Adding upstream version 1:115.7.0.upstream/1%115.7.0upstream

Signed-off-by: Daniel Baumann <daniel.baumann@progress-linux.org>

1 files changed, 1473 insertions, 0 deletions

diff --git a/servo/components/style/values/animated/transform.rs b/servo/components/style/values/animated/transform.rs
new file mode 100644
index 0000000000..d30f8b74f8
--- /dev/null
+++ b/servo/components/style/values/animated/transform.rs
@@ -0,0 +1,1473 @@
+/* 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/. */
+
+//! Animated types for transform.
+// There are still some implementation on Matrix3D in animated_properties.mako.rs
+// because they still need mako to generate the code.
+
+use super::animate_multiplicative_factor;
+use super::{Animate, Procedure, ToAnimatedZero};
+use crate::values::computed::transform::Rotate as ComputedRotate;
+use crate::values::computed::transform::Scale as ComputedScale;
+use crate::values::computed::transform::Transform as ComputedTransform;
+use crate::values::computed::transform::TransformOperation as ComputedTransformOperation;
+use crate::values::computed::transform::Translate as ComputedTranslate;
+use crate::values::computed::transform::{DirectionVector, Matrix, Matrix3D};
+use crate::values::computed::Angle;
+use crate::values::computed::{Length, LengthPercentage};
+use crate::values::computed::{Number, Percentage};
+use crate::values::distance::{ComputeSquaredDistance, SquaredDistance};
+use crate::values::generics::transform::{self, Transform, TransformOperation};
+use crate::values::generics::transform::{Rotate, Scale, Translate};
+use crate::values::CSSFloat;
+use crate::Zero;
+use std::cmp;
+
+// ------------------------------------
+// Animations for Matrix/Matrix3D.
+// ------------------------------------
+/// A 2d matrix for interpolation.
+#[derive(Clone, ComputeSquaredDistance, Copy, Debug)]
+#[cfg_attr(feature = "servo", derive(MallocSizeOf))]
+#[allow(missing_docs)]
+// FIXME: We use custom derive for ComputeSquaredDistance. However, If possible, we should convert
+// the InnerMatrix2D into types with physical meaning. This custom derive computes the squared
+// distance from each matrix item, and this makes the result different from that in Gecko if we
+// have skew factor in the Matrix3D.
+pub struct InnerMatrix2D {
+    pub m11: CSSFloat,
+    pub m12: CSSFloat,
+    pub m21: CSSFloat,
+    pub m22: CSSFloat,
+}
+
+impl Animate for InnerMatrix2D {
+    fn animate(&self, other: &Self, procedure: Procedure) -> Result<Self, ()> {
+        Ok(InnerMatrix2D {
+            m11: animate_multiplicative_factor(self.m11, other.m11, procedure)?,
+            m12: self.m12.animate(&other.m12, procedure)?,
+            m21: self.m21.animate(&other.m21, procedure)?,
+            m22: animate_multiplicative_factor(self.m22, other.m22, procedure)?,
+        })
+    }
+}
+
+/// A 2d translation function.
+#[cfg_attr(feature = "servo", derive(MallocSizeOf))]
+#[derive(Animate, Clone, ComputeSquaredDistance, Copy, Debug)]
+pub struct Translate2D(f32, f32);
+
+/// A 2d scale function.
+#[derive(Clone, ComputeSquaredDistance, Copy, Debug)]
+#[cfg_attr(feature = "servo", derive(MallocSizeOf))]
+pub struct Scale2D(f32, f32);
+
+impl Animate for Scale2D {
+    fn animate(&self, other: &Self, procedure: Procedure) -> Result<Self, ()> {
+        Ok(Scale2D(
+            animate_multiplicative_factor(self.0, other.0, procedure)?,
+            animate_multiplicative_factor(self.1, other.1, procedure)?,
+        ))
+    }
+}
+
+/// A decomposed 2d matrix.
+#[derive(Clone, Copy, Debug)]
+#[cfg_attr(feature = "servo", derive(MallocSizeOf))]
+pub struct MatrixDecomposed2D {
+    /// The translation function.
+    pub translate: Translate2D,
+    /// The scale function.
+    pub scale: Scale2D,
+    /// The rotation angle.
+    pub angle: f32,
+    /// The inner matrix.
+    pub matrix: InnerMatrix2D,
+}
+
+impl Animate for MatrixDecomposed2D {
+    /// <https://drafts.csswg.org/css-transforms/#interpolation-of-decomposed-2d-matrix-values>
+    fn animate(&self, other: &Self, procedure: Procedure) -> Result<Self, ()> {
+        // If x-axis of one is flipped, and y-axis of the other,
+        // convert to an unflipped rotation.
+        let mut scale = self.scale;
+        let mut angle = self.angle;
+        let mut other_angle = other.angle;
+        if (scale.0 < 0.0 && other.scale.1 < 0.0) || (scale.1 < 0.0 && other.scale.0 < 0.0) {
+            scale.0 = -scale.0;
+            scale.1 = -scale.1;
+            angle += if angle < 0.0 { 180. } else { -180. };
+        }
+
+        // Don't rotate the long way around.
+        if angle == 0.0 {
+            angle = 360.
+        }
+        if other_angle == 0.0 {
+            other_angle = 360.
+        }
+
+        if (angle - other_angle).abs() > 180. {
+            if angle > other_angle {
+                angle -= 360.
+            } else {
+                other_angle -= 360.
+            }
+        }
+
+        // Interpolate all values.
+        let translate = self.translate.animate(&other.translate, procedure)?;
+        let scale = scale.animate(&other.scale, procedure)?;
+        let angle = angle.animate(&other_angle, procedure)?;
+        let matrix = self.matrix.animate(&other.matrix, procedure)?;
+
+        Ok(MatrixDecomposed2D {
+            translate,
+            scale,
+            angle,
+            matrix,
+        })
+    }
+}
+
+impl ComputeSquaredDistance for MatrixDecomposed2D {
+    #[inline]
+    fn compute_squared_distance(&self, other: &Self) -> Result<SquaredDistance, ()> {
+        // Use Radian to compute the distance.
+        const RAD_PER_DEG: f64 = std::f64::consts::PI / 180.0;
+        let angle1 = self.angle as f64 * RAD_PER_DEG;
+        let angle2 = other.angle as f64 * RAD_PER_DEG;
+        Ok(self.translate.compute_squared_distance(&other.translate)? +
+            self.scale.compute_squared_distance(&other.scale)? +
+            angle1.compute_squared_distance(&angle2)? +
+            self.matrix.compute_squared_distance(&other.matrix)?)
+    }
+}
+
+impl From<Matrix3D> for MatrixDecomposed2D {
+    /// Decompose a 2D matrix.
+    /// <https://drafts.csswg.org/css-transforms/#decomposing-a-2d-matrix>
+    fn from(matrix: Matrix3D) -> MatrixDecomposed2D {
+        let mut row0x = matrix.m11;
+        let mut row0y = matrix.m12;
+        let mut row1x = matrix.m21;
+        let mut row1y = matrix.m22;
+
+        let translate = Translate2D(matrix.m41, matrix.m42);
+        let mut scale = Scale2D(
+            (row0x * row0x + row0y * row0y).sqrt(),
+            (row1x * row1x + row1y * row1y).sqrt(),
+        );
+
+        // If determinant is negative, one axis was flipped.
+        let determinant = row0x * row1y - row0y * row1x;
+        if determinant < 0. {
+            if row0x < row1y {
+                scale.0 = -scale.0;
+            } else {
+                scale.1 = -scale.1;
+            }
+        }
+
+        // Renormalize matrix to remove scale.
+        if scale.0 != 0.0 {
+            row0x *= 1. / scale.0;
+            row0y *= 1. / scale.0;
+        }
+        if scale.1 != 0.0 {
+            row1x *= 1. / scale.1;
+            row1y *= 1. / scale.1;
+        }
+
+        // Compute rotation and renormalize matrix.
