How can I assign material colors to a color attribute?

Question

For the record I'm asking this question because it was brought up by a member recently and deleted under my eyes while I was clicking on the submit answer button, but I think it was a worthy question and I'll share my answer.

The premise was :

I have an object with materials assigned to its material slots. The materials are assigned to the object's polygons. Each material has a simple node tree with a single principled BSDF node with a specific base color.

I want to create a new color attribute on the object's mesh which will assign each polygon with its corresponding material color.

Answer 1

Here's an example script. I'll assume you use V3.6+

1. Create a new color attribute
2. Iterate over all materials and fetch their color
3. Iterate over all polygons and fill out the correct colors into the polygon corners.

Note non-assigned materials or materials with no principled bsdf node will have a black color.

You should be interested in reading bpy.types.Mesh, bpy.types.MeshPolygon, bpy.types.AttributeGroup and bpy.types.Attribute.

import bpy


def main():
    obj = bpy.data.objects.get("House")
    if obj is None or obj.type != "MESH":
        return

    mesh = obj.data   
    color_attribute = mesh.color_attributes.get("Color")
    if not color_attribute:
        color_attribute = mesh.color_attributes.new("Color", "FLOAT_COLOR", "CORNER")

    materials_colors = []
    for material_slot in obj.material_slots:
        material = material_slot.material
        if material is None:
            materials_colors.append((0, 0, 0, 0))
            continue
        principled = next((n for n in material.node_tree.nodes if n.type == "BSDF_PRINCIPLED"), None)
        if principled is None:
            materials_colors.append((0, 0, 0, 0))
            continue
        materials_colors.append(principled.inputs["Base Color"].default_value[:])

    for polygon in mesh.polygons:
        for corner_index in polygon.loop_indices:
            color_attribute.data[corner_index].color = materials_colors[polygon.material_index]


if __name__ == "__main__":
    main()