Creating a movie

[Kevin Fitch <kfitch@xxxxxxxxxxx>]

I want to be able to create a movie/animation of what my program 
displays, so I have some code that grabs the frambuffer when I want it.
Like this:

void
Visualization::renderGL(int l,int b,int w,int h,unsigned char* buf=0) {

  SbViewportRegion vp(w+l,h+b);
  vp.setViewportPixels(l,b,w,h);
  SoGLRenderAction myRenderAction(vp);
  myRenderAction.apply(root);
  if(buf) {
    glReadBuffer(GL_BACK);
    glReadPixels(l,b,w,h,GL_RGB,GL_UNSIGNED_BYTE,buf);
  }
}

But, it seems that no matter what I get back straight 0's in buf. Is 
there some magical incantation to get glReadPixels to work with 
Inventor, or do I not understand how to use glReadPixels?
Kevin