Add colour to a cut meshface in animation

Question

EDIT NOTE: This thread had two question in it. One was answered, the the other was not. I split the question into to separate questions and the other one can be found here.

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I want to animate cutting open a machine with a mesh with a boolean modifier (or something that does the same). And I want to colour the cut face.

What I want to create is something like this:

In the animation I want to create, eg. the walls should be red, and other parts of it should have different colours.

• https://streamable.com/uwbif

This is actually what I want to get. An animation, then some kind of boolean modifier cutting away the mesh and the cutface has a different colour.

This video is a part the How It's Made show ( https://www.youtube.com/watch?v=6qnuQw3DFj0 )

I maybe think about this in the wrong way, but what I would like to have it that so I can specify that obejct X has has the matrial Y and if cut, the new face should be colour Z.

I can imagine how it could work: You could create a very thin layer over a mesh that is made up of a different material. So the coloured part is not really the colour of the cut face but the actual original mesh colour. The colour you saw before the cut is just the thin coat that serves no function.

As you can see in this prove of concept video ( https://streamable.com/5s6dv ), it would work up to some point. These are two cubes inside each other. The white one is a tiny bit larger as the red one (solidify modifier with a thickness of 0.01). The white cube is hollowed out using the red cube and a boolean modifier. However, as you can see in the end there seem to be some problems. Moreover, I think it's more like flub work, since you need an extra mesh that actually does nothing.

My Question:

If not clear, my questionis: How do I do this?

How can I add a colour to a cutface that is created with something like the boolean tool and can be used in an animation.

Answer 1

This works out of the box with the boolean modifer:

1. add your boolean modifier, don't apply it (this way you can use it in animations)
2. add a second material, the one to be assigned to the cut-off parts
3. change the order of the material so that the one created in 2. becomes the first material in the list.

This way, it will be assigned only to the cut-off faces created dynamically by the boolean modifier:

Answer 2

I think a boolean modifier will apply the other object's materials to any parts it cuts from the modified object. So make sure that each cutter object has the right material applied, and it should transfer to the cutout. If you're wondering about UV mapping: that's not going to be easy here, but you can make a procedural material to do it instead or just use global coordinates for your texture.

I just tested the above: you do need to ensure that the cutter object's material is also present in the cutaway object's material list, but it works fine. No need to apply the modifier.