cc : Use screen space transform for computing translation

cc/trees/property_tree.cc

 // Copyright 2014 The Chromium Authors. All rights reserved.
 // Use of this source code is governed by a BSD-style license that can be
 // found in the LICENSE file.
 
 #include <stddef.h>
 
 #include <set>
 #include <vector>
 
 #include "base/logging.h"
@@ skipping 143 matching lines @@
     CombineTransformsBetween(source_id, dest_id, transform);
     return true;
   }
 
   return CombineInversesBetween(source_id, dest_id, transform);
 }
 
 bool TransformTree::ComputeTranslation(int source_id,
                                        int dest_id,
                                        gfx::Transform* transform) const {
-  bool success = ComputeTransform(source_id, dest_id, transform);
+  // Since the transform between source and destination is at most translation,
+  // flattenning will not affect the result and we can use the screen space
+  // transforms.
+  transform->MakeIdentity();
+  if (source_id == dest_id)
+    return true;
+
+  const TransformNode* dest = Node(dest_id);
+  if (dest->ancestors_are_invertible) {
+    if (source_id != kInvalidNodeId)
+      transform->ConcatTransform(ToScreen(source_id));
+    if (dest_id != kInvalidNodeId)
+      transform->ConcatTransform(FromScreen(dest_id));
+  } else {
+    return false;
+  }
   DCHECK(
       transform->IsApproximatelyIdentityOrTranslation(SkDoubleToMScalar(1e-4)));
-  return success;
+  return true;
 }
 
 bool TransformTree::NeedsSourceToParentUpdate(TransformNode* node) {
   return (source_to_parent_updates_allowed() &&
           node->parent_id != node->source_node_id);
 }
 
 void TransformTree::ResetChangeTracking() {
   for (int id = 1; id < static_cast<int>(size()); ++id) {
     TransformNode* node = Node(id);
@@ skipping 1826 matching lines @@
     DCHECK(effect_node->has_render_surface ||
            effect_node->id == EffectTree::kRootNodeId);
     target_transform_id = effect_node->transform_id;
   }
 
   return transform_tree.ComputeTransform(target_transform_id, transform_id,
                                          transform);
 }
 
 }  // namespace cc