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I've had a hard time to figure out why I couldn't set a different material for a given face on a mesh used by two objects (duplicate by AltD).

As soon as I was assigning a material to a face on one object, the face was also changed on the other object, in spite the material was 'object-linked'.

enter image description here

I initially set the material slots exactly the same for the two objects, including materials used by both objects. What I realized (and the documentation doesn't help) is that one doesn't assign a material to a face, one rather assign the material slot. This slight difference has indeed a lot of consequences.

The solution was to set different materials to the same slot in each object and not changing the material-face assignment.

Slots have a taste of C pointers for the mass... whatever, this leaves me with more questions:

  • Is that correct the slots are the actual entities which have the 'storage location' attribute and are stored either at mesh ('data') level or at 'object' level?

  • Is that correct slots (not materials) are assigned to faces, and in the case of object duplicates sharing the same mesh, the face-slot assignment done with the 'Assign' button is shared between duplicates?

  • Is it true that a face is assigned by reference to the absolute position of a slot in the list? The evidence is using the up/down arrow of the slot list in the Material tab, to change the order materials appear, actually changes which material is assigned to a face .

That's the general idea, do I miss an important point regarding the use of the material slots? Any reference to good documentation to go further appreciated, especially a description of the model.

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  • $\begingroup$ When you look at the mesh name you see the number 2. 2 users means sharing. $\endgroup$
    – atomicbezierslinger
    Commented Aug 24, 2018 at 18:13
  • $\begingroup$ A face has an integer material index property.. Will be the active material slot index when set from UI. $\endgroup$
    – batFINGER
    Commented Aug 24, 2018 at 21:31
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    $\begingroup$ blender.stackexchange.com/questions/75816/… $\endgroup$
    – Duarte Farrajota Ramos ♦
    Commented Aug 25, 2018 at 1:44
  • $\begingroup$ @DuarteFarrajotaRamos: Your answer to the linked question would be valid here too. Very educative. $\endgroup$
    – mins
    Commented Aug 25, 2018 at 19:58

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enter image description here

Questions

  • Q1. Slots can be stored at Object or Mesh level to suit your needs.

  • Q2. Slots color faces. Slots at the Mesh Data level can be shared.

  • Q3. I do not believe your statement is true in all cases. I reordered slots and did not see same result as stated in your question.

  • Your probably know this. You can switch to shared data by choosing the existing data item. You can stop sharing by clicking on the number which is greater than 1 in the mesh data panel, seen in the image below. The image above shows Duplicate and Duplicate Linked. The User Preferences can also fine tune these settings. A Blender Material contains a sequence of slots which can be assigned to faces.

enter image description here

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