cc: Compute draw transforms dynamically.

cc/trees/draw_property_utils.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 "cc/trees/draw_property_utils.h"
 
 #include <stddef.h>
 
 #include <vector>
 
@@ skipping 879 matching lines @@
         if (clip_node->target_effect_id != EffectTree::kContentsRootNodeId) {
           const EffectNode* target_effect_node =
               effect_tree.Node(clip_node->target_effect_id);
           PostConcatSurfaceContentsScale(target_effect_node, &source_to_target);
 #if DCHECK_IS_ON()
           VerifySurfaceContentsScalesMatch(clip_node->target_effect_id,
                                            clip_node->target_transform_id,
                                            effect_tree, transform_tree);
 #endif
         }
-        // source_to_target computation should be successful as target is an
-        // ancestor of the transform node.
-        DCHECK(success);

[ajuma, 2016/09/07 14:51:23]

In which cases is this DCHECK now failing?

[sunxd, 2016/09/07 15:20:02]

It only fails when the target's surface content scale is 0, or partly 0. The
test that triggers this is
PaintArtifactCompositorTestWithPropertyTrees.NestedClips.

       }
 
       gfx::RectF source_clip_in_target_space =
           MathUtil::MapClippedRect(source_to_target, clip_node->clip);
 
       // With surfaces disabled, the only case where we use only the local clip
       // for layer clipping is the case where no non-viewport ancestor node
       // applies a local clip.
       bool layer_clipping_uses_only_local_clip =
           non_root_surfaces_enabled
@@ skipping 678 matching lines @@
 void UpdateElasticOverscroll(PropertyTrees* property_trees,
                              const Layer* overscroll_elasticity_layer,
                              const gfx::Vector2dF& elastic_overscroll) {
   UpdateElasticOverscrollInternal(property_trees, overscroll_elasticity_layer,
                                   elastic_overscroll);
 }
 
 }  // namespace draw_property_utils
 
 }  // namespace cc