# XBG7 — mesh geometry (inside XPR2 model containers)

- **Confidence:** 🟡 `PROBABLE` for the single-stream layout (below); ❔ `HYPOTHESIS` / undecoded
  for the complex multi-stream body layout.
- **Parser in:** `sylpheed-formats/src/mesh.rs` (`Xbg7Model::from_xpr2`), tests
  `tests/mesh_disc.rs`. Container parsing reused from `src/texture.rs` (`Xpr2Header` /
  `Xpr2ResourceEntry`).
- **Applies to:** ship / weapon / prop models in `hidden/resource3d/*.xpr` (166 files).
- **Method:** clean-room — static hex inspection of the retail disc + geometric validation of the
  recovered triangles (non-degenerate area, indices in range, bbox matches the descriptor's stored
  size). No game code decompiled or copied.

## Where XBG7 lives

Models are ordinary **`XPR2`** containers (see the XPR2 texture doc / `texture.rs`). The 16-byte
resource-directory entries (from file offset `0x10`) carry `TX2D` texture resources **and** one or
more `XBG7` geometry resources:

```
entry = [ tag:4 ][ data_offset:u32 ][ descriptor_size:u32 ][ name_offset:u32 ]   (big-endian)
```

Offsets are relative to the directory base `0x10`. The `XBG7` *descriptor* (at `data_offset+0x10`,
`descriptor_size` bytes) is a **scene / material / node graph** — it holds node names
(`rou_f001_mnt1_root`, `Light`, …), a bounding value (`0x41F00000` = 30.0 ≈ the ship's ~30-unit
length), material names matching the `TX2D` channels (`_col` albedo, `_spc` specular, `_gls` gloss,
`_lum` luminance), and per-sub-mesh records. The **vertex / index buffers** live in the container's
shared **data section** (from `header_size`).

## Sub-mesh records (in the descriptor)

Read in file order by a sliding 4-byte scan; each is a big-endian tuple:

```
[ vtx_count:u32 ][ 0:u32 ][ idx_count:u32 ][ tail:u32 ]
   3..=65535        ==0     mult. of 3        1..=64
```

(For `rou_f001_wep_00`: `vtx_count=215`, `idx_count=1092` — matches the recovered geometry exactly.)

## The single-stream data layout — 🟡 PROBABLE (decoded, GPU-cross-checked)

For 36 of the 166 models (weapons, simple props) the data section is a straight sequence of
sub-meshes, carved from `header_size` in record order:

```
per sub-mesh block:
  [ 12-byte header  (contents undecoded) ]
  [ index buffer  : idx_count × u16 BE ]        triangle list (prim=4, GPU-confirmed)
  [ vertex buffer : vtx_count × stride bytes ]  ← declaration-driven
  (pad to 16 bytes → next sub-mesh)
```

The 12-byte header precedes the **index** buffer (the same block shape as stage
resources — see below); the vertex buffer follows the indices with no further
gap. (Earlier this was mis-modelled as `[index][12-byte gap][vertex]`, which put
the vertex buffer at the identical offset but read the index buffer 12 bytes too
early — turning the 12 header bytes into 6 junk indices = **2 leading degenerate
triangles** (a stray-triangle artifact) and dropping the last 6 real indices.
Skipping the header fixes the triangle list with no change to vertex coverage.)

**Vertex declaration.** The layout is **not fixed-stride**. The descriptor holds a declaration table
(right after the `(index_bytes, index_count)` marker) of `{offset:u32, format-code:u32, usage<<16:u32}`
big-endian triples, terminated by `offset == 0x00FF0000` / `code == 0xFFFFFFFF`:

| usage | element  | format code | format | size |
|-------|----------|-------------|--------|------|
| 0x00  | POSITION | `0x2A23B9`   | f32×3  | 12 B |
| 0x03  | NORMAL   | `0x1A2360`   | f16×4 (use xyz) | 8 B |
| 0x05  | TEXCOORD | `0x2C235F`   | f16×2 (u,v) | 4 B |

Stride = max element extent. Models **omit elements** → variable stride (20 = pos+normal, no UV;
24 = pos+normal+uv). Each element is read in **naive big-endian component order** (the raw file
bytes; see the endianness note below). Assuming a fixed stride-24 was why the old decoder mis-aligned
and declined the pos+normal-only models.

