VR-Mess/research/ragdoll-player-step2.md

# Body step 2 — the muscle-driven ragdoll player (2026-10-02)

Implementation record for Nimso's Project TX player rebuilt on Box3D (design source:
`nimso-project-tx.md` §2; joint mapping: `godot-box3d-6dof.md`). Code: `scripts/PlayerRagdoll.cs` (input-agnostic
body), `scripts/PlayerRig.cs` (XR driver + stick locomotion), regression `scenes/ragdoll_spike.tscn`
("RAGDOLL: overall PASS", 14 phases). It replaces PhysicsHands / PhysicsTorso / PhysicsArms / BodyRig in
`main.tscn`. Those scripts stay for their own spikes.

## What was built

| Part | Implementation | Why |
|---|---|---|
| Bodies | 9 boxes, each with its origin at its centre of mass: pelvis (pelvis + abdomen + both legs' Dempster mass, 36 kg at 60 kg total), chest 13 kg, head + neck 4.9 kg, upper arm 1.7, forearm 1.0, hand 0.36 | Legs aren't physical yet (Nimso's leg collision is off by default too), but their mass belongs to the body the ground carries |
| Spine, neck | `Generic6DOFJoint3D` BALL (cone/twist, twist axis = up) + muscle motor | Local muscles, Nimso: "each bone drives itself" |
| Shoulder | `Generic6DOFJoint3D` DRIVE chest→upper arm. The linear velocity motor (2000 N) holds the humeral head on the chest point + the True Body girdle offset; the angular velocity motor (45 N·m) is the shoulder muscle | One joint carries both the moving girdle (shrug/protraction = real reach) and the muscle. See "light links" below |
| Elbow | `HingeJoint3D` 0..150° with a muscle motor (45 N·m) on the flexion angle | A real hinge can't flip or hyper-extend |
| Wrist | BALL, cone 80°, twist ±90°, no motor | The hand guide orients the hand |
| Hand guide | DRIVE chest→hand. Linear velocity motor 40 N with controller-velocity feed-forward, angular 25 N·m | "Tracking is a guide". The reaction goes into the chest, so pushing a wall pushes you |
| Balance | DRIVE from a shapeless world anchor to the pelvis, angular motor only (400 N·m grounded, 30% airborne) | Upright/yaw hold. See "explicit torques" below |
| Ground control | Ray down from the pelvis. If ground is within hip height + 0.3 m (normal.y > 0.6): critically damped 4 Hz PD on the pelvis toward its pose height above the actual ground, with gravity compensation for the whole body mass. Support is clamped to [0, 2 × weight], horizontal force to friction × support | Nimso's "leg strength" scales with mass and only pushes. No ground → falls |
| Muscle law | Motor target velocity = feed-forward (the target's own relative spin) + Kp·RotationError(target, current) in joint frame A, capped at 15 rad/s; force/torque cap = strength | Proven in `sixdof_spike`. Feed-forward keeps moving poses from lagging by speed/Kp |
| Targets | From `TrueBodySolver`: trunk rebuilt top-down (upper chest → lumbar → hips, hips take 50% of the crouch lean). Head = camera orientation tilted so the body's eye lands on the camera. Arms = ArmSolver shoulder/elbow/wrist; hinge axis = Pole × arm direction | A pose the physical chain can actually reach |
| Camera / play space | Camera stays 1:1. Each tick the driver applies `OriginCorrection`: a 3 cm sphere swept from the head body's eye to the camera. If geometry is in the way, the view is put back at the free point (walls stop both stick and real walking). If the path is free, the camera may lead the body by up to 0.2 m (leaning over a table whose edge holds your hips), beyond that it follows. Grounded: horizontal only, the floor eases toward the ground under the pelvis. Airborne: 3D (falls carry the view) | Owner: Bonelab push-back, no fade |
| Discrete moves | Origin jumps > 0.5 m or > 0.1 rad in one tick (spawn, teleport markers) and snap turns carry the body with `TeleportBy` + `TrueBodySolver.Shift` | No body flung across the gap |
| Locomotion | Left stick head-relative 2.2 m/s (8 m/s² accel, no air control); right stick snap 30° (`PlayerRig.SmoothTurn` → 2.5 rad/s). A stick whose hand points at the menu scrolls it instead | Owner scope: Boneworks-style stick only, no jump |

