OpenGL Pre-render Step
Allow a rendered OpenGL scene to act as input to our renderer which can then overlay onto that scene using the depth buffer. It is not possible to go the other way and render opengl onto our output. One use case is to render grid lines, bounding boxes and other meta data into the 3D scene. Alternatively an entire 3D model could be integrated with our scene. We could also do an OpenGL overlay post render for a HUD etc.
Edited by Nicolas Pope