Weight paint is broken with Armature Multi Modifier (backport request to 3.3 LTS) #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.

**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 9823a8f72be80887898782978012298031466ccf. **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](https://archive.blender.org/developer/F9877829/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.)

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:

Blender 2.83.x LTS is now end-of-life, but does exhibit the bug.
Blender 2.93.17 LTS is still supported, and does exhibit the bug.
Blender 3.3.6 LTS is still supported, and does exhibit the bug.
Latest main does 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.

Working to confirm the bug again: - Blender 2.83.x LTS is now end-of-life, but does exhibit the bug. - Blender 2.93.17 LTS is still supported, and **does** exhibit the bug. - Blender 3.3.6 LTS is still supported, and **does** exhibit the bug. - Latest `main` **does 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 & Rigging module if this should/could be backported to 3.3 LTS

Seems this got "fixed" by 9823a8f72be80887898782978012298031466ccf. So 3.5 is also fine. Since this looks more or less like an "unintentional" fix, I would like this discussed within the `Animation & Rigging` module if this should/could be backported 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 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:

Combine the two armatures into one (as mentioned above) and use a single armature modifier. This is almost certainly a preferable setup anyway, even aside from this bug. Splitting your deformations up between two armatures just adds complexity with no additional deformation capabilities.
Disable all but one of the armature modifiers while weight painting. This is a little annoying since you can't quite see what you're doing on the vertex group that determines the blend between the two modifiers. But with a little patience it's workable, particularly since it's only a single vertex group that you're working with at that stage.

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.

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: 1. Combine the two armatures into one (as mentioned above) and use a single armature modifier. This is almost certainly a preferable setup anyway, even aside from this bug. Splitting your deformations up between two armatures just adds complexity with no additional deformation capabilities. 2. Disable all but one of the armature modifiers while weight painting. This is a little annoying since you can't quite see what you're doing on the vertex group that determines the blend between the two modifiers. But with a little patience it's workable, particularly since it's only a single vertex group that you're working with at that stage. ### 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.

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Weight paint is broken with Armature Multi Modifier (backport request to 3.3 LTS) #86371

Backport Decision

Workarounds

Additional Notes