WebGL: clarify which Front or Back buffer is used by each API.

Source/platform/graphics/gpu/DrawingBuffer.cpp

 /*
  * Copyright (c) 2010, Google Inc. All rights reserved.
  *
  * Redistribution and use in source and binary forms, with or without
  * modification, are permitted provided that the following conditions are
  * met:
  *
  *     * Redistributions of source code must retain the above copyright
  * notice, this list of conditions and the following disclaimer.
  *     * Redistributions in binary form must reproduce the above
@@ skipping 869 matching lines @@
 void DrawingBuffer::setPackAlignment(GLint param)
 {
     m_packAlignment = param;
 }
 
 void DrawingBuffer::paintRenderingResultsToCanvas(ImageBuffer* imageBuffer)
 {
     paintFramebufferToCanvas(framebuffer(), size().width(), size().height(), !m_actualAttributes.premultipliedAlpha, imageBuffer);
 }
 
-PassRefPtr<Uint8ClampedArray> DrawingBuffer::paintRenderingResultsToImageData(int& width, int& height)
+PassRefPtr<Uint8ClampedArray> DrawingBuffer::paintRenderingResultsToImageData(int& width, int& height, SourceBuffer sourceBuffer)
 {
     if (m_actualAttributes.premultipliedAlpha)
         return nullptr;
 
     width = size().width();
     height = size().height();
 
     Checked<int, RecordOverflow> dataSize = 4;
     dataSize *= width;
     dataSize *= height;
     if (dataSize.hasOverflowed())
         return nullptr;
 
     RefPtr<Uint8ClampedArray> pixels = Uint8ClampedArray::createUninitialized(width * height * 4);
 
-    m_context->bindFramebuffer(GL_FRAMEBUFFER, framebuffer());
+    GLint fbo = 0;
+    if (sourceBuffer == Front && m_frontColorBuffer.texInfo.textureId) {
+        GLint fbo = m_context->createFramebuffer();
+        m_context->bindFramebuffer(GL_FRAMEBUFFER, fbo);
+        m_context->framebufferTexture2D(GL_FRAMEBUFFER, GL_COLOR_ATTACHMENT0, GL_TEXTURE_2D, m_frontColorBuffer.texInfo.textureId, 0);
+    } else {
+        m_context->bindFramebuffer(GL_FRAMEBUFFER, framebuffer());
+    }
+
     readBackFramebuffer(pixels->data(), width, height, ReadbackRGBA, WebGLImageConversion::AlphaDoNothing);
     flipVertically(pixels->data(), width, height);
 
+    if (fbo) {
+        m_context->framebufferTexture2D(GL_FRAMEBUFFER, GL_COLOR_ATTACHMENT0, GL_TEXTURE_2D, 0, 0);
+        m_context->deleteFramebuffer(fbo);

[Ken Russell (switch to Gerrit), 2014/11/26 00:49:10]

Please bind either the 0 framebuffer or framebuffer() after deleting this
temporary FBO.

[dshwang, 2014/11/26 13:37:30]

I intentionally don't bind FBO because
WebGLRenderingContextBase::paintRenderingResultsToImageData() restores FBO,
which is only one caller.
However, after reading your comment, I think
DrawingBuffer::paintRenderingResultsToImageData() should clean up by itself. So
I move the FBO restoration code from WebGLRenderingContextBase to DrawingBuffer.

+    }
+
     return pixels.release();
 }
 
 void DrawingBuffer::paintFramebufferToCanvas(int framebuffer, int width, int height, bool premultiplyAlpha, ImageBuffer* imageBuffer)
 {
     unsigned char* pixels = 0;
 
     const SkBitmap& canvasBitmap = imageBuffer->bitmap();
     const SkBitmap* readbackBitmap = 0;
     ASSERT(canvasBitmap.colorType() == kN32_SkColorType);
@@ skipping 101 matching lines @@
 void DrawingBuffer::deleteChromiumImageForTexture(TextureInfo* info)
 {
     if (info->imageId) {
         m_context->releaseTexImage2DCHROMIUM(GL_TEXTURE_2D, info->imageId);
         m_context->destroyImageCHROMIUM(info->imageId);
         info->imageId = 0;
     }
 }
 
 } // namespace blink