cc: Partial tile update for one-copy raster.

cc/resources/raster_source_helper.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/resources/raster_source_helper.h"
 
 #include "base/trace_event/trace_event.h"
 #include "cc/debug/debug_colors.h"
 #include "third_party/skia/include/core/SkCanvas.h"
 #include "ui/gfx/geometry/rect_conversions.h"
 #include "ui/gfx/skia_util.h"
 
 namespace cc {
 
 void RasterSourceHelper::PrepareForPlaybackToCanvas(
     SkCanvas* canvas,
-    const gfx::Rect& canvas_rect,
+    const gfx::Rect& canvas_bitmap_rect,
+    const gfx::Rect& canvas_playback_rect,
     const gfx::Rect& source_rect,
     float contents_scale,
     SkColor background_color,
     bool clear_canvas_with_debug_color,
     bool requires_clear) {
-  canvas->discard();
+  bool partial_update = canvas_bitmap_rect != canvas_playback_rect;
+  gfx::Rect canvas_rect =
+      canvas_playback_rect - canvas_bitmap_rect.OffsetFromOrigin();
+
+  if (!partial_update)
+    canvas->discard();
   if (clear_canvas_with_debug_color) {
     // Any non-painted areas in the content bounds will be left in this color.
-    canvas->clear(DebugColors::NonPaintedFillColor());
+    if (!partial_update) {
+      canvas->clear(DebugColors::NonPaintedFillColor());
+    } else {
+      canvas->save();
+      canvas->clipRect(gfx::RectToSkRect(canvas_rect));
+      canvas->drawColor(DebugColors::NonPaintedFillColor());
+      canvas->restore();
+    }
   }
 
   // If this raster source has opaque contents, it is guaranteeing that it will
   // draw an opaque rect the size of the layer.  If it is not, then we must
   // clear this canvas ourselves.
   if (requires_clear) {
     TRACE_EVENT_INSTANT0("cc", "SkCanvas::clear", TRACE_EVENT_SCOPE_THREAD);
     // Clearing is about ~4x faster than drawing a rect even if the content
     // isn't covering a majority of the canvas.
-    canvas->clear(SK_ColorTRANSPARENT);
+    if (!partial_update) {
+      canvas->clear(SK_ColorTRANSPARENT);
+    } else {
+      canvas->save();
+      canvas->clipRect(gfx::RectToSkRect(canvas_rect));
+      canvas->drawColor(SK_ColorTRANSPARENT);
+      canvas->restore();
+    }
   } else {
     // Even if completely covered, for rasterizations that touch the edge of the
     // layer, we also need to raster the background color underneath the last
     // texel (since the recording won't cover it) and outside the last texel
     // (due to linear filtering when using this texture).
     gfx::Rect content_rect =
         gfx::ToEnclosingRect(gfx::ScaleRect(source_rect, contents_scale));
 
     // The final texel of content may only be partially covered by a
     // rasterization; this rect represents the content rect that is fully
@@ skipping 25 matching lines @@
                        SkRegion::kReplace_Op);
       canvas->clipRect(gfx::RectToSkRect(deflated_content_rect),
                        SkRegion::kDifference_Op);
       canvas->drawColor(background_color, SkXfermode::kSrc_Mode);
       canvas->restore();
     }
   }
 }
 
 }  // namespace cc