Add "alpha" option to PaintWorklet

third_party/WebKit/Source/platform/graphics/RecordingImageBufferSurface.cpp

 // 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 "platform/graphics/RecordingImageBufferSurface.h"
 
 #include "platform/Histogram.h"
 #include "platform/graphics/CanvasMetrics.h"
 #include "platform/graphics/ExpensiveCanvasHeuristicParameters.h"
 #include "platform/graphics/GraphicsContext.h"
@@ skipping 11 matching lines @@
     , m_imageBuffer(0)
     , m_currentFramePixelCount(0)
     , m_previousFramePixelCount(0)
     , m_frameWasCleared(true)
     , m_didRecordDrawCommandsInCurrentFrame(false)
     , m_currentFrameHasExpensiveOp(false)
     , m_previousFrameHasExpensiveOp(false)
     , m_fallbackFactory(std::move(fallbackFactory))
 {
     initializeCurrentFrame();
+    clear();

[ikilpatrick, 2016/06/22 16:00:24]

Why is this needed?

[Gleb Lanbin, 2016/06/22 21:23:45]

clear() method is the one which actually sets the background to be transparent
or opaque. All other ImageBufferSurface child classes call clear() method inside
of their constructor. So I'm surprised that RecordingImageBufferSurface doesn't
do that.

IMHO we should call clear() every time when we resetCanvas but I decided to call
clear() from the constructor only to be consistent with other ImageBufferSurface
implementations.

 }
 
 RecordingImageBufferSurface::~RecordingImageBufferSurface()
 { }
 
 void RecordingImageBufferSurface::initializeCurrentFrame()
 {
     static SkRTreeFactory rTreeFactory;
     m_currentFrame = wrapUnique(new SkPictureRecorder);
     m_currentFrame->beginRecording(size().width(), size().height(), &rTreeFactory);
@@ skipping 306 matching lines @@
 
 void RecordingImageBufferSurface::setIsHidden(bool hidden)
 {
     if (m_fallbackSurface)
         m_fallbackSurface->setIsHidden(hidden);
     else
         ImageBufferSurface::setIsHidden(hidden);
 }
 
 } // namespace blink