cc: Improve raster performance of PicturePileImpl

cc/picture_pile_impl.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 "base/debug/trace_event.h"
 #include "cc/picture_pile_impl.h"
 #include "cc/rendering_stats.h"
 #include "third_party/skia/include/core/SkCanvas.h"
 #include "third_party/skia/include/core/SkSize.h"
 #include "ui/gfx/rect_conversions.h"
@@ skipping 41 matching lines @@
     float contents_scale,
     RenderingStats* stats) {
 
   if (!pile_.size())
     return;
 
   DCHECK(contents_scale >= min_contents_scale_);
 
   base::TimeTicks rasterizeBeginTime = base::TimeTicks::Now();
 
-  // TODO(enne): do this more efficiently, i.e. top down with Skia clips
   canvas->save();
   canvas->translate(-content_rect.x(), -content_rect.y());
   canvas->clipRect(gfx::RectToSkRect(content_rect));
 
-  for (PicturePile::Pile::const_iterator i = pile_.begin();
-       i != pile_.end(); ++i) {
+  // Raster through the pile top down, using clips to make sure that
+  // pictures on top are not overdrawn by pictures on the bottom.
+  Region unclipped(content_rect);
+  for (PicturePile::Pile::reverse_iterator i = pile_.rbegin();
+       i != pile_.rend(); ++i) {
     // This is intentionally *enclosed* rect, so that the clip is aligned on
     // integral post-scale content pixels and does not extend past the edges of
     // the picture's layer rect.  The min_contents_scale enforces that enough
     // buffer pixels have been added such that the enclosed rect encompasses all
     // invalidated pixels at any larger scale level.
     gfx::Rect content_clip = gfx::ToEnclosedRect(
         gfx::ScaleRect((*i)->LayerRect(), contents_scale));
-    if (!content_rect.Intersects(content_clip))
+    if (!unclipped.Intersects(content_clip))
       continue;
-
     (*i)->Raster(canvas, content_clip, contents_scale);
 
-    SkISize deviceSize = canvas->getDeviceSize();
-    stats->totalPixelsRasterized += deviceSize.width() * deviceSize.height();
+    // Don't allow pictures underneath to draw where this picture did.
+    canvas->clipRect(gfx::RectToSkRect(content_clip), SkRegion::kDifference_Op);
+    unclipped.Subtract(content_clip);
+
+    stats->totalPixelsRasterized +=
+        content_clip.width() * content_clip.height();
   }
   canvas->restore();
 
   stats->totalRasterizeTimeInSeconds += (base::TimeTicks::Now() -
                                          rasterizeBeginTime).InSecondsF();
 }
 
 void PicturePileImpl::GatherPixelRefs(
     const gfx::Rect& rect, std::list<skia::LazyPixelRef*>& pixel_refs) {
   std::list<skia::LazyPixelRef*> result;
   for (PicturePile::Pile::const_iterator i = pile_.begin();
       i != pile_.end(); ++i) {
     (*i)->GatherPixelRefs(rect, result);
     pixel_refs.splice(pixel_refs.end(), result);
   }
 }
 
 }  // namespace cc