Interchange GLTF importer truncates skeletal mesh bone influences to 4 per vertex

cf https://forums.unrealengine.com/t/interchange-gltf-importer-truncates-skeletal-mesh-bone-influences-to-4-per-vertex/2745391/2

Original message

The UE5 Interchange GLTF importer appears to truncate skeletal mesh bone influences to a maximum of 4 influences per vertex, even when the source glTF asset contains additional valid JOINTS_n / WEIGHTS_n attribute sets representing more than four influences.

As a result, importing a glTF skeletal mesh through the Interchange GLTF pipeline can produce a skeletal mesh whose skinning data differs from the source asset, with additional bone influences being silently discarded.

Only the first 4 bone influences per vertex are imported. Additional influences present in the source glTF are discarded, resulting in incorrect skinning compared to the original asset.

Flavien reply

Flavien argue that this is what the spec said but the user said I misread the spec

It is the glTF specification that limits mesh bone influences to 4 per vertex. UE is aligned on the format spec.

Where do you get your glTF from? Are you exporting it from a specific DCC?

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User reply

The glTF spec limits each JOINTS_n / WEIGHTS_n attribute to 4 joints/weights, but it does not necessarily limit a vertex to 4 total influences. Additional influences can be stored in additional attribute sets, for example:

• JOINTS_0 / WEIGHTS_0 → influences 0–3
• JOINTS_1 / WEIGHTS_1 → influences 4–7
• and so on…

The spec explicitly describes this mechanism for vertices influenced by more than four joints.

The asset was exported from Blender 5.2 using its built-in glTF 2.0 exporter.

1. Import the test.glb asset into UE5.
2. Inspect the imported skeletal mesh’s skin weights.

You can see all bones assigned to a vertex in blender: Edit Mode, select the Icosphere and look in the Item meny for the Vertex Weights

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Only 4 in UE

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