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description: "Audit the SpeedTree (.spt) TLV walker + placeholder-billboard fallback shipped in Session 33 Phase 1" argument-hint: "--focus <dimensions> --depth shallow|deep"

SpeedTree Subsystem Audit

Audit the byroredux-spt crate — a young, deliberately small subsystem (Session 33 Phase 1, "S1"). It does two things: (1) walks the .spt parameter section as a tag-length-value stream for FNV / FO3 / Oblivion, and (2) emits a placeholder billboard ImportedScene so TREE cells render something instead of panicking or going treeless. The real geometry tail is not decoded — everything past tail_offset is left on the floor by design.

Keep the audit proportionate. The highest-risk surface is the walker's byte accounting (one mis-sized payload desyncs the whole stream), then the placeholder fallback correctness, then TREE record → billboard wiring, then per-game .spt differences. The tag dictionary is large but each entry is a fixed-size decode — low risk individually.

Architecture: Single-pass — small enough to run all dimensions inline rather than spawning Tasks.

See .claude/commands/_audit-common.md for project layout, methodology, deduplication, context rules, severity, and finding format. Do not re-derive any of those here.

Scope

Crate: crates/spt/src/ — parser.rs, tag.rs, version.rs, stream.rs, scene.rs, crates/spt/src/import/mod.rs, plus the feature-gated crates/spt/src/recon/mod.rs. Public surface re-exported from crates/spt/src/lib.rs: parse_spt, import_spt_scene, SptImportParams, SptScene, detect_variant, dispatch_tag. (compute_billboard_size is a private helper in the import module, not an entry point.)

Cross-cuts (the wiring that actually invokes the crate):

• byroredux/src/cell_loader/references/mod.rs — the production route. An is_spt extension check (model_path … eq_ignore_ascii_case("spt")) dispatches to parse_and_import_spt (implemented in the sibling byroredux/src/cell_loader/references/import.rs), which looks up the matching TREE record from record_index.trees and threads its metadata into SptImportParams. byroredux/src/cell_loader/refr.rs does not carry a .spt route.
• byroredux/src/scene/nif_loader.rs — the --tree / loose-file direct-visualiser route (parse_import_and_merge, is_spt branch). This is a parallel path: it calls import_spt_scene with SptImportParams::default() — no TREE metadata (no ICON override, no OBND/MODB/BNAM sizing). Verify the two routes don't silently diverge in ways that matter (Dimension 4).
• crates/plugin/src/esm/records/tree.rs — parse_tree → TreeRecord. Captures OBND / ICON / MODB / SNAM / CNAM / BNAM / PFIG. has_speedtree_binary() is the case-insensitive .spt predicate. Pre-S1 TREE fell into the generic MODL-only path and dropped every field but MODL.
• byroredux/src/systems/billboard.rs — make_billboard_system rotates any entity carrying a Billboard component. BsRotateAboutUp (the mode the spt placeholder uses) currently falls back to the world-up yaw lock — confirm that's still the behaviour and that the -Z front-face convention in import/mod.rs matches the rotation arc used here.
• byroredux/src/cell_loader/spawn.rs — attaches the Billboard component from cached.placement_root_billboard, and routes the placeholder ImportedMesh through the NIFAL boundary (crate::material_translate::translate_material).

Phase 1 ("S1") acceptance (ground truth — verify before reporting):

• TLV walker recovered against the FNV/FO3/Oblivion .spt corpus (133 vanilla files; Oblivion ≈ 113).
• Acceptance gate ≥ 95 % unknown-tag-clean rate, asserted in crates/spt/tests/parse_real_spt.rs (env-var gated, #[ignore]).
• Placeholder fallback: un-decoded trees render as a billboard card, never an Err out of the cell loader.
• .spt REFRs route to the SpeedTree importer, not NIF.
• --tree smoke path parses + imports.

Future phases (NOT shipped — do not flag as missing unless --focus explicitly includes them): real branch/leaf mesh recovery from the geometry tail, wind-bone animation from SNAM/CNAM, distance-LOD swap, baked-shadow lookup. SNAM/CNAM are parsed but not consumed (TD5-011 gate) — that's intentional, not a drop.

Parameters (from $ARGUMENTS)

• --focus <dimensions>: comma-separated dimension numbers. Default: all.
• --depth shallow|deep: shallow = walker contract + wiring review from source only; deep = also run the corpus harness against on-disk BSAs. Default: deep.

