How can I preserve both materials from two meshes that undergo boolean union?

Question

I want to Boolean union a cube (“A”) and a sphere (“B”) to created unified object (“U”). “U” should be a single mesh object with the faces of the new object inheriting the material properties of the corresponding faces from both "A" and "B" as explained in the documentation:

When I try to recreate this my unified object "U" ends up with either just the material from "A" or "B" but not both. I.e I want it to end up still yellow and red rather than totally red or totally yellow.

My process is simply create new mesh objects, assign separate materials, change the Diffuse property of each material, then go to add modifier > boolean > union > apply. Am I doing it wrong?

Answer 1

I've posted this question elsewhere and got this answer "The object with the boolean modifier needs to have both materials in its material list"

I tried exactly that, first when I make my two objects with separate materials, I make sure the second mesh also lists the material of the other object, even though it is not in use:

then when I request that second mesh to be boolean unified to the other mesh, the result is multicolor:

That object in the screenshot is definitely a single mesh with two materials. So that seems the way to do it.

Answer 2

I just tested this and I did it with textures mapped onto each object. At first, I though it did not work. One texture was now mapped onto both parts. I clicked to wire frame to see if all internal elements of the ball and cube were removed as they should be in the union-ed object. When I clicked on textured view i got the correct texture on each part as is should be. After applying the modifier, it looked right in texture view, but had only texture in material view. My experience is if you want to export the item for something like 3d printing, you need it to look right in material view. The only way I found so far to get this is to be sure one object is fully selected in edit mode and one is not selected. After you apply the modifier, the selected vertex groups are still in that state. That makes it easy to create 2 proper textures in the union-ed product. If you find an easier way, please share :)