+        let mut angle = row0y.atan2(row0x);
+        if angle != 0.0 {
+            let sn = -row0y;
+            let cs = row0x;
+            let m11 = row0x;
+            let m12 = row0y;
+            let m21 = row1x;
+            let m22 = row1y;
+            row0x = cs * m11 + sn * m21;
+            row0y = cs * m12 + sn * m22;
+            row1x = -sn * m11 + cs * m21;
+            row1y = -sn * m12 + cs * m22;
+        }
+
+        let m = InnerMatrix2D {
+            m11: row0x,
+            m12: row0y,
+            m21: row1x,
+            m22: row1y,
+        };
+
+        // Convert into degrees because our rotation functions expect it.
+        angle = angle.to_degrees();
+        MatrixDecomposed2D {
+            translate: translate,
+            scale: scale,
+            angle: angle,
+            matrix: m,
+        }
+    }
+}
+
+impl From<MatrixDecomposed2D> for Matrix3D {
+    /// Recompose a 2D matrix.
+    /// <https://drafts.csswg.org/css-transforms/#recomposing-to-a-2d-matrix>
+    fn from(decomposed: MatrixDecomposed2D) -> Matrix3D {
+        let mut computed_matrix = Matrix3D::identity();
+        computed_matrix.m11 = decomposed.matrix.m11;
+        computed_matrix.m12 = decomposed.matrix.m12;
+        computed_matrix.m21 = decomposed.matrix.m21;
+        computed_matrix.m22 = decomposed.matrix.m22;
+
+        // Translate matrix.
+        computed_matrix.m41 = decomposed.translate.0;
+        computed_matrix.m42 = decomposed.translate.1;
+
+        // Rotate matrix.
+        let angle = decomposed.angle.to_radians();
+        let cos_angle = angle.cos();
+        let sin_angle = angle.sin();
+
+        let mut rotate_matrix = Matrix3D::identity();
+        rotate_matrix.m11 = cos_angle;
+        rotate_matrix.m12 = sin_angle;
+        rotate_matrix.m21 = -sin_angle;
+        rotate_matrix.m22 = cos_angle;
+
+        // Multiplication of computed_matrix and rotate_matrix
+        computed_matrix = rotate_matrix.multiply(&computed_matrix);
+
+        // Scale matrix.
+        computed_matrix.m11 *= decomposed.scale.0;
+        computed_matrix.m12 *= decomposed.scale.0;
+        computed_matrix.m21 *= decomposed.scale.1;
+        computed_matrix.m22 *= decomposed.scale.1;
+        computed_matrix
+    }
+}
+
+impl Animate for Matrix {
+    #[cfg(feature = "servo")]
+    fn animate(&self, other: &Self, procedure: Procedure) -> Result<Self, ()> {
+        let this = Matrix3D::from(*self);
+        let other = Matrix3D::from(*other);
+        let this = MatrixDecomposed2D::from(this);
+        let other = MatrixDecomposed2D::from(other);
+        Matrix3D::from(this.animate(&other, procedure)?).into_2d()
+    }
+
+    #[cfg(feature = "gecko")]
+    // Gecko doesn't exactly follow the spec here; we use a different procedure
+    // to match it
+    fn animate(&self, other: &Self, procedure: Procedure) -> Result<Self, ()> {
+        let this = Matrix3D::from(*self);
+        let other = Matrix3D::from(*other);
+        let from = decompose_2d_matrix(&this)?;
+        let to = decompose_2d_matrix(&other)?;
+        Matrix3D::from(from.animate(&to, procedure)?).into_2d()
+    }
+}
+
+/// A 3d translation.
+#[cfg_attr(feature = "servo", derive(MallocSizeOf))]
+#[derive(Animate, Clone, ComputeSquaredDistance, Copy, Debug)]
+pub struct Translate3D(pub f32, pub f32, pub f32);
+
+/// A 3d scale function.
+#[derive(Clone, ComputeSquaredDistance, Copy, Debug)]
+#[cfg_attr(feature = "servo", derive(MallocSizeOf))]
+pub struct Scale3D(pub f32, pub f32, pub f32);
+
+impl Scale3D {
+    /// Negate self.
+    fn negate(&mut self) {
+        self.0 *= -1.0;
+        self.1 *= -1.0;
+        self.2 *= -1.0;
+    }
+}
+
+impl Animate for Scale3D {
+    fn animate(&self, other: &Self, procedure: Procedure) -> Result<Self, ()> {
+        Ok(Scale3D(
+            animate_multiplicative_factor(self.0, other.0, procedure)?,
+            animate_multiplicative_factor(self.1, other.1, procedure)?,
+            animate_multiplicative_factor(self.2, other.2, procedure)?,
+        ))
+    }
+}
+
+/// A 3d skew function.
+#[cfg_attr(feature = "servo", derive(MallocSizeOf))]
+#[derive(Animate, Clone, Copy, Debug)]
+pub struct Skew(f32, f32, f32);
+
+impl ComputeSquaredDistance for Skew {
+    // We have to use atan() to convert the skew factors into skew angles, so implement
+    // ComputeSquaredDistance manually.
+    #[inline]
+    fn compute_squared_distance(&self, other: &Self) -> Result<SquaredDistance, ()> {
+        Ok(self.0.atan().compute_squared_distance(&other.0.atan())? +
+            self.1.atan().compute_squared_distance(&other.1.atan())? +
+            self.2.atan().compute_squared_distance(&other.2.atan())?)
+    }
+}
+
+/// A 3d perspective transformation.
+#[derive(Clone, ComputeSquaredDistance, Copy, Debug)]
+#[cfg_attr(feature = "servo", derive(MallocSizeOf))]
+pub struct Perspective(pub f32, pub f32, pub f32, pub f32);
+
+impl Animate for Perspective {
+    fn animate(&self, other: &Self, procedure: Procedure) -> Result<Self, ()> {
+        Ok(Perspective(
+            self.0.animate(&other.0, procedure)?,
+            self.1.animate(&other.1, procedure)?,
+            self.2.animate(&other.2, procedure)?,
+            animate_multiplicative_factor(self.3, other.3, procedure)?,
+        ))
+    }
+}
+
+/// A quaternion used to represent a rotation.
+#[derive(Clone, Copy, Debug)]
+#[cfg_attr(feature = "servo", derive(MallocSizeOf))]
+pub struct Quaternion(f64, f64, f64, f64);
+
+impl Quaternion {
+    /// Return a quaternion from a unit direction vector and angle (unit: radian).
+    #[inline]
+    fn from_direction_and_angle(vector: &DirectionVector, angle: f64) -> Self {
+        debug_assert!(
+            (vector.length() - 1.).abs() < 0.0001,
+            "Only accept an unit direction vector to create a quaternion"
+        );
+        // Reference:
+        // https://en.wikipedia.org/wiki/Quaternions_and_spatial_rotation
+        //
+        // if the direction axis is (x, y, z) = xi + yj + zk,
+        // and the angle is |theta|, this formula can be done using
+        // an extension of Euler's formula:
+        //   q = cos(theta/2) + (xi + yj + zk)(sin(theta/2))
+        //     = cos(theta/2) +
+        //       x*sin(theta/2)i + y*sin(theta/2)j + z*sin(theta/2)k
+        Quaternion(
+            vector.x as f64 * (angle / 2.).sin(),
+            vector.y as f64 * (angle / 2.).sin(),
+            vector.z as f64 * (angle / 2.).sin(),
+            (angle / 2.).cos(),
+        )
+    }
+
+    /// Calculate the dot product.
+    #[inline]
+    fn dot(&self, other: &Self) -> f64 {
+        self.0 * other.0 + self.1 * other.1 + self.2 * other.2 + self.3 * other.3
+    }
+
+    /// Return the scaled quaternion by a factor.
