VRM1: Missing first thumb bone in the generated IK Rigs' Retarget Chains

[andreiagmu]

Unreal Engine v5.4.4
VRM4U v20241007

Describe the bug
After importing a VRM1 avatar, I noticed that in all of the generated IK Rigs (Mannequin, MannequinBone, VrmHumanoid, UEFN), the first thumb bone (on both thumbs) isn't part of the thumbs' Retarget Chains.
In VRM0, the first thumb bone is included in the thumbs' Retarget Chains, in all of the generated IK Rigs.

Is there any reason for this change in VRM1?
Based on my animation retargeting experiments, the retargeted thumbs look unnatural when the first thumb bone is excluded from the retargeting.
After I add it again to the Retarget Chains (just like in VRM0), the thumbs look more natural in the retargeted animations.

VRM0 IK Retargeter

VRM1 IK Retargeter

VRM0 IK Rig (Mannequin)

VRM1 IK Rig (Mannequin)

[andreiagmu]

Here are screenshots of some retargeted animations.

On each screenshot pair, the first image has the first thumb bone excluded from the Retarget Chain (the current behavior from VRM1 import), and the second image has the bone added again to the Retarget Chain (just like in VRM0 import).

In my opinion, the thumbs look more natural when the first thumb bone is added back to the retargeting process.
It's very noticeable in the last animation I display here (Anim_KA_Idle08_ComeUpWithAnIdea, from Kawaii Animations 100).

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Reference pose - from the VRM4U IK Retargeter (generated on avatar import)

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MF_Idle - from UE5 Third-Person Template
(ignore the thumb clipping into the clothes)

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MM_Walk_InPlace - from UE5 Third-Person Template

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Anim_KA_Idle08_ComeUpWithAnIdea - from Kawaii Animations 100
(when the first thumb bone is excluded from the Retarget Chain, the retargeted thumb goes to the avatar's mouth)

[ruyo]

Hi.
Thanks for the report. This will be fixed in the next release.

[andreiagmu]

The thumb bone Retarget Chains have been fixed in the IK_avatarName_Mannequin and IK_UEFN_avatarName_Mannequin IK Rigs, thanks!

But the IK_avatarName_MannequinBone and IK_avatarName_VrmHumanoid IK Rigs are still inconsistent with their VRM0 equivalents.

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In the following screenshot pairs, the first image shows the VRM0 IK Rig, and the second image shows the VRM1 IK Rig.

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IK_avatarName_MannequinBone

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IK_avatarName_VrmHumanoid