Avoid GPU readback in tex(Sub)Image2D(ImageBitmap)

third_party/WebKit/Source/core/frame/ImageBitmap.cpp

 // Copyright 2013 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 "core/frame/ImageBitmap.h"
 
 #include "core/html/HTMLCanvasElement.h"
 #include "core/html/HTMLVideoElement.h"
 #include "core/html/ImageData.h"
 #include "platform/graphics/skia/SkiaUtils.h"
@@ skipping 448 matching lines @@
 }
 
 unsigned long ImageBitmap::height() const
 {
     if (!m_image)
         return 0;
     ASSERT(m_image->height() > 0);
     return m_image->height();
 }
 
+bool ImageBitmap::isTextureBacked() const
+{
+    return (!m_image->hasSkImage() || m_image->hasContext3DProvider());

[Justin Novosad, 2016/06/07 15:11:07]

This is hard to read. The "!m_image->hasSkImage()" condition implies that
m_image has a mailbox, which is what we really want to know here.  You should
put an isTextureBacked() method directly on StaticBitmapImage and call that
here.

[xidachen, 2016/06/07 17:49:53]

I actually tried that before and it is a bit complicated...

When texImage2D consumes the ImageBitmap, this method tells texImage2D that the
ImageBitmap is texture-backed either:
1. it doesn't have a SkImage, which means it has a mailbox
2. or it has a contextProvider.

But if I put this method in StaticBitmapImage, it should look like this, this
StaticBitmapImage is texture-backed if either:
1. it doesn't have a SkImage, which means it has a mailbox
2. or it has a contextProvider
3. or the SkImage is texture-backed.

Now if put this method in StaticBitmapImage, and texImage2D calls it, all those
texImage-ImageBitmap layout tests will fail under virtual/gpu/. In those layout
tests, ImageBitmap is created from different sources, so it has an SkImage and
that SkImage is texture-backed. But it doesn't have a mailbox, nor does it have
a context provider. In this case, isTextureBacked() should tell texImage2D that
this ImageBitmap is not texture-backed. I understand that this will trigger a
read-back, how should I solve this case?

+}
+
 IntSize ImageBitmap::size() const
 {
     if (!m_image)
         return IntSize();
     ASSERT(m_image->width() > 0 && m_image->height() > 0);
     return IntSize(m_image->width(), m_image->height());
 }
 
 ScriptPromise ImageBitmap::createImageBitmap(ScriptState* scriptState, EventTarget& eventTarget, int sx, int sy, int sw, int sh, const ImageBitmapOptions& options, ExceptionState& exceptionState)
 {
@@ skipping 33 matching lines @@
 FloatSize ImageBitmap::elementSize(const FloatSize&) const
 {
     return FloatSize(width(), height());
 }
 
 DEFINE_TRACE(ImageBitmap)
 {
 }
 
 } // namespace blink