+    #[inline]
+    fn scale(&self, factor: f64) -> Self {
+        Quaternion(
+            self.0 * factor,
+            self.1 * factor,
+            self.2 * factor,
+            self.3 * factor,
+        )
+    }
+}
+
+impl Animate for Quaternion {
+    fn animate(&self, other: &Self, procedure: Procedure) -> Result<Self, ()> {
+        let (this_weight, other_weight) = procedure.weights();
+        debug_assert!(
+            // Doule EPSILON since both this_weight and other_weght have calculation errors
+            // which are approximately equal to EPSILON.
+            (this_weight + other_weight - 1.0f64).abs() <= f64::EPSILON * 2.0 ||
+                other_weight == 1.0f64 ||
+                other_weight == 0.0f64,
+            "animate should only be used for interpolating or accumulating transforms"
+        );
+
+        // We take a specialized code path for accumulation (where other_weight
+        // is 1).
+        if let Procedure::Accumulate { .. } = procedure {
+            debug_assert_eq!(other_weight, 1.0);
+            if this_weight == 0.0 {
+                return Ok(*other);
+            }
+
+            let clamped_w = self.3.min(1.0).max(-1.0);
+
+            // Determine the scale factor.
+            let mut theta = clamped_w.acos();
+            let mut scale = if theta == 0.0 { 0.0 } else { 1.0 / theta.sin() };
+            theta *= this_weight;
+            scale *= theta.sin();
+
+            // Scale the self matrix by this_weight.
+            let mut scaled_self = *self;
+            scaled_self.0 *= scale;
+            scaled_self.1 *= scale;
+            scaled_self.2 *= scale;
+            scaled_self.3 = theta.cos();
+
+            // Multiply scaled-self by other.
+            let a = &scaled_self;
+            let b = other;
+            return Ok(Quaternion(
+                a.3 * b.0 + a.0 * b.3 + a.1 * b.2 - a.2 * b.1,
+                a.3 * b.1 - a.0 * b.2 + a.1 * b.3 + a.2 * b.0,
+                a.3 * b.2 + a.0 * b.1 - a.1 * b.0 + a.2 * b.3,
+                a.3 * b.3 - a.0 * b.0 - a.1 * b.1 - a.2 * b.2,
+            ));
+        }
+
+        // Straight from gfxQuaternion::Slerp.
+        //
+        // Dot product, clamped between -1 and 1.
+        let dot = (self.0 * other.0 + self.1 * other.1 + self.2 * other.2 + self.3 * other.3)
+            .min(1.0)
+            .max(-1.0);
+
+        if dot.abs() == 1.0 {
+            return Ok(*self);
+        }
+
+        let theta = dot.acos();
+        let rsintheta = 1.0 / (1.0 - dot * dot).sqrt();
+
+        let right_weight = (other_weight * theta).sin() * rsintheta;
+        let left_weight = (other_weight * theta).cos() - dot * right_weight;
+
+        let left = self.scale(left_weight);
+        let right = other.scale(right_weight);
+
+        Ok(Quaternion(
+            left.0 + right.0,
+            left.1 + right.1,
+            left.2 + right.2,
+            left.3 + right.3,
+        ))
+    }
+}
+
+impl ComputeSquaredDistance for Quaternion {
+    #[inline]
+    fn compute_squared_distance(&self, other: &Self) -> Result<SquaredDistance, ()> {
+        // Use quaternion vectors to get the angle difference. Both q1 and q2 are unit vectors,
+        // so we can get their angle difference by:
+        // cos(theta/2) = (q1 dot q2) / (|q1| * |q2|) = q1 dot q2.
+        let distance = self.dot(other).max(-1.0).min(1.0).acos() * 2.0;
+        Ok(SquaredDistance::from_sqrt(distance))
+    }
+}
+
+/// A decomposed 3d matrix.
+#[derive(Animate, Clone, ComputeSquaredDistance, Copy, Debug)]
+#[cfg_attr(feature = "servo", derive(MallocSizeOf))]
+pub struct MatrixDecomposed3D {
+    /// A translation function.
+    pub translate: Translate3D,
+    /// A scale function.
+    pub scale: Scale3D,
+    /// The skew component of the transformation.
+    pub skew: Skew,
+    /// The perspective component of the transformation.
+    pub perspective: Perspective,
+    /// The quaternion used to represent the rotation.
+    pub quaternion: Quaternion,
+}
+
+impl From<MatrixDecomposed3D> for Matrix3D {
+    /// Recompose a 3D matrix.
+    /// <https://drafts.csswg.org/css-transforms/#recomposing-to-a-3d-matrix>
+    fn from(decomposed: MatrixDecomposed3D) -> Matrix3D {
+        let mut matrix = Matrix3D::identity();
+
+        // Apply perspective
+        matrix.set_perspective(&decomposed.perspective);
+
+        // Apply translation
+        matrix.apply_translate(&decomposed.translate);
+
+        // Apply rotation
+        {
+            let x = decomposed.quaternion.0;
+            let y = decomposed.quaternion.1;
+            let z = decomposed.quaternion.2;
+            let w = decomposed.quaternion.3;
+
+            // Construct a composite rotation matrix from the quaternion values
+            // rotationMatrix is a identity 4x4 matrix initially
+            let mut rotation_matrix = Matrix3D::identity();
+            rotation_matrix.m11 = 1.0 - 2.0 * (y * y + z * z) as f32;
+            rotation_matrix.m12 = 2.0 * (x * y + z * w) as f32;
+            rotation_matrix.m13 = 2.0 * (x * z - y * w) as f32;
+            rotation_matrix.m21 = 2.0 * (x * y - z * w) as f32;
+            rotation_matrix.m22 = 1.0 - 2.0 * (x * x + z * z) as f32;
+            rotation_matrix.m23 = 2.0 * (y * z + x * w) as f32;
+            rotation_matrix.m31 = 2.0 * (x * z + y * w) as f32;
+            rotation_matrix.m32 = 2.0 * (y * z - x * w) as f32;
+            rotation_matrix.m33 = 1.0 - 2.0 * (x * x + y * y) as f32;
+
+            matrix = rotation_matrix.multiply(&matrix);
+        }
+
+        // Apply skew
+        {
+            let mut temp = Matrix3D::identity();
+            if decomposed.skew.2 != 0.0 {
+                temp.m32 = decomposed.skew.2;
+                matrix = temp.multiply(&matrix);
+                temp.m32 = 0.0;
+            }
+
+            if decomposed.skew.1 != 0.0 {
+                temp.m31 = decomposed.skew.1;
+                matrix = temp.multiply(&matrix);
+                temp.m31 = 0.0;
+            }
+
+            if decomposed.skew.0 != 0.0 {
+                temp.m21 = decomposed.skew.0;
+                matrix = temp.multiply(&matrix);
+            }
+        }
+
+        // Apply scale
+        matrix.apply_scale(&decomposed.scale);
+
+        matrix
+    }
+}
+
+/// Decompose a 3D matrix.
+/// https://drafts.csswg.org/css-transforms-2/#decomposing-a-3d-matrix
+/// http://www.realtimerendering.com/resources/GraphicsGems/gemsii/unmatrix.c
+fn decompose_3d_matrix(mut matrix: Matrix3D) -> Result<MatrixDecomposed3D, ()> {
+    // Combine 2 point.
+    let combine = |a: [f32; 3], b: [f32; 3], ascl: f32, bscl: f32| {
+        [
+            (ascl * a[0]) + (bscl * b[0]),
+            (ascl * a[1]) + (bscl * b[1]),
+            (ascl * a[2]) + (bscl * b[2]),
+        ]
+    };
+    // Dot product.
+    let dot = |a: [f32; 3], b: [f32; 3]| a[0] * b[0] + a[1] * b[1] + a[2] * b[2];
+    // Cross product.
+    let cross = |row1: [f32; 3], row2: [f32; 3]| {
+        [
+            row1[1] * row2[2] - row1[2] * row2[1],
+            row1[2] * row2[0] - row1[0] * row2[2],
+            row1[0] * row2[1] - row1[1] * row2[0],
+        ]
+    };
+
+    if matrix.m44 == 0.0 {
+        return Err(());
+    }
+
+    let scaling_factor = matrix.m44;
+
+    // Normalize the matrix.
