99 lines
9.2 KiB
Markdown
99 lines
9.2 KiB
Markdown
# Research: avatar arm length vs the player's real arm length (2026-10-02)
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Question (owner): get arm length right first time — should the player's body be fitted to the player, or should
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the player embody an existing (stylized/anime) avatar with its own proportions, VRChat-style? Decide once.
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[S] = sourced, [I] = inference.
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## 1. What the research says
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- [S] **The one mismatch that breaks embodiment: the REAL arm reaching full extension while the virtual arm is
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still bent** (virtual arm longer than real, or offset forward). Porssut et al., *Reaching articular limits can
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negatively impact embodiment in VR*, PLOS ONE 2022 (https://journals.plos.org/plosone/article?id=10.1371%2Fjournal.pone.0255554):
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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
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its limit first; the opposite (avatar arm straight, real arm bent) was tolerated. Advice: "the user never reaches
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full arm extension by surprise"; if calibration can't be perfect, "choose to have the virtual limbs slightly
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SHORTER than the real ones". The elbow's own end-stop is "internal haptic feedback" the brain trusts.
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- [S] **Virtual arm shorter than real → the hand can't reach the controller.** Ponton et al., *Stretch your reach*,
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CHI 2024 (https://arxiv.org/html/2407.08011, https://dl.acm.org/doi/10.1145/3613904.3642268): avatars scaled
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uniformly from height left an average 26.3 cm (SD 4.7) reach shortfall; of five strategies, STRETCHING the arm
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to keep the hand on the controller was best for embodiment, proprioception and preference; a free-floating
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controller kept location but broke continuity; capping the hand at avatar reach was worst. Rendering the
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controller doesn't matter. A too-LONG virtual arm (untested there) only bends the elbow — reach is unaffected.
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- [S] **Fitted skeletons beat uniform scaling.** Ponton et al., *Fitted avatars: automatic skeleton adjustment for
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self-avatars in VR*, Virtual Reality 2023 (https://arxiv.org/pdf/2307.09558): uniform scaling by eye height
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"assume[s] … the same ratios between bone lengths as the user" → controller-to-wrist errors; their fitting
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reduced hand error by 4–14 cm (male) / 2.6–8 cm (female). Method: (1) uniformly scale the avatar by eye-height
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ratio, (2) resize upper-arm/forearm bones to the measured arm length. **Arm length measurement: the user swings
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both STRAIGHT arms in circles; a sphere fitted to the controller positions gives the SHOULDER JOINT centre (sphere
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centre) and ARM LENGTH (radius, shoulder→wrist).** Neck from head rotations. Embodiment gains appeared "only when
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the initial avatar proportions differed from the user's" — i.e. exactly the stylized-avatar case. Some avatars
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needed arms stretched by 12–23 cm.
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- [S] Body ownership tolerates large DELIBERATE changes when movement stays synchronous — up to ~3× arm length
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(Kilteni et al., *Very Long Arm Illusion*, PLOS ONE 2012, https://journals.plos.org/plosone/article?id=10.1371%2Fjournal.pone.0040867).
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Proportion is not the problem; a reach/extension mismatch is.
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- [S] **VRChat** measures avatars by ARM SPAN by default (scales so the avatar's arms match yours — hands right,
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floor/feet often wrong, worse with long-legged anime/MMD avatars) or by HEIGHT (feet right, recommended for
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full-body tracking) (https://docs.vrchat.com/docs/ik-20-features-and-options). Community limb-scaling prefabs
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stretch avatar arm/leg bones to the user (https://nanochip.gumroad.com/l/avatarlimbscaling); shoulder-mobility
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add-ons extend reach when real arms exceed the avatar's.
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- [S] **Bonelab** asks height + T-shirt size with fine-tuning of wingspan, chest, waist, hips, inseam, and runs a
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"body remapping system" so differently-shaped avatars keep a realistic range of motion
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(https://www.uploadvr.com/bonelab-avatar-system/, https://roadtovr.com/bonelab-custom-avatar-mod-levels-items-vehicles/).
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Players report: arms too short for them → "my arm held straight out keeps wanting to lift my character up off the
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ground" and climbing feels broken because hands aren't 1:1 — the PHYSICS-BODY version of the Ponton shortfall.
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- [S] **Nimso (Project TX)** keeps a TRUE BODY from the user's metrics (height, floor, arm length = shoulder joint
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→ wrist, set so real full stretch = in-game full stretch) and a separate SCALED BODY for avatars, scaling reach
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from the shoulder so full stretch still maps to full stretch (see `nimso-project-tx.md`).
