Weight paint is broken with Armature Multi Modifier (backport request to 3.3 LTS) #86371
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Reference: blender/blender#86371
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System Information
Operating system: Ubuntu 20.10
Graphics card: Intel i915
Blender Version
Broken: 2.83.12 and 2.92.0, 2.93 LTS, 3.3 LTS, 3.4.1
Works: main and 3.5.1
Worked: 2.79
Seems this got "fixed" by
9823a8f72b.Short description of error
When using multiple Armature modifiers with Multi Modifier option Weight Paint mode doesn't work as expected, It doesn't paint vertices I click at and sometimes paints wrong vertices.
Exact steps for others to reproduce the error
Here is the blend file: weightpaint-problem.blend
Steps to recreate it:
Create two armatures, place them to different poses, parent the default cube to one of then with automatic weights. Then add another Armature modifier, set it to the second armature and enable Multi Modifier. Add an empty vertex group and set it to the second armature modifier. Then when I try to paint this new vertex group it paints wrong vertices.
Added subscriber: @upliner
Added subscriber: @lichtwerk
Changed status from 'Needs Triage' to: 'Confirmed'
Can confirm, this was working in 2.79.
Added subscriber: @zhart
Confirm, I have the same problem. Moreover, it is enough to have one armature and simply duplicate the armature modifier and checking the "Multi Modifier" option in the second one.
(This approach may be necessary to disable the "Preserve Volume" option for selected Vertex Group.)
Working to confirm the bug again:
maindoes not exhibit the bug.So some time after Blender 3.3 the bug got fixed.
To anyone watching this issue: do you need this fix in one of the LTS releases?
If so, I'll investigate when the fix happened and back-port. Otherwise this will (at least) be in the next main Blender release, and I can close as fixed.
For me it's sufficient have it fixed it in latest release.
I will check if I can find the commit that fixed this (to see if this can be backported to - at least - 3.3 LTS)
Thanks @lichtwerk (Philipp Oeser)! If you end up not having the time/energy to find and backport it after all, please let me know so I can take over. But otherwise I'll leave it up to you!
Seems this got "fixed" by
9823a8f72b.So 3.5 is also fine.
Since this looks more or less like an "unintentional" fix, I would like this discussed within the
Animation & Riggingmodule if this should/could be backported to 3.3 LTSWeight paint is broken with Armature Multi Modifierto Weight paint is broken with Armature Multi Modifier (backport request to 3.3 LTS)I suspect unless someone specifically asks for a backport, it's probably fine to leave as-is. But I've added it to the Animation & Rigging project board and will ask the module.
Thanks!
I've tested on the releases since 3.3.x. As noted by @lichtwerk above, the fix is in Blender 3.5. It's also fixed in 3.4. So using any release after 3.3.x doesn't exhibit the bug.
Additionally, even in releases affected by the bug, this only seems to happen when the two modifiers use two different armatures, which is both functionally useless (you could instead just combine the armatures into one, and do weight painting as normal) and also not the feature's intended use case.
Backport Decision
We discussed this in the module meeting, and due to the niche nature of this and the probable complexity of backporting, the consensus is to not backport the fix (unless specifically requested by someone who has a real need for it).
Workarounds
Additionally, there are at least two workarounds available to users:
Additional Notes
In additional to everything said above, it's perhaps worth pointing out that most users don't seem use the multi-modifier, or indeed even know what it does or what it's for.
I'm actually an exception: I use the multi-modifier in almost all of my rigs. In fact, I'm the user that originally requested the feature, while working on Big Buck Bunny. So if anyone is blocked by this bug, it would be me. But I've never been affected by this when using the multi-modifier, and as far as I can tell it only manifests in functionally useless (e.g. two armature) configurations.