[2/5] Disable transformed rasterization if layer is opaque

cc/layers/picture_layer.cc

 // Copyright 2012 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/layers/picture_layer.h"
 
 #include "base/auto_reset.h"
 #include "base/trace_event/trace_event.h"
 #include "cc/layers/content_layer_client.h"
 #include "cc/layers/picture_layer_impl.h"
@@ skipping 85 matching lines @@
     recording_source_->SetNeedsDisplayRect(layer_rect);
   Layer::SetNeedsDisplayRect(layer_rect);
 }
 
 bool PictureLayer::Update() {
   update_source_frame_number_ = layer_tree_host()->SourceFrameNumber();
   bool updated = Layer::Update();
 
   gfx::Size layer_size = paint_properties().bounds;
 
-  recording_source_->SetBackgroundColor(SafeOpaqueBackgroundColor());
+  // When transformed rasterization is used, we try to match the rendering

[enne (OOO), 2017/03/29 13:01:42]

I'm not sure I agree with this.  I think if a layer is marked as opaque then
that should take priority over transformed rasterization.  In other words,
ShouldUseTransformedRasterization should take into account contents_opaque, and
then this change here wouldn't be needed.

[trchen, 2017/03/29 21:50:36]

I thought layer opaqueness is more of a hint than a directive. i.e. The layer
client is guaranteed to return a paint record that is opaque within the layer
bound. Thus cc could potentially apply optimization like dropping the alpha
channel, using kSrc mode, not clearing raster buffer, that kind of stuff.

Actually I feel background overfill does more harm than good with translated
layers. Maybe I don't fully understand the rationale of it. Do you remember why
was it introduced in the first place?

+  // without a compositing layer, thus we don't over draw background to fill
+  // snapped edges.
+  SkColor background_color = ShouldUseTransformedRasterization()
+                                 ? SK_ColorTRANSPARENT
+                                 : SafeOpaqueBackgroundColor();
+  recording_source_->SetBackgroundColor(background_color);
   recording_source_->SetRequiresClear(
       !contents_opaque() &&
       !picture_layer_inputs_.client->FillsBoundsCompletely());
 
   TRACE_EVENT1("cc", "PictureLayer::Update", "source_frame_number",
                layer_tree_host()->SourceFrameNumber());
   devtools_instrumentation::ScopedLayerTreeTask update_layer(
       devtools_instrumentation::kUpdateLayer, id(), layer_tree_host()->GetId());
 
   // UpdateAndExpandInvalidation will give us an invalidation that covers
@@ skipping 151 matching lines @@
     return false;
 
   return true;
 }
 
 const DisplayItemList* PictureLayer::GetDisplayItemList() {
   return picture_layer_inputs_.display_list.get();
 }
 
 }  // namespace cc