Black is zero in firefly, many softwares over the years produce 16 bit middle grey as 0 displacement maps... most notably older versions of Zbrush. So Bagginsbill suggested fixing this difference with a Math Functions node Subtract with value 1 set at 1 and value 2 set at 0.5. You then run your map into value 1 and run the math node into the displacement socket of the root.

Not doing so with grey as 0 maps will produce bloating and other artifacts.

If you're using software like mudbox it produces 32 bit maps that are black as base point and values less than zero. 32 bit maps produce ranges beyond what a monitor can display.

Black as 0 maps require no math node adjustment.

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