As the most of us know one of the disadvantages of displacement-maps this the fact that you can only have a positive or a negative value, so in other words you can only raise  but not lower a surface with the same map.I don't know if someone mentioned this before but I discovered a solution with a simple node-trick in the material room which I like to share with you.
For the examples I made a scene with two high resolution planes at the same location and rotation: One plane is for reference, colored red and made a bit transparant. The other plane is colored gray will have the displacement map. In the folowing message I'll show the material room setup.

Bopperthijs

-How can you improve things when you don't make mistakes?