Extra Per-Finding Fields

• Dimension: Walker Byte-Accounting | Placeholder Fallback | TREE→Billboard Wiring | Per-Game Variants | Tag Dictionary | NIFAL Material Translation

Phase 1: Setup

1. mkdir -p /tmp/audit/speedtree
2. Dedup query (per _audit-common.md): gh issue list --repo matiaszanolli/ByroRedux --limit 200 --json number,title,state,labels --search "speedtree OR spt OR TREE" > /tmp/audit/speedtree/issues.json
3. Read the most recent docs/audits/AUDIT_SPEEDTREE_*.md report (sort by date — do not hardcode a filename here, it rots every cycle) and diff direction against it rather than re-deriving from scratch. Findings SPT-D4-01/02/03/04, SPT-D5-01/02, SPT-D2-01, SPT-D3-01, SPT-D1-01 are all closed (#994/#995/#996/#997/#998/#999/#1000/#1001/#1002). Of the later SPT-NEW batch, SPT-NEW-01 (dead-code detect_variant, #1820) and SPT-NEW-06 (format-notes byte-align, #1821) are also closed — only SPT-NEW-07 (MaybeStringElseBare misparse risk on a bare tag-13005 immediately before the geometry tail, #1822) remains open. Treat closed findings as regression guards, not open items; only re-flag if a guard has actually broken.
4. deep only — corpus + recon harness:
   • corpus location: Fallout - Meshes.bsa (FNV/FO3), Oblivion - Meshes.bsa.
   • acceptance run: BYROREDUX_FNV_DATA=… cargo test -p byroredux-spt --release --test parse_real_spt -- --ignored --nocapture (and _FO3_DATA / _OBL_DATA).
   • per-file dumps: the recon examples are feature-gated — cargo run -p byroredux-spt --features recon --example spt_walk (also spt_tagmap, spt_transitions, spt_dissect, spt_recon). Findings log to crates/spt/docs/format-notes.md.

Phase 2: Dimensions

Ordered by SpeedTree risk.

Dimension 1: Walker Byte-Accounting

Highest risk. The walker has no length-prefixed framing for most tags — it advances by the payload kind's fixed size. A single wrong size in the dictionary desyncs every subsequent read. Entry points: crates/spt/src/parser.rs (parse_spt, read_payload, TAG_MIN/TAG_MAX), crates/spt/src/stream.rs (SptStream LE readers + is_eof/remaining guards), crates/spt/src/parser.rs tests. Checklist:

• Each SptTagKind decode advances the cursor by exactly the bytes it claims: U8=1, U32=4, Vec3=12, FixedBytes(n)=n, String=4+len, ArrayBytes{stride}=4+count×stride, Bare=0. Cross-check read_payload against the dictionary in tag.rs for any size mismatch.
• The MaybeStringElseBare branch (tag 13005, #999) consumes the tag, then peeks the next u32 to decide Bare vs String. Confirm it re-syncs cleanly on both arms and that a None peek (EOF) can't panic — the tag_13005_at_eof_does_not_panic regression guard.
• Walker stops cleanly at is_eof() (sets reached_eof) and at an out-of-range / Unknown tag (records tail_offset + unknown_tags), never reads one tag past EOF.
• Pathological lengths: read_string_lp and ArrayBytes both cap at 64 KiB and bail with Err, not OOM. Confirm the cap is on the byte count (count × stride), not just count.
• parse_spt returns Err only on the two fatal conditions (missing magic, mid-payload underflow). In-range-but-unknown tags are non-fatal (recorded, walker stops) — this is the contract the placeholder relies on. Any new fatal-error path is a HIGH finding (it kills the cell-loader fallback).
• Endian: LE-only, unconditional (no version-gated readers — every .spt is __IdvSpt_02_). Flag any big-endian assumption or host-endian read.

