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crosstyan ab728c7e9a fix: use shortest-path rotation interpolation in playback

Switch viewer playback from Magnum Math::slerp() to Math::slerpShortestPath() when interpolating adjacent OpenSim frame orientations.

Why:

- Adjacent OpenSim quaternions can cross sign while representing nearly identical orientations.

- Non-shortest-path interpolation can create artificial long-arc spins between valid sampled poses.

- That makes playback exaggerate or invent visible bone flips that are not present in the sampled frame states.

What changed:

- Updated playback interpolation in src/ViewerApp.cpp to use shortest-path quaternion slerp.

- Added docs/motion-troubleshooting.md documenting the distinction between viewer interpolation artifacts and upstream IK discontinuities.

- Added a README pointer to the troubleshooting note.

Investigation log:

- Verified the viewer loads .mot through OpenSim state storage and renders PhysicalFrame transforms directly.

- Reproduced the target Sports2D/Pose2Sim/OpenSim clip and confirmed the .mot already contains large coordinate discontinuities and limit clamping, indicating upstream IK failure.

- Confirmed the viewer also had a separate interpolation issue due to non-shortest-path quaternion slerp.

Validation:

- Rebuilt with: cmake --build build -j

- Relaunched the viewer successfully against the problematic .osim/.mot pair after the fix.

2026-03-11 11:54:41 +08:00

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Motion Troubleshooting

Bone Flips During Playback

The viewer loads the .mot file through OpenSim, converts it to model states, and renders each body using PhysicalFrame::getTransformInGround(). It does not solve IK, rebuild bones from markers, or apply any custom joint logic.

Because of that, a visible flip can come from two different places:

The upstream motion data already contains a discontinuity.
Playback interpolation between two valid sampled poses introduces an artificial long-arc rotation.

Viewer Behavior

Playback now uses shortest-path quaternion interpolation between sampled OpenSim poses. This avoids false in-between spins when adjacent quaternions have opposite signs but represent nearly the same orientation.

If a flip is still visible after this fix, the sampled OpenSim motion itself is already discontinuous.

Upstream IK Failures

For Sports2D / Pose2Sim / OpenSim pipelines, broken IK usually shows up as one or more of the following:

sudden large frame-to-frame jumps in joint coordinates
joint values snapping to model limits
marker RMS staying low while anatomically implausible body orientations appear
left/right ambiguity or bad depth reconstruction in the source TRC

Sports2D ultimately delegates IK generation to Pose2Sim, which then calls opensim.InverseKinematicsTool(...). If the generated _ik.mot already contains large discontinuities, the viewer is only displaying that failure.

Practical Check

To separate viewer issues from upstream IK issues:

Load the .osim and .mot in the viewer.
Scrub near the suspicious frame while paused.
If the sampled pose itself is wrong, the IK output is broken upstream.
If only the in-between motion looks wrong, interpolation is the likely cause.

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