Treat calls to CanvasRenderingContext2D.clearRect as operations that clear the canvas

Treat calls to CanvasRenderingContext2D.clearRect as operations that clear the canvas

Currently RecordingImageBufferSurface determines whether
the canvas was cleared by tracking opaque draws.  For this
purpose we need all draw operations that completely
overwrite the destination pixels to be treated as opaque
even though the resulting pixels may not be opaque
(alpha = 1). To solve this problem we introduce an alternate
mode of operation in OpaqueRegionSkia, which was renamed to
RegionTracker, to reflect its new more generic vocation.

BUG=397205
TEST=blink platform unit test RecordingImageBufferSurface.clearRect

Committed: https://src.chromium.org/viewvc/blink?view=rev&revision=179074

[Justin Novosad, 2014-07-24 21:07:39]

PTAL

[Stephen White, 2014-07-25 19:29:07]

https://codereview.chromium.org/413313002/diff/1/Source/platform/graphics/Gra...
File Source/platform/graphics/GraphicsContext.h (right):

https://codereview.chromium.org/413313002/diff/1/Source/platform/graphics/Gra...
Source/platform/graphics/GraphicsContext.h:227: void
setTreatOverwriteAsOpaque(bool value) {
m_opaqueRegion.setTreatOverwriteAsOpaque(value); }
As discussed, let's make this a setter that specifies the mode of the region
tracker, since the difference is subtle but important. Something like:

setTrackRegion({OPAQUE | OVERWRITE | NONE});

https://codereview.chromium.org/413313002/diff/1/Source/platform/graphics/ski...
File Source/platform/graphics/skia/OpaqueRegionSkia.cpp (right):

https://codereview.chromium.org/413313002/diff/1/Source/platform/graphics/ski...
Source/platform/graphics/skia/OpaqueRegionSkia.cpp:335: if (!preservesOpaque)
Nit: unless I'm missing something, this could all be inline:

if (!m_treatOverwriteAsOpaque && !xfermodePreservesOpaque(paint, drawsOpaque)) {
    markRectAsNonOpaque(targetRect);
}

[Justin Novosad, 2014-07-28 19:21:43]

Done. New Patch.

+jamesr for Source/web/ OWNER

[jamesr, 2014-07-28 19:29:56]

/web/ lgtm

[Stephen White, 2014-07-28 20:32:17]

LGTM, assuming the bots are happy

Optional: add a "regionTrackingEnabled()" accessor, just so that the
GraphicsContext.cpp code reads a bit more clearly.

[Justin Novosad, 2014-07-28 20:59:58]

The CQ bit was checked by junov@chromium.org

[commit-bot: I haz the power, 2014-07-28 21:00:55]

CQ is trying da patch. Follow status at
 https://chromium-status.appspot.com/cq/junov@chromium.org/413313002/80001

[commit-bot: I haz the power, 2014-07-28 22:11:09]

FYI, CQ is re-trying this CL (attempt #1).
The failing builders are:
  win_gpu_triggered_tests on tryserver.chromium.gpu
(http://build.chromium.org/p/tryserver.chromium.gpu/builders/win_gpu_triggered...)

[commit-bot: I haz the power, 2014-07-29 00:15:14]

Change committed as 179074