] Number),
    /// scaleX(factor)
    #[css(function = "scaleX")]
    ScaleX(Number),
    /// scaleY(factor)
    #[css(function = "scaleY")]
    ScaleY(Number),
    /// scaleZ(factor)
    #[css(function = "scaleZ")]
    ScaleZ(Number),
    /// scale3D(factorX, factorY, factorZ)
    #[css(comma, function = "scale3d")]
    Scale3D(Number, Number, Number),
    /// Describes a 2D Rotation.
    ///
    /// In a 3D scene `rotate(angle)` is equivalent to `rotateZ(angle)`.
    #[css(function)]
    Rotate(Angle),
    /// Rotation in 3D space around the x-axis.
    #[css(function = "rotateX")]
    RotateX(Angle),
    /// Rotation in 3D space around the y-axis.
    #[css(function = "rotateY")]
    RotateY(Angle),
    /// Rotation in 3D space around the z-axis.
    #[css(function = "rotateZ")]
    RotateZ(Angle),
    /// Rotation in 3D space.
    ///
    /// Generalization of rotateX, rotateY and rotateZ.
    #[css(comma, function = "rotate3d")]
    Rotate3D(Number, Number, Number, Angle),
    /// Specifies a perspective projection matrix.
    ///
    /// Part of CSS Transform Module Level 2 and defined at
    /// [§ 13.1. 3D Transform Function](https://drafts.csswg.org/css-transforms-2/#funcdef-perspective).
    ///
    /// The value must be greater than or equal to zero.
    #[css(function)]
    Perspective(GenericPerspectiveFunction<Length>),
    /// A intermediate type for interpolation of mismatched transform lists.
    #[allow(missing_docs)]
    #[css(comma, function = "interpolatematrix")]
    InterpolateMatrix {
        from_list: GenericTransform<
            GenericTransformOperation<Angle, Number, Length, Integer, LengthPercentage>,
        >,
        to_list: GenericTransform<
            GenericTransformOperation<Angle, Number, Length, Integer, LengthPercentage>,
        >,
        progress: computed::Percentage,
    },
    /// A intermediate type for accumulation of mismatched transform lists.
    #[allow(missing_docs)]
    #[css(comma, function = "accumulatematrix")]
    AccumulateMatrix {
        from_list: GenericTransform<
            GenericTransformOperation<Angle, Number, Length, Integer, LengthPercentage>,
        >,
        to_list: GenericTransform<
            GenericTransformOperation<Angle, Number, Length, Integer, LengthPercentage>,
        >,
        count: Integer,
    },
}

pub use self::GenericTransformOperation as TransformOperation;

#[derive(
    Clone,
    Debug,
    Deserialize,
    MallocSizeOf,
    PartialEq,
    Serialize,
    SpecifiedValueInfo,
    ToComputedValue,
    ToCss,
    ToResolvedValue,
    ToShmem,
)]
#[repr(C)]
/// A value of the `transform` property
pub struct GenericTransform<T>(#[css(if_empty = "none", iterable)] pub crate::OwnedSlice<T>);

pub use self::GenericTransform as Transform;

impl<Angle, Number, Length, Integer, LengthPercentage>
    TransformOperation<Angle, Number, Length, Integer, LengthPercentage>
where
    Angle: Zero,
    LengthPercentage: Zero + ZeroNoPercent,
    Number: PartialEq,
{
    /// Check if it is any rotate function.
    pub fn is_rotate(&self) -> bool {
        use self::TransformOperation::*;
        matches!(
            *self,
            Rotate(..) | Rotate3D(..) | RotateX(..) | RotateY(..) | RotateZ(..)
        )
    }

    /// Check if it is any translate function
    pub fn is_translate(&self) -> bool {
        use self::TransformOperation::*;
        match *self {
            Translate(..) | Translate3D(..) | TranslateX(..) | TranslateY(..) | TranslateZ(..) => {
                true
            },
            _ => false,
        }
    }

    /// Check if it is any scale function
    pub fn is_scale(&self) -> bool {
        use self::TransformOperation::*;
        match *self {
            Scale(..) | Scale3D(..) | ScaleX(..) | ScaleY(..) | ScaleZ(..) => true,
            _ => false,
        }
    }
}

