I’m playing around with GLX and xlib and I’m curious about rendering using straight X calls on top of an openGL buffer. The glx intro clearly says that:
GLX extended X servers make a subset of their visuals available for OpenGL rendering. Drawables created with these visual can also be rendered into using the core X renderer and or any other X extension that is compatible with all core X visuals.
And, indeed, I’m able to render a simple quad colored with some rainbow effects and then draw on top of it with xlib calls. However, GLX extends the X window with a back buffer, which I have to swap to the front before I can then directly render to the window. My question is: is it possible to use X to render to the back buffer after openGL is done with it and then swap that buffer wholesale to the front, thus giving me flicker free animation on both my openGL and X graphics?
I think the answer is no, but there are maybe some alternatives.
You could do another layer of double-buffering with a pixmap (render X and GL to a pixmap, then draw the pixmap to your X window). It probably wrecks your framerate if you were doing an FPS game but for what you describe might not matter.
You could also use Cairo to draw to a client-side memory buffer, with alpha channel for the background to show through. Then upload the result as a texture to GL and paint it over your background. The Clutter toolkit does this for some of its drawing.