Dimension 2: Placeholder Fallback Correctness

Entry points: crates/spt/src/import/mod.rs (import_spt_scene, compute_billboard_size, placeholder_billboard_mesh, placeholder_root_node, DEFAULT_BILLBOARD_WIDTH/_HEIGHT), byroredux/src/systems/billboard.rs. Checklist:

• import_spt_scene always returns a one-node / one-mesh ImportedScene — there is no Err path out of it. The only way the cell loader gets None is parse_spt returning Err (magic / underflow); confirm that path logs + skips the REFR without aborting the rest of the cell (graceful-degradation contract).
• Leaf-texture precedence: TREE.ICON override → .spt tag 4003 (first wins, #997) → unset (renderer magenta placeholder). Regression guards: leaf_texture_override_wins_over_spt_tag, empty_texture_leaves_path_unset_for_renderer_placeholder.
• Billboard sizing precedence in compute_billboard_size: OBND → BNAM → MODB → 256×512 default, every path clamped to [16, 8192] (#1001/#1002). OBND beats BNAM intentionally (BNAM clamps tall trees, e.g. WhiteOak01 OBND 802×1567 vs BNAM 768×768). Vanilla Oblivion ships MODB-only / no OBND, so an OBND-first-or-default path would render Cyrodiil pines at half scale. Guard the ordering.
• Normal / winding convention (#1000): front-face normal is -Z, indices [0, 3, 2, 2, 1, 0]. The billboard system rotates via Quat::from_rotation_arc(-Z, look_dir), so object -Z ends up facing the camera — the textured face must be -Z. Pre-#1000 normals were +Z and two_sided: true masked it. Guards: placeholder_normals_point_negative_z_for_billboard_arc, placeholder_index_winding_produces_negative_z_geometric_normal.
• bs_bound Z-up → Y-up swap (#995): center via byroredux_core::math::coord::zup_to_yup_pos, half-extents reshuffled (hx, hz, hy). Guard: placeholder_uses_obnd_bounds_when_present.
• Two-sided + alpha-test cutout: two_sided: true, alpha_test: true, alpha_threshold: 0.5, alpha_test_func: 6 (GREATEREQUAL), has_alpha: false (cutout and blend are exclusive).
• BsRotateAboutUp handling in billboard.rs::compute_billboard_rotation currently falls back to the world-up yaw lock (it lacks the local frame). Verify that approximation is still acceptable for tree imposters and documented — drifting it silently into pitch would tilt every tree.

Dimension 3: TREE → Billboard Wiring

Entry points: byroredux/src/cell_loader/references/mod.rs (is_spt dispatch) + byroredux/src/cell_loader/references/import.rs (parse_and_import_spt), byroredux/src/cell_loader/spawn.rs (placement_root_billboard → Billboard::new), crates/plugin/src/esm/records/tree.rs (parse_tree, TreeRecord, has_speedtree_binary). Checklist:

• The .spt route fires when the REFR's TREE base's MODL ends in .spt; TREE record is fetched from record_index.trees by the same form id resolved against index.statics. Mixed .nif + .spt REFRs in one cell must coexist.
• parse_and_import_spt returns the same CachedNifImport shape as every other model, with synthetic defaults the generic spawn path must not mis-read as NIF-rooted: placement_root_billboard = Some(BsRotateAboutUp) (#994), bsx_flags = 0 (#1214), root_flags = 0 (#1235), flame_attach_offset = None. Confirm the spawn site never assumes the placeholder carries a real NiAVObject root / BSXFlags / flame marker.
• spawn.rs actually inserts the Billboard component when placement_root_billboard.is_some() — without it the quad spawns static (this was SPT-D4-01, now closed; it's the regression guard for the whole dimension).
• TreeRecord field capture is lossless for the fields the importer reads (OBND→bounds, ICON→leaf_texture, MODB→bound_radius, BNAM→billboard_size). SNAM/CNAM are parsed-but-not-consumed (TD5-011) — don't flag as a drop, but DO flag if they're silently mis-parsed (e.g. CNAM length not shape-tolerant across the 5-float Oblivion vs 8-float FO3/FNV split).
• .spt files in BSAs resolve through the same extract_mesh lookup chain as .nif (sibling-BSA auto-load, AE pipeline-path strip if relevant) — no parallel "spt resolver".
• Cell unload despawns the placeholder entities cleanly; no leaked BLAS entries for the billboard quad.

