how to get a mask that only contains the inside of a bowl I am trying to texture a bowl (procedurally) and I want a part of a texture to only work on the inside walls of the bowl and vise versa I tried using the back facing option in the geometry node but that seems to work in opposite directions of the normals (I think) whereas in this bowl I want the texture to be on the same side of the surface just not on the outside walls of the mesh.
basically, I want the circular white color only on the inside walls and not on the outside.
True normal channel
Given a similar face distribution, you can spot a difference between the normal of the faces on the concave side and the ones on the convex side.

Just sort them with a math node:

And exclude the ones who are pointing straight up if you want to get rid of the border.

Obiuvsly this trick works only for a simple vase like the one you posted, for any other model I would use vertex paint: