WebGL 2: validate read buffer attachment when reading from FBO

Source/core/html/canvas/WebGLRenderingContextBase.h

Index: Source/core/html/canvas/WebGLRenderingContextBase.h
diff --git a/Source/core/html/canvas/WebGLRenderingContextBase.h b/Source/core/html/canvas/WebGLRenderingContextBase.h
index 0f6d7275e4c260c67cdaff6525afe92277df88d8..59fec9afc0334264d7215e5a1680f38a8f96941c 100644
--- a/Source/core/html/canvas/WebGLRenderingContextBase.h
+++ b/Source/core/html/canvas/WebGLRenderingContextBase.h
@@ -569,6 +569,8 @@ protected:
     bool m_drawBuffersWebGLRequirementsChecked;
     bool m_drawBuffersSupported;
 
+    GLenum m_readbufferOfFBO;

[Zhenyao Mo, 2015/07/01 15:21:27]

Now from our discussion, this should be part of the WebGLFramebuffer, as it is a
per fbo state.

[Ken Russell (switch to Gerrit), 2015/07/01 17:24:06]

Right. Thanks Mo for finding this. Yunchao, could you make this change, and also
cache the readBuffer for the default framebuffer here?

Could you please also add some tests? For example, have two framebuffers, one
with COLOR_ATTACHMENT0 and one with both COLOR_ATTACHMENT0 and
COLOR_ATTACHMENT1. Set READ_BUFFER to COLOR_ATTACHMENT0 for the first and
COLOR_ATTACHMENT1 for the second. Ensure that calling bindFramebuffer and then
readPixels for each has the expected behavior.

[yunchao, 2015/07/02 15:24:53]

Ken, I updated the patch locally, found that caching readbuffer for the default
fb is not necessary (but caching readbuffer for fbo will be used to check
whether the attachment exists before reading from fbo by
copyTexImage2D/readPixels/...), so I prefer to not cache readbuffer for default
fb, WDYT? 

[yunchao, 2015/07/02 15:24:53]

Acknowledged.

+
     GLint m_packAlignment;
     GLint m_unpackAlignment;
     bool m_unpackFlipY;
@@ -862,6 +864,10 @@ protected:
     // the given format.
     bool validateCompressedTexSubDimensions(const char* functionName, GLenum target, GLint level, GLint xoffset, GLint yoffset, GLsizei width, GLsizei height, GLenum format, WebGLTexture*);
 
+    // Helper function to validate that the image is attached to read buffer
+    // when reading from FBO (readPixels/copyTexImage2D/copyTexSubImage2D).
+    bool validateReadBufferAttachment(const char* functionName, const WebGLFramebuffer* readFramebufferBinding);
+
     // Helper function to validate mode for draw{Arrays/Elements}.
     bool validateDrawMode(const char* functionName, GLenum);