How to apply a displacement modifier to one area of an object from an image? [duplicate]

Question

Say I have a trapezoidal prism (a six-sided object with 4 trapezoidal faces and a rectangular top and bottom) like this one here.

I need to apply a displacement modifier to one trapezoidal face using an image containing the texture/normals, such as this crude example.

In other words, I need to take an image that is the same shape as the face and use it to create the actual 3D texture using a displacement modifier (the resulting object will be 3D printed).

I know how to do this on a simple plane/surface, but I'm running into trouble in this scenario. The main problems seem to be:

1. How do I apply the displacement modifier to only one selected face, without affecting the rest of the object, and

2. When I apply the image, how do I force the image to fit correctly onto the surface, as they have the same proportions but may not be the same size?

Edit: This has been flagged as a possible duplicate of this question. I did see that question before posting mine, but it did not help me. It does not cover how to apply displacement modifiers to only one face without altering the object, and does not touch upon the second part of my question at all.

Answer 1

I think you'll need to use a UV map here as using a vertex group does not seem to give a good result.

As you want to use a displace modifier you'll need to subdivide your shape a lot. But, whatever the subdivision level is, the precision of the displacement will be limited by the texture resolution itself. So, if the model is to be 3D printed, both resolutions (subdivisions and texture) will matter.

First, make your UV map :

• Select your face in edit mode
• Go to front view (or more generally in front of the face)
• UV unwrap : U then choose "project from view (bounds)"

• Adjust your UV map

Note : you may also UV unwrap all the faces and / or make your texture after the unwrapping (so, or adjust the UV geometry to the texture, or make a texture adapted to the UV geometry)

• Set your texture as displacement
• And use UV as texture coordinates

• Subdivide your face (a lot)

• Adjust the displacement

Note

If you need to do this on several faces :

• UV unwrap each face and eventually use the vertex group option of the displace modifier (a vertex group per face, but taking care of the border vertices). Alternative : use several UV maps.
• I also suggest to apply the subdivision to all faces at the same time.

Again, I am not sure that the precision will be good enough for a 3D printed object.