Implement transferToImageBitmap() in WebGLRenderingContext

third_party/WebKit/Source/modules/webgl/WebGLRenderingContextBase.cpp

Index: third_party/WebKit/Source/modules/webgl/WebGLRenderingContextBase.cpp
diff --git a/third_party/WebKit/Source/modules/webgl/WebGLRenderingContextBase.cpp b/third_party/WebKit/Source/modules/webgl/WebGLRenderingContextBase.cpp
index ceb485bdbf6c7e4ddd84cb242e0e556ae3c88ebf..e86e22bac5a50c567cb9a6d4e84a66bdd05a5aa1 100644
--- a/third_party/WebKit/Source/modules/webgl/WebGLRenderingContextBase.cpp
+++ b/third_party/WebKit/Source/modules/webgl/WebGLRenderingContextBase.cpp
@@ -633,6 +633,14 @@ void WebGLRenderingContextBase::forceNextWebGLContextCreationToFail()
     shouldFailContextCreationForTesting = true;
 }
 
+ImageBitmap* WebGLRenderingContextBase::transferToImageBitmapBase()
+{
+    ImageData* imageData = paintRenderingResultsToImageData(BackBuffer);

[Ken Russell (switch to Gerrit), 2016/05/10 23:11:42]

This needs to be much better optimized in the future. This operation should
transfer the DrawingBuffer's texture into a texture mailbox, retrieve that
texture into a new texture object (presumably), and create an ImageBitmap from
that texture ID. Is this how the 2D canvas context works, or will work? Is there
a plan for this? Please, minimally, add a TODO to this that this needs to avoid
readbacks from the GPU, which is how the canvas is getting the data into an
ImageData object.

[xidachen, 2016/05/11 00:17:52]

In the case of 2d canvas context, it contains a ImageBuffer private member, and
creating an ImageBitmap from a ImageBuffer is easy. I will dig into your
suggestion here. If it is not hard to get a new texture object and create an
ImageBitmap, then I will do that in this CL. I agree with you, accessing the
entire image data and creating from that is really in-efficient.

+    if (!imageData)
+        return nullptr;
+    return ImageBitmap::create(imageData, IntRect(0, 0, imageData->width(), imageData->height()));
+}
+
 namespace {
 
 // ES2 enums
@@ -1164,6 +1172,9 @@ void WebGLRenderingContextBase::markContextChanged(ContentChangeType changeType)
 
     drawingBuffer()->markContentsChanged();
 
+    if (!canvas())
+        return;
+
     LayoutBox* layoutBox = canvas()->layoutBox();
     if (layoutBox && layoutBox->hasAcceleratedCompositing()) {
         m_markedCanvasDirty = true;