you plug the dN/dU (et al.) nodes into reflect node softness, and you colour-multiply reflect node by those first-derivative nodes?

Almost. If I colour-multiply reflect with first-derivative, and the first-derivative is 0 (flat) then it would be black. Instead, what I'm doing is driving a Blender with the first-derivative. The Blender returns one of two colors to use as the reflection color that gets multipled with the reflect. One is dark brown, the other is goldish. For softness, I modulated it as 3 - 2 * c, where c is the curvature detector output (0 to 1). Thus the flat parts are softness = 3, and the curved parts are softness = 1. (Still not mirror sharp, but much sharper)

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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)