Discard correct drawing buffer

third_party/WebKit/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 270 matching lines @@
     if (!frontColorBufferMailbox)
         frontColorBufferMailbox = createNewMailbox(createTextureAndAllocateMemory(m_size));
 
     if (m_preserveDrawingBuffer == Discard) {
         std::swap(frontColorBufferMailbox->textureInfo, m_colorBuffer);
         attachColorBufferToReadFramebuffer();
 
         if (m_discardFramebufferSupported) {
             // Explicitly discard framebuffer to save GPU memory bandwidth for tile-based GPU arch.
             const GLenum attachments[3] = { GL_COLOR_ATTACHMENT0, GL_DEPTH_ATTACHMENT, GL_STENCIL_ATTACHMENT };
+            // It appears safe to overwrite the context's framebuffer binding in the Discard case since there will always be a

[Zhenyao Mo, 2016/07/14 17:27:03]

This comment is unnecessary because there is a restoreFramebufferBindings() call
right below.

[qiankun, 2016/07/14 23:40:00]

Thanks for reminding. The comments was added previously when
restoreFramebufferBindings() was not always called.

+            // WebGLRenderingContext::clearIfComposited() call made before the next draw call which restores the framebuffer binding.
+            // If this stops being true at some point, we should track the current framebuffer binding in the DrawingBuffer and restore
+            // it after attaching the new back buffer here.
+            m_gl->BindFramebuffer(GL_FRAMEBUFFER, m_fbo);
             m_gl->DiscardFramebufferEXT(GL_FRAMEBUFFER, 3, attachments);

[qiankun, 2016/07/14 12:37:10]

This only makes framebuffer undefined to save some wasted efforts to load/store
logical buffer (color, depth, stencil) from/to fast memory. See
https://www.khronos.org/registry/gles/extensions/EXT/EXT_discard_framebuffer.txt.
WebGLRenderingContext::clearIfComposited() is still required to do real
clearance.

         }
     } else {
         m_gl->CopySubTextureCHROMIUM(m_colorBuffer.textureId, frontColorBufferMailbox->textureInfo.textureId,
             0, 0, 0, 0, m_size.width(), m_size.height(), GL_FALSE, GL_FALSE, GL_FALSE);
     }
 
     restoreFramebufferBindings();
     m_contentsChanged = false;
 
     m_gl->ProduceTextureDirectCHROMIUM(frontColorBufferMailbox->textureInfo.textureId, frontColorBufferMailbox->textureInfo.parameters.target, frontColorBufferMailbox->mailbox.name);
@@ skipping 799 matching lines @@
 
 void DrawingBuffer::restoreTextureBindings()
 {
     // This class potentially modifies the bindings for GL_TEXTURE_2D and
     // GL_TEXTURE_RECTANGLE. Only GL_TEXTURE_2D needs to be restored since
     // the public interface for WebGL does not support GL_TEXTURE_RECTANGLE.
     m_gl->BindTexture(GL_TEXTURE_2D, m_texture2DBinding);
 }
 
 } // namespace blink