Ok, I have several models that because various joining and un joining, have material slots that are there, but not assigned to any faces. Im exporting and would like these gone but there are too many to delete by hand:
Is there a script/way to get materials not assigned to a face to be eliminated?
Another method
How to get indices of the faces to which a material has been applied, using Python?
If not assigned a slot, or slot is empty assign to material "None". Make sure there is no material named "None" in file
If more than one slot has same material it merges them
Will end up with a dictionary of materials by name to corresponding list of face indices, and those assigned to None.
Note if a material index is found on a face that is outside the range of slots it is also assigned to "None".
Make sure the slots list is the same length.
Reassign slots and faces materials to dictionary values.
Test script. As pointed out by @Ratt always test on a copy, or don't save until happy with result. Simplifies material slot list of context object to those used, and an empty slot for outliers.
from collections import defaultdict
import bpy
context = bpy.context
ob = context.object
me = ob.data
slots = ob.material_slots
def getmname(idx):
if idx < len(slots):
name = slots[idx].name
name = name if name else "None"
else:
name = "None"
return name
matdic = defaultdict(list)
for f in me.polygons:
matdic[getmname(f.material_index)].append(f.index)
while len(slots) > len(matdic):
ob.active_material_index = 1
bpy.ops.object.material_slot_remove()
while len(slots) < len(matdic):
bpy.ops.object.material_slot_add()
for i, (name, faces) in enumerate(matdic.items()):
slots[i].material = bpy.data.materials.get(name)
for f in faces:
me.polygons[f].material_index = i
for index, s in enumerated(curve_object.data.splines) for the case of curves.
$\endgroup$
Commented
Oct 6, 2020 at 20:11
For future landers. This works on selected objects and its using new APIs so 2.80+ only.
import bpy
objs = [obj for obj in bpy.context.selected_objects if obj.type == 'MESH']
for obj in objs:
bpy.context.view_layer.objects.active = obj
bpy.ops.object.material_slot_remove_unused()
Here's a method per request that doesn't involve using an operator. It will remove all unused material slots from all mesh objects in the file.
import bpy
for obj in bpy.data.objects:
material_indices = set()
if not obj.material_slots or not obj.type == "MESH":
continue
for f in obj.data.polygons:
material_indices.add(f.material_index)
for i in range(len(obj.material_slots) - 1, -1, -1):
if i not in material_indices:
obj.data.materials.pop(index=i)
Thanks to this thread over on Blenderartists.
Is there a script/way to get materials not assigned to a face to be eliminated?
I guess you're open to non-scripting solutions, which are simpler for anybody else finding this page in the future. (I'm sure that your problem is long solved.) Click on the materials specials dropdown and select "Removed Unused Slots".
This feature hasn't always been around, not natively, and may not have existed in 2019. It was implemented from the "Materials Extras" addon, distributed with Blender, which I believe has existed for a long, long time.
I would recommend testing this on a copy of your file first but based on my limited testing the following script should work for Blender 2.7x. Note this has to check every polygon of every mesh based on your description and may take some time. Script removes material slots which are not assigned to any polygons of a mesh. If a specific material is not assigned to any polygon of any mesh it will become orphan data and removed automatically on file save.
import bpy
objs = [obj for obj in bpy.data.objects if obj.type == 'MESH']
for obj in objs:
mats = obj.material_slots
used_slots = []
for poly in obj.data.polygons:
if poly.material_index not in used_slots:
used_slots.append(poly.material_index)
for i, mat in reversed(list(enumerate((mats)))):
if i not in used_slots:
obj.active_material_index = i
bpy.ops.object.material_slot_remove()