/// Convert a length type into the absolute lengths.
pub trait ToAbsoluteLength {
    /// Returns the absolute length as pixel value.
    fn to_pixel_length(&self, containing_len: Option<ComputedLength>) -> Result<CSSFloat, ()>;
}

impl ToAbsoluteLength for SpecifiedLength {
    // This returns Err(()) if there is any relative length or percentage. We use this when
    // parsing a transform list of DOMMatrix because we want to return a DOM Exception
    // if there is relative length.
    #[inline]
    fn to_pixel_length(&self, _containing_len: Option<ComputedLength>) -> Result<CSSFloat, ()> {
        match *self {
            SpecifiedLength::NoCalc(len) => len.to_computed_pixel_length_without_context(),
            SpecifiedLength::Calc(ref calc) => calc.to_computed_pixel_length_without_context(),
        }
    }
}

impl ToAbsoluteLength for SpecifiedLengthPercentage {
    // This returns Err(()) if there is any relative length or percentage. We use this when
    // parsing a transform list of DOMMatrix because we want to return a DOM Exception
    // if there is relative length.
    #[inline]
    fn to_pixel_length(&self, _containing_len: Option<ComputedLength>) -> Result<CSSFloat, ()> {
        use self::SpecifiedLengthPercentage::*;
        match *self {
            Length(len) => len.to_computed_pixel_length_without_context(),
            Calc(ref calc) => calc.to_computed_pixel_length_without_context(),
            Percentage(..) => Err(()),
        }
    }
}

impl ToAbsoluteLength for ComputedLength {
    #[inline]
    fn to_pixel_length(&self, _containing_len: Option<ComputedLength>) -> Result<CSSFloat, ()> {
        Ok(self.px())
    }
}

impl ToAbsoluteLength for ComputedLengthPercentage {
    #[inline]
    fn to_pixel_length(&self, containing_len: Option<ComputedLength>) -> Result<CSSFloat, ()> {
        Ok(self
            .maybe_percentage_relative_to(containing_len)
            .ok_or(())?
            .px())
    }
}

/// Support the conversion to a 3d matrix.
pub trait ToMatrix {
    /// Check if it is a 3d transform function.
    fn is_3d(&self) -> bool;

    /// Return the equivalent 3d matrix.
    fn to_3d_matrix(
        &self,
        reference_box: Option<&Rect<ComputedLength>>,
    ) -> Result<Transform3D<f64>, ()>;
}

/// A little helper to deal with both specified and computed angles.
pub trait ToRadians {
    /// Return the radians value as a 64-bit floating point value.
    fn radians64(&self) -> f64;
}

impl ToRadians for computed::angle::Angle {
    #[inline]
    fn radians64(&self) -> f64 {
        computed::angle::Angle::radians64(self)
    }
}

impl ToRadians for SpecifiedAngle {
    #[inline]
    fn radians64(&self) -> f64 {
        computed::angle::Angle::from_degrees(self.degrees()).radians64()
    }
}

impl<Angle, Number, Length, Integer, LoP> ToMatrix
    for TransformOperation<Angle, Number, Length, Integer, LoP>
where
    Angle: Zero + ToRadians + Copy,
    Number: PartialEq + Copy + Into<f32> + Into<f64>,
    Length: ToAbsoluteLength,
    LoP: Zero + ToAbsoluteLength + ZeroNoPercent,
{
    #[inline]
    fn is_3d(&self) -> bool {
        use self::TransformOperation::*;
        match *self {
            Translate3D(..) | TranslateZ(..) | Rotate3D(..) | RotateX(..) | RotateY(..) |
            RotateZ(..) | Scale3D(..) | ScaleZ(..) | Perspective(..) | Matrix3D(..) => true,
            _ => false,
        }
    }

    /// If |reference_box| is None, we will drop the percent part from translate because
    /// we cannot resolve it without the layout info, for computed TransformOperation.
    /// However, for specified TransformOperation, we will return Err(()) if there is any relative
    /// lengths because the only caller, DOMMatrix, doesn't accept relative lengths.
    #[inline]
    fn to_3d_matrix(
        &self,
        reference_box: Option<&Rect<ComputedLength>>,
    ) -> Result<Transform3D<f64>, ()> {
        use self::TransformOperation::*;

