Quite right pj. The cavities arc past 180 degrees. To do them would require vector displacement, not just displacement along the normal. In vector displacement, you move any point to anywhere, not just on a line along the normal. Firefly actually has this ability, but it is not exposed to us in the material room.

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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)