Why Aren't Weapon Bones Animating Along with the Character in RealityKit?

Below is some guidance for troubleshooting this issue:

Check the Rig & Skinning

RealityKit applies its animations to entities that have skinning information (bone–mesh binding) baked into the USDZ file. In your Blender file, even though you see the attachments follow the character in a Blender preview, verify that the weapons’ bones are truly part of the character’s skinned hierarchy when exported. It’s possible that the weapons are parented or linked differently so that the skinning data isn’t being exported properly.

• Ensure Weapons Are Attached as Skinned Meshes:
  Make sure that the sword and shield are not separate objects with independent transforms. They should be part of the same skeletal hierarchy so that their transforms are driven by the animated joints.

• Bake the Animation:
  Some export workflows require baking the animation so that every transformation is captured in the USDZ file. Consider baking the animation in Blender prior to export.

USDZ Export Settings

USDZ export from Blender (or with an intermediary conversion tool) sometimes needs careful configuration:

• Skinning Data:
  Confirm that you’re including proper skinning data in the USDZ export options. This ensures bone influences are carried over into RealityKit.

• Hierarchy Preservation:
  Verify that the hierarchy (character → bones → attached objects) is maintained. If the weapons are only “linked” but not parented to the specific bones, they may not animate in RealityKit.

RealityKit Considerations

When you use the following code to play the animation, you are essentially requesting the animation resource associated with the character’s node:

SWIFTCopied
private func loadAnimations() {
    let unifiedAnimations = children[0].availableAnimations.first!.definition
    let animationResource = try! AnimationResource.generate(with: unifiedAnimations)
    self.children[0].playAnimation(
        animationResource.repeat()
    )
}

Ensure that the animated entity (children[0]) is indeed the root of the entire skinned hierarchy. If the weapons are attached elsewhere in the scene graph, their transforms might not be updated by the character’s animation.

Workarounds

If after verifying the export settings you still experience issues:

• Manually Reattach the Weapons:
  Consider reparenting the sword and shield to the corresponding bone nodes on the character entity within RealityKit. Although this may involve reading joint transforms (or constraints) at runtime, it gives you fine control over the attachments.

• Merge Meshes:
  If possible, merging the weapons with the character’s mesh (and ensuring the weights are correct) can simplify the skeletal animation and guarantee that all parts animate together.

Final Thoughts

The issue likely stems from a mismatch between how the Blender rig (with its attached weapons) is being exported to USDZ and how RealityKit interprets the skinning data. Double-check the export settings and hierarchy structure in your Blender file, and if needed, adjust your RealityKit scene graph so that the weapons are moved by the corresponding bone transforms.

Feel free to ask more questions if you need further clarification on the animation pipeline or export settings in RealityKit.