**Alignment pinned by the normals.** The vertex buffer starts at `index_end + 12` (a fixed 12-byte
header — NOT `align16`, which lands 4 bytes early on most models). The correct offset is the unique
one where recovered normals are exactly unit-length.

**Safety gate:** every index is validated `< vtx_count`, and when the declaration has a normal
element the mean recovered `|normal|` must be ≈1 (`[0.5, 2.0]`). A model failing either is rejected
(`MeshError::UnsupportedLayout`) rather than emitting garbage.

### Endianness — file bytes are naive-BE, `k8in32` is a red herring ✅

The Canary GPU capture reports every vertex stream with fetch `endian = 2` (`k8in32`, each 32-bit
word byte-reversed). This describes the **guest-memory** copy the GPU fetches — **not** the `.xpr`
file bytes. Reading the file with a `k8in32` transform breaks the normals (mean `|normal|` → 1.33);
the plain per-element big-endian read yields exactly unit normals. So the game **rearranges** the
vertex data between the on-disc `.xpr` and the uploaded buffer; the decoder reads the file directly
and must use naive BE.

## Stage containers — multi-resource, grouped pools ✅ (content-anchored)

`hidden/resource3d/Stage_S*.xpr` are not single models but **collections of
enemy / prop sub-models** — up to ~400 `XBG7` resources each (e.g. `Stage_S07` =
378). Their data layout differs from the weapon files:

- Each resource is a block `[12-byte header][index buffer][vertex buffer]`.
  Unlike weapons there is **no 12-byte gap between index and vertex** — the
  vertex buffer directly follows the indices.
- The **index count** is the descriptor's `(index_bytes, index_count)` marker
  (the *total* for the resource — a resource may have several sub-meshes summing
  to it), **not** the first sub-mesh record. The **vertex count** is a `u32`
  stored **32 bytes before** the marker.
- The blocks are **scattered through the data section, interleaved with the
  container's texture data**, in an allocation order that is **not** directory
  order and is **not stored** in any descriptor field we could find (the
  descriptor holds sizes — `rel 160 ≈ index_bytes+3`, `rel 164 =
  0x1000_0000 | (vertex_bytes+2)` — but no data offset). Reconstructing that
  allocation order is unsolved.

Because the offset is not stored, each resource's block is located by
**content**: parser `Xbg7Model::stage_models` does one `O(file)` pass per
distinct stride to find every **vertex-buffer start** (an offset whose NORMAL —
`f16×4` at vertex `+12` — is unit length while the *previous* stride slot's is
not, i.e. a run boundary; ~one candidate per block, not millions), then pins each
resource to the unique candidate where the `index_count` indices ending just
before it are all `< vertex_count`, reference nearly all vertices, and yield
non-degenerate triangles with real extent. This is fast (≤ ~2 s on a 70 MB
stage) and unambiguous (no two resources collide). Blocks that fail validation —
the few quantized hero bodies — are **skipped**, never emitted as garbage.

**Coverage:** 5662 sub-models decode across the 22 stage files (e.g. `Stage_S07`
366/378, `Stage_S10` 7/9 — including the main enemy bodies `e003`/`e005`, their
LODs, weapons, and props). The viewer (`spawn_stage_models`) lays the decoded
sub-models out as a side-by-side "cast sheet", skipping the few huge skybox-plane
resources (300 k-unit quads). Cross-checked: `e003` = 2383 v / 1436 t bbox
23.5×9.5×32.5; `e005` = 2566 v / 1507 t; both 0 degenerate.

