typedef int (* XPLMDrawCallback_f)( XPLMDrawingPhase inPhase, int inIsBefore, void * inRefcon);
This is the prototype for a low level drawing callback. You are passed in the phase and whether it is before or after. If you are before the phase, return 1 to let x-plane draw or 0 to suppress x-plane drawing. If you are after the phase the return value is ignored.
Refcon is a unique value that you specify when registering the callback, allowing you to slip a pointer to your own data to the callback.
Upon entry the OpenGL context will be correctly set up for you and OpenGL will be in 'local' coordinates for 3d drawing and panel coordinates for 2d drawing. The OpenGL state (texturing, etc.) will be unknown.