VR-Mess/research/godot-box3d-6dof.md

# Addon patch 2: Generic6DOFJoint3D on Box3D (2026-10-02)

Second local patch to the godot-box3d fork (after the ConeTwist→weld patch, `godot-box3d-weld.md`). Purpose:
the joint primitives for the Project-TX-style muscle ragdoll (`nimso-project-tx.md`, `vr-physics-rig-hexabody.md` §4).

- Fork branch `generic-6dof-joint` (on top of `rigid-weld-joint`) in `.vendor/godot-box3d`, commit "Expose Box3D
  joints through Godot's Generic6DOFJoint3D"; patch file `.vendor/box3d-generic-6dof.patch`. NOT pushed to
  `github.com/Foxyz1302/godot-box3d` yet (owner decision). Linux binary only (same rebuild recipe as the weld
  patch: `cmake --build build --parallel 12` in `.vendor/godot-box3d`, copy `bin/libgodot-box3d.so` to
  `addons/godot-box3d/bin/`; stock binaries stay in `addons/godot-box3d/bin-stock/`).
- Files: `src/joints/box3d_generic_6dof_joint_impl_3d.{hpp,cpp}` (new), `src/servers/box3d_physics_server_3d.cpp`
  (`_joint_make_generic_6dof` + param/flag get/set dispatch; was a hard error).

## Mapping (configuration-based — Box3D has no general 6-DOF constraint)

| Godot configuration | Box3D joint | Notes |
|---|---|---|
| all 3 linear axes locked (limit on, lower ≥ upper, no linear spring/motor), angular not fully locked or angular motor/spring on | `b3SphericalJoint` = **BALL** | joint **X = twist axis** (angular X limits = twist range); swing cone symmetric, half-angle = largest \|limit\| of angular Y/Z (warned if Y/Z differ); angular spring → rotation spring to the angular equilibrium (Euler, Godot default YXZ); angular **motor = torque-capped velocity motor** |
| everything locked, no angular motor/spring (the Godot default) | `b3WeldJoint` = **WELD** | rigid at the creation pose |
| any linear axis free/springy/motored | `b3MotorJoint` = **DRIVE** | springs pull frame B onto frame A shifted by the equilibrium points (linear + Euler); force/torque caps = Linear/Angular **MotorForceLimit** (0 = uncapped); motor target velocities add a velocity drive; linear limits NOT enforced (warned); **angular limits enforced** (patch 3, below) once any angular axis has a real range |

- **Units differ from Godot/Jolt**: spring STIFFNESS = frequency in **Hz**, DAMPING = **damping ratio** (Box3D
  soft constraints are mass-independent). Choose ~5–15 Hz, ratio ~1.
- Motor target velocities are per joint axis in **joint frame A**; the impl converts to Box3D's world-space relative
  angular/linear velocity at set time using body A's current rotation → set them EVERY TICK (they are a PD output).
- Flags and limits **rebuild** the Box3D joint (warm start lost — set them once); motor velocities, force limits,
  springs and equilibrium points apply **live**. Configure before `AddChild` (as with every joint here).
- Bullet-era params (limit softness/restitution/damping, ERP, angular force limit) are ignored.

## Muscle recipe (C#, used by `scripts/SixDofSpike.cs`)

```csharp
// BALL joint: angular limits as the anatomical range, EnableMotor on X/Y/Z, AngularMotorForceLimit = strength (N·m)
Quaternion rel = frameAWorld.Inverse() * frameBWorld;            // current joint rotation, frame-A coordinates
Vector3 err = BodyMath.RotationError(targetRel, rel);            // axis*angle in frame-A coordinates
Vector3 w = (err * Kp).LimitLength(maxSpin);                     // PD velocity
joint.SetParamX(Generic6DofJoint3D.Param.AngularMotorTargetVelocity, w.X); // …Y, …Z
```
The solver applies at most `AngularMotorForceLimit` torque per step — strength is a real cap, leverage emerges.

## Proof — `scenes/sixdof_spike.tscn` ("6DOF: overall PASS", 3× identical)

server type `Type6Dof`; BALL pivot holds (≤ 0.7 mm) while swinging 100°; cone 30° / twist 10° hold under hard torque
(31.3° / 11.6° peak — Box3D soft-limit overshoot); muscle PD holds a 1 kg × 0.6 m lever (2.94 N·m gravity load)
horizontal with a 10 N·m cap (0.0°) and FAILS with 1.5 N·m (drops 107°); tracks a 60° raise to 0.0°; DRIVE follows a
moving target to 0.000 m; a hand driven into a wall never penetrates it and pushes the 10 kg anchor body AWAY
(−0.82 m — push-off reaction; pulling toward a fixed point is the climbing case); all-locked = rigid (0.13°); 90
live flag toggles stay finite. All existing spikes (weld, physics, grab, body, collapse, arm) PASS 3× on the new binary.

