In the above picture - there are two cubes with boolean difference operation.
The newly created geometry (created by boolean operation) is red - original geometry is grey. (I changed viewport display of the boolean difference object to wire frame so you can see generated geometry clearly).
If this shape is created with geometry nodes - How to identify or select only the red faces - the generated geometry from boolean operation using geometry nodes only?
Probably one of the best solutions is to use the node Capture Attribute with a boolean value.
With this you select the faces before the node Mesh Boolean, which remain afterwards.
You can then use this selection, for example, to apply a specific material, or similar.
Since the Mesh Boolean node doesn't output if a face is part of the original mesh or the boolean mesh, we need another way:
If you just want to change the material for the new parts, you can just set the Material Index of the boolean mesh sources to the desired material's index:
Result:
If you want to do something else besides setting the material, like extruding, just use the Material Index node to select the faces:
Result:
If you want to set the material, but still know what faces comes from the boolean operation, you can capture the Material Index attribute to use it after changing the Material Index in the mesh:
Important:
For using Material indexes, you need to have the material slots for the indexes in your object. Ex: if you want to use three indexes (0 to 2), you need three slots:
They don't need to be linked to a material.
You can raycast from the modified mesh to the original with a very small ray length:
Result, extruding from raycast selection:
With raycast, you cannot use multiple indexes unless you raycast all the meshes being used in the boolean operation.
