# ADR-0018: Quaternius Stage 2 — adopt source proportions at runtime

- **Status:** Proposed
- **Date:** 2026-11-05
- **Supersedes:** ADR-0017 decision 2 (Stage 1 keeps the 13-core contract)

## Context

ADR-0017 made Quaternius the NPC base and staged the migration: Stage 1 bakes a
proportion-warped mesh onto the existing 13-core contract; Stage 2 adopts the
62-joint rig at runtime only if the warp fails.

The Stage 1 proportion warp was executed and measured (`docs/npc-model-sourcing.md`
§Rest-pose and proportion-warp measurement). The arm-down rest pose bakes cleanly, but the
warp to the contract proportions does not:

| Joint | Quaternius | Contract | Delta |
| --- | --- | --- | --- |
| Hips height | 0.869 | 0.62 | −0.249 |
| Shoulder x | 0.148 | 0.27 | +0.122 |
| Arm (shoulder→hand tip) | 0.429 | 0.70 | ×1.63 |
| Leg (hip→ankle) | 0.835 | 0.62 | ×0.74 |
| Torso (hip→head) | 0.745 | 1.20 | ×1.61 |

The vision judge reads the warped mesh as distorted: paddle/claw hands, forearms lost into the
torso, stretched shins, "long legs + short torso". The arm either stays short (a shortened arm
against the contract hand) or stretches 1.63× (a distorted deltoid + hand). Both are the gate's
rejection conditions.

## Decision

Stop the Stage 1 rollout. Adopt the **Quaternius source proportions** as the runtime character
scale, rather than warping them to the old contract numbers. Two sub-options are open and must
be chosen by the owner:

1. **Keep the 13-core bone names, change their rest positions to the Quaternius joints.**
   The contract armature keeps 14 bones, but the rest positions and gear attach points are
   re-derived from the Quaternius skeleton (hip ≈0.87, shoulder ≈0.148/1.39, wrist ≈0.99,
   knee ≈0.52). `NpcAvatar`'s bone count and pose composition stay; gear offsets and the
   contract tests are re-baked. This is the smallest runtime change that keeps the Quaternius
   silhouette.
2. **Adopt the 62-joint rig at runtime.** Ship the Quaternius skeleton (including the optional
   30 finger + 2 foot bones) and retarget Idle/Walk onto it. `NpcAvatar` bone mapping, gear
   attach points and the pose composition change together. Largest change, closest to the
   source.

Whichever is chosen, the mesh is **not** proportion-warped: it keeps its source LOD (3,199 welded
verts / 6,206 tris) and only the arm-down rest pose is baked.

## Consequences

### Positive

- The Quaternius silhouette, deltoid, hand and leg proportions are preserved — no paddle hands,
  no shortened arm, no torso/leg distortion.
- The mesh stays at source LOD (far under the 24,000 cap), so no decimation is needed.

### Negative

- The runtime contract numbers (hip 0.62, shoulder 0.27, hand 0.78) change; every gear attach
  point, `npc_proportions` guide, fit measurement and pose-composition constant must be
  re-derived and re-tested.
- Option 2 additionally rewrites `NpcAvatar`'s bone mapping and animation retarget, which is a
  larger, riskier change than the mesh swap.
- The 12-variant rollout and the 12×6 baseline are still blocked until the new rest positions
  and gear fit are owner-approved.

## Alternatives

- **Ship the warped mesh.** Rejected by the gate: the 1.63× arm / 0.74× leg warp visibly
  distorts and the owner directed keeping the Quaternius as close to source as possible.
- **Keep the old contract and accept a short arm.** Rejected for the same reason: the hand ends
  at ≈0.99 instead of 0.78, so handheld gear floats unless attach points move — which is option 1.
- **Abandon Quaternius and keep the procedural generator.** Rejected in ADR-0017.

## Open question

Choose option 1 (13-core names, Quaternius rest positions) or option 2 (62-joint runtime).
Recommendation: **option 1** first — it preserves the silhouette, keeps `NpcAvatar`'s bone count
and pose composition, and only re-derives gear offsets and contract tests.