Here is one way of edge detecting a mask.  The function is:

Clamp(7 * (Bias(x, .7) - Bias(x, .6)))

It's not the best edge detector but it's cool how you can use two bias nodes for something interesting eh?

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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)