## Not yet decoded — ❔ the complex body layout

The hero-ship *body* meshes (`DeltaSaber_A/_T/_W.xpr` resource `f004`, and ~100 other models) still
decline. Two open sub-problems: (1) **multi-sub-mesh models** whose *first* sub-mesh decodes but a
later one's inter-mesh offset isn't yet handled (the `align16` advance is a guess) — these are
declined whole; (2) the big body meshes, where the data section does not start with an index buffer
and the geometry sits at descriptor-addressed offsets. NB the GPU capture showed **every** rendered
mesh is *single-stream* (just wider strides, e.g. 44 bytes = pos + f32×3 + colour + 2×f16×4), so the
body is likely single-stream-with-a-richer-declaration rather than the "separate streams" first
guessed — it was simply not rendered in the captured session (menu only). A capture taken *in a
mission* (where `DeltaSaber` renders) would hand over its exact declaration directly. `DeltaSaber_A`'s
5 XBG7 blocks are `f004` (body) + `_rou_f004_mnv01_L/_R`, `_mnv02`, `_turn180` (maneuver / pose).

## Evidence log

- 2026-07-12 — `rou_f001_wep_00.xpr`: XPR2 dir = 1×XBG7 (`f001_wep_00`) + 3×TX2D
  (`_col/_gls/_spc`). Data section starts with a u16-BE index run (max 214), then stride-24
  vertices. Descriptor record `[215,0,1092,4]` at desc `+0x2B0`; index-buffer byte size `0x888`
  (=2184=1092×2) at desc `+0x1A0`. Recovered mesh = 215 v / 364 t, 362 non-degenerate, median tri
  area 0.015 — coherent. → single-stream layout **PROBABLE**.
- 2026-07-12 — descriptor-driven sequential carving over all 166 models: **42 carve** under the
  index-only check. Real 3D extent confirmed on `rou_f001_wep_04` (bbox 3.7×1.2×3.5).
- 2026-07-12 (refine) — attribute ranges differed per model (`wep_00` UV≈attr0/2, `wep_04`
  attr4/5, `wep_03` attr1≈±60000 = garbage) → **refuted the fixed "6 half attrs, UV=attr0/2"
  reading.** Found the descriptor **vertex declaration** (offsets 0x0C/0x14, usages 0x03 NORMAL /
  0x05 TEXCOORD). Solved the vertex-base offset with a **unit-normal validator**: `index_end + 12`
  gives median `|normal| = 1.000` on every weapon model (`wep_00/02/03/04`), vs `align16` landing 4
  bytes early. UVs then land in `[0,1]`. Adding the normal gate: **25 models decode clean +
  normal-valid** (the rest — incl. `Stage_S*` degenerate blobs — correctly declined).
- 2026-07-12 (DYNAMIC) — added a cvar-gated draw logger to Canary
  (`command_processor.cc::LogDrawForRE`, cvar `log_draws`), captured the Ready Room / Briefings.
  **GPU ground truth confirmed the static layout exactly**: primitive `prim=4` = **triangle LIST**
  (settles the list-vs-strip question), and a stream with `f32x3 @offset0` + `f16x4 @3dw` +
  `f16x2 @5dw`, stride 6 dwords = 24 bytes — matching `POSITION@0, NORMAL@0x0C, TEXCOORD@0x14`.
  Revealed **stride varies** (24, 20, 28, 44 …) and that all streams are **single-stream** →
  parsed the declaration for variable stride: coverage **25 → 36** (e.g. `wep_05` is pos+normal,
  stride 20, no UV — previously mis-aligned). Also confirmed the endianness note above: fetch
  `endian=2` (k8in32) is the *guest* copy; file stays naive-BE. `Stage_S*` now decode (stride 20).
- 2026-07-12 (STAGE) — `Stage_S*.xpr` decoded as multi-resource containers. Found each geometry
  block is `[12B hdr][index buffer][vertex buffer]`, index count = descriptor marker (total, e.g.
  `e003` = 4308 spanning 2 sub-meshes), vertex count = `u32` at marker−32 (`e003` = 2383 → verified
  by max-index 2382 and 0 degenerate tris, bbox 23.5×9.5×32.5). Blocks are scattered among texture
  data with **no stored offset** (descriptor rel 160 = idx_bytes+3, rel 164 = `0x1000_0000 |
