Fix for bug where composited rotate3d should SLERP

cc/animation/transform_operation.cc

Index: cc/animation/transform_operation.cc
diff --git a/cc/animation/transform_operation.cc b/cc/animation/transform_operation.cc
index 7421924aedcea56ef9658d47bc2e4c77225404d2..e3b28d965a46733496e25d46bf1af5735d53b3f9 100644
--- a/cc/animation/transform_operation.cc
+++ b/cc/animation/transform_operation.cc
@@ -32,6 +32,12 @@ static bool IsOperationIdentity(const TransformOperation* operation) {
   return !operation || operation->IsIdentity();
 }
 
+static bool Is3DRotation(const TransformOperation* operation) {
+  if (IsOperationIdentity(operation))
+    return false;
+  return operation->rotate.axis.x != 0 || operation->rotate.axis.y != 0;
+}
+
 static bool ShareSameAxis(const TransformOperation* from,
                           const TransformOperation* to,
                           SkMScalar* axis_x,
@@ -124,7 +130,8 @@ bool TransformOperation::BlendTransformOperations(
     SkMScalar axis_z = 1;
     SkMScalar from_angle = 0;
     SkMScalar to_angle = IsOperationIdentity(to) ? 0 : to->rotate.angle;
-    if (ShareSameAxis(from, to, &axis_x, &axis_y, &axis_z, &from_angle)) {
+    if (ShareSameAxis(from, to, &axis_x, &axis_y, &axis_z, &from_angle) &&
+        !Is3DRotation(from) && !Is3DRotation(to)) {

[ajuma, 2015/06/23 13:24:58]

I'm not sure I follow how this corresponds to the spec. This says that 3d
rotations should never be interpolated numerically, but the spec seems to say
that they should be interpolated numerically when they have the same normalized
direction vector. So maybe it's ShareSameAxis that needs fixing? Please add
tests (to transform_operations_unittest.cc) that show that we're interpolating
according to the spec in both cases (numerical interpolation for 3d rotations
with the same direction vector, and matrix interpolation when the direction
vectors are different).

[soonm, 2015/06/23 23:43:42]

Thanks ajuma, I think you're right. I misunderstood the spec, thinking that
rotate3d should always be interpolated with SLERP. With reference to the bug,
the error is on the main thread RotateTransformOperation.cpp because that does
not check the normalized axis correctly.

Closing this patch.

       result->RotateAbout(gfx::Vector3dF(axis_x, axis_y, axis_z),
                           BlendSkMScalars(from_angle, to_angle, progress));
     } else {