Quote -      Blend the displacement map with a solid grey (matching the displacement map's background), and adjust the blend.

This works - blending with the neutral value before the subtract .5 will do the same as what I showed - blending with black after the subtract .5.

The equations - starting with one I can derive the other

Blender(map, .5, mask) - .5

= (1-mask) * map + mask * .5 - .5

= map - mask * map + mask * .5 - .5

= map - mask * map - .5 + mask * .5

= (1 - mask) * (map - .5)

= (1 - mask) * (map - .5) + mask * 0

= Blender(map - .5, 0, mask)

However, the difficulty lies in matching the gray. When gamma correction is enabled with gamma = 2.2, the color chips are assumed gamma = 2.2 and you cannot override that as is done with the displacement map gamma. It is easier to deal with blending with black than blending with .5.

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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)