Dimension 4: Per-Game Variants & Route Divergence

Entry points: crates/spt/src/version.rs (detect_variant, SpeedTreeVariant, MAGIC_HEAD), byroredux/src/scene/nif_loader.rs (parse_import_and_merge is_spt branch). Checklist:

• detect_variant recognises any __IdvSpt_02_-prefixed file but cannot tell V4Oblivion from V5Fnv at the magic level — it defaults to V5Fnv and the caller is meant to override via game context. Verify nothing downstream actually depends on the variant being correct today (the placeholder path is variant-agnostic); if a consumer branches on it, that's a real bug. Guards: detect_variant_recognises_idvspt_magic, detect_variant_unknown_for_non_speedtree_inputs.
• MAGIC_HEAD is the exact 20 bytes (u32 1000, u32 12, "__IdvSpt_02_"). A one-byte flip must reject → placeholder. Confirm no partial-prefix leakage (input shorter than 20 bytes rejects).
• Route divergence (the real per-game risk here): the cell-loader route threads TREE metadata; the --tree / loose route uses SptImportParams::default(). That means a loose-loaded Oblivion .spt gets the 256×512 default (no MODB sizing) and no ICON override. Confirm this is understood/documented and isn't masking a sizing bug that would also bite the cell route. Flag if the two routes have drifted in the parse call (they must both call parse_spt + import_spt_scene).
• Oblivion (SpeedTree 4.x) vs FO3/FNV (5.x): the parameter walker is assumed unified across all three (same magic, same tag dictionary). The geometry-tail layout is not confirmed unified — but the tail is out-of-scope for Phase 1, so only flag a tail-decode assumption if --focus includes the future phases.

Dimension 5: Tag Dictionary

Lower risk (fixed-size decodes), but a wrong size here is the Dimension-1 desync trigger, so spot-check rather than skip. Entry points: crates/spt/src/tag.rs (SptTagKind, dispatch_tag), crates/spt/docs/format-notes.md, recon examples (spt_tagmap, spt_transitions). Checklist:

• dispatch_tag currently maps ~120 tag values across the payload kinds. This is conservative-by-design: any tag not in the table → Unknown → walker stops cleanly. The old "~14 tags / 40-tag aspirational target" framing is stale — do not report dictionary size as a gap.
• Cross-check a sample of fixed-size assignments against the format-notes.md 2026-05-09 table and the tag.rs unit tests (fixed_byte_payload_tags, string_payload_tags, etc.): e.g. 8003/8005/8009 = 52 B, 13008 = 11 B, 13013 = 7 B, 12002 = 16 B, 12003 = 20 B, ArrayBytes 10002 stride 1 / 10003 stride 8. A size that contradicts the observed histogram is a Dimension-1 desync waiting to happen → MEDIUM.
• Confounder tags (4096, 5376, string-length values that fell in the tag band) must stay Unknown so the walker bails rather than misparses (unknown_for_out_of_dictionary_tags).
• Any tag observed in the corpus at ≥1 % frequency that's still Unknown should have a format-notes.md rationale; an undocumented common-tag bail is a LOW finding.

Dimension 6: NIFAL Material Translation for Placeholders

The placeholder ImportedMesh flows through the single NIFAL boundary like any other mesh. Cross-cuts /audit-nifal — report single-boundary / no-fabrication findings there, not here. Entry points: the material defaults in crates/spt/src/import/mod.rs (placeholder_billboard_mesh), byroredux/src/material_translate.rs (translate_material, consumed at the spawn.rs call site), crates/core/src/ecs/components/material.rs (Material::resolve_pbr). Checklist:

• The placeholder is canonicalised at the single translate_material boundary — no parallel "spt material" path that bypasses it (the --tree loose route and the cell route must both land here via spawn.rs).
• Non-PBR defaults survive translation: is_pbr: false, from_bgsm: false (#1076/#1077); metalness_override: Some(0.0) / roughness_override: Some(0.85) — explicit foliage defaults set at import (#1819/SPT-NEW-05, placeholder_billboard_mesh in crates/spt/src/import/mod.rs), NOT None. A regression to None re-opens the keyword-classifier substring collision (Boxwood→wood, Elderberry→glass). Guard: placeholder_billboard_sets_foliage_pbr_overrides_regardless_of_texture_path. resolve_pbr must fill the canonical metalness/roughness f32 from the non-PBR keyword path, never promote the billboard to metallic-roughness. SpeedTree never resolves a BGSM/BGEM (#1241/#1353) — guard that import-side PBR plumbing (a82366e9-style) left the billboard non-PBR.
• emissive_source: EmissiveSource::None (#1280) holds — a tree billboard must not pick up an emissive lobe.
• The two-sided alpha-test cutout maps to the correct canonical Material flags after translation (foliage silhouette preserved, not opaque-blitted).

Phase 3: Output

Write findings to docs/audits/AUDIT_SPEEDTREE_<TODAY>.md using the base finding format from _audit-common.md. Mark anything already covered by #994–#1002 as a regression guard, not a new finding. Suggest /audit-publish on completion.