GLTF Export Animation Wrong Bone Transforms (Rigify)

[InnPatron]

Problem

• Certain area in exported animation are not correct
• Rig generated from Rigify
  • Relevant bone group: "Dress (Upper)"
  • Area generated with skin.basic_chain
  • As far as I can tell, bendy bones in the relavent area have already been disabled (set to 1 segment)
• Uses a custom (but not crazy) export script (game-export.py)
  • export_reset_pose_bones does not appear to affect the issue (#2104)
• Looks like the deform bones in the original animation behave similarly to the exported version. The exported version looks more exaggerated. Maybe this is a math error in the baking calculations? Something to do with parenting?

Export operation:

bpy.ops.export_scene.gltf(
        filepath=exportPath,
        export_format="GLTF_SEPARATE",
        export_bake_animation=True,
        # Does not appear to be the issue?
        export_reset_pose_bones=False,
        use_renderable=True,
        export_def_bones=True,
        export_apply=True,
        export_anim_slide_to_zero=True)

Reproducing

1. Download min-repo.zip and export with export.sh (calls a custom game-export.py script) that outputs to ./export/
2. Import GLTF file and observe around the armpit area (should only be one action: idle-stretch_hannah-rig)

import-annotated.blend has what I see under the ERROR bone group

Minimum reproduction: min-repo.zip

Expected Behavior

Animations should match

Original:

Exported:

Machine Config

• Kubuntu 22.04
• GLTF exporter version: 4.0.44
• Blender from Snapstore
  • Blender 4.0.2
  • Build hash: 9be62e85b727

[InnPatron]

Looks like setting export_hierarchy_flatten_bones=True ~fixes the issue~ reduces the magnitude of the issue, but I am not sure why that is the case.

The rig around that area is pretty simple.

• Trying to remove the retargeted constraint on sleeve.anchor.B.R and sleeve.anchor.B.L didn't affect anything
• Changing the parent of the dress sleve to the spine instead of shoulder didn't affect anything either

[julienduroure]

There are multiple things here:

• Some non uniform scaling can lead to non decomposable matrices, that cannot be written in glTF files. See #1683, for example, for another case of this issue. Using export_hierarchy_flatten_bones=True avoid this issue by flatten the hierarchy of bones at export.
• You have some vertices that are influenced by more than 4 bones. By default at export, only the 4 biggest weights are used. You can change this value, or decide to use all influences at export.

Using these options solve your issues:

Here is repo.blend with reimport of the export, in wire. Seems that everything is ok:

[InnPatron]

Thanks for the reply!

Looks like my export target (Godot) supports 8 max, but I still have too many influences there. I'll rework the rig and bump up the export maximum instead of allowing unlimited influences.