Active material slot

Question

I have 200 material slots. I know how to iterate through them, but I cannot make them active.

for o in bpy.data.objects:
    for m in o.material_slots:
        if "screen" in m.name:
            bpy.ops.mesh.select_similar(type='MATERIAL', threshold=0.01)

But I cannot execute the last operator, as I cannot make them active material.

I have to just select similar materials by name and separate them as a new objects. Unfortunately, I cannot way of selecting textures by name, so I cannot execute select_similar and separate operators.

To be more precise, I have a .obj file. It contains 200 materials. I should separate it into 2 groups, group 1 is wall, and group 2 is furniture. I am able to iterate through material_slots and find certain materials which I need. But I cannot select them in edit mode. I have every material's name, I should select 20 or 30 of them and separate them into new objects.

Answer 1

Here is a script that checks the material slots of all mesh objects. If there is the word screen in the material's name then it selects the vertices of this material and checks the next material. After the check, the selection is separated into a new object if there were matches.

I think what you were actually looking for are lines 31 and 32:

31:         bpy.context.object.active_material_index = idx
32:         bpy.ops.object.material_slot_select()

but you also need to select the object to have the correct context (line 21) and do a few other safety checks.

The complete script looks like this:

import bpy
save objects because the operation 'Separate Selection' creates new objects
objects = set(obj for obj in bpy.data.objects)
ensure we are in object mode
if bpy.context.active_object.mode != 'OBJECT':
    bpy.ops.object.mode_set(mode='OBJECT', toggle=False)
clear object selection so the context is correct
bpy.ops.object.select_all(action='DESELECT')
for obj in objects:

    if obj.type != 'MESH':
        print(f"Skipping {obj.name} of type {obj.type}")
        continue
print(f"Checking materials of {obj.name} ")

# select object, switch to edit mode & clear vertex selection
bpy.context.view_layer.objects.active = obj
bpy.ops.object.editmode_toggle()
bpy.ops.mesh.select_all(action='DESELECT')

# check material slots
found = 0
for idx, slot in enumerate(obj.material_slots):
    if "screen" in slot.material.name:              #  <-- materials to look for
        print(f"Found material slot #{idx}: {slot.name}")
        # select the material slot, then select all vertices with this material
        bpy.context.object.active_material_index = idx
        bpy.ops.object.material_slot_select()
        found += 1

# do 'Separate Selection' if we have a selection from at least one match
if found > 0:
    bpy.ops.mesh.separate(type='SELECTED')

# switch back to object mode 
bpy.ops.object.editmode_toggle()

Example output for materials with "Blue" in their name:

Checking materials of Cube
Found material slot #1: Ocean Blue
Found material slot #2: Deep Blue
Found material slot #3: Blue Sky
Checking materials of Cylinder
Skipping Camera of type CAMERA
Skipping Light of type LIGHT
Checking materials of Cube default
Checking materials of Sphere
Found material slot #1: Night Blue Sphere