WebGL 2: add readPixels API to read pixels into pixel pack buffer

Source/modules/webgl/WebGLRenderingContextBase.idl

Index: Source/modules/webgl/WebGLRenderingContextBase.idl
diff --git a/Source/modules/webgl/WebGLRenderingContextBase.idl b/Source/modules/webgl/WebGLRenderingContextBase.idl
index ad3c1b81635569b9403c3d6d2981ed2022995a47..c379f235b172bf8ac37e6b16d0c5cdef8d161bfe 100644
--- a/Source/modules/webgl/WebGLRenderingContextBase.idl
+++ b/Source/modules/webgl/WebGLRenderingContextBase.idl
@@ -598,6 +598,7 @@ typedef unrestricted float GLclampf;
     void polygonOffset(GLfloat factor, GLfloat units);
 
     void readPixels(GLint x, GLint y, GLsizei width, GLsizei height, GLenum format, GLenum type, ArrayBufferView? pixels);
+    void readPixels(GLint x, GLint y, GLsizei width, GLsizei height, GLenum format, GLenum type, GLintptr offset);

[Zhenyao Mo, 2015/08/17 17:02:47]

No, you can't expose this to WebGL 1.  It is a WebGL 2 only function.

[yunchao, 2015/08/18 02:30:00]

In the implementation, we can directly return and do nothing under WebGL 1
rendering context. So this API will not be exposed to the web developers. WDYT? 

Another solution is that we can use the [Custom] tag in the IDL file, and
manually write the binding code under WebKit/Source/bindings/core/v8/custom/. 

Final solution I can imagine: 
1. create a new attribute, say [CrossClassOverload], in IDL file. 
2. write python code to generate appropriate js binding code to support
cross-class overloading, although cross-class overloading is not syntactically
correct in C++. 

Any comments? Maybe I am wrong... 

[Ken Russell (switch to Gerrit), 2015/08/18 02:55:48]

I'm not sure that this would be completely invisible to JavaScript.
Why is this necessary?
Why can this not just be added to WebGL2RenderingContextBase.idl? Is there a
deficiency in the code generator?
What is the issue with cross-class overloading? WebGLRenderingContextBase.h
would define one overloading of readPixels and WebGL2RenderingContextBase.h
would define the other overload.

[yunchao, 2015/08/18 05:35:58]

cross-class overloading is not allowed, it will always call the function in the
sub-class, making the JS binding code failed (parameter mismatch:
'DOMArrayBufferView' to 'long long' in this case). 

Well, we can make the original WebGLRenderingContextBase::readPixels virtual,
and override it in WebGL2RenderingContext. then
WebGL2RenderingContext::readPixels(,,,, DOMArrayBufferView* pixels ) can
correctly overloads with WebGL2RenderingContext(,,,,, long long offset), because
these two readPixels are in the same class (WebGL2RenderingContext) now.

Code has been updated, PTAL when you have time.

 
     void renderbufferStorage(GLenum target, GLenum internalformat, GLsizei width, GLsizei height);
     void sampleCoverage(GLclampf value, GLboolean invert);