  (vtx_bytes+2)` are sizes, not offsets; block starts e.g. `e003`@0x10230, `e003_l`@0x5d000,
  `e005_l`@0x9b000 are not directory-ordered). Solved by **content anchoring**: a single per-stride
  pass finds vertex-run starts (unit NORMAL at +12 whose previous slot isn't), then match each
  resource by strict index+triangle validation. **5662 sub-models decode across 22 stages** (S07
  366/378), incl. the previously-declined main bodies `e005` (2566 v) and weapons — ≤2 s on 70 MB.
  Parser `Xbg7Model::stage_models`, tests `stage_models_{decode,sweep,quality_audit}`.
- 2026-07-17 — **triangle-LIST re-confirmed; a strip interlude refuted; winding-consistency gate
  added.** A 2026-07 change had briefly re-read the index buffers as triangle *strips* (to "fill
  holes"). Refuted objectively with a new `XVERIFY` diagnostic that compares both readings by
  **stored-normal agreement** (each triangle's cross-product face normal vs the sum of its vertices'
  stored normals): the LIST reading gives agreement **1.000** on every clean weapon (`wep_00/03/04/19`
  = only possible with correct topology + winding), the STRIP reading **~0.49** (random). The strip
  reading also over-generated ~2.5× the triangles (wep_00: 938 vs 364) — a hole-filling garbage soup.
  Reverted to LIST in both paths (`from_xpr2`, `read_pool_mesh`), matching the `prim=4` GPU capture.
  Added an objective **winding-consistency gate** `max(na, 1-na)`: a correct carve is internally
  consistent (agreement ≈1.0, or ≈0.0 for inverted-but-consistent winding — a real single-sided
  mesh), a mis-carve scatters to the ≈0.5 middle. `from_xpr2` declines sub-meshes below 0.90 (e.g.
  `wep_23` na=0.398 → declined instead of a spike-mess); the single-model **content-anchor fallback**
  gates at 0.85; the large multi-resource **stage** path stays ungated (its enemy meshes span a
  continuous 0.5–1.0 consistency range — a hard gate there dropped ~48/314 legit S07 blocks).
  **Routing fixed:** `decode_models` (CLI) and the viewer now route by `count_xbg7` (1 → validated
  records-based list decode, fallback to strict-gated anchor; >1 → stage anchor) instead of the old
  "whichever decoder yields more verts" rule — that rule let stage content-anchoring win on
  single-model weapon files and fabricate **phantom** blocks (a `wep_00` clone appearing inside
  `wep_19`), duplicates, and spike-mess anchors. Weapons now: 33 clean-decode / 26 declined (declined
  = genuinely multi-stream or un-carvable, shown as nothing rather than garbage); stage coverage
  unchanged (S07 314). `expand_triangle_strip` retained as an `XVERIFY`-only diagnostic.
- 2026-07-12 — `DeltaSaber_A.xpr` body: data does **not** begin with indices; plain-`f32×3` runs
  with ship-scale extent (span ≈27–34, matching bbox 30.0) found only at high offsets
  (`data+0x28634C`, …) → multi-stream, **undecoded**.
- 2026-07-18 — **GROUPED-POOL layout cracked → the hero ship (Delta Saber) fully decodes.** The
  detailed models (`DeltaSaber_*.xpr` + ~100 others) were declined for **location**, not format —
  their vertex format is the standard stride-24 triangle list. A resource's *several* sub-meshes
  don't interleave `[idx][vtx]` per block; they share **two grouped pools**: an **index pool**
  (buffers concatenated in descriptor-marker order, each **4-byte aligned**) followed by a **vertex
  pool** (each sub-pool `vtx_count × stride`, same order), with the index pool ending **exactly**
  where the vertex pool begins. So the whole resource pivots on one unknown, the first vertex-pool
  start `vb0` (= index-pool end, found by the unit-normal vertex-run scan); everything else is
  derived: `ib0 = vb0 − span`, `ib[i] = align4(ib[i-1] + idx_count[i-1]·2)`,
