Nobody is an idiot for not knowing something in a the material room. It is arcane.

The answer to the question, while simple, has nothing to do with your use case. But I'll show you how to invert a map, as well as how to do what you desired which will not involve inverting a map at all.

I'll be back shortly.

[cross-posted with tgx] He's correct - that's how you invert  map. But you don't want to invert a map.

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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)