fix: correct rig deformation — arms no longer cross body, feet no longer stretch to center

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The model animated but deformed wrong: arms crossed the torso and foot heels
stretched to the model's center-bottom. Two root causes, both fixed:

1. Pose mismatch. Sketchfab models are modelled in an A-pose (arms angled
   down) but the Quaternius library rests in a T-pose. Binding the A-pose mesh
   to T-pose arm bones misweighted the arms, and the library clips (which
   rotate arms ~70° down from the T rest) then shoved the already-drooped arms
   across the body.
   Fix: autorig.fit_arms_to_mesh() reorients each arm bone chain along the
   model's real arm direction; merge_animations retargets every clip from the
   library's T-pose rest onto the fitted rest via world-space constraint bake
   (Copy Rotation/Location + nla.bake), so world motion is reproduced
   regardless of the rest difference.

2. Feet-to-center stretch. The library's deform-flagged `root` bone runs up
   the body centre-line; nearest-bone weighting assigned inner-foot/heel verts
   to it, stretching them to center when animated.
   Fix: exclude root/control/mechanism bones from weighting (_NON_DEFORM_HINTS).

Also upgraded nearest-bone weighting from rigid (1 bone/vert) to a smooth
inverse-distance blend across the nearest 4 bones, so joints deform cleanly.
Bone-heat still fails on Miku's joined hair/clothing mesh, so this is the
primary path.

Verified by rendering Idle/Walk/Run poses (Blender): natural stance, arms at
sides swinging correctly, feet planted, proper running stride. Smoke test
still 30/30.

Co-Authored-By: Claude Fable 5 <[email protected]>

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This commit is contained in:

Nicholas Butzke

2026-07-06 19:38:59 -04:00

co-authored by Claude Fable 5

parent 22cb0a58b7

commit 9939e7e524

4 changed files with 179 additions and 30 deletions

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assets/characters/skins/miku_test.glb

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