+    matrix.scale_by_factor(1.0 / scaling_factor);
+
+    // perspective_matrix is used to solve for perspective, but it also provides
+    // an easy way to test for singularity of the upper 3x3 component.
+    let mut perspective_matrix = matrix;
+
+    perspective_matrix.m14 = 0.0;
+    perspective_matrix.m24 = 0.0;
+    perspective_matrix.m34 = 0.0;
+    perspective_matrix.m44 = 1.0;
+
+    if perspective_matrix.determinant() == 0.0 {
+        return Err(());
+    }
+
+    // First, isolate perspective.
+    let perspective = if matrix.m14 != 0.0 || matrix.m24 != 0.0 || matrix.m34 != 0.0 {
+        let right_hand_side: [f32; 4] = [matrix.m14, matrix.m24, matrix.m34, matrix.m44];
+
+        perspective_matrix = perspective_matrix.inverse().unwrap().transpose();
+        let perspective = perspective_matrix.pre_mul_point4(&right_hand_side);
+        // NOTE(emilio): Even though the reference algorithm clears the
+        // fourth column here (matrix.m14..matrix.m44), they're not used below
+        // so it's not really needed.
+        Perspective(
+            perspective[0],
+            perspective[1],
+            perspective[2],
+            perspective[3],
+        )
+    } else {
+        Perspective(0.0, 0.0, 0.0, 1.0)
+    };
+
+    // Next take care of translation (easy).
+    let translate = Translate3D(matrix.m41, matrix.m42, matrix.m43);
+
+    // Now get scale and shear. 'row' is a 3 element array of 3 component vectors
+    let mut row = matrix.get_matrix_3x3_part();
+
+    // Compute X scale factor and normalize first row.
+    let row0len = (row[0][0] * row[0][0] + row[0][1] * row[0][1] + row[0][2] * row[0][2]).sqrt();
+    let mut scale = Scale3D(row0len, 0.0, 0.0);
+    row[0] = [
+        row[0][0] / row0len,
+        row[0][1] / row0len,
+        row[0][2] / row0len,
+    ];
+
+    // Compute XY shear factor and make 2nd row orthogonal to 1st.
+    let mut skew = Skew(dot(row[0], row[1]), 0.0, 0.0);
+    row[1] = combine(row[1], row[0], 1.0, -skew.0);
+
+    // Now, compute Y scale and normalize 2nd row.
+    let row1len = (row[1][0] * row[1][0] + row[1][1] * row[1][1] + row[1][2] * row[1][2]).sqrt();
+    scale.1 = row1len;
+    row[1] = [
+        row[1][0] / row1len,
+        row[1][1] / row1len,
+        row[1][2] / row1len,
+    ];
+    skew.0 /= scale.1;
+
+    // Compute XZ and YZ shears, orthogonalize 3rd row
+    skew.1 = dot(row[0], row[2]);
+    row[2] = combine(row[2], row[0], 1.0, -skew.1);
+    skew.2 = dot(row[1], row[2]);
+    row[2] = combine(row[2], row[1], 1.0, -skew.2);
+
+    // Next, get Z scale and normalize 3rd row.
+    let row2len = (row[2][0] * row[2][0] + row[2][1] * row[2][1] + row[2][2] * row[2][2]).sqrt();
+    scale.2 = row2len;
+    row[2] = [
+        row[2][0] / row2len,
+        row[2][1] / row2len,
+        row[2][2] / row2len,
+    ];
+    skew.1 /= scale.2;
+    skew.2 /= scale.2;
+
+    // At this point, the matrix (in rows) is orthonormal.
+    // Check for a coordinate system flip.  If the determinant
+    // is -1, then negate the matrix and the scaling factors.
+    if dot(row[0], cross(row[1], row[2])) < 0.0 {
+        scale.negate();
+        for i in 0..3 {
+            row[i][0] *= -1.0;
+            row[i][1] *= -1.0;
+            row[i][2] *= -1.0;
+        }
+    }
+
+    // Now, get the rotations out.
+    let mut quaternion = Quaternion(
+        0.5 * ((1.0 + row[0][0] - row[1][1] - row[2][2]).max(0.0) as f64).sqrt(),
+        0.5 * ((1.0 - row[0][0] + row[1][1] - row[2][2]).max(0.0) as f64).sqrt(),
+        0.5 * ((1.0 - row[0][0] - row[1][1] + row[2][2]).max(0.0) as f64).sqrt(),
+        0.5 * ((1.0 + row[0][0] + row[1][1] + row[2][2]).max(0.0) as f64).sqrt(),
+    );
+
+    if row[2][1] > row[1][2] {
+        quaternion.0 = -quaternion.0
+    }
+    if row[0][2] > row[2][0] {
+        quaternion.1 = -quaternion.1
+    }
+    if row[1][0] > row[0][1] {
+        quaternion.2 = -quaternion.2
+    }
+
+    Ok(MatrixDecomposed3D {
+        translate,
+        scale,
+        skew,
+        perspective,
+        quaternion,
+    })
+}
+
+/// Decompose a 2D matrix for Gecko.
+// Use the algorithm from nsStyleTransformMatrix::Decompose2DMatrix() in Gecko.
+#[cfg(feature = "gecko")]
+fn decompose_2d_matrix(matrix: &Matrix3D) -> Result<MatrixDecomposed3D, ()> {
+    // The index is column-major, so the equivalent transform matrix is:
+    // | m11 m21  0 m41 |  =>  | m11 m21 | and translate(m41, m42)
+    // | m12 m22  0 m42 |      | m12 m22 |
+    // |   0   0  1   0 |
+    // |   0   0  0   1 |
+    let (mut m11, mut m12) = (matrix.m11, matrix.m12);
+    let (mut m21, mut m22) = (matrix.m21, matrix.m22);
+    // Check if this is a singular matrix.
+    if m11 * m22 == m12 * m21 {
+        return Err(());
+    }
+
+    let mut scale_x = (m11 * m11 + m12 * m12).sqrt();
+    m11 /= scale_x;
+    m12 /= scale_x;
+
+    let mut shear_xy = m11 * m21 + m12 * m22;
+    m21 -= m11 * shear_xy;
+    m22 -= m12 * shear_xy;
+
+    let scale_y = (m21 * m21 + m22 * m22).sqrt();
+    m21 /= scale_y;
+    m22 /= scale_y;
+    shear_xy /= scale_y;
+
+    let determinant = m11 * m22 - m12 * m21;
+    // Determinant should now be 1 or -1.
+    if 0.99 > determinant.abs() || determinant.abs() > 1.01 {
+        return Err(());
+    }
+
+    if determinant < 0. {
+        m11 = -m11;
+        m12 = -m12;
+        shear_xy = -shear_xy;
+        scale_x = -scale_x;
+    }
+
+    Ok(MatrixDecomposed3D {
+        translate: Translate3D(matrix.m41, matrix.m42, 0.),
+        scale: Scale3D(scale_x, scale_y, 1.),
+        skew: Skew(shear_xy, 0., 0.),
+        perspective: Perspective(0., 0., 0., 1.),
+        quaternion: Quaternion::from_direction_and_angle(
+            &DirectionVector::new(0., 0., 1.),
+            m12.atan2(m11) as f64,
+        ),
+    })
+}
+
+impl Animate for Matrix3D {
+    #[cfg(feature = "servo")]
+    fn animate(&self, other: &Self, procedure: Procedure) -> Result<Self, ()> {
+        if self.is_3d() || other.is_3d() {
+            let decomposed_from = decompose_3d_matrix(*self);
+            let decomposed_to = decompose_3d_matrix(*other);
+            match (decomposed_from, decomposed_to) {
+                (Ok(this), Ok(other)) => Ok(Matrix3D::from(this.animate(&other, procedure)?)),
+                // Matrices can be undecomposable due to couple reasons, e.g.,
+                // non-invertible matrices. In this case, we should report Err
+                // here, and let the caller do the fallback procedure.