        let reference_width = reference_box.map(|v| v.size.width);
        let reference_height = reference_box.map(|v| v.size.height);
        let matrix = match *self {
            Rotate3D(ax, ay, az, theta) => {
                let theta = theta.radians64();
                let (ax, ay, az, theta) =
                    get_normalized_vector_and_angle(ax.into(), ay.into(), az.into(), theta);
                Transform3D::rotation(
                    ax as f64,
                    ay as f64,
                    az as f64,
                    euclid::Angle::radians(theta),
                )
            },
            RotateX(theta) => {
                let theta = euclid::Angle::radians(theta.radians64());
                Transform3D::rotation(1., 0., 0., theta)
            },
            RotateY(theta) => {
                let theta = euclid::Angle::radians(theta.radians64());
                Transform3D::rotation(0., 1., 0., theta)
            },
            RotateZ(theta) | Rotate(theta) => {
                let theta = euclid::Angle::radians(theta.radians64());
                Transform3D::rotation(0., 0., 1., theta)
            },
            Perspective(ref p) => {
                let px = match p {
                    PerspectiveFunction::None => f32::INFINITY,
                    PerspectiveFunction::Length(ref p) => p.to_pixel_length(None)?,
                };
                create_perspective_matrix(px).cast()
            },
            Scale3D(sx, sy, sz) => Transform3D::scale(sx.into(), sy.into(), sz.into()),
            Scale(sx, sy) => Transform3D::scale(sx.into(), sy.into(), 1.),
            ScaleX(s) => Transform3D::scale(s.into(), 1., 1.),
            ScaleY(s) => Transform3D::scale(1., s.into(), 1.),
            ScaleZ(s) => Transform3D::scale(1., 1., s.into()),
            Translate3D(ref tx, ref ty, ref tz) => {
                let tx = tx.to_pixel_length(reference_width)? as f64;
                let ty = ty.to_pixel_length(reference_height)? as f64;
                Transform3D::translation(tx, ty, tz.to_pixel_length(None)? as f64)
            },
            Translate(ref tx, ref ty) => {
                let tx = tx.to_pixel_length(reference_width)? as f64;
                let ty = ty.to_pixel_length(reference_height)? as f64;
                Transform3D::translation(tx, ty, 0.)
            },
            TranslateX(ref t) => {
                let t = t.to_pixel_length(reference_width)? as f64;
                Transform3D::translation(t, 0., 0.)
            },
            TranslateY(ref t) => {
                let t = t.to_pixel_length(reference_height)? as f64;
                Transform3D::translation(0., t, 0.)
            },
            TranslateZ(ref z) => Transform3D::translation(0., 0., z.to_pixel_length(None)? as f64),
            Skew(theta_x, theta_y) => Transform3D::skew(
                euclid::Angle::radians(theta_x.radians64()),
                euclid::Angle::radians(theta_y.radians64()),
            ),
            SkewX(theta) => Transform3D::skew(
                euclid::Angle::radians(theta.radians64()),
                euclid::Angle::radians(0.),
            ),
            SkewY(theta) => Transform3D::skew(
                euclid::Angle::radians(0.),
                euclid::Angle::radians(theta.radians64()),
            ),
            Matrix3D(m) => m.into(),
            Matrix(m) => m.into(),
            InterpolateMatrix { .. } | AccumulateMatrix { .. } => {
                // TODO: Convert InterpolateMatrix/AccumulateMatrix into a valid Transform3D by
                // the reference box and do interpolation on these two Transform3D matrices.
                // Both Gecko and Servo don't support this for computing distance, and Servo
                // doesn't support animations on InterpolateMatrix/AccumulateMatrix, so
                // return an identity matrix.
                // Note: DOMMatrix doesn't go into this arm.
                Transform3D::identity()
            },
        };
        Ok(matrix)
    }
}

impl<T> Transform<T> {
    /// `none`
    pub fn none() -> Self {
        Transform(Default::default())
    }
}

impl<T: ToMatrix> Transform<T> {
    /// Return the equivalent 3d matrix of this transform list.
    ///
    /// We return a pair: the first one is the transform matrix, and the second one
    /// indicates if there is any 3d transform function in this transform list.
    #[cfg_attr(rustfmt, rustfmt_skip)]
    pub fn to_transform_3d_matrix(
        &self,
        reference_box: Option<&Rect<ComputedLength>>
    ) -> Result<(Transform3D<CSSFloat>, bool), ()> {
        Self::components_to_transform_3d_matrix(&self.0, reference_box)
    }

    /// Converts a series of components to a 3d matrix.
    #[cfg_attr(rustfmt, rustfmt_skip)]
    pub fn components_to_transform_3d_matrix(
        ops: &[T],
        reference_box: Option<&Rect<ComputedLength>>,
    ) -> Result<(Transform3D<CSSFloat>, bool), ()> {
        let cast_3d_transform = |m: Transform3D<f64>| -> Transform3D<CSSFloat> {
            use std::{f32, f64};
            let cast = |v: f64| v.min(f32::MAX as f64).max(f32::MIN as f64) as f32;
            Transform3D::new(
                cast(m.m11), cast(m.m12), cast(m.m13), cast(m.m14),
                cast(m.m21), cast(m.m22), cast(m.m23), cast(m.m24),
                cast(m.m31), cast(m.m32), cast(m.m33), cast(m.m34),
                cast(m.m41), cast(m.m42), cast(m.m43), cast(m.m44),
            )
        };

        let (m, is_3d) = Self::components_to_transform_3d_matrix_f64(ops, reference_box)?;
        Ok((cast_3d_transform(m), is_3d))
    }