## Patch 3 (2026-10-02): angular limits on the DRIVE — the shoulder range of motion

Problem: the shoulder is a DRIVE (linear motor = moving girdle socket, angular motor = muscle), and Box3D's
`b3MotorJoint` had no angular limits — an external force could bend the arm into the chest or behind the back.
A second joint can't add them (`b3SphericalJoint` always pins its anchor and would fight the moving socket), so the
limits went into Box3D's motor joint itself.

- Fork branch `shoulder-cone-limit` (on `generic-6dof-joint`), commit "Angular limits for drive-mode
  Generic6DOFJoint3D". The Box3D change is `patches/box3d/0001-motor-joint-angular-limits.patch` in the fork, applied
  to the `box3d/` checkout by CMake at configure time (marker `b3MotorJointSwingLimit`, idempotent) — the submodule
  stays pinned to upstream. Rebuild recipe unchanged (`cmake -B build …`, `cmake --build build`). Linux binary only.
- Box3D side (`b3MotorJointDef`): `limitRotationA/B` (own limit frames, so moving the drive target never moves the
  range), `enableSwingLimit` + `lower/upperSwingX/Y` (per-quadrant ELLIPSE on the swing rotation vector — asymmetric
  bounds), `enableTwistLimit` + `lower/upperTwistAngle`. Solved after the angular motor/spring, Jacobians rebuilt
  every sub-step: swing `J = −n + r′(φ)(z − cot θ · m)` (the radius change along an eccentric boundary — without
  it a fast arm sank 36° into the corners), exact twist Jacobian (Box3D's spherical `tan(θ/2)` form lags ~5° at 100°
  swing). Warm-started, included in joint torque reports and recordings (def size tripwire 168 → 232).
- Wrapper mapping: a DRIVE gets limits when any angular axis has the limit flag on with lower < upper. Joint
  **X = twist** (X limits), **Y/Z = swing bounds**: the swing rotation-vector components about joint Y and Z must stay
  inside the ellipse whose semi-axes are the per-sign bounds (lower ≤ 0 ≤ upper, up to ±0.99π). Flag off = that axis
  free; lower ≥ upper = locked. Measured between the joint frames AS CREATED (relative rotation identity = centre),
  not the equilibrium-shifted target. Limit softness 120 Hz (joint default 60 Hz let the twist sink ~5° while the arm
  slid along the swing boundary under 25 N·m). Limits rebuild the joint — configure before `AddChild`.
- Plain drives (angular limit flags off, or Godot's locked default with no real range) are byte-for-byte unchanged:
  `ragdoll_spike` prints identical numbers on the old and new binary.

### Shoulder values (for `PlayerRagdoll`)

Joint frame at build: X = the humerus as built (`dir`), Y = world up made square to it, Z = X × Y. Fitted against
target arm directions (chest frame): bounds Y ∈ [−90°, +100°] (right arm; left mirrors to [−100°, +90°]),
Z ∈ [−85°, +155°], twist ±80°. Inside with ≥ 2.6° margin: hanging, 45° extension, forward, overhead, overhead 10°
behind, lateral, lateral 30° behind, 45°-across hug, hand on the opposite shoulder, hand behind the back, up-and-across.
Out by ≥ 8°: medial horizontal, 80° extension, straight back, behind-up, behind the head across, medial behind-down.
(Arm hanging medially in front of the torso stays allowed — real, and own-body collision is off anyway.)

### Proof (added to `sixdof_spike`, 3× identical; weld_spike 3× PASS on the same binary)

Shoulder-style drive (2 kg × 0.3 m arm, socket = linear velocity motor 2000 N, bounds Y [−20°, 60°], Z [−40°, 100°],
twist ±45°): a 25 N·m torque sweeping every swing direction + 6 N·m alternating twist → swing overshoot 1.1° max,
every bound reached (61/20/101/41°), twist max 46.3°, socket 1.8 mm. A 30 N·m muscle PD reaches targets inside the
range (0.0 / 0.0 / 1.3° error) and stops at 60.2° when commanded to 85° (beyond the 60° bound). Moving the socket
5.8 cm (shrug) tracks to 0.0 mm with the arm still (0.0° swing).