USD: preserve bone lengths on skeleton IO #116615

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Michael Kowalski · 2023-12-29T00:08:03+01:00

Michael Kowalski commented

2023-12-29 00:08:03 +01:00

This issue was raised during the review for USD skeleton export pull request: #111931 (comment)

The underlying problem is that exporting bones from Blender to USD does not preserve the original bone lengths. This is because USD skeletons are composed of joints, not bones, and the joints don't encode length. When importing the skeleton back into Blender, the bone lengths often appear incorrect.

Currently, the skeleton import code tries to guess what the bone length should be based on distance from parent bone to child bone. For leaf joints, the guess is based on additional heuristics, such as the average of the estimated lengths for non-leaf bones computed for the entire skeleton. In the case where there is a single bone in the skeleton, the bone is assigned a default length of 1.0.

This goal of this design task is to provide a way to preserve the original bone lengths in the USD skeleton. The following two approaches have been suggested so far:

@mont29 suggested adding leaf bones on export:

This is similar to the FBX handling of skeletons... In FBX exporter there is the Add Leaf Bones option (enabled by default), which adds an extra joint at the tip of each Blender leaf bone. The importer has a matching Ignore Leaf Bones (although disabled by default), to remove these on import. These extra leaves are always fully non-deforming joints. Could this be considered as an USD-valid solution as well? Or would that go against USD standard and/or common expectations from other DCCs?

Alternatively, on export we could record the original bone lengths as a custom property on the USD skeleton, and these lengths could be restored when importing the skeleton back into Blender.

This issue was raised during the review for USD skeleton export pull request: https://projects.blender.org/blender/blender/pulls/111931#issuecomment-1088602 The underlying problem is that exporting bones from Blender to USD does not preserve the original bone lengths. This is because USD skeletons are composed of joints, not bones, and the joints don't encode length. When importing the skeleton back into Blender, the bone lengths often appear incorrect. Currently, the skeleton import code tries to guess what the bone length should be based on distance from parent bone to child bone. For leaf joints, the guess is based on additional heuristics, such as the average of the estimated lengths for non-leaf bones computed for the entire skeleton. In the case where there is a single bone in the skeleton, the bone is assigned a default length of 1.0. This goal of this design task is to provide a way to preserve the original bone lengths in the USD skeleton. The following two approaches have been suggested so far: @mont29 suggested adding **leaf bones** on export: > This is similar to the FBX handling of skeletons... In FBX exporter there is the Add Leaf Bones option (enabled by default), which adds an extra joint at the tip of each Blender leaf bone. The importer has a matching Ignore Leaf Bones (although disabled by default), to remove these on import. These extra leaves are always fully non-deforming joints. Could this be considered as an USD-valid solution as well? Or would that go against USD standard and/or common expectations from other DCCs? Alternatively, on export we could record the original bone lengths as a **custom property** on the USD skeleton, and these lengths could be restored when importing the skeleton back into Blender.

Michael Kowalski added the

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USD: preserve bone lengths on skeleton IO #116615