Legacy Compatibility Audit

Read _audit-common.md and _audit-severity.md for shared protocol (layout, game-data + legacy-source locations, severity, NIFAL severity rows, dedup, report format).

Purpose and scope

Compare Gamebryo 2.3 / Creation behaviour with Redux and surface mapping gaps — content the source engines render or simulate that Redux drops, mis-maps or diverges on — in the seams no layer or per-game owner covers. Each abstraction layer has its own deep audit; findings about a layer's internals go there, not here:

This skill keeps three dimensions: the coordinate/placement math (no other owner), the legacy-subsystem cross-check (the only audit that reads the 2.3 headers) and the cross-game translation-pattern spot-check. The former NIFAL, material-boundary, PHYSAL and EXAL dimensions were re-homed to the layer owners above: PHYSAL and the material boundary yielded only duplicates of /audit-physics / /audit-nifal; EXAL/WATAL yielded real boundary-completeness finds, which is exactly /audit-exterior's charter.

Documented limitations are not findings (FO4+ packed Havok blobs, phantoms, NiFogProperty's deliberate non-dispatch — no Material landing site, cell-scope CellLighting only — and the measured ~1.0 shared emissive scale, nifal.md §4). Cross-check the layer specs' leak inventories so closed leaks are not re-filed. If the Gamebryo 2.3 drive is unmounted, /mnt/data/src/reference/gamebyro-v26 / gamebyro-v32 and docs/legacy/ carry the same class shapes.

Dimensions

1. Coordinate + placement fidelity (Z-up → Y-up)

The most catastrophic mapping-bug class: one wrong transform mis-places or mirrors all content silently. Reference: docs/engine/coordinate-system.md.
Paths: crates/core/src/math/coord.rs, crates/nif/src/import/coord.rs, crates/nif/src/rotation.rs, crates/nif/src/blocks/strip.rs, byroredux/src/cell_loader/euler.rs, crates/core/src/ecs/components/camera.rs
First step: git log --since=<last report> --format='%h %cs %s' -- crates/core/src/math crates/nif/src/import/coord.rs byroredux/src/cell_loader/euler.rs and grep -rn '(x, z, -y)\|4096.0' byroredux/src crates --include='*.rs' for new duplicated swaps / cell literals.
Guards: coord.rs tests euler_multi_axis_matches_openmw_objectpaging + euler_zyx_order_pinned_by_rx_then_rz; cell_loader/euler_zup_to_quat_yup_tests.rs; strip.rs::even_odd_winding_matches_opengl_convention.

• One implementation — the (x, z, -y) swap and quaternion conversions live in coord.rs (zup_to_yup_pos, zup_to_yup_quat_wxyz, euler_zup_to_quat_yup, normalize_quat, cell_grid_to_world_yup); the NIF-typed flavour (zup_point_to_yup, zup_matrix_to_yup_quat) wraps them in import/coord.rs. Any new duplicated swap or matrix→quat path that bypasses them is a finding (the pre-#1044 five-site duplication kept the #333 unit-quaternion fix at only one).
• REFR Euler — Bethesda CW-positive, ZYX product from_rotation_y(-rz) * from_rotation_z(ry) * from_rotation_x(-rx) (the XYZ variant only matches Z-only REFRs and skews multi-axis architecture). REFR placement goes through the --rotation-mode diagnostic dispatcher euler_zup_to_quat_yup_refr (default mode 1); XCLL lighting calls the canonical helper directly. Flag any caller that hardcodes a mode or re-derives the formula. --rotation-mode reaches the library unclamped (boot::cli::rotation_mode_arg, #4126/#4764), so an out-of-range value falls back to the shipping formula — a CLI-side clamp is the regression.
• Winding — NIF CW → projection Y-flip (Camera::projection_matrix, Vulkan clip space) → CCW front face. Strip de-stitch has ONE implementation (strip::destrip, #2298): odd triangles swap the last two indices; a D3D-style first-two swap re-introduces inside-out geometry, and a hand-copied destrip is the regression.
• Exterior grid — EXTERIOR_CELL_UNITS = 4096.0 + cell_grid_to_world_yup are the sole source (#1112 collapsed six divergent literals, one with a Z-flip sign bug); new literal 4096 cell math is a regression (distance literals are not). The one sanctioned re-declaration is crates/bsa/src/uvd.rs (the bsa crate cannot depend on core), re-exported as UVD_CELL_UNITS and const-asserted equal to the core constant in byroredux/src/cell_loader/precombined.rs (27002ed99, #4939). Also check frame composition, not just the literal: FO4 exterior _oc.nif precombine instances are already world-absolute, so bakes spawn at a Vec3::ZERO root — composing cell_grid_to_world_yup on top placed every exterior bake one cell out (#5228, f3e1bba62).
• Transform model — Gamebryo's NiTransform scale is uniform by format (docs/legacy/api-deep-dive.md), so Redux's Transform.scale: f32 is not a collapse — a "non-uniform scale is dropped" finding is a false premise. Matrix3→Quat at import (Shepperd + SVD repair for degenerate rotations) and local * parent propagation are the fidelity surface. Non-finite input is neutralised at the boundary: rotation.rs::sanitize_rotation (identity) + the read_ni_transform* readers (translation 0, scale 1, #4549); animation rotation keys go through the one sanitizer anim/keys.rs::normalized_rotation_sample (#4396). Core normalize_quat is deliberately lenient — a new caller using it as a sanitizer is a finding.
• Unit scale — Starfield authors metric; crates/plugin/src/esm/records/spatial_units.rs::normalize lifts ESM placement/light/fog/weather/water/navmesh distances to 70 units/m once per plugin walk (FO76 is BU-native; its only lift is the shared-decoder WATR per-metre absorption lane, #5169), and NIF import does the matching mesh/bone conversion. A second game-dependent distance multiplier downstream (lighting, skinning, camera) is the regression; per-field coverage is /audit-esm Dim 5.

