VR-Mess/research/vr-avatar-arm-length.md

Research: avatar arm length vs the player's real arm length (2026-10-02)

Question (owner): get arm length right first time — should the player's body be fitted to the player, or should the player embody an existing (stylized/anime) avatar with its own proportions, VRChat-style? Decide once. [S] = sourced, [I] = inference.

1. What the research says

• [S] The one mismatch that breaks embodiment: the REAL arm reaching full extension while the virtual arm is still bent (virtual arm longer than real, or offset forward). Porssut et al., Reaching articular limits can negatively impact embodiment in VR, PLOS ONE 2022 (https://journals.plos.org/plosone/article?id=10.1371%2Fjournal.pone.0255554): a 5.5 cm offset; ownership fell from ~0.66 to 0.45 and "more than one body" feelings rose when the real elbow hit its limit first; the opposite (avatar arm straight, real arm bent) was tolerated. Advice: "the user never reaches full arm extension by surprise"; if calibration can't be perfect, "choose to have the virtual limbs slightly SHORTER than the real ones". The elbow's own end-stop is "internal haptic feedback" the brain trusts.
• [S] Virtual arm shorter than real → the hand can't reach the controller. Ponton et al., Stretch your reach, CHI 2024 (https://arxiv.org/html/2407.08011, https://dl.acm.org/doi/10.1145/3613904.3642268): avatars scaled uniformly from height left an average 26.3 cm (SD 4.7) reach shortfall; of five strategies, STRETCHING the arm to keep the hand on the controller was best for embodiment, proprioception and preference; a free-floating controller kept location but broke continuity; capping the hand at avatar reach was worst. Rendering the controller doesn't matter. A too-LONG virtual arm (untested there) only bends the elbow — reach is unaffected.
• [S] Fitted skeletons beat uniform scaling. Ponton et al., Fitted avatars: automatic skeleton adjustment for self-avatars in VR, Virtual Reality 2023 (https://arxiv.org/pdf/2307.09558): uniform scaling by eye height "assume[s] … the same ratios between bone lengths as the user" → controller-to-wrist errors; their fitting reduced hand error by 4–14 cm (male) / 2.6–8 cm (female). Method: (1) uniformly scale the avatar by eye-height ratio, (2) resize upper-arm/forearm bones to the measured arm length. Arm length measurement: the user swings both STRAIGHT arms in circles; a sphere fitted to the controller positions gives the SHOULDER JOINT centre (sphere centre) and ARM LENGTH (radius, shoulder→wrist). Neck from head rotations. Embodiment gains appeared "only when the initial avatar proportions differed from the user's" — i.e. exactly the stylized-avatar case. Some avatars needed arms stretched by 12–23 cm.
• [S] Body ownership tolerates large DELIBERATE changes when movement stays synchronous — up to ~3× arm length (Kilteni et al., Very Long Arm Illusion, PLOS ONE 2012, https://journals.plos.org/plosone/article?id=10.1371%2Fjournal.pone.0040867). Proportion is not the problem; a reach/extension mismatch is.
• [S] VRChat measures avatars by ARM SPAN by default (scales so the avatar's arms match yours — hands right, floor/feet often wrong, worse with long-legged anime/MMD avatars) or by HEIGHT (feet right, recommended for full-body tracking) (https://docs.vrchat.com/docs/ik-20-features-and-options). Community limb-scaling prefabs stretch avatar arm/leg bones to the user (https://nanochip.gumroad.com/l/avatarlimbscaling); shoulder-mobility add-ons extend reach when real arms exceed the avatar's.
• [S] Bonelab asks height + T-shirt size with fine-tuning of wingspan, chest, waist, hips, inseam, and runs a "body remapping system" so differently-shaped avatars keep a realistic range of motion (https://www.uploadvr.com/bonelab-avatar-system/, https://roadtovr.com/bonelab-custom-avatar-mod-levels-items-vehicles/). Players report: arms too short for them → "my arm held straight out keeps wanting to lift my character up off the ground" and climbing feels broken because hands aren't 1:1 — the PHYSICS-BODY version of the Ponton shortfall.
• [S] Nimso (Project TX) keeps a TRUE BODY from the user's metrics (height, floor, arm length = shoulder joint → wrist, set so real full stretch = in-game full stretch) and a separate SCALED BODY for avatars, scaling reach from the shoulder so full stretch still maps to full stretch (see nimso-project-tx.md).

2. Decision for VR Messs

The physics body is the player's real body; any avatar is a skin fitted onto it.

1. Physics ragdoll proportions = the player's calibrated metrics (Nimso's True Body; the research's "fitted" method). Shoulder joint positions and arm length come from measurement, never from current head height, so standing, seated or lying play all use the same body.
2. Calibration (one-time, saved profile):
   • Stand straight → eye height + floor (Nimso's "stand straight, touch the ground").
   • "Arm circles" ~10 s: swing both straight arms; sphere-fit each controller trail → shoulder centres (sphere centres, gives shoulder width and height) and arm length (radius − wrist-to-grip offset).
   • Skip / failure fallback: Winter/Drillis-Contini ratios from eye height (nimso-project-tx.md §3).
   • Store arm length ~1–2 cm SHORT of the measured value (Porssut: never let the real elbow lock first); the physics hand then trails the controller by that margin only at full stretch.
3. Stylized / anime avatar = visual skin on the fitted skeleton (the fitted-avatar pipeline): uniform scale by eye-height ratio, then resize the avatar's upper-arm/forearm bones to the calibrated arm length (hands not stretched). The anime look survives (head/torso/legs proportions untouched); only arm bone lengths adapt. No bespoke player model needed — any humanoid works.
4. "Become a different-sized character" (Bonelab/Nimso Scaled Body) stays on the back-burner; if built, it must preserve the extension invariant (scale reach from the shoulder so real full stretch = virtual full stretch).
5. Shoulder girdle: model the clavicle as a driven segment (shrug/protraction ~ a few cm) so overhead/forward reach beyond the arm comes from the shoulder, as in real bodies and in Nimso's "shoulder lifts as you raise the arm".