Here is a simple black-gray-white image map applied to a one-sided square. There are three other squares as well. All four are perfectly lined up. There are two in the middle - one that is the image map, and one that is semi-transparent and unshifted. Thus you can see where the central square is supposed to be based on the idea that gray (.5) is desired to be no shift.

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Renderosity forum reply notifications are wonky. If I read a follow-up in a thread, but I don't myself reply, then notifications no longer happen AT ALL on that thread. So if I seem to be ignoring a question, that's why. (Updated September 23, 2019)