GLTF import alters scene hierarchy for skinned items

[Mondkalb1]

Describe the bug
When importing a GLTF file in Blender, it forces all objects that are skinned to a specific Armature to be children of that Armature in the hierarchy.

Contextual image:
The left side shows the scene how it was constructed in blender natively.
The right side shows that same scene after being saved to gltf, and then imported again into a new empty scene in Blender.

To Reproduce
Steps to reproduce the behavior:

1. Download the attached cubeBones.zip and extract it
2. Open the cubeBones.blend file in Blender
3. Observe the hierarchy. We have "Cube" under LOD_0 node, and "Cube.001" under LOD_GEO node.
4. Export this complete scene as .gltf
5. Clear the workspace so that you have an empty scene without default cube
6. Import that newly saved .gltf file
7. Observe the hierarchy. We now have both "Cube" and "Cube.001" under the "skel" armature. They don't belong there.

Expected behavior
The scene hierarchy of the objects shall be preserved. They must not be force-moved to be in the hierarchy under the skeleton, but remain where they were.

.blend file/ .gltf
cubeBones.zip

Version

• OS: Win 10
• Blender Version 3.1.0

Additional context
This may be by convention and design, as we noticed the FBX import does this also. However, it is critical for us that the actual structure is maintained. We were also able to determine that the gltf export accurately writes this hierarchy correctly, only during the import into blender does this reshuffling happen.

Ideally this would be an optional boolean toggle to disable this default behaviour.

[julienduroure]

Hello,
There is no easy answer.
Current way of working, with scene hierarchy change, is an intentional behaviour, trying ot follow both guidelines.
glTF and Blender are not using the 100% same convention for skinned mesh.

• Blender should have skinned mesh as children of armature (this is the reason why importer do it, to follow Blender guidelines)
• glTF should have mesh at root of scene. This is a warning, not a blocking error. Currently, this is not done at export, but there are discussion about it on Skin/armature export produces NODE_SKINNED_MESH_NON_ROOT #1626 and Fix #1626 Set skinned mesh at root #1627

[Mondkalb1]

@julienduroure Thank you for the insight. I had a suspicion it would be by convention.

We may look into a small modification ourselves then, would imp\gltf2_blender_node.py be the place to start looking? Any recommendations/suggestions? Thanks again.

If we get anywhere we'll shoot a PR back for review.

[scurest]

This is done for correctness. The main reason is it achieves the glTF requirement that only the joints in its skin (not the node where it is instantiated) affects the transform of a skinned mesh. The notion of a skinned mesh being a child of LOD_0 and affected by LOD_0's transforms does not exist in glTF.

The place where it is actually moved is move_skinned_meshes. node.py and mesh.py files also currently assume an arma will have been created before the mesh it affects, and mesh.py skins meshes into its arma space on the assumption it will be a child of the arma.

[julienduroure]

No issue here, not sure there is something that can be done. Closing. Don't hesitate to comment if you want to add some extra infos.