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

Source/platform/graphics/gpu/DrawingBuffer.cpp

Index: Source/platform/graphics/gpu/DrawingBuffer.cpp
diff --git a/Source/platform/graphics/gpu/DrawingBuffer.cpp b/Source/platform/graphics/gpu/DrawingBuffer.cpp
index 60a9d9862c845d8daf7e086ded9f085bd0552242..5996ff8feb6ead894f07fc13219b828fe4c38253 100644
--- a/Source/platform/graphics/gpu/DrawingBuffer.cpp
+++ b/Source/platform/graphics/gpu/DrawingBuffer.cpp
@@ -459,7 +459,7 @@ bool DrawingBuffer::initialize(const IntSize& size)
 }
 
 bool DrawingBuffer::copyToPlatformTexture(WebGraphicsContext3D* context, Platform3DObject texture, GLenum internalFormat,
-    GLenum destType, GLint level, bool premultiplyAlpha, bool flipY, SourceBuffer source)
+    GLenum destType, GLint level, bool premultiplyAlpha, bool flipY, SourceDrawingBuffer sourceBuffer)
 {
     if (m_contentsChanged) {
         if (m_multisampleMode != None) {
@@ -478,7 +478,7 @@ bool DrawingBuffer::copyToPlatformTexture(WebGraphicsContext3D* context, Platfor
     // Contexts may be in a different share group. We must transfer the texture through a mailbox first
     WebExternalTextureMailbox mailbox;
     GLint textureId = 0;
-    if (source == Front && m_frontColorBuffer.texInfo.textureId) {
+    if (sourceBuffer == FrontBuffer && m_frontColorBuffer.texInfo.textureId) {
         textureId = m_frontColorBuffer.texInfo.textureId;
         mailbox = m_frontColorBuffer.mailbox;
     } else {
@@ -887,7 +887,7 @@ 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, SourceDrawingBuffer sourceBuffer)
 {
     if (m_actualAttributes.premultipliedAlpha)
         return nullptr;
@@ -903,10 +903,29 @@ PassRefPtr<Uint8ClampedArray> DrawingBuffer::paintRenderingResultsToImageData(in
 
     RefPtr<Uint8ClampedArray> pixels = Uint8ClampedArray::createUninitialized(width * height * 4);
 
-    m_context->bindFramebuffer(GL_FRAMEBUFFER, framebuffer());
+    GLint fbo = 0;
+    if (sourceBuffer == FrontBuffer && m_frontColorBuffer.texInfo.textureId) {
+        GLint fbo = m_context->createFramebuffer();

[Yuki, 2014/11/27 04:30:22]

Hi dongseong.hwang,

It seems that you're shadowing |fbo| variable here, so the other |fbo| on line
906 will not be updated.  It seems a if-clause on line 918 never be true.

Is this right?  Is this what you intended?

[dshwang, 2014/11/27 09:46:30]

Thank you very much for noting! it's a mistake. 

+        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);
+    }
+
+    if (m_framebufferBinding) {
+        restoreFramebufferBinding();
+    } else {
+        bind();
+    }
+
     return pixels.release();
 }