## Measured findings (RagdollSpike, Box3D fork, 90 Hz, 4 substeps)

1. **Light links between two strong motors oscillate.** The first build had a 0.6 kg neck link (two neck joints)
   and a 0.8 kg clavicle body per arm (chest→clavicle drive + clavicle→upper-arm ball). The neck pitch oscillated
   with growing amplitude (±11 rad/s head spin while standing still). Raising the shoulder to 200 N·m made the arms
   *worse* (0.94 m hand lag vs 0.47 m). Box3D runs one solver iteration per substep, and a light body squeezed
   between two motors with large caps doesn't converge. **Fix: no light intermediate bodies.** The neck merged into
   the head (single joint at the neck base); the clavicle was replaced by one shoulder DRIVE carrying both the girdle
   translation and the muscle. Result: standing error ~0, swing lag 0.13 m, random-motion eye error 2.3 cm. Rule
   for the legs and anything later: keep every body that sits between two motors ≳ 1 kg, or merge it.
2. **Explicit world torques on the pelvis vs an implicit motor.** The explicit 3 Hz PD torque balance looked fine
   until the arms got strong. With the light links gone, the implicit balance motor is stable and holds the pelvis
   rigidly against arm reaction torques.
3. **Hand-guide strength is the "cheat budget".** With a 120 N guide the arms looked perfect, but 12 kg at arm's
   length sagged only 2 cm (Nimso: it must not be holdable). At 40 N guide + 45 N·m shoulder/elbow (≈ human):
   12 kg held close sags 1.3 cm, at arm's length 25 cm. Fast swings still track (0.13 m peak lag at 1.5 Hz ±0.25 m).
4. **Test poses must be anatomically possible.** Controllers pointing forward with the arms out sideways need a 90°
   wrist bend. The old strong guide hid that; with honest strength the wrist limit wins. RagdollSpike's reach poses
   now aim the controllers along the arm.
5. **Unreachable targets stop at the arm's end.** That is correct, not a bug: a push target 0.91 m from a 0.57 m
   arm leaves the hands at full reach.
6. Hinge sign: positive `HingeJoint3D` motor velocity rotates the child about the joint's +Z (spike phase 0 measures
   it; `PlayerRagdoll.HingeSign`).

## Not done yet / known gaps

- **Legs**: no physical or visual legs; the pelvis floats on ground control. Step 3 = procedural legs (Nimso:
  "feet don't hold the player up" — visuals + optional collision).
- **Shoulder has no cone limit** (Box3D's motor joint has none). A strong external force can bend the arm into the
  chest; muscles return it. If it shows, add a BALL limit via a second, collision-only constraint, or a clavicle
  heavy enough (≥ 1 kg) to be stable.
- **Climbing**: grabs are props only (`PropGrabber`); welding to static geometry is not wired. The physics would
  allow it: the hand guide reacts into the chest.
- **Strength values are guesses** at human scale (Nimso's are unpublished). One global strength scalar (Nimso's
  "Crysis mode") is not exposed yet.
- **144 Hz**: not switched. Physics runs at the headset rate. Decoupling it needs physics interpolation, and today
  `AnchorMoveMode`/`Locomotion` move the XR origin in `_Process`, which would jitter under interpolation.
- **Anchor-grab / teleport legacy locomotion** still drive the origin. Teleports carry the body. Grab-flying upward
  now fights gravity (the body falls and the view follows it), so it effectively only pulls you along the ground.

## Step 2.1 — owner playtest fixes + sandbox (2026-10-02)

Owner playtest of step 2: "feels amazing for the most part". It found three problems:
- You could walk onto the table: the body detached for a second and wobbled before settling.
- You couldn't climb onto the table by pushing yourself up.
- You couldn't crawl under the table.

