crosstyan
ab728c7e9a
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.
46 lines
1.7 KiB
Markdown
46 lines
1.7 KiB
Markdown
# Motion Troubleshooting
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## Bone Flips During Playback
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The viewer loads the `.mot` file through OpenSim, converts it to model states,
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and renders each body using `PhysicalFrame::getTransformInGround()`. It does
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not solve IK, rebuild bones from markers, or apply any custom joint logic.
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Because of that, a visible flip can come from two different places:
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1. The upstream motion data already contains a discontinuity.
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2. Playback interpolation between two valid sampled poses introduces an
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artificial long-arc rotation.
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## Viewer Behavior
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Playback now uses shortest-path quaternion interpolation between sampled OpenSim
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poses. This avoids false in-between spins when adjacent quaternions have
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opposite signs but represent nearly the same orientation.
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If a flip is still visible after this fix, the sampled OpenSim motion itself is
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already discontinuous.
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## Upstream IK Failures
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For Sports2D / Pose2Sim / OpenSim pipelines, broken IK usually shows up as one
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or more of the following:
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- sudden large frame-to-frame jumps in joint coordinates
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- joint values snapping to model limits
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- marker RMS staying low while anatomically implausible body orientations appear
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- left/right ambiguity or bad depth reconstruction in the source TRC
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Sports2D ultimately delegates IK generation to Pose2Sim, which then calls
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`opensim.InverseKinematicsTool(...)`. If the generated `_ik.mot` already
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contains large discontinuities, the viewer is only displaying that failure.
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## Practical Check
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To separate viewer issues from upstream IK issues:
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1. Load the `.osim` and `.mot` in the viewer.
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2. Scrub near the suspicious frame while paused.
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3. If the sampled pose itself is wrong, the IK output is broken upstream.
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4. If only the in-between motion looks wrong, interpolation is the likely cause.
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