Windowless OpenGL Context in Apache2 Module

Question

I'm trying to develop an Apache2 module that utilizes OpenGL to perform off-screen rendering and dynamically generate images that I can then send back to the client.

Apache2 is running on an Ubuntu 12.04 machine and I created a test module that renders a quad and stores the frame as an image to disk using OpenGL/GLX. But when the module receives a client request, it crashes at XOpenDisplay(0) with a segmentation fault. Any ideas what could be going wrong?

Edit: All the examples I have seen talk about using a pixel buffer (PBuffer). As far as I know, these are deprecated and FBOs should be used instead. Can someone explain how to create a context and use FBOs to perform off-screen rendering?

Answer 1

While technically it's perfectly possible to do windowless, display server less off-screen GPU accelerated rendering with OpenGL, practically it's impossible these days because you need a display environment to actually get access to the GPU. Fortunately the structure of graphics systems is changing these days (Hybrid graphics, display compositors). Already Mesa provides an off-screen context creation mode (OSMesa), but it's far from being feature complete.

So right now, you'll need some kind of display server drawable to work with on which you can bind a context. X11 offers two kinds of GPU accelerated drawables: Windows and PBuffers. You can use FBOs with either (PBuffers are technically Windows that can not be mapped to the root window and have an off-screen canvas). The easiest way to go is to create a regular window on an X server but not showing it; you can still create an OpenGL context on it and create FBOs, like shown in numerous tutorials. But for OpenGL to work the X server you use must be active hold the console and be configured to use the GPU (theoretically with newer Hybrid graphics capable X servers and drivers it should be possible to configure the X server to use a dummy display device and configure the GPU as a secondary device for accelerated rendering, but I never tried that, so far).