Quote - ZBrush has a function called "Mid" in the Displacement Map panel, I think that's what you're getting at.  It's a slider which lets you set the mid-point of the displacement amplitude.  It's at 0.5 by default meaning that mid-grey equals no displacement, but it can be adjusted by sliding the control between 0 and 1.

And Poser has a math:subtract node that can make the midpoint 0, .5, .7571575, 1, 2014, or any other number you want. However, if you intend to just plug the displacement in straight without an offset (via math node) then configure ZBrush to use 0. You will be unable to represent a negative displacement in that case.

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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)