Limit pixels that get cleared to repaint area.

cc/resources/content_layer_updater.cc

 // Copyright 2011 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/resources/content_layer_updater.h"
 
 #include "base/debug/trace_event.h"
 #include "base/time/time.h"
 #include "cc/debug/rendering_stats_instrumentation.h"
 #include "cc/resources/layer_painter.h"
@@ skipping 44 matching lines @@
     canvas->scale(SkFloatToScalar(contents_width_scale),
                   SkFloatToScalar(contents_height_scale));
 
     layer_rect = gfx::ScaleToEnclosingRect(
         content_rect, 1.f / contents_width_scale, 1.f / contents_height_scale);
   }
 
   SkRect layer_sk_rect = SkRect::MakeXYWH(
       layer_rect.x(), layer_rect.y(), layer_rect.width(), layer_rect.height());
 
+  canvas->clipRect(layer_sk_rect);
+
   // If the layer has opaque contents then there is no need to
   // clear the canvas before painting.
-  if (!layer_is_opaque_)
-    canvas->clear(SK_ColorTRANSPARENT);
+  if (!layer_is_opaque_) {
+    TRACE_EVENT0("cc", "Clear");
 
-  canvas->clipRect(layer_sk_rect);
+    // If the canvas is substantially larger than the area that will be painted
+    // then it's faster to clear only the pixels within the clipRect.
+    const float kClearFasterThanDrawRatio = 0.8f;
+    int canvasArea =

[enne (OOO), 2013/11/06 01:59:45]

canvasArea => canvas_area

+        canvas->getDevice()->width() * canvas->getDevice()->height();
+    int clipArea = layer_rect.width() * layer_rect.height();

[enne (OOO), 2013/11/06 01:59:45]

clipArea => clip_area

+    if (kClearFasterThanDrawRatio * canvasArea < clipArea)
+      canvas->clear(SK_ColorTRANSPARENT);
+    else
+      canvas->drawColor(SK_ColorTRANSPARENT, SkXfermode::kSrc_Mode);
+  }
 
   gfx::RectF opaque_layer_rect;
   painter_->Paint(canvas, layer_rect, &opaque_layer_rect);
   canvas->restore();
 
   gfx::Rect opaque_content_rect = gfx::ToEnclosedRect(gfx::ScaleRect(
       opaque_layer_rect, contents_width_scale, contents_height_scale));
   *resulting_opaque_rect = opaque_content_rect;
 
   content_rect_ = content_rect;
 }
 
 void ContentLayerUpdater::SetOpaque(bool opaque) {
   layer_is_opaque_ = opaque;
 }
 
 }  // namespace cc