displacement Mod displacing black with no shift

[andrewjohn81]

I'm using 2D mapping, filter texmap checked with filter blur at .001
Amount, 0.4mm
Shift 0.0mm

Resolution 1024
Precision 8

It's giving me the quality I want, only black is also being bumped out a little. The image I am using is 16bit, if that makes a difference for this, but I have confirmed that the tiff is actually 0,0,0 where it is black, and 1.0,1.0,1.0 for white.

Anyone know why it's doing this?
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edit:

Just for $%^s and giggles I though I should try shifting the map exactly opposite the amount it should go out. That cause it to stop bumping out where it shouldn't. It's giving me the results I expected now, but I don't understand why.
Anyone care to explain why? I'm drawing a blank here.

[vlado]

Hard to say whats going on without the scene. Is your geometry flat or rounded?

Best regards,
Vlado

[andrewjohn81]

it's rounded, but the normals are all perpendicular to their respective face. It's displacing properly, it just seems that the edge is going up .4mm, and white is going up another .4mm.
Shifting the map a negative amount fixed it, but I understood it to work differently than that.

On one product it's on glass and the lines actually looked nice because I put the edges where the parting lines would be. It actually looks like a nice imperfection like manufacturing creates. For my other surfaces I wasn't so lucky.