Bug 1488414 - Use decomposition to interpolate matched perspective transform operations. r=birtles

Looks like this produces sensible results for interpolation with 0, though I'm
not really convinced about the results from, let's say, 1px to 2000px in the
attached test-case, I would've expected a linear interpolation from that to go
through normal length interpolation.

css-transforms-1 says:

  > Two transform functions with the same name and the same number of arguments
  > are interpolated numerically without a former conversion. The calculated
  > value will be of the same transform function type with the same number of
  > arguments.
  >
  > Special rules apply to <matrix()>.

Which is what we do... I was going to file a spec issue but turns out that it's
already addressed in css-transforms-2:

  https://drafts.csswg.org/css-transforms-2/#interpolation-of-transform-functions

Which says:

  > The transform functions <matrix()>, matrix3d() and perspective() get
  > converted into 4x4 matrices first and interpolated as defined in section
  > Interpolation of Matrices afterwards.

Differential Revision: https://phabricator.services.mozilla.com/D4942

--HG--
extra : moz-landing-system : lando

servo/components/style/properties/helpers/animated_properties.mako.rs

@@ -1296,9 +1296,27 @@ impl Animate for ComputedTransformOperation {
                 &TransformOperation::Perspective(ref fd),
                 &TransformOperation::Perspective(ref td),
             ) => {
-                Ok(TransformOperation::Perspective(
-                    fd.animate(td, procedure)?
-                ))
+                use values::computed::CSSPixelLength;
+                use 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.px());
+                let to = create_perspective_matrix(td.px());
+
+                let interpolated =
+                    Matrix3D::from(from).animate(&Matrix3D::from(to), procedure)?;
+
+                let decomposed = decompose_3d_matrix(interpolated)?;
+                let perspective_z = decomposed.perspective.2;
+                let used_value =
+                    if perspective_z == 0. { 0. } else { -1. / perspective_z };
+                Ok(TransformOperation::Perspective(CSSPixelLength::new(used_value)))
             },
             _ if self.is_translate() && other.is_translate() => {
                 self.to_translate_3d().animate(&other.to_translate_3d(), procedure)

servo/components/style/values/computed/transform.rs

@@ -42,6 +42,7 @@ impl TransformOrigin {
 
 /// computed value of matrix3d()
 pub type Matrix3D = generic::Matrix3D<Number>;
+
 /// computed value of matrix()
 pub type Matrix = generic::Matrix<Number>;