Armature Modifier: Multi Modifier changes the meaning of the mask vertex group. #103074

New Issue

Alexander Gavrilov · 2022-12-09T16:20:14+01:00

2022-12-09 16:20:14 +01:00

The Armature modifier provides a vertex group option, which allows limiting the area of effect of the modifier.

The modifier also has a Multi Modifier option, which when used on an Armature modifier that immediately follows another Armature modifier makes the subsequent modifier evaluate on top of the same input vertex coordinates as the first one. This effectively makes the modifiers evaluate their deformation in parallel rather than in sequence.

Main Issue

The main issue is that the Multi-Modifier evaluation mode of the modifier interprets the vertex group opposite to the normal mode.

test-armature-mask-mm.blend

{F14046805,size=full}

{F14055443,size=full}

This test has two rigs with a single bone, and a number of meshes weighted to give that bone full influence over the whole mesh. However, they also have vertex group A and B, painted to cover the top and bottom of the mesh correspondingly.

The first two meshes only use a single Armature modifier, using the corresponding rig and mask vertex group. This interprets the group to specify the area affected by the corresponding modifier.

The third mesh uses two modifiers in a Multi-Modifier setup to combine the effect of the two rigs. However, unexpectedly, the vertex group on the second modifier has to be inverted. Effectively, the vertex group in the Multi-Modifier mode specifies the area not affected by the corresponding modifier.

Secondary Issue

However, the worst part is that this difference depends not on the value of the Multi Modifier checkbox, but on whether the modifier is actually evaluated in this mode.

In the last example the first modifier (using rig A with group A) is muted, switching the evaluation of the second one (rig B, inverted group B) to normal mode - and this interprets the vertex group regularly, deforming the top (i.e. inverse of bottom marked by B) using rig B.

Compatibility Concerns

This is easy to fix, but the main concern is backward compatibility.

The most conservative solution is to address only the secondary issue, making the behavior depend purely on the Multi Modifier setting. This only affects cases where the Multi Modifier option is enabled, but doesn't actually work. Versioning could disable the Multi-Modifier option in such cases to upgrade the files in a backward and forward compatible way.

A full fix would make files non forward compatible, and would affect Python code that generates Armature modifier setups in a way that can only be detected by checking the Blender version.

The Armature modifier provides a vertex group option, which allows limiting the area of effect of the modifier. The modifier also has a Multi Modifier option, which when used on an Armature modifier that immediately follows another Armature modifier makes the subsequent modifier evaluate on top of the same input vertex coordinates as the first one. This effectively makes the modifiers evaluate their deformation in parallel rather than in sequence. # Main Issue The main issue is that the Multi-Modifier evaluation mode of the modifier interprets the vertex group opposite to the normal mode. [test-armature-mask-mm.blend](https://archive.blender.org/developer/F14046828/test-armature-mask-mm.blend) {[F14046805](https://archive.blender.org/developer/F14046805/test-armature-mask-mm.png),size=full} {[F14055443](https://archive.blender.org/developer/F14055443/2022-12-13_06.21.35.gif),size=full} This test has two rigs with a single bone, and a number of meshes weighted to give that bone full influence over the whole mesh. However, they also have vertex group A and B, painted to cover the top and bottom of the mesh correspondingly. The first two meshes only use a single Armature modifier, using the corresponding rig and mask vertex group. This interprets the group to specify the area *affected* by the corresponding modifier. The third mesh uses two modifiers in a Multi-Modifier setup to combine the effect of the two rigs. However, unexpectedly, the vertex group on the second modifier has to be inverted. Effectively, the vertex group in the Multi-Modifier mode specifies the area *not affected* by the corresponding modifier. # Secondary Issue However, the worst part is that this difference depends not on the value of the Multi Modifier checkbox, but on whether the modifier is actually evaluated in this mode. In the last example the first modifier (using rig A with group A) is muted, switching the evaluation of the second one (rig B, inverted group B) to normal mode - and this interprets the vertex group regularly, deforming the top (i.e. inverse of bottom marked by B) using rig B. # Compatibility Concerns This is easy to fix, but the main concern is backward compatibility. The most conservative solution is to address only the secondary issue, making the behavior depend purely on the Multi Modifier setting. This only affects cases where the Multi Modifier option is enabled, but doesn't actually work. Versioning could disable the Multi-Modifier option in such cases to upgrade the files in a backward and forward compatible way. A full fix would make files non forward compatible, and would affect Python code that generates Armature modifier setups in a way that can only be detected by checking the Blender version.