+                _ => Err(()),
+            }
+        } else {
+            let this = MatrixDecomposed2D::from(*self);
+            let other = MatrixDecomposed2D::from(*other);
+            Ok(Matrix3D::from(this.animate(&other, procedure)?))
+        }
+    }
+
+    #[cfg(feature = "gecko")]
+    // Gecko doesn't exactly follow the spec here; we use a different procedure
+    // to match it
+    fn animate(&self, other: &Self, procedure: Procedure) -> Result<Self, ()> {
+        let (from, to) = if self.is_3d() || other.is_3d() {
+            (decompose_3d_matrix(*self)?, decompose_3d_matrix(*other)?)
+        } else {
+            (decompose_2d_matrix(self)?, decompose_2d_matrix(other)?)
+        };
+        // Matrices can be undecomposable due to couple reasons, e.g.,
+        // non-invertible matrices. In this case, we should report Err here,
+        // and let the caller do the fallback procedure.
+        Ok(Matrix3D::from(from.animate(&to, procedure)?))
+    }
+}
+
+impl ComputeSquaredDistance for Matrix3D {
+    #[inline]
+    #[cfg(feature = "servo")]
+    fn compute_squared_distance(&self, other: &Self) -> Result<SquaredDistance, ()> {
+        if self.is_3d() || other.is_3d() {
+            let from = decompose_3d_matrix(*self)?;
+            let to = decompose_3d_matrix(*other)?;
+            from.compute_squared_distance(&to)
+        } else {
+            let from = MatrixDecomposed2D::from(*self);
+            let to = MatrixDecomposed2D::from(*other);
+            from.compute_squared_distance(&to)
+        }
+    }
+
+    #[inline]
+    #[cfg(feature = "gecko")]
+    fn compute_squared_distance(&self, other: &Self) -> Result<SquaredDistance, ()> {
+        let (from, to) = if self.is_3d() || other.is_3d() {
+            (decompose_3d_matrix(*self)?, decompose_3d_matrix(*other)?)
+        } else {
+            (decompose_2d_matrix(self)?, decompose_2d_matrix(other)?)
+        };
+        from.compute_squared_distance(&to)
+    }
+}
+
+// ------------------------------------
+// Animation for Transform list.
+// ------------------------------------
+fn is_matched_operation(
+    first: &ComputedTransformOperation,
+    second: &ComputedTransformOperation,
+) -> bool {
+    match (first, second) {
+        (&TransformOperation::Matrix(..), &TransformOperation::Matrix(..)) |
+        (&TransformOperation::Matrix3D(..), &TransformOperation::Matrix3D(..)) |
+        (&TransformOperation::Skew(..), &TransformOperation::Skew(..)) |
+        (&TransformOperation::SkewX(..), &TransformOperation::SkewX(..)) |
+        (&TransformOperation::SkewY(..), &TransformOperation::SkewY(..)) |
+        (&TransformOperation::Rotate(..), &TransformOperation::Rotate(..)) |
+        (&TransformOperation::Rotate3D(..), &TransformOperation::Rotate3D(..)) |
+        (&TransformOperation::RotateX(..), &TransformOperation::RotateX(..)) |
+        (&TransformOperation::RotateY(..), &TransformOperation::RotateY(..)) |
+        (&TransformOperation::RotateZ(..), &TransformOperation::RotateZ(..)) |
+        (&TransformOperation::Perspective(..), &TransformOperation::Perspective(..)) => true,
+        // Match functions that have the same primitive transform function
+        (a, b) if a.is_translate() && b.is_translate() => true,
+        (a, b) if a.is_scale() && b.is_scale() => true,
+        (a, b) if a.is_rotate() && b.is_rotate() => true,
+        // InterpolateMatrix and AccumulateMatrix are for mismatched transforms
+        _ => false,
+    }
+}
+
+/// <https://drafts.csswg.org/css-transforms/#interpolation-of-transforms>
+impl Animate for ComputedTransform {
+    #[inline]
+    fn animate(&self, other: &Self, procedure: Procedure) -> Result<Self, ()> {
+        use std::borrow::Cow;
+
+        // Addition for transforms simply means appending to the list of
+        // transform functions. This is different to how we handle the other
+        // animation procedures so we treat it separately here rather than
+        // handling it in TransformOperation.
+        if procedure == Procedure::Add {
+            let result = self.0.iter().chain(&*other.0).cloned().collect();
+            return Ok(Transform(result));
+        }
+
+        let this = Cow::Borrowed(&self.0);
+        let other = Cow::Borrowed(&other.0);
+
+        // Interpolate the common prefix
+        let mut result = this
+            .iter()
+            .zip(other.iter())
+            .take_while(|(this, other)| is_matched_operation(this, other))
+            .map(|(this, other)| this.animate(other, procedure))
+            .collect::<Result<Vec<_>, _>>()?;
+
+        // Deal with the remainders
+        let this_remainder = if this.len() > result.len() {
+            Some(&this[result.len()..])
+        } else {
+            None
+        };
+        let other_remainder = if other.len() > result.len() {
+            Some(&other[result.len()..])
+        } else {
+            None
+        };
+
+        match (this_remainder, other_remainder) {
+            // If there is a remainder from *both* lists we must have had mismatched functions.
+            // => Add the remainders to a suitable ___Matrix function.
+            (Some(this_remainder), Some(other_remainder)) => {
+                result.push(TransformOperation::animate_mismatched_transforms(
+                    this_remainder,
+                    other_remainder,
+                    procedure,
+                )?);
+            },
+            // If there is a remainder from just one list, then one list must be shorter but
+            // completely match the type of the corresponding functions in the longer list.
+            // => Interpolate the remainder with identity transforms.
+            (Some(remainder), None) | (None, Some(remainder)) => {
+                let fill_right = this_remainder.is_some();
+                result.append(
+                    &mut remainder
+                        .iter()
+                        .map(|transform| {
+                            let identity = transform.to_animated_zero().unwrap();
+
+                            match transform {
+                                TransformOperation::AccumulateMatrix { .. } |
+                                TransformOperation::InterpolateMatrix { .. } => {
+                                    let (from, to) = if fill_right {
+                                        (transform, &identity)
+                                    } else {
+                                        (&identity, transform)
+                                    };
+
+                                    TransformOperation::animate_mismatched_transforms(
+                                        &[from.clone()],
+                                        &[to.clone()],
+                                        procedure,
+                                    )
+                                },
+                                _ => {
+                                    let (lhs, rhs) = if fill_right {
+                                        (transform, &identity)
+                                    } else {
+                                        (&identity, transform)
+                                    };
+                                    lhs.animate(rhs, procedure)
+                                },
+                            }
+                        })
+                        .collect::<Result<Vec<_>, _>>()?,
+                );
+            },
+            (None, None) => {},
+        }
+
+        Ok(Transform(result.into()))
+    }
+}
+
+impl ComputeSquaredDistance for ComputedTransform {
+    #[inline]
+    fn compute_squared_distance(&self, other: &Self) -> Result<SquaredDistance, ()> {
+        let squared_dist = super::lists::with_zero::squared_distance(&self.0, &other.0);
+
+        // Roll back to matrix interpolation if there is any Err(()) in the
+        // transform lists, such as mismatched transform functions.
+        //
+        // FIXME: Using a zero size here seems a bit sketchy but matches the
+        // previous behavior.
+        if squared_dist.is_err() {
+            let rect = euclid::Rect::zero();
+            let matrix1: Matrix3D = self.to_transform_3d_matrix(Some(&rect))?.0.into();
+            let matrix2: Matrix3D = other.to_transform_3d_matrix(Some(&rect))?.0.into();
+            return matrix1.compute_squared_distance(&matrix2);
+        }
+
+        squared_dist
+    }
+}
+
+/// <http://dev.w3.org/csswg/css-transforms/#interpolation-of-transforms>
+impl Animate for ComputedTransformOperation {
+    fn animate(&self, other: &Self, procedure: Procedure) -> Result<Self, ()> {
+        match (self, other) {
+            (&TransformOperation::Matrix3D(ref this), &TransformOperation::Matrix3D(ref other)) => {
+                Ok(TransformOperation::Matrix3D(
+                    this.animate(other, procedure)?,
+                ))
+            },
+            (&TransformOperation::Matrix(ref this), &TransformOperation::Matrix(ref other)) => {
+                Ok(TransformOperation::Matrix(this.animate(other, procedure)?))
