[PyOpenGL-Users] Having a problem with glBufferData

[Gijs <in...@bs...>]

Hey List,

I'm trying to use PBOs for the first time so I'm not completely aware of 
what I'm doing with the code I found on this site 
<http://www.mathematik.uni-dortmund.de/%7Egoeddeke/gpgpu/tutorial3.html>. One 
thing I do know is that the code I can execute in C, I cannot execute in 
Python. This has to do with a piece of code that makes use of 
glBufferData and its 4th argument, namely "size".

The C code:
glBindBuffer(GL_PIXEL_PACK_BUFFER_ARB, thebuffer);
glBufferData(GL_PIXEL_PACK_BUFFER_ARB,4*sizeof(float), NULL, 
GL_STREAM_READ);
glReadPixels (0, 0, 1, 1, GL_RGBA, GL_FLOAT, 0);
void* mem = glMapBuffer(GL_PIXEL_PACK_BUFFER_ARB, GL_READ_ONLY);
assert(mem);
glUnmapBuffer(GL_PIXEL_PACK_BUFFER_ARB);
glBindBuffer(GL_PIXEL_PACK_BUFFER_ARB, 0);

Python Code:
glBindBuffer(GL_PIXEL_PACK_BUFFER_ARB, thebuffer)
glBufferData(GL_PIXEL_PACK_BUFFER_ARB,None, GL_STREAM_READ)
glReadPixels (0, 0, 1, 1, GL_RGBA, GL_FLOAT, 0)
mem = glMapBuffer(GL_PIXEL_PACK_BUFFER_ARB, GL_READ_ONLY)
glUnmapBuffer(GL_PIXEL_PACK_BUFFER_ARB)
glBindBuffer(GL_PIXEL_PACK_BUFFER_ARB, 0)

Since I cannot specify 4 arguments for glBufferData, as I can do in C, 
the code I execute in Python is doing something different than the C 
version. This results in an OpenGL error 1282 (invalid operation) when I 
call glReadPixels. I guess when the interface for this function was 
written, it was never expected to be called like this. A normal call 
would supply a variable, and PyOpenGL would be able to determine the 
size, so the size argument wouldn't be needed. Too bad in a case like 
this, it is needed, as far as I can see. Or is there some other way of 
accessing this function?

Regards, Gijs