Part of the Camera transform
Convert things in the viewing volume of the camera (which is a Truncated pyramid) to Image space (which is a Cube)
This transformation is done so that the Stereographic projection that would have been necessary to project into the image plane in the original space, now becomes a Perpendicular projection in the image space!
This can be done with a 4x4 matrix – this is the beginning of the matrix – this is the rest of the matrix