I've got the "easy" half finished. This render pass encodes the surface normal to the green and blue channels, while it encodes the distance between the camera and the plane in red. End result is that all triangles with the same normal and distance are rendered the same color. I can then hopefully have the GPU read each pixel and make a list of everything that shows up a significant amount of times. This will determine all reflection planes that are visible and important in the current frame.
