VR-Mess/research/vr-arm-ik-deep.md

Research: arm IK stability, elbow poles, torso yaw, overextension (follow-up)

Second research pass for VR Messs, after v1.1 of the placeholder rig showed: residual shaking when the arm tugs the body near (but not at) full extension, twitchy elbows driven by wrist rotation, and pure-HMD chest yaw. Label: [S] = sourced, [I] = inference. See also vr-arm-ik.md (prior art) and vr-grabbing.md (grab architecture).

1. Full-extension elbow jitter = the classic 2-bone singularity

• [S] At cos(±1) the elbow solution is bistable and tiny target noise flips it (robotics "elbow singularity"; FINAL IK threads confirm the "elbow pops to fully straight" symptom: https://discussions.unity.com/t/final-ik-full-body-ik-aim-look-at-fabrik-ccd-ik-1-0-released/525769).
• [S] The established fix is SOFT IK: clamp the effective target distance to a fraction of chain length (recommended 0.98) and ease over the last percent of reach (MSMotion docs "Soft IK Ratio": https://maharajastudiocustomerservice-cell.github.io/MaharajaStudioWeb/ms-motion-docs.html; Blender soft-IK threads: https://blenderartists.org).
• [S] VRIK docs: elbow bones should be slightly bent in their natural bend direction — production solvers NEVER allow the perfectly-straight configuration (http://www.root-motion.com/finalikdox/html/page16.html).
• [I] Pole smoothing alone does NOT fix a singular solve — the pole is undefined exactly at full extension.
• Recommendation applied/usable: clamp shoulder→hand distance to reach×0.98 before the law-of-cosines solve; keep ~5–8° minimum elbow bend; slerp the elbow over 2–3 frames; hysteresis 0.97/0.99 on the straight-arm state so the transition can't oscillate.

2. Elbow direction (pole) heuristics

• [S] VRIK does NOT use hand rotation as the primary pole driver — it samples the avatar's start pose for the natural bend direction, then uses twist weights (shoulderTwistWeight etc.) and optional bend goals (docs above; Resonite VRIK mirror: https://wiki.resonite.com/Component:VRIK).
• [S] Community practice for aiming: a bend goal at the elbow's down/back/outward position as a SOFT bias (Unreal forums on driving the middle joint from hand orientation: https://forums.unrealengine.com).
• [I] Controller-rotation-driven poles (ours) are common in physics-hands recreations but are the twitchiest variant; Boneworks-style arms derive elbow direction mostly from body-relative geometry.
• Recommendation: build the pole in a stable torso frame ("down + back + outward"), PROJECT OUT the shoulder→hand axis so wrist spin can't rotate it, add hand-forward only as a small weight (~0.2–0.3) that ramps in when aiming, and exponentially smooth the final pole; never let a blend weight cross zero (flip risk).

3. Chest/torso yaw from HMD + hands

• [S] Academic prior art: Ponton et al. 2022, "Combining Motion Matching and Orientation Prediction to Animate Avatars for Consumer-Grade VR Devices" (https://arxiv.org/abs/2209.11478) — HMD-forward alone has 14.5° mean body-orientation error vs 5.4° for a learned predictor; AvatarPoser (https://arxiv.org/abs/2107.04044), HMD-NeMo (https://arxiv.org/html/2308.11261v1), MirrorPoser are the NN lineage.
• [S] Classic game heuristic: torso yaw from the controller-to-controller line blended with HMD yaw (https://forums.unrealengine.com).
• [S] Ponton et al. smooth predicted orientation with a spring-damper/slerp and DECOUPLE orientation from the head (never hard-blended against head yaw).
• Recommendation (non-NN): yaw = HMD yaw base blended toward a hands-derived yaw with weight proportional to how far forward/raised the hands are (0 idle → ~0.6 aiming), applied through a critically damped spring with ~150–250 ms response.

4. Overextension with physics hands

• [S] VRIF-style frameworks render an "arm stretch" but clamp within a limit; soft IK is the standard clamp (VRIF: https://discussions.unity.com; MSMotion above).
• [S] Boneworks is fully physics-driven: at max reach the arm simply straightens and the whole physics body is pulled/leaning rather than the arm stretching (GDC coverage: https://www.gamedeveloper.com/design/how-classical-game-mechanics-and-physics-converge-in-vr-hit-i-boneworks-i/; https://www.reddit.com/r/StressLevelZero/comments/frucx5/).
• [I] No documented game stretches the visual arm beyond bone length — everyone clamps the target to a sphere around the shoulder; the physics hand continuing past reach reads fine because the straight arm still points at it.
• Recommendation: clamp the VISUAL hand target to a sphere around the shoulder (~98% reach, soft-eased) while the physics hand body keeps moving freely; the reach-budget torso shift covers the remainder.

Applied to VR Messs (BodyRig.cs)

Soft IK factor 0.98; pole projected in torso frame with hand weight 0.25 and exponential smoothing; torso yaw spring (HMD base + hands-derived yaw × forwardness, exp smoothing ~12/s); reach-budget hysteresis unchanged. Next dial if shaking persists: SoftIKFactor → 0.95.