delete unused materials on a single object using python

Question

After merging and separating objects, I have 69 materials on a single objects but only 50% of them are used by this object. How can I create a list of unused material per object?

I have tried the solution on the page (https://blender.stackexchange.com/questions/4817/how-to-know-which-object-is-using-a-material) but my problem is that materials are assigned to slots but slots are not used on any face of the mesh. I have to delete those.

Asked By: Student

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Source

Answer 1

A list of materials used by the object can be generated by going through the mesh polygons.

used_mats = []
for f in obj.data.polygons:
    used_mats.append(obj.material_slots[f.material_index].material)

If we use sets we can get a unique list and subtract used materials from listed materials.

import bpy

for obj in [o for o in bpy.data.objects if o.type=='MESH']:
    mat_list = set(ms.material for ms in obj.material_slots)
    used_mats = set(obj.material_slots[f.material_index].material
                     for f in obj.data.polygons)

    unused_mats = mat_list - used_mats

    print(obj.name, unused_mats)

    for ms in obj.material_slots:
        if ms.material in unused_mats:
            ms.material = None

You may also want to go through and remove duplicates of the same material in different slots.

for obj in [o for o in bpy.data.objects if o.type=='MESH']:
    for ms in obj.material_slots:
        for cs in [s for s in obj.material_slots if s != ms]:
            if cs.material == ms.material:
                cs.material = None

While that just clears unused and duplicate slots, there is the step of removing any now unused slots. There is bpy.ops.object.material_slot_remove() which will remove the last slot in the list, unless you find the instance of the UI item and adjust its properties.

Answered By: sambler

Answer 2

This is a new op added in blender 2.9+

import bpy

for ob in bpy.context.selected_objects:
    ob= bpy.context.active_object
    bpy.ops.object.material_slot_remove_unused()

Then save the file and reopen

Answered By: Fahad Hasan Pathik

delete unused materials on a single object using python

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