Mutable Generating Incorrect Skin Weights in 5.8

General Issues and Bug Reporting (Unreal Engine)
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Summary

Mutable seems to be generating meshes with incorrect skin weights in 5.8, especially around the pelvis/lower stomach/upper thighs. The characters look fine standing still, but any animation affecting those areas such as crouching results in a badly deformed area.

I created a new third person project and tested it with the default Quinn model to confirm no other plugin was causing it. The attached image shows the metahuman from my original project.

The video shows the default Quinn project with the animation MM_Rifle_Jump_Fall_Loop. The actor made with Mutable is on the left. The regular skeleton mesh actor on the right doesn’t have the issue. I also show my Mutable CO file, so I assume this is just a bug that shipped with the 5.8 version of the plugin.

What Type of Bug are you experiencing?

Animation

Steps to Reproduce

Create an actor using a generated mutable asset and attempt to use animations that bends the pelvis/thigh areas.

Expected Result

The mutable actors should animate correctly like their original skeletal mesh models.

Observed Result

The mutable actors have badly deformed regions all around the pelvis.

Affects Versions

5.8

Platform(s)

Windows

Upload an image

2026-07-07 22_38_09-MH58 - Unreal Editor.png
2026-07-07 22_38_09-MH58 - Unreal Editor.png735×755 457 KB

Video

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Hi Rakashal,

Mutable dev here. Could you check whether the number of bone influences in the generated mesh matches that of the original mesh? Mutable clamps and normalizes skin weights to either four (the current default), eight, or twelve influences. This can be configured by adding a DefaultMutable.ini file to the project’s Config folder with the following two lines:

[Features]
CustomizableObjectNumBoneInfluences=Eight

Note: You may have to restart the editor and recompile the CO to see the changes.

Let me know if it helps
Pere

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Ah, turns out it was user error on my part!

I turned on the 16 bit bone weights in the CO file, but somehow skipped the wiki line about having to create the DefaultMutable.ini file and adding the lines to it.

I can now confirm that everything works as expected. I only wish that the forum would have alerted me to the response. I had to manually browse to this thread to see a reply was ever given.

Thanks for the help!

1 Like