Alexander Gavrilov commented

2022-12-09 16:20:14 +01:00

Added subscriber: @angavrilov

Alexander Gavrilov self-assigned this 2022-12-09 16:21:02 +01:00

Alexander Gavrilov commented

2022-12-09 16:21:02 +01:00

Added subscribers: @dr.sybren, @jpbouza-4, @Mets

Alexander Gavrilov commented

2022-12-09 16:21:16 +01:00

Changed status from 'Needs Triage' to: 'Confirmed'

Ivan Cappiello commented

2022-12-09 18:36:04 +01:00

Added subscriber: @icappiello

Brad Clark commented

2022-12-09 21:00:57 +01:00

Added subscriber: @BClark

Sybren A. Stüvel commented

2022-12-13 16:35:56 +01:00

This was discussed in today's module meeting. The conclusion:

Yes, this behaviour seems rather buggy & arbitrary.
The chance of this property being set by Python code (for example in a rig generation script) is expected to be rather low. I did a quick search, and none of the Blender addons in A and blender/blender-addons@C mention the property. Nobody on Blender Stack Exchange asks about it.
There is one question on Blender Stack Exchange about the checkbox (the above link checks for the Python property name), and in the comments you can see that this person had to invert the weight group.

So it seems like a great thing to fix, especially with nice versioning code to adjust existing files.

This was discussed in [today's module meeting](https://devtalk.blender.org/t/2022-12-13-animation-rigging-off-schedule-module-meeting/26875). The conclusion: - Yes, this behaviour seems rather buggy & arbitrary. - The chance of this property being set by Python code (for example in a rig generation script) is expected to be rather low. I did a quick search, and none of the Blender addons in A and blender/blender-addons@C mention the property. Nobody on [Blender Stack Exchange](https://blender.stackexchange.com/search?q=use_multi_modifier) asks about it. - There is [one question on Blender Stack Exchange](https://blender.stackexchange.com/a/140708) about the checkbox (the above link checks for the Python property name), and in the comments you can see that this person had to invert the weight group. So it seems like a great thing to fix, especially with nice versioning code to adjust existing files.

Juan Pablo Bouza commented

2022-12-13 17:33:21 +01:00

The vgroup option being inverted has always been confusing... I can only think of the Big Buck Bunny rigs using this option, hehe, so I guess it won't be too harmful to do the fix!

To tell you the truth, I was never able to understand what this multi-modifier option really does, I only know I have to use it on certain scenarios, like using mesh deform in dynamic mode, but it's really an obscure option to me.

The vgroup option being inverted has always been confusing... I can only think of the Big Buck Bunny rigs using this option, hehe, so I guess it won't be too harmful to do the fix! To tell you the truth, I was never able to understand what this multi-modifier option really does, I only know I have to use it on certain scenarios, like using mesh deform in dynamic mode, but it's really an obscure option to me.

Alexander Gavrilov commented

2022-12-13 18:29:25 +01:00

In #103074#1459716, @jpbouza-4 wrote:
To tell you the truth, I was never able to understand what this multi-modifier option really does, I only know I have to use it on certain scenarios, like using mesh deform in dynamic mode, but it's really an obscure option to me.

It really isn't complicated at all, it is a hack to evaluate armatures in parallel in a linear modifier stack. Here it is expressed as a node mockup (with this bug fixed):

{F14055897,size=full}

Edit: this bug is basically swapping the two vector inputs on the second Mix node, and I suspect it may have been caused by essentially doing that accidentally in the code.

> In #103074#1459716, @jpbouza-4 wrote: > To tell you the truth, I was never able to understand what this multi-modifier option really does, I only know I have to use it on certain scenarios, like using mesh deform in dynamic mode, but it's really an obscure option to me. It really isn't complicated at all, it is a hack to evaluate armatures in parallel in a linear modifier stack. Here it is expressed as a node mockup (with this bug fixed): {[F14055897](https://archive.blender.org/developer/F14055897/multi-modifier.png),size=full} Edit: this bug is basically swapping the two vector inputs on the second Mix node, and I suspect it may have been caused by essentially doing that accidentally in the code.

blender-admin commented

2022-12-15 22:37:14 +01:00

This issue was referenced by ea1c31a244

This issue was referenced by ea1c31a24438442a4fa866d13af1563d76ffa3fc

Alexander Gavrilov commented

2022-12-15 23:19:35 +01:00

I actually searched history and found that this behavior was seemingly introduced in 1199744188, but the reason is still unclear.

I actually searched history and found that this behavior was seemingly introduced in 1199744188, but the reason is still unclear.

Alexander Gavrilov commented

2022-12-23 12:04:44 +01:00

So I made a build for this patch D16787 here for testing: https://builder.blender.org/download/patch/

It would be appreciated if the riggers here could test it and reply.

So I made a build for this patch [D16787](https://archive.blender.org/developer/D16787) here for testing: https://builder.blender.org/download/patch/ It would be appreciated if the riggers here could test it and reply.

Alexander Gavrilov commented

2023-01-12 11:47:21 +01:00

Changed status from 'Confirmed' to: 'Resolved'

Alexander Gavrilov closed this issue

2023-01-12 11:47:21 +01:00

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Armature Modifier: Multi Modifier changes the meaning of the mask vertex group. #103074

Main Issue

Secondary Issue

Compatibility Concerns