the “tech” shader

If you’re using mental ray as your renderer, chances are that you aren’t going to get a whole lot out of the passes system, especially if you’re trying to write custom color buffers. It’s a slow, buggy, work-intensive process to get a lot of passes out of mental ray that Vray has absolutely no trouble with. You could use render layers instead of custom passes, but mental ray also has a particularly long translation time for complex scenes (think of any scene where you see mental ray hang for about 10 minutes before it even starts to render a frame). So you can’t exactly add render layers haphazardly… you need to condense things as much as possible.

Someone told me about a neat trick that they saw at a studio they were freelancing at where three data channels would be written to RGB channels, almost like an RGB matte pass except with “technical” passes instead of mattes or beauty or whatever. Since the data being written only needs a single channel, you can get three kinds of data written to one image and then split them apart later in post. Simple enough when you think about it…