[drudru11]

Interestingly, many old workstations that did 3d (primarily SGI), had what was called an overlay plane. This was essentially a separate frame buffer using indexed color into a palette. One of the colors would indicate transparency. The overlay was traditionally used for all UI or debug output on top of a 3d scene. For example, the Alias/Wavefront 3d modeler required this feature in order to run. It allowed the slower 3d hardware of that era to focus on a complex scene vs the scene AND the UI.