Changes and the measured reasons:

| Change | Why (RagdollSpike evidence) |
|---|---|
| **Step rule**: ground more than `MaxStepHeight` 0.35 m above the feet isn't stood on unless the hips got up there: airborne/landing, legs tucked, or hips > 60% of hip height above it, and a second ray 12 cm behind the hips also finds it | The single pelvis ray treated a 0.725 m table under the hips as a "step" → walking onto the table |
| **Lower-body collider** (`Legs`, 6 kg box from the hips down to a step above the feet, DRIVE from the pelvis). It slides up when crouching, and its shape is **disabled** (not just layer-cleared) and tucked into the pelvis when crawling or when the arms lift the body (mantle) | Knee-high obstacles and tables now block; steps ≤ 0.35 m pass under it. Box3D kept the old contact when only the collision layer changed, which jammed a crawl against a tunnel roof |
| **Structural hand strength**: guide cap = lerp(40 N, 400 N, axial share of the hand error), with a 3 cm deadband | Force along the arm (pressing down on a ledge, hanging, carrying at your side) is bone-strong; across the arm it stays weak. Measured: two-hand mantle onto 0.9 m ✓, one hand ✗ (5 cm). Pull-up on a grabbed bar: two hands lift the pelvis 0.69 m, one hand 0.24 m. 12 kg at arm's length still sags (6.6 cm vs 0.9 cm close) |
| **Arm aim from the physical shoulder**, horizontal only: the arm reaches for the real hand when the body is held away horizontally (ledge, lean-over), but keeps its True Body pose vertically | Purely camera-relative arms left the hands 26 cm short when the body was held back at a ledge. Purely physical arms fold to wherever a sagging chest is and stop being pillars, so the crawl collapsed (eye 0.14 m vs 0.42). The split fixes both |
| **Ground feed-forward from the GOAL** (pose + ground offset) | When the floor follows up onto a ledge, pose and offset move opposite; differentiating the pose alone launched the body 1 m up |
| **Crawl pose** (`TrueBodySolver`): when the hips would go below 0.15 m, the trunk tips toward ~83°, hips kick back; ragdoll hips take the full lean in a crawl | Crouch lean capped at 43° put the hips through the floor, so the head could never get low |
| **Crawl carry** 300 N + torso friction 0.3 | With the torso resting on the floor, μ·weight exceeded the support-scaled carry, so stick crawling stalled (Nimso has no stick crawl at all; ours is a comfort concession) |
| **Braced air control** 150 N, **AirSlack** 0.05 m | While mantling/hanging, the hips drift after the head; airborne, the view rides the body tightly |
| **Climbing grabs**: `PropGrabber` welds the hand to any `StaticBody3D` under the palm (5 cm sphere) except group `no_grab` (the floor) | Pull-ups, ledges, climbing-wall holds, monkey bars |
| **Procedural legs** (`ProceduralLegs.cs`): visual only. Planted feet, alternate stepping on 20 cm / 34° error, stride leads velocity by 0.25 s, 0.2–0.32 s steps; airborne they dangle, crawling they trail | Nimso: feet don't hold the body up |
| **144 Hz physics + physics interpolation**. `XROrigin3D`, `TrueBody` and `PlayerRig` (ghosts) are excluded; the ragdoll is explicitly interpolated. Legacy chase-body spikes pin 90 Hz | Owner's earlier choice; the old origin-moving code runs per frame, so its subtree must not interpolate |
| **Strength slider** (0.25–3×, scales every muscle + hand guides), **Spectator** toggle (no body, the old grab-drag flight), **Turning** snap/smooth toggle | Owner: drag movement "doesn't fit this anymore", keep it as a spectator mode |
| **Playground** (`Playground.cs`): mantling ledges 0.3–1.5 m, a 1 m platform with stairs, an 18° ramp and a 40° ramp, a climbing wall, a pull-up bar, monkey bars, a plank bridge, loose planks, a seesaw, a hanging swing board, a crate pyramid, a 25 kg box, barrels, a balance beam, a 0.6 m crawl tunnel, a duck bar, a doorway. "Reset props" resets it | Owner: a Nimso-sandbox-like level |

Ground steeper than `MaxSlopeDegrees` 35° gives no support, so the 40° ramp can't be walked up; the 18° one can.
Still open: the shoulder cone limit (needs a fork addition). `tools/playground_shots.tscn` renders overview PNGs for
layout checks.