How do I assign materials to vertex groups in Python

Question

I have an object with vertex groups already set up and materials already added. I want to run through all of vertex groups in that object and randomly assign one of the materials to it. How do I do this? I can run through the vertex groups but I'm struggling to assign the material to it.

Thanks

You can't assign materials to vertex, materials go into faces only, and vertex groups don't unequivocally define mutually exclusive groups of faces. See https://blender.stackexchange.com/questions/51296/is-there-a-way-to-save-selection-by-faces/ — Duarte Farrajota Ramos, Oct 07 '22 at 09:03
Thank you! At least that explains why I couldn't work it out!! :) — SaintPaulie, Oct 07 '22 at 09:05
You might be able to convert a vertex group into a selection of faces, but there will likely be some overlap — Duarte Farrajota Ramos, Oct 07 '22 at 09:14

Markus von Broady · Accepted Answer · 2022-10-07T09:36:04.030

A vertex group, as the name says, is a group of vertices. A material is assigned to a face, not to a group. So how do you translate between the two? This will cause issues in a moment.

How to count the number of faces of each vertex group?

I think the most reasonable approach is the "select a face if all its vertices are selected" - it's also intuitive because that's how selection in Blender works. So I'm using the Bmesh - All Corners solution. First let's prepare Suzanne for make-up:

Whoops! You can't select lips without selecting the mouth as well. Again, this will cause a problem in a moment. Also, notice how the vertex groups and the material slots appear in the same order - this is needed, because the script will operate on indices rather than names - bmesh doesn't even store the names (neither does a mesh, only an object stores this information, to my knowledge).

import bpy, bmesh
from bpy import context as C, data as D
from collections import Counter
me = C.object.data
bm = bmesh.from_edit_mesh(me)
layer = bm.verts.layers.deform.active
for f in bm.faces:
    groups = (v[layer].keys() for v in f.verts)
    groups_set = set(next(groups))
    for g in groups:
        groups_set.intersection_update(g)
    if groups_set:
        f.material_index = groups_set.pop() + 1
    else:
        f.material_index = 0

Whoops! The entire mouth (the inside) has the lip color. Or at least for me this is the case, since I'm just popping from a set, I'm getting the first vertex group found for a given face - Lips is indexed before Mouth so it was found first, and is popped first. However, fixing this by swapping both vertex groups would be a mistake - while current CPython implementation is deterministic in this regard, there's no formal guarantee that this will always be so:

A set is an unordered collection

source

Instead, just explicitly prioritize the index of the Mouth vertex group:

import bpy, bmesh
from bpy import context as C, data as D
from collections import Counter
me = C.object.data
bm = bmesh.from_edit_mesh(me)
layer = bm.verts.layers.deform.active
for f in bm.faces:
    groups = (v[layer].keys() for v in f.verts)
    groups_set = set(next(groups))
    for g in groups:
        groups_set.intersection_update(g)
    if 3 in groups_set:  # prioritize Mouth
        f.material_index = 4
    elif groups_set:
        f.material_index = groups_set.pop() + 1
    else:
        f.material_index = 0

How do I assign materials to vertex groups in Python

1 Answers1