+            },
+            (
+                &TransformOperation::Skew(ref fx, ref fy),
+                &TransformOperation::Skew(ref tx, ref ty),
+            ) => Ok(TransformOperation::Skew(
+                fx.animate(tx, procedure)?,
+                fy.animate(ty, procedure)?,
+            )),
+            (&TransformOperation::SkewX(ref f), &TransformOperation::SkewX(ref t)) => {
+                Ok(TransformOperation::SkewX(f.animate(t, procedure)?))
+            },
+            (&TransformOperation::SkewY(ref f), &TransformOperation::SkewY(ref t)) => {
+                Ok(TransformOperation::SkewY(f.animate(t, procedure)?))
+            },
+            (
+                &TransformOperation::Translate3D(ref fx, ref fy, ref fz),
+                &TransformOperation::Translate3D(ref tx, ref ty, ref tz),
+            ) => Ok(TransformOperation::Translate3D(
+                fx.animate(tx, procedure)?,
+                fy.animate(ty, procedure)?,
+                fz.animate(tz, procedure)?,
+            )),
+            (
+                &TransformOperation::Translate(ref fx, ref fy),
+                &TransformOperation::Translate(ref tx, ref ty),
+            ) => Ok(TransformOperation::Translate(
+                fx.animate(tx, procedure)?,
+                fy.animate(ty, procedure)?,
+            )),
+            (&TransformOperation::TranslateX(ref f), &TransformOperation::TranslateX(ref t)) => {
+                Ok(TransformOperation::TranslateX(f.animate(t, procedure)?))
+            },
+            (&TransformOperation::TranslateY(ref f), &TransformOperation::TranslateY(ref t)) => {
+                Ok(TransformOperation::TranslateY(f.animate(t, procedure)?))
+            },
+            (&TransformOperation::TranslateZ(ref f), &TransformOperation::TranslateZ(ref t)) => {
+                Ok(TransformOperation::TranslateZ(f.animate(t, procedure)?))
+            },
+            (
+                &TransformOperation::Scale3D(ref fx, ref fy, ref fz),
+                &TransformOperation::Scale3D(ref tx, ref ty, ref tz),
+            ) => Ok(TransformOperation::Scale3D(
+                animate_multiplicative_factor(*fx, *tx, procedure)?,
+                animate_multiplicative_factor(*fy, *ty, procedure)?,
+                animate_multiplicative_factor(*fz, *tz, procedure)?,
+            )),
+            (&TransformOperation::ScaleX(ref f), &TransformOperation::ScaleX(ref t)) => Ok(
+                TransformOperation::ScaleX(animate_multiplicative_factor(*f, *t, procedure)?),
+            ),
+            (&TransformOperation::ScaleY(ref f), &TransformOperation::ScaleY(ref t)) => Ok(
+                TransformOperation::ScaleY(animate_multiplicative_factor(*f, *t, procedure)?),
+            ),
+            (&TransformOperation::ScaleZ(ref f), &TransformOperation::ScaleZ(ref t)) => Ok(
+                TransformOperation::ScaleZ(animate_multiplicative_factor(*f, *t, procedure)?),
+            ),
+            (
+                &TransformOperation::Scale(ref fx, ref fy),
+                &TransformOperation::Scale(ref tx, ref ty),
+            ) => Ok(TransformOperation::Scale(
+                animate_multiplicative_factor(*fx, *tx, procedure)?,
+                animate_multiplicative_factor(*fy, *ty, procedure)?,
+            )),
+            (
+                &TransformOperation::Rotate3D(fx, fy, fz, fa),
+                &TransformOperation::Rotate3D(tx, ty, tz, ta),
+            ) => {
+                let animated = Rotate::Rotate3D(fx, fy, fz, fa)
+                    .animate(&Rotate::Rotate3D(tx, ty, tz, ta), procedure)?;
+                let (fx, fy, fz, fa) = ComputedRotate::resolve(&animated);
+                Ok(TransformOperation::Rotate3D(fx, fy, fz, fa))
+            },
+            (&TransformOperation::RotateX(fa), &TransformOperation::RotateX(ta)) => {
+                Ok(TransformOperation::RotateX(fa.animate(&ta, procedure)?))
+            },
+            (&TransformOperation::RotateY(fa), &TransformOperation::RotateY(ta)) => {
+                Ok(TransformOperation::RotateY(fa.animate(&ta, procedure)?))
+            },
+            (&TransformOperation::RotateZ(fa), &TransformOperation::RotateZ(ta)) => {
+                Ok(TransformOperation::RotateZ(fa.animate(&ta, procedure)?))
+            },
+            (&TransformOperation::Rotate(fa), &TransformOperation::Rotate(ta)) => {
+                Ok(TransformOperation::Rotate(fa.animate(&ta, procedure)?))
+            },
+            (&TransformOperation::Rotate(fa), &TransformOperation::RotateZ(ta)) => {
+                Ok(TransformOperation::Rotate(fa.animate(&ta, procedure)?))
+            },
+            (&TransformOperation::RotateZ(fa), &TransformOperation::Rotate(ta)) => {
+                Ok(TransformOperation::Rotate(fa.animate(&ta, procedure)?))
+            },
+            (
+                &TransformOperation::Perspective(ref fd),
+                &TransformOperation::Perspective(ref td),
+            ) => {
+                use crate::values::computed::CSSPixelLength;
+                use crate::values::generics::transform::create_perspective_matrix;
+
+                // From https://drafts.csswg.org/css-transforms-2/#interpolation-of-transform-functions:
+                //
+                //    The transform functions matrix(), matrix3d() and
+                //    perspective() get converted into 4x4 matrices first and
+                //    interpolated as defined in section Interpolation of
+                //    Matrices afterwards.
+                //
+                let from = create_perspective_matrix(fd.infinity_or(|l| l.px()));
+                let to = create_perspective_matrix(td.infinity_or(|l| l.px()));
+
+                let interpolated = Matrix3D::from(from).animate(&Matrix3D::from(to), procedure)?;
+
+                let decomposed = decompose_3d_matrix(interpolated)?;
+                let perspective_z = decomposed.perspective.2;
+                // Clamp results outside of the -1 to 0 range so that we get perspective
+                // function values between 1 and infinity.
+                let used_value = if perspective_z >= 0. {
+                    transform::PerspectiveFunction::None
+                } else {
+                    transform::PerspectiveFunction::Length(CSSPixelLength::new(
+                        if perspective_z <= -1. {
+                            1.
+                        } else {
+                            -1. / perspective_z
+                        },
+                    ))
+                };
+                Ok(TransformOperation::Perspective(used_value))
+            },
+            _ if self.is_translate() && other.is_translate() => self
+                .to_translate_3d()
+                .animate(&other.to_translate_3d(), procedure),
+            _ if self.is_scale() && other.is_scale() => {
+                self.to_scale_3d().animate(&other.to_scale_3d(), procedure)
+            },
+            _ if self.is_rotate() && other.is_rotate() => self
+                .to_rotate_3d()
+                .animate(&other.to_rotate_3d(), procedure),
+            _ => Err(()),
+        }
+    }
+}
+
+impl ComputedTransformOperation {
+    /// If there are no size dependencies, we try to animate in-place, to avoid
+    /// creating deeply nested Interpolate* operations.
