center of screen is simple enough,but for light source: opengl right? multiply by modelview and projection matrices and drop the z value I believe for it's screen coords. then you are left with nothing but simple 2d work. not sure how q3 engine is set up, but in mine it's as simple as querying the matrix info from the active camera and not the opengl calls because the getfloatfv() calls to get the current modelview and projection matrices tends to stall the render pipe.