Material shading nodes: Different shaders for flat vs. curved faces

Question

I have a cylinder and I want the curved outside to have one simple diffuse shader, and the two flat sides to have a different diffuse shader. How would I do that?

I was stuck on believing that this is connected to the "Auto Smooth" (including the angle limit) function that already comes with Blender. I thought the basics are there because, the way I understand it, Auto Smooth almost does the thing I want: It shades faces differently, depending on their outer vertices' normals.

I tried playing around with the normals Input:

But this is dependant on the rotation of the object. The lower flat face, the one that's not visible in this screenshot, is still blue. And if I rotate the object, everything is messed up.

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EDIT: Possible solution

Okay so I asked elsewhere, and I got a promising response. Now I'll be honest, I just barely passed linear algebra back in university, more than 10 years ago by sheer dumb luck, so I don't know exactly what these nodes do. But using vector absolute and vector dot product nodes seems like it might be the solution to my problem. This is the current node setup:

And this is the result. I have to verify if it works like I want it to more thoroughly, but maybe this is the solution!

Also, I updated the title. It's more fitting to what I'm actually trying to find out here now.

Answer 1

I think you're confused about how "auto smooth" works. It doesn't make some parts smooth and some parts flat. Everything is smoothed, but the effect (as Gordon already said) is limited across the edges above the "Auto smooth angle" threshold. This is why one alternative to Auto smooth is using "Split Edges" modifier, and that's how you should think about it.

Take a look at this cylinder with poked flat faces (just for explicit triangulation, not related to the issue) and then bent so now there are still 3 separate surfaces: top, side, bottom, you can still see the top and side, and they are still separated - both are smooth, but (visually) separated:

Now, a shader is run for each point separately. The point somewhere in the middle of an area couldn't possible know, that somewhere there is an edge, and this edge makes a full "circle" on the surface of the mesh, therefore separating the area of this point, out of the rest of the mesh, and the shader of this area should be some specific shader… Consider the case below - the threshold has been increased, so now a few edges no longer stop the flow of the normal:

So are the areas above still separate areas or now they are one area? Assuming the latter, you can use geometry nodes to physically split the edge using the angle threshold, and then assign materials based on island index:

How did I know which material will go where? I didn't. I first named them Top, Side, Bottom, in that order, then I tested, saw it's wrong, and renamed them to be correct. You could improve the geometry nodes setup to somehow sort the mesh islands…

Answer 2

Very similar to Markus' answer .. again..

For a more general definition of what constitutes a 'flat' region .. (all the faces are flat), you could reach for something like 3DS 's 'Smoothing Groups'

This GN modifier splits a copy of the mesh by edge angle, and stores the normalized index of the resulting mesh-islands on face-corners of the original:

The resulting 0-1 attribute could be used in a material:

Separating islands:

Unlike the 3DS case, where groups are manually defined using faces, this is vulnerable to islands not being completely bounded by sharp edges, so won't be the same, but could still have its uses.