2. Legacy subsystem coverage

What the 2.3 engine drives that Redux maps incompletely or not at all: cross-check SDK/Win32/Include/, CoreLibs/Ni*/ (see _audit-common.md Legacy Source; /legacy-lookup) against the Redux side. Audit fidelity, not existence — the import-critical set exists.
Paths: docs/legacy/api-deep-dive.md (mapping table), crates/core/src/ecs/components/, crates/core/src/animation/, byroredux/src/anim_convert.rs, crates/nif/src/import/{material,walk}/
First step: read the mapping table against crates/core/src/ecs/components/; gh issue list --search 'SUBSYS' --state open for what is already tracked.

• Property → pipeline — for each NiProperty type: does its authored effect land in dynamic pipeline state (cull, blend, stencil two-sided) or a Material/per-object component? Flag any property whose authored effect is parsed and dropped.
• Animation model — NiTimeController → NiInterpolator → keys converge at import (nifal.md Animation; B-spline / Euler / TBC → quaternion keys; KF + embedded through convert_nif_clip; text keys → AnimationTextKeyEvents). Only per-light ambient colour channels (ColorTarget::LightAmbient, no light-buffer slot, #983) are parked; morph-weight channels reach AnimatedMorphWeights and deform on the GPU in skin_vertices.comp (#3231/#4399). The timing envelope (cycle type, frequency, phase) and the NiControllerManager / KFM sequence state machine are the known-fragile spots (#4128 open).
• Scene graph — each NiAVObject field → Redux component (Parent, Children, GlobalTransform, WorldBound, Name, flags) with fidelity, not existence.
• String interning — NiFixedString + GlobalStringTable ↔ StringPool + FixedString: semantic equivalence; flag any gap that breaks bone-name → entity resolution (load-bearing for skinning and PHYSAL bone binding).
• Typical yield is the shape "authored input parsed, no canonical sink" — weapon reach/speed, XLOC lock state, the controller timing envelope and Oblivion's parallax mode were each MEDIUM in past reports and are since fixed (#3096–#3098). Confirm against /audit-gameplay scope before filing gameplay-facing gaps.

3. Cross-game translation-pattern spot-check

Reference: docs/engine/per-game-translation-survey.md §4 (per-layer inventory), §5 (Patterns A/B/C), §7 (why Fallout is the stress case). Verify a survey claim against code before citing it — the doc last changed 2026-10-06 (a comment-drift fix) and has carried stale examples before.
Paths: crates/nif/src/version.rs, crates/plugin/src/esm/reader.rs (GameKind), crates/plugin/src/esm/records/
First step: grep -rnE 'bsver(\(\))? *[<>=]+ *[0-9]{2,3}\b' crates/nif/src --include='*.rs' | grep -v test then drop // comment lines (grep -vE '^[^:]+:[0-9]+: *//') — literal thresholds; at 2026-10-10 the only non-comment hits are three test asserts in blocks/base.rs. Then grep -rn 'game == \|GameKind::' byroredux/src/cell_loader byroredux/src/scene --include='*.rs'.

• The renderer/shader side is clean; per-game branches live upstream (parser, importer, cell loader). Look there.
• Pattern A — hardcoded BSVER threshold literals that should be a named version::bsver::* constant. This is not a bypassed NifVariant helper: those feature-flag predicates were removed deliberately (#938/#1511/#1840/#1897) and version.rs records the raw-bsver doctrine as fully enforced (search raw-bsver doctrine). Do not flag a raw bsver() comparison as "should call a helper".
• Pattern B — feature flag on an enum where the wire format already discriminates the game (the correct shape; flag mis-dispatch, not "needs a trait").
• Pattern C — divergent decode of one wire structure across games/eras (same record, different offsets or sizes): FO4 WEAP with no DATA and WATR simulator blocks decoded one field off were the HIGH finds. Depth belongs to /audit-esm / /audit-nif; this dimension checks the cross-game shape (a new per-game size/offset split with no test per era).
• Scattered new if game == … in the cell loader / exterior assembly is a finding (one GameKind-keyed decision or a table-shaped match returning data is fine).

Process

1. Read the live spec for the seam under audit; its leak inventory is the baseline — do not re-file what it records as closed or a limitation.
2. Trace each claimed boundary to its callers (grep the symbol) and confirm single-producer.
3. Cross-reference Redux components / translate sites against the legacy headers and the survey.
4. Classify per _audit-severity.md (a wrong/divergent canonical out of a translate() = HIGH; a translatable input silently dropped = MEDIUM, escalate if it removes visible content). Disprove every finding before keeping it.
5. Finalize per _audit-common.md "Report Finalization" (save to docs/audits/AUDIT_LEGACY_COMPAT_<TODAY>.md; do not open issues directly). Suggest /audit-publish (label legacy-compat + the domain label).