Re: [PyOpenGL-Users] access violation when calling glGenFramebuffers

[David V. <cod...@ha...>]

Hello, everyone!

I'm having a very similar problem to this one and I'd like to help out 
however I can.

I am on Windows 7, I have an Intel HD 4000 chipset, and I have both 
32-bit and 64-bit version of Python installed. I get:

 > "WindowsError: exception: access violation writing 0xFFFFFFFFE1FEFA20"

when I call GL.glActiveTexture(GL.GL_TEXTURE0). I can reproduce this 
with a very small sample program that only calls pygame.set_mode and 
glActiveTexture, and confirm that the crash only happens with the 64-bit 
Python.


Also, while I did not write a little C program, I can observe something 
that tells me the graphics drivers aren't outright broken: I can play 
Minecraft on top of a 64-bit Java runtime, and Minecraft has no problems 
calling glActiveTexture (via LWJGL, which has to be calling the C natives).

glActiveTexture is part of the OpenGL 1.3 feature set. Someone mentioned 
that features not part of GL 1.1 were being loaded from the wrong DLL 
(or something?):

> In fact I think to remember that, when I was investigating this issue, 
> it was as if the Python bindings were incorrect, in that PyOpenGL 
> tried to link to the *native* Windows OpenGL 1.1 drivers functions and 
> not the graphics card drivers ones. So any call to an OpenGL function 
> available in, say, OpenGL 2.1 and not OpenGL 1.1 would result in a 
> crash. glGenFramebuffers is such an example, but there were a lot of 
> similar other OpenGL commands which yielded the same issue. You could 
> try to look for such commands not available in OpenGL 1.1 and see if 
> this observation is correct.

So, I looked at a few function objects (GL.glEnable, GL.glTexImage, 
GL.glActiveTexture and others) using the 64-bit debugger. Each one has a 
DLL attribute which references the open DLL handle, and every one I 
looked at refers to "C:\Windows\System32\opengl.dll". I also noticed 
that different functions had different object types:

glActiveTexture = <OpenGL.platform.baseplatform.glActiveTexture object 
at 0x03DCC5F0>
glGenFramebuffers = <OpenGL.platform.baseplatform.glGenFramebuffers 
object at 0x03E695F8>
glEnable = <WinFunctionType object at 0x03CD27B0>
glTexImage2d = <ctypes.glTexImage2D object at 0x03DB28A0>

I wondered what the difference is, stepped around in the debugger some 
more, and found out the first two are "null function" placeholders. They 
are null at first because a GL context is required to check for 
extensions and GL versions. I stepped through glActiveTexture's lazy 
loading the first time it is called, and I think I found out what the 
problem is.

In OpenGL/platform/baseplatform.py, constructFunction calls 
self.getExtensionProcedure and gets a small negative number as a result. 
I looked at getExtensionProcedure and traced it back to the 
wglGetProcAddress that is implicitly created in 
OpenGL/platform/win32.py. This wglGetProcAddress is a _FuncPtr whose 
restype attribute is ctypes.c_long.

If I add another line to OpenGL/platform/win32.py after the 
`getExtensionProcedure = ` line, that says:

OpenGL.wglGetProcAddress.restype = ctypes.c_void_p

Then the result of getExtensionProcedure is now a longer positive 
number, and better yet the call to glActiveTexture doesn't crash with an 
Access Violation any more!

So, my conclusion is that the function pointer to wglGetProcAddress used 
by the PyOpenGL's win32 platform module has the wrong result type (it is 
c_long which is only 32 bits when it should be c_void_p which is 64 bits 
long).

Hope this helps,

- David Vierra