Skip clearing the canvas before painting the contents of opaque layers

The SkCanvas doesn't need to be cleared before painting the layer contents
if the layer is known to be opaque (e.g. the root layer). Bypassing the clear
gives a sizeable perf boost.

This patch also switches to calling clear() instead of drawRect() for clearing the canvas pixels as clear has been shown to be significantly faster. 

BUG=279612

Review URL: https://chromiumcodereview.appspot.com/23484005

git-svn-id: svn://svn.chromium.org/chrome/trunk/src@219894 0039d316-1c4b-4281-b951-d872f2087c98

1 files changed, 19 insertions, 8 deletions

diff --git a/cc/resources/content_layer_updater.cc b/cc/resources/content_layer_updater.cc
index c9c2ecc..50ea86b 100644
--- a/cc/resources/content_layer_updater.cc
+++ b/cc/resources/content_layer_updater.cc
@@ -9,6 +9,7 @@
 #include "cc/debug/rendering_stats_instrumentation.h"
 #include "cc/resources/layer_painter.h"
 #include "third_party/skia/include/core/SkCanvas.h"
+#include "third_party/skia/include/core/SkDevice.h"
 #include "third_party/skia/include/core/SkPaint.h"
 #include "third_party/skia/include/core/SkRect.h"
 #include "third_party/skia/include/core/SkScalar.h"
@@ -23,7 +24,8 @@ ContentLayerUpdater::ContentLayerUpdater(
     int layer_id)
     : rendering_stats_instrumentation_(stats_instrumentation),
       layer_id_(layer_id),
-      painter_(painter.Pass()) {}
+      painter_(painter.Pass()),
+      layer_is_opaque_(false) {}
 
 ContentLayerUpdater::~ContentLayerUpdater() {}
 
@@ -33,14 +35,17 @@ void ContentLayerUpdater::set_rendering_stats_instrumentation(
 }
 
 void ContentLayerUpdater::PaintContents(SkCanvas* canvas,
-                                        gfx::Rect content_rect,
+                                        gfx::Point origin,
                                         float contents_width_scale,
                                         float contents_height_scale,
                                         gfx::Rect* resulting_opaque_rect) {
   TRACE_EVENT0("cc", "ContentLayerUpdater::PaintContents");
   canvas->save();
-  canvas->translate(SkFloatToScalar(-content_rect.x()),
-                    SkFloatToScalar(-content_rect.y()));
+  canvas->translate(SkFloatToScalar(-origin.x()),
+                    SkFloatToScalar(-origin.y()));
+
+  SkDevice* device = canvas->getDevice();
+  gfx::Rect content_rect(origin, gfx::Size(device->width(), device->height()));
 
   gfx::Rect layer_rect = content_rect;
 
@@ -52,12 +57,14 @@ void ContentLayerUpdater::PaintContents(SkCanvas* canvas,
         content_rect, 1.f / contents_width_scale, 1.f / contents_height_scale);
   }
 
-  SkPaint paint;
-  paint.setAntiAlias(false);
-  paint.setXfermodeMode(SkXfermode::kClear_Mode);
   SkRect layer_sk_rect = SkRect::MakeXYWH(
       layer_rect.x(), layer_rect.y(), layer_rect.width(), layer_rect.height());
-  canvas->drawRect(layer_sk_rect, paint);
+
+  // 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);
+
   canvas->clipRect(layer_sk_rect);
 
   gfx::RectF opaque_layer_rect;
@@ -71,4 +78,8 @@ void ContentLayerUpdater::PaintContents(SkCanvas* canvas,
   content_rect_ = content_rect;
 }
 
+void ContentLayerUpdater::SetOpaque(bool opaque) {
+  layer_is_opaque_ = opaque;
+}
+
 }  // namespace cc