+    fn try_animate_mismatched_transforms_in_place(
+        left: &[Self],
+        right: &[Self],
+        procedure: Procedure,
+    ) -> Result<Self, ()> {
+        let (left, _left_3d) = Transform::components_to_transform_3d_matrix(left, None)?;
+        let (right, _right_3d) = Transform::components_to_transform_3d_matrix(right, None)?;
+        Ok(Self::Matrix3D(
+            Matrix3D::from(left).animate(&Matrix3D::from(right), procedure)?,
+        ))
+    }
+
+    fn animate_mismatched_transforms(
+        left: &[Self],
+        right: &[Self],
+        procedure: Procedure,
+    ) -> Result<Self, ()> {
+        if let Ok(op) = Self::try_animate_mismatched_transforms_in_place(left, right, procedure) {
+            return Ok(op);
+        }
+        let from_list = Transform(left.to_vec().into());
+        let to_list = Transform(right.to_vec().into());
+        Ok(match procedure {
+            Procedure::Add => {
+                debug_assert!(false, "Addition should've been handled earlier");
+                return Err(());
+            },
+            Procedure::Interpolate { progress } => Self::InterpolateMatrix {
+                from_list,
+                to_list,
+                progress: Percentage(progress as f32),
+            },
+            Procedure::Accumulate { count } => Self::AccumulateMatrix {
+                from_list,
+                to_list,
+                count: cmp::min(count, i32::max_value() as u64) as i32,
+            },
+        })
+    }
+}
+
+// This might not be the most useful definition of distance. It might be better, for example,
+// to trace the distance travelled by a point as its transform is interpolated between the two
+// lists. That, however, proves to be quite complicated so we take a simple approach for now.
+// See https://bugzilla.mozilla.org/show_bug.cgi?id=1318591#c0.
+impl ComputeSquaredDistance for ComputedTransformOperation {
+    fn compute_squared_distance(&self, other: &Self) -> Result<SquaredDistance, ()> {
+        match (self, other) {
+            (&TransformOperation::Matrix3D(ref this), &TransformOperation::Matrix3D(ref other)) => {
+                this.compute_squared_distance(other)
+            },
+            (&TransformOperation::Matrix(ref this), &TransformOperation::Matrix(ref other)) => {
+                let this: Matrix3D = (*this).into();
+                let other: Matrix3D = (*other).into();
+                this.compute_squared_distance(&other)
+            },
+            (
+                &TransformOperation::Skew(ref fx, ref fy),
+                &TransformOperation::Skew(ref tx, ref ty),
+            ) => Ok(fx.compute_squared_distance(&tx)? + fy.compute_squared_distance(&ty)?),
+            (&TransformOperation::SkewX(ref f), &TransformOperation::SkewX(ref t)) |
+            (&TransformOperation::SkewY(ref f), &TransformOperation::SkewY(ref t)) => {
+                f.compute_squared_distance(&t)
+            },
+            (
+                &TransformOperation::Translate3D(ref fx, ref fy, ref fz),
+                &TransformOperation::Translate3D(ref tx, ref ty, ref tz),
+            ) => {
+                // For translate, We don't want to require doing layout in order
+                // to calculate the result, so drop the percentage part.
+                //
+                // However, dropping percentage makes us impossible to compute
+                // the distance for the percentage-percentage case, but Gecko
+                // uses the same formula, so it's fine for now.
+                let basis = Length::new(0.);
+                let fx = fx.resolve(basis).px();
+                let fy = fy.resolve(basis).px();
+                let tx = tx.resolve(basis).px();
+                let ty = ty.resolve(basis).px();
+
+                Ok(fx.compute_squared_distance(&tx)? +
+                    fy.compute_squared_distance(&ty)? +
+                    fz.compute_squared_distance(&tz)?)
+            },
+            (
+                &TransformOperation::Scale3D(ref fx, ref fy, ref fz),
+                &TransformOperation::Scale3D(ref tx, ref ty, ref tz),
+            ) => Ok(fx.compute_squared_distance(&tx)? +
+                fy.compute_squared_distance(&ty)? +
+                fz.compute_squared_distance(&tz)?),
+            (
+                &TransformOperation::Rotate3D(fx, fy, fz, fa),
+                &TransformOperation::Rotate3D(tx, ty, tz, ta),
+            ) => Rotate::Rotate3D(fx, fy, fz, fa)
+                .compute_squared_distance(&Rotate::Rotate3D(tx, ty, tz, ta)),
+            (&TransformOperation::RotateX(fa), &TransformOperation::RotateX(ta)) |
+            (&TransformOperation::RotateY(fa), &TransformOperation::RotateY(ta)) |
+            (&TransformOperation::RotateZ(fa), &TransformOperation::RotateZ(ta)) |
+            (&TransformOperation::Rotate(fa), &TransformOperation::Rotate(ta)) => {
+                fa.compute_squared_distance(&ta)
+            },
+            (
+                &TransformOperation::Perspective(ref fd),
+                &TransformOperation::Perspective(ref td),
+            ) => fd
+                .infinity_or(|l| l.px())
+                .compute_squared_distance(&td.infinity_or(|l| l.px())),
+            (&TransformOperation::Perspective(ref p), &TransformOperation::Matrix3D(ref m)) |
+            (&TransformOperation::Matrix3D(ref m), &TransformOperation::Perspective(ref p)) => {
+                // FIXME(emilio): Is this right? Why interpolating this with
+                // Perspective but not with anything else?
+                let mut p_matrix = Matrix3D::identity();
+                let p = p.infinity_or(|p| p.px());
+                if p >= 0. {
+                    p_matrix.m34 = -1. / p.max(1.);
+                }
+                p_matrix.compute_squared_distance(&m)
+            },
+            // Gecko cross-interpolates amongst all translate and all scale
+            // functions (See ToPrimitive in layout/style/StyleAnimationValue.cpp)
+            // without falling back to InterpolateMatrix
+            _ if self.is_translate() && other.is_translate() => self
+                .to_translate_3d()
+                .compute_squared_distance(&other.to_translate_3d()),
+            _ if self.is_scale() && other.is_scale() => self
+                .to_scale_3d()
+                .compute_squared_distance(&other.to_scale_3d()),
+            _ if self.is_rotate() && other.is_rotate() => self
+                .to_rotate_3d()
+                .compute_squared_distance(&other.to_rotate_3d()),
+            _ => Err(()),
+        }
+    }
+}
+
+// ------------------------------------
+// Individual transforms.
+// ------------------------------------
+/// <https://drafts.csswg.org/css-transforms-2/#propdef-rotate>
+impl ComputedRotate {
+    fn resolve(&self) -> (Number, Number, Number, Angle) {
+        // According to the spec:
+        // https://drafts.csswg.org/css-transforms-2/#individual-transforms
+        //
+        // If the axis is unspecified, it defaults to "0 0 1"
+        match *self {
+            Rotate::None => (0., 0., 1., Angle::zero()),
+            Rotate::Rotate3D(rx, ry, rz, angle) => (rx, ry, rz, angle),
+            Rotate::Rotate(angle) => (0., 0., 1., angle),
+        }
+    }
+}
+
+impl Animate for ComputedRotate {
+    #[inline]
+    fn animate(&self, other: &Self, procedure: Procedure) -> Result<Self, ()> {
+        match (self, other) {
+            (&Rotate::None, &Rotate::None) => Ok(Rotate::None),
+            (&Rotate::Rotate3D(fx, fy, fz, fa), &Rotate::None) => {
+                // We always normalize direction vector for rotate3d() first, so we should also
+                // apply the same rule for rotate property. In other words, we promote none into
+                // a 3d rotate, and normalize both direction vector first, and then do
+                // interpolation.
+                let (fx, fy, fz, fa) = transform::get_normalized_vector_and_angle(fx, fy, fz, fa);
+                Ok(Rotate::Rotate3D(
+                    fx,
+                    fy,
+                    fz,
+                    fa.animate(&Angle::zero(), procedure)?,
+                ))
+            },
+            (&Rotate::None, &Rotate::Rotate3D(tx, ty, tz, ta)) => {
+                // Normalize direction vector first.