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## 2. Shoulders — Nimso's evolution (all shoulder mentions in the transcripts) and real anatomy
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- [S] 2017: hands limited to "shoulder arm length". 2018 (hzShV7qqbAE): derived from 3 points only — "you put
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your hand up, you can see your shoulder moves up and down", "he puts his shoulders into" punches. 2019 Test
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Chamber: "I haven't got a shoulder system yet". 2019 ragdoll: the shoulder is a physical surface (lean hammers/
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guns on it). 2021 rewrite: "derived elbow positions, logical sort of shoulder control … the shoulder rotation
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based on all of that structure including the head, the chest position and rotation". 2022 (yPsCuuNM3lM): rewrote
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the body because of "the lack of shoulder control" — before, "the shoulder just kind of stays straight and the
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arm becomes very folded up"; now "the shoulders … actually care about where the elbow is and the arm is just about
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pushing on the limits of the shoulder". 2024 (i7F4V9K6SlU): "it's not just an arm length — as you lift your arm
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the shoulder moves up and down, it lifts, so you have to figure out all the mechanical structures … as accurate as
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possible in order for the limit of your arm to be accurate"; crouching tilts the shoulders with the chest.
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Calibration still sets ARM LENGTH = shoulder joint → wrist ("these two bones"); the shoulder motion is DERIVED
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on top. Avatar mapping scales "from the shoulder point" off a chest centre line so hands still meet.
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- [S] Anatomy: the arm hangs off the shoulder girdle (clavicle + scapula); the glenohumeral joint itself moves —
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elevation (shrug), protraction/retraction (reach forward/back) — and arm raising is shared between glenohumeral
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and scapular rotation (the classic "scapulohumeral rhythm", ≈2:1 above the first ~30° of abduction; Inman et al.
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1944). Winter Table 4.1 lists the "shoulder mass" segment as sternoclavicular joint → glenohumeral axis; Fig 4.1
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gives neck→shoulder 0.129H. [I] Girdle motion adds very roughly 5–10 cm of reach (forward/up) over a fixed
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shoulder — the part of "arm length" a straight-arm calibration does NOT include, which is correct: it must be
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modelled as motion, not as longer bones.
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- [I] VR Messs: arm length (bones) from calibration; shoulder girdle as its own derived + physical segment
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(sternoclavicular ball joint with tight limits and a muscle): elevates with arm raise (rhythm), protracts toward
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far reach targets, tilts with the chest; only past the girdle's limits does the chest lean (current "lean").
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## 3. Decision for VR Messs (revised after owner's "embody a short avatar?" question)
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The conflict ("fit the avatar to me" vs "I become the avatar") goes away by fixing the order of operations:
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**One body model, two layers, one calibration — Nimso's True Body → Scaled Body, Bonelab's "calibrate once".**
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1. TRUE BODY = you. Measured once, saved, reused for every avatar and every posture (standing/seated/lying).
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Zero visible setup by default (VRChat-like): eye height + floor captured from the first standing moments; arm
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length starts at the anthropometric estimate (0.332·H, slightly short — the safe side per Porssut) and is
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refined PASSIVELY from the furthest controller-to-shoulder distances you reach during play (high percentile,
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only ever growing toward the true value). [I] An explicit 10 s "arm circles" sphere-fit lives in settings for
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people who want it exact immediately (Bonelab-style fine-tuning, done once, forgotten).
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2. SCALED BODY = the avatar you embody, derived from the True Body:
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- **uniform scale s = avatar eye height / your eye height** applied to tracking around your body: a short avatar
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makes YOU short (world looks bigger, floor still where your feet are, crouching still touches the floor),
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camera rotation stays 1:1 — this is what "embodying" a character means and is what VRChat/Bonelab/Nimso do;
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- **reach remap r = avatar arm / (s · your arm)**, applied to the hand target's offset from the (scaled)
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shoulder: your full stretch is always the avatar's full stretch (the Porssut/Ponton invariant), so anime
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short/long arms never cause the elbow-lock conflict nor the can't-reach problem; the residual hand offset is
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only the proportion difference (typically a few %, Ponton/Kilteni say tolerable);
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- the PHYSICS ragdoll uses the avatar's proportions (Mario is physically Mario-sized; Nimso's scaled ragdoll).
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3. "Your own body" avatar (default) = an avatar generated/fitted to your True Body → s = r = 1, zero offsets.
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Any stylized avatar can alternatively be shown "fitted to you" (arm bones stretched) — that's just r = 1 with
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visual bone scaling; one flag per avatar, no new system.
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So no box goes unchecked: true scale and floor come from s, reach and extension from r, perfect hands when the avatar
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matches you, and setup is invisible by default with optional exact calibration.
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