When you take color data and desaturate it, hoping to create a bump map that makes sense, you accidentally pick up lighting in that bump map. This lighting produces gradients in shape that are not there.

Here's an example: I grabbed an eye wrinkle photo.

When I place a grayscale version of that on a sphere (left) you can see there are gradients where the light hits the convex curves at different angles of incidence.

The resulting gradient, when used as a height map (right), gives inaccurate curves, producing not convex compression zones, but terraces. There's a sharp rise at the highlight, and a dip on the shadow side.

I see these everywhere and they look like crap.

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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)