    /// Same as Transform::to_transform_3d_matrix but a f64 version.
    fn components_to_transform_3d_matrix_f64(
        ops: &[T],
        reference_box: Option<&Rect<ComputedLength>>,
    ) -> Result<(Transform3D<f64>, bool), ()> {
        // We intentionally use Transform3D<f64> during computation to avoid
        // error propagation because using f32 to compute triangle functions
        // (e.g. in rotation()) is not accurate enough. In Gecko, we also use
        // "double" to compute the triangle functions. Therefore, let's use
        // Transform3D<f64> during matrix computation and cast it into f32 in
        // the end.
        let mut transform = Transform3D::<f64>::identity();
        let mut contain_3d = false;

        for operation in ops {
            let matrix = operation.to_3d_matrix(reference_box)?;
            contain_3d = contain_3d || operation.is_3d();
            transform = matrix.then(&transform);
        }

        Ok((transform, contain_3d))
    }
}

/// Return the transform matrix from a perspective length.
#[inline]
pub fn create_perspective_matrix(d: CSSFloat) -> Transform3D<CSSFloat> {
    if d.is_finite() {
        Transform3D::perspective(d.max(1.))
    } else {
        Transform3D::identity()
    }
}

/// Return the normalized direction vector and its angle for Rotate3D.
pub fn get_normalized_vector_and_angle<T: Zero>(
    x: CSSFloat,
    y: CSSFloat,
    z: CSSFloat,
    angle: T,
) -> (CSSFloat, CSSFloat, CSSFloat, T) {
    use crate::values::computed::transform::DirectionVector;
    use euclid::approxeq::ApproxEq;
    let vector = DirectionVector::new(x, y, z);
    if vector.square_length().approx_eq(&f32::zero()) {
        // https://www.w3.org/TR/css-transforms-1/#funcdef-rotate3d
        // A direction vector that cannot be normalized, such as [0, 0, 0], will cause the
        // rotation to not be applied, so we use identity matrix (i.e. rotate3d(0, 0, 1, 0)).
        (0., 0., 1., T::zero())
    } else {
        let vector = vector.robust_normalize();
        (vector.x, vector.y, vector.z, angle)
    }
}

#[derive(
    Clone,
    Copy,
    Debug,
    Deserialize,
    MallocSizeOf,
    PartialEq,
    Serialize,
    SpecifiedValueInfo,
    ToAnimatedZero,
    ToComputedValue,
    ToResolvedValue,
    ToShmem,
)]
#[repr(C, u8)]
/// A value of the `Rotate` property
///
/// <https://drafts.csswg.org/css-transforms-2/#individual-transforms>
pub enum GenericRotate<Number, Angle> {
    /// 'none'
    None,
    /// '<angle>'
    Rotate(Angle),
    /// '<number>{3} <angle>'
    Rotate3D(Number, Number, Number, Angle),
}

pub use self::GenericRotate as Rotate;

/// A trait to check if the current 3D vector is parallel to the DirectionVector.
/// This is especially for serialization on Rotate.
pub trait IsParallelTo {
    /// Returns true if this is parallel to the vector.
    fn is_parallel_to(&self, vector: &computed::transform::DirectionVector) -> bool;
}

impl<Number, Angle> ToCss for Rotate<Number, Angle>
where
    Number: Copy + ToCss + Zero,
    Angle: ToCss,
    (Number, Number, Number): IsParallelTo,
{
    fn to_css<W>(&self, dest: &mut CssWriter<W>) -> fmt::Result
    where
        W: fmt::Write,
    {
        use crate::values::computed::transform::DirectionVector;
        match *self {
            Rotate::None => dest.write_str("none"),
            Rotate::Rotate(ref angle) => angle.to_css(dest),
            Rotate::Rotate3D(x, y, z, ref angle) => {
                // If the axis is parallel with the x or y axes, it must serialize as the
                // appropriate keyword. If a rotation about the z axis (that is, in 2D) is
                // specified, the property must serialize as just an <angle>
                //
                // https://drafts.csswg.org/css-transforms-2/#individual-transform-serialization
                let v = (x, y, z);
                let axis = if x.is_zero() && y.is_zero() && z.is_zero() {
                    // The zero length vector is parallel to every other vector, so
                    // is_parallel_to() returns true for it. However, it is definitely different
                    // from x axis, y axis, or z axis, and it's meaningless to perform a rotation
                    // using that direction vector. So we *have* to serialize it using that same
                    // vector - we can't simplify to some theoretically parallel axis-aligned
                    // vector.
                    None
                } else if v.is_parallel_to(&DirectionVector::new(1., 0., 0.)) {
                    Some("x ")
                } else if v.is_parallel_to(&DirectionVector::new(0., 1., 0.)) {
                    Some("y ")
                } else if v.is_parallel_to(&DirectionVector::new(0., 0., 1.)) {
                    // When we're parallel to the z-axis, we can just serialize the angle.
                    return angle.to_css(dest);
                } else {
                    None
                };
                match axis {
                    Some(a) => dest.write_str(a)?,
                    None => {
                        x.to_css(dest)?;
                        dest.write_char(' ')?;
                        y.to_css(dest)?;
                        dest.write_char(' ')?;
                        z.to_css(dest)?;
                        dest.write_char(' ')?;
                    },
                }
                angle.to_css(dest)
            },
        }
    }
}