+                let (tx, ty, tz, ta) = transform::get_normalized_vector_and_angle(tx, ty, tz, ta);
+                Ok(Rotate::Rotate3D(
+                    tx,
+                    ty,
+                    tz,
+                    Angle::zero().animate(&ta, procedure)?,
+                ))
+            },
+            (&Rotate::Rotate3D(_, ..), _) | (_, &Rotate::Rotate3D(_, ..)) => {
+                let (from, to) = (self.resolve(), other.resolve());
+                let (mut fx, mut fy, mut fz, fa) =
+                    transform::get_normalized_vector_and_angle(from.0, from.1, from.2, from.3);
+                let (mut tx, mut ty, mut tz, ta) =
+                    transform::get_normalized_vector_and_angle(to.0, to.1, to.2, to.3);
+
+                if fa == Angle::from_degrees(0.) {
+                    fx = tx;
+                    fy = ty;
+                    fz = tz;
+                } else if ta == Angle::from_degrees(0.) {
+                    tx = fx;
+                    ty = fy;
+                    tz = fz;
+                }
+
+                if (fx, fy, fz) == (tx, ty, tz) {
+                    return Ok(Rotate::Rotate3D(fx, fy, fz, fa.animate(&ta, procedure)?));
+                }
+
+                let fv = DirectionVector::new(fx, fy, fz);
+                let tv = DirectionVector::new(tx, ty, tz);
+                let fq = Quaternion::from_direction_and_angle(&fv, fa.radians64());
+                let tq = Quaternion::from_direction_and_angle(&tv, ta.radians64());
+
+                let rq = Quaternion::animate(&fq, &tq, procedure)?;
+                let (x, y, z, angle) = transform::get_normalized_vector_and_angle(
+                    rq.0 as f32,
+                    rq.1 as f32,
+                    rq.2 as f32,
+                    rq.3.acos() as f32 * 2.0,
+                );
+
+                Ok(Rotate::Rotate3D(x, y, z, Angle::from_radians(angle)))
+            },
+            (&Rotate::Rotate(_), _) | (_, &Rotate::Rotate(_)) => {
+                // If this is a 2D rotation, we just animate the <angle>
+                let (from, to) = (self.resolve().3, other.resolve().3);
+                Ok(Rotate::Rotate(from.animate(&to, procedure)?))
+            },
+        }
+    }
+}
+
+impl ComputeSquaredDistance for ComputedRotate {
+    #[inline]
+    fn compute_squared_distance(&self, other: &Self) -> Result<SquaredDistance, ()> {
+        match (self, other) {
+            (&Rotate::None, &Rotate::None) => Ok(SquaredDistance::from_sqrt(0.)),
+            (&Rotate::Rotate3D(_, _, _, a), &Rotate::None) |
+            (&Rotate::None, &Rotate::Rotate3D(_, _, _, a)) => {
+                a.compute_squared_distance(&Angle::zero())
+            },
+            (&Rotate::Rotate3D(_, ..), _) | (_, &Rotate::Rotate3D(_, ..)) => {
+                let (from, to) = (self.resolve(), other.resolve());
+                let (mut fx, mut fy, mut fz, angle1) =
+                    transform::get_normalized_vector_and_angle(from.0, from.1, from.2, from.3);
+                let (mut tx, mut ty, mut tz, angle2) =
+                    transform::get_normalized_vector_and_angle(to.0, to.1, to.2, to.3);
+
+                if angle1 == Angle::zero() {
+                    fx = tx;
+                    fy = ty;
+                    fz = tz;
+                } else if angle2 == Angle::zero() {
+                    tx = fx;
+                    ty = fy;
+                    tz = fz;
+                }
+
+                if (fx, fy, fz) == (tx, ty, tz) {
+                    angle1.compute_squared_distance(&angle2)
+                } else {
+                    let v1 = DirectionVector::new(fx, fy, fz);
+                    let v2 = DirectionVector::new(tx, ty, tz);
+                    let q1 = Quaternion::from_direction_and_angle(&v1, angle1.radians64());
+                    let q2 = Quaternion::from_direction_and_angle(&v2, angle2.radians64());
+                    q1.compute_squared_distance(&q2)
+                }
+            },
+            (&Rotate::Rotate(_), _) | (_, &Rotate::Rotate(_)) => self
+                .resolve()
+                .3
+                .compute_squared_distance(&other.resolve().3),
+        }
+    }
+}
+
+/// <https://drafts.csswg.org/css-transforms-2/#propdef-translate>
+impl ComputedTranslate {
+    fn resolve(&self) -> (LengthPercentage, LengthPercentage, Length) {
+        // According to the spec:
+        // https://drafts.csswg.org/css-transforms-2/#individual-transforms
+        //
+        // Unspecified translations default to 0px
+        match *self {
+            Translate::None => (
+                LengthPercentage::zero(),
+                LengthPercentage::zero(),
+                Length::zero(),
+            ),
+            Translate::Translate(ref tx, ref ty, ref tz) => (tx.clone(), ty.clone(), tz.clone()),
+        }
+    }
+}
+
+impl Animate for ComputedTranslate {
+    #[inline]
+    fn animate(&self, other: &Self, procedure: Procedure) -> Result<Self, ()> {
+        match (self, other) {
+            (&Translate::None, &Translate::None) => Ok(Translate::None),
+            (&Translate::Translate(_, ..), _) | (_, &Translate::Translate(_, ..)) => {
+                let (from, to) = (self.resolve(), other.resolve());
+                Ok(Translate::Translate(
+                    from.0.animate(&to.0, procedure)?,
+                    from.1.animate(&to.1, procedure)?,
+                    from.2.animate(&to.2, procedure)?,
+                ))
+            },
+        }
+    }
+}
+
+impl ComputeSquaredDistance for ComputedTranslate {
+    #[inline]
+    fn compute_squared_distance(&self, other: &Self) -> Result<SquaredDistance, ()> {
+        let (from, to) = (self.resolve(), other.resolve());
+        Ok(from.0.compute_squared_distance(&to.0)? +
+            from.1.compute_squared_distance(&to.1)? +
+            from.2.compute_squared_distance(&to.2)?)
+    }
+}
+
+/// <https://drafts.csswg.org/css-transforms-2/#propdef-scale>
+impl ComputedScale {
+    fn resolve(&self) -> (Number, Number, Number) {
+        // According to the spec:
+        // https://drafts.csswg.org/css-transforms-2/#individual-transforms
+        //
+        // Unspecified scales default to 1
+        match *self {
+            Scale::None => (1.0, 1.0, 1.0),
+            Scale::Scale(sx, sy, sz) => (sx, sy, sz),
+        }
+    }
+}
+
+impl Animate for ComputedScale {
+    #[inline]
+    fn animate(&self, other: &Self, procedure: Procedure) -> Result<Self, ()> {
+        match (self, other) {
+            (&Scale::None, &Scale::None) => Ok(Scale::None),
+            (&Scale::Scale(_, ..), _) | (_, &Scale::Scale(_, ..)) => {
+                let (from, to) = (self.resolve(), other.resolve());
+                // For transform lists, we add by appending to the list of
+                // transform functions. However, ComputedScale cannot be
+                // simply concatenated, so we have to calculate the additive
+                // result here.
+                if procedure == Procedure::Add {
+                    // scale(x1,y1,z1)*scale(x2,y2,z2) = scale(x1*x2, y1*y2, z1*z2)
+                    return Ok(Scale::Scale(from.0 * to.0, from.1 * to.1, from.2 * to.2));
+                }
+                Ok(Scale::Scale(
+                    animate_multiplicative_factor(from.0, to.0, procedure)?,
+                    animate_multiplicative_factor(from.1, to.1, procedure)?,
+                    animate_multiplicative_factor(from.2, to.2, procedure)?,
+                ))
+            },
+        }
+    }
+}
+
+impl ComputeSquaredDistance for ComputedScale {
+    #[inline]
+    fn compute_squared_distance(&self, other: &Self) -> Result<SquaredDistance, ()> {
+        let (from, to) = (self.resolve(), other.resolve());
+        Ok(from.0.compute_squared_distance(&to.0)? +
+            from.1.compute_squared_distance(&to.1)? +
+            from.2.compute_squared_distance(&to.2)?)
+    }
+}