MultiMaterial UNDO [ solved ]

#1

I append new material to multi-material, but after undoing this operation all faces that initially used ID#1 material now suddenly seen as black in viewports and still look correctly on render.
Is there anything I can do to fix or prevent that?

Here’s the code that demonstrates the issue



	delete objects
	gc()

	b1 = convertToMesh (box lengthsegs:1 widthsegs:1 heightsegs:1)
	b2 = box lengthsegs:1 widthsegs:1 heightsegs:1 pos:[40,0,0]
	addModifier b2 (Materialmodifier materialID:2)
	meshop.attach b1 b2

	mm = Multimaterial numsubs:1
	mm.materiallist[1].diffuse = yellow
	b1.material = mm

	redrawViews() -- both first and second boxes use first material
	sleep 0.5


	mm.numsubs += 1 -- add yet another material for ID 2
	redrawViews() -- second box uses second mtl as expected

	sleep 0.5
	max undo -- emulate user undo

	-- faces with ID 2 now ignore ID 1 material and seen as black in viewports, but render correctly
gif

gwrIEHSLOk
gwrIEHSLOk.gif693x648 28.3 KB

One thing I found to ‘fix’ it is to save & reload the scene, but that’s not a solution…
Another one is

replaceInstances mm (copy mm)
#2 
b1 = convertToMesh (box lengthsegs:1 widthsegs:1 heightsegs:1)
b2 = box lengthsegs:1 widthsegs:1 heightsegs:1 pos:[40,0,0]
addModifier b1 (Materialmodifier materialID:1)
addModifier b2 (Materialmodifier materialID:2)
converttomesh b1
converttomesh b2
meshop.attach b1 b2

this makes all undoable.

It is unsafe (from my practice point of view) to use a multi-material with fewer sub-materials than the material IDs used in a mesh object. I see my old scripts in the archive that check this situation and fix it in two ways: a) - fix the multi-material; or b) - fix material IDs.
There is also another “cue” to solve this problem - when you attach meshes, you also have the option of choosing what you should do with material IDs - mapping materials to IDs or IDs to material. In any case, the problem should be resolved.

#3

I checked it in 2014 & 2020 and the issue still remains.
Undoing mm.numsubs += 1 makes object black in both.

gif

Y4MOBN31nK
Y4MOBN31nK.gif1327x576 168 KB

I’m not always in control of what kind of geometry it is to condense mesh IDs to match multimat. Or even if I could do something object may have stack of modifiers that make it impossible
.
What I need is an undoable append/delete for multi-material like the one you have in ‘Assign Material to Selection’ when you assign materials to subobj selection.
.
seems like wrapping it with an undo context solves it

undo on
(
	mm.numsubs += 1	
	replaceInstances mm (copy mm)
)