security-game/docs/adr/0018-quaternius-stage2-runtime-rig.md

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.