#[derive(
    Clone,
    Copy,
    Debug,
    Deserialize,
    MallocSizeOf,
    PartialEq,
    Serialize,
    SpecifiedValueInfo,
    ToAnimatedZero,
    ToComputedValue,
    ToResolvedValue,
    ToShmem,
)]
#[repr(C, u8)]
/// A value of the `Scale` property
///
/// <https://drafts.csswg.org/css-transforms-2/#individual-transforms>
pub enum GenericScale<Number> {
    /// 'none'
    None,
    /// '<number>{1,3}'
    Scale(Number, Number, Number),
}

pub use self::GenericScale as Scale;

impl<Number> ToCss for Scale<Number>
where
    Number: ToCss + PartialEq + Copy,
    f32: From<Number>,
{
    fn to_css<W>(&self, dest: &mut CssWriter<W>) -> fmt::Result
    where
        W: fmt::Write,
        f32: From<Number>,
    {
        match *self {
            Scale::None => dest.write_str("none"),
            Scale::Scale(ref x, ref y, ref z) => {
                x.to_css(dest)?;

                let is_3d = f32::from(*z) != 1.0;
                if is_3d || x != y {
                    dest.write_char(' ')?;
                    y.to_css(dest)?;
                }

                if is_3d {
                    dest.write_char(' ')?;
                    z.to_css(dest)?;
                }
                Ok(())
            },
        }
    }
}

#[inline]
fn y_axis_and_z_axis_are_zero<LengthPercentage: Zero + ZeroNoPercent, Length: Zero>(
    _: &LengthPercentage,
    y: &LengthPercentage,
    z: &Length,
) -> bool {
    y.is_zero_no_percent() && z.is_zero()
}

#[derive(
    Clone,
    Debug,
    Deserialize,
    MallocSizeOf,
    PartialEq,
    Serialize,
    SpecifiedValueInfo,
    ToAnimatedZero,
    ToComputedValue,
    ToCss,
    ToResolvedValue,
    ToShmem,
)]
#[repr(C, u8)]
/// A value of the `translate` property
///
/// https://drafts.csswg.org/css-transforms-2/#individual-transform-serialization:
///
/// If a 2d translation is specified, the property must serialize with only one
/// or two values (per usual, if the second value is 0px, the default, it must
/// be omitted when serializing; however if 0% is the second value, it is included).
///
/// If a 3d translation is specified and the value can be expressed as 2d, we treat as 2d and
/// serialize accoringly. Otherwise, we serialize all three values.
/// https://github.com/w3c/csswg-drafts/issues/3305
///
/// <https://drafts.csswg.org/css-transforms-2/#individual-transforms>
pub enum GenericTranslate<LengthPercentage, Length>
where
    LengthPercentage: Zero + ZeroNoPercent,
    Length: Zero,
{
    /// 'none'
    None,
    /// <length-percentage> [ <length-percentage> <length>? ]?
    Translate(
        LengthPercentage,
        #[css(contextual_skip_if = "y_axis_and_z_axis_are_zero")] LengthPercentage,
        #[css(skip_if = "Zero::is_zero")] Length,
    ),
}

pub use self::GenericTranslate as Translate;

#[allow(missing_docs)]
#[derive(
    Clone,
    Copy,
    Debug,
    MallocSizeOf,
    Parse,
    PartialEq,
    SpecifiedValueInfo,
    ToComputedValue,
    ToCss,
    ToResolvedValue,
    ToShmem,
)]
#[repr(u8)]
pub enum TransformStyle {
    Flat,
    #[css(keyword = "preserve-3d")]
    Preserve3d,
}