[PyOpenGL-Users] second call to glGenFramebuffersEXT() crashes

[Joshua R. D. <jd...@ca...>]

I am trying to cast shadows from multiple lights using shaders. I use one framebuffer object per light. When I have just one light, it works. When I try two lights, the program survives the first call to glGenFramebuffersEXT() but crashes ("Bus error") upon the second call to it.

What follows is a minimal demo. If I run it as-is, then it prints out the two FBO identifiers and terminates normally. If I switch the "cow = ..." and "print framebuffer" lines, then it prints out the first FBO identifier and then crashes ("Bus error"). Why would the location of the print statement matter?

from OpenGL.GL import *
from OpenGL.GLU import *
from OpenGL.GLUT import *
# adapted from http://www.pygame.org/wiki/GLSLExample
try:
    from OpenGL import platform
    gl = platform.OpenGL
except ImportError:
    try:
        gl = cdll.LoadLibrary('libGL.so')
    except OSError:
        from ctypes.util import find_library
        path = find_library('OpenGL')
        gl = cdll.LoadLibrary(path)
glutInit(0)
glutInitDisplayMode(GLUT_RGB|GLUT_DOUBLE|GLUT_DEPTH)
glutCreateWindow('test')
framebuffer = gl.glGenFramebuffersEXT(1)
cow = gl.glGenFramebuffersEXT(1)
print framebuffer
print cow

Other questions:
1. Why do I not need to call glInitFramebufferObjectEXT()?
2. Does glGenFramebuffersEXT(1) return a framebuffer identifier or a list of them? (When I try glGenFramebuffersEXT(1)[0], I get "TypeError: 'int' object is unsubscriptable".)

I'm new to both Python and framebuffer objects, so I may have fundamental misconceptions. My setup is Mac OS 10.4.11, ATI Radeon 9600, Python 2.5, PyOpenGL 3.0.0b6.

-- Josh