Displacement Decals?

So we can create an image decal for a mesh and place the decal with an Empty.
Everyone following so far?

The conventional application of this is to assign color data to one area of a mesh.
What about assigning displacement data instead?

I tried it. There’s a problem: I can’t get it to affect just one area of the mesh.
See screen shot:

The influence of color data only affects that isolated area of the mesh, while the influence of displacement data spans the entire object. And there seems to be no way to use black areas of the image to mean zero displacement and white areas of my image to mean full displacement.

Any ideas on how to get this working so I can displace just one part of a mesh?
Thanks in advance.

zero displacement would be grey, .5 values for R G and B. black means it gets indented.

Never mind - I answered my own question.
Has to be set up as a Displacement Modifier, not as the object’s Texture data.

This will prove very useful for adding detail to a mesh without being constrained to multi-res sculpting or UV map displacement, both of which break every time you delete or add verts from your mesh.