cc: Partial tile update for one-copy raster.

cc/resources/raster_source_helper.cc

Index: cc/resources/raster_source_helper.cc
diff --git a/cc/resources/raster_source_helper.cc b/cc/resources/raster_source_helper.cc
index 34436e4a3f04c4d33cdc0afc28885581c4472a3b..0efc394f7d230a4abae154be0fcaba9bac49789f 100644
--- a/cc/resources/raster_source_helper.cc
+++ b/cc/resources/raster_source_helper.cc
@@ -14,16 +14,28 @@ 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;
+
+  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_playback_rect - canvas_bitmap_rect.OffsetFromOrigin()));

[vmpstr, 2015/05/15 20:54:22]

Can you leave a comment at the top of this function that explains the spaces
|canvas|, |canvas_playback_rect|  |canvas_bitmap_rect| are in?

What I mean to say it's not really clear to me, why this clip subtracts
bitmap_rect origin from playback_Rect, but say on line 70, we don't subtract
things when checking if content rect contains it, plus later we do a bunch of
translates /o\

This is possibly has nothing to do with your patch; this is probably just me not
really understanding what this code is doing.

[danakj, 2015/05/15 21:52:15]

OK sure. They're in the same space.

The thing is the canvas here hasn't been translated so we subtract the bitmap
rect to move into an untranslated space relative to the canvas instead of
relative to the recording/playback.

+      canvas->drawColor(DebugColors::NonPaintedFillColor());
+      canvas->restore();
+    }
   }
 
   // If this raster source has opaque contents, it is guaranteeing that it will
@@ -33,7 +45,15 @@ void RasterSourceHelper::PrepareForPlaybackToCanvas(
     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_playback_rect - canvas_bitmap_rect.OffsetFromOrigin()));
+      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
@@ -47,14 +67,14 @@ void RasterSourceHelper::PrepareForPlaybackToCanvas(
     // covered by content.
     gfx::Rect deflated_content_rect = content_rect;
     deflated_content_rect.Inset(0, 0, 1, 1);
-    if (!deflated_content_rect.Contains(canvas_rect)) {
+    if (!deflated_content_rect.Contains(canvas_playback_rect)) {
       if (clear_canvas_with_debug_color) {
         // Any non-painted areas outside of the content bounds are left in
         // this color.  If this is seen then it means that cc neglected to
         // rerasterize a tile that used to intersect with the content rect
         // after the content bounds grew.
         canvas->save();
-        canvas->translate(-canvas_rect.x(), -canvas_rect.y());
+        canvas->translate(-canvas_bitmap_rect.x(), -canvas_bitmap_rect.y());
         canvas->clipRect(gfx::RectToSkRect(content_rect),
                          SkRegion::kDifference_Op);
         canvas->drawColor(DebugColors::MissingResizeInvalidations(),
@@ -65,7 +85,7 @@ void RasterSourceHelper::PrepareForPlaybackToCanvas(
       // Drawing at most 2 x 2 x (canvas width + canvas height) texels is 2-3X
       // faster than clearing, so special case this.
       canvas->save();
-      canvas->translate(-canvas_rect.x(), -canvas_rect.y());
+      canvas->translate(-canvas_bitmap_rect.x(), -canvas_bitmap_rect.y());
       gfx::Rect inflated_content_rect = content_rect;
       inflated_content_rect.Inset(0, 0, -1, -1);
       canvas->clipRect(gfx::RectToSkRect(inflated_content_rect),