  `vb[i] = vb[i-1] + vtx_count[i-1]·stride`. Reversed statically from `DeltaSaber_T.xpr` and
  **cross-checked against a Canary GPU draw-log capture** (mission ship = `DeltaSaber_T.xpr`, found
  via the `--log_file_io` kernel hook): `f001` = body (idx@`data+0xC` = 0x5500C, vtx@0x61ACC, 10891 v
  / 8187 t) + **7 detail parts** (fins/cockpit/wingtips, markers at descriptor 0x3BEC…0x58FC) =
  **8650 tris**, and **every sub-mesh decodes at 0 degenerate / full coverage / winding-agreement
  1.000**. This is the layout the per-block adjacency anchor (`ib = vb − idx_bytes`) rendered as a
  **spiky phantom** (it read 24561 indices starting 2782 B too late, agree 0.64, 1277 degenerate).
  Insight: a single index marker reduces the grouped model to `index_end = vb0`, i.e. the existing
  adjacency `ib = vb − idx_bytes` — so grouped **generalises** the single-block anchor (n=1 is
  identical). Implemented as `anchor_grouped_meshes` (mesh.rs): `anchor_models` routes resources with
  >1 index marker to it (validated per sub-mesh; on failure falls back to the old first-marker
  adjacency anchor so stage coverage never regresses); single-marker stages/props keep the exact
  prior path. The shared acceptance test is factored into `validate_block` (the connectivity
  heuristic is relaxed for *derived* grouped parts, which are pinned by in-range + consistency, so
  small flat fins aren't mis-rejected). Render self-check: `sylpheed-cli mesh render DeltaSaber_T.xpr
  --only f001` (exact-name match excludes the `_rou_f001_mnv*` animation poses) → clean complete
  fighter. Test `hero_ship_grouped_pool_decodes`. Colours/UVs still pending the running-game oracle.
- 2026-07-18 (refinement) — **4-byte vertex-pool alignment + weapon recovery.** The grouped-pool
  rule "index pool ends exactly where the vertex pool begins" is really "the vertex pool is **4-byte
  aligned** after the index pool": `vb0 = align4(ib0 + span)`, so 0..=3 bytes of padding can sit
  between them. DeltaSaber's index pool ended already-aligned (pad 0), which hid this; **19
  weapon/`*_hangar` models** (single- and multi-marker: `wep_08/11/34/58/62/69/81/83…`) have pad 2
  and so decoded to *nothing* — the viewer then showed them as a flat 2D texture instead of a model.
  Fix: both anchors try `pad ∈ 0..=3` (`ib = vb − idx_bytes − pad` for the single-block adjacency
  anchor; `ib0 = vb0 − span − pad` for the grouped pivot), validated — a wrong pad reads shifted
  indices → agreement collapses < 0.85, so only the true pad passes. pad>0 in the ungated stage path
  is gated at a strict 0.85 to avoid a false anchor; pad 0 keeps its exact prior behaviour (stages
  unchanged). Result: all 19 now decode as clean models (e.g. `wep_34` 1243 v / 1233 t, a
  long-barrelled gun-pod; `wep_08` 3 sub-meshes / 478 t). Viewer routing already falls through
  `from_xpr2` → `anchor_models(0.85)` for single-XBG7 files, so the recovered grouped/padded weapons
  now preview as meshes.
- 2026-07-18 (refinement 2) — **pivot on the largest sub-mesh; all 19 recovered.** Three weapons
  (`wep_81`, `wep_81_hangar`, `wep_30_hangar`) still declined because the grouped pivot validated
  `markers[0]`, which for these is a tiny *elongated* lead bracket that fails the connectivity gate
  even when perfectly placed. Fixed by pivoting the alignment check on the **largest** marker (max
  index count) — the sub-mesh whose triangle-quality/connectivity signature most reliably confirms
  `(ib0, vb0)`. Once the pivot validates, markers up to it are read unconditionally (a legitimately
  tiny/flat lead part may fail the quality gates yet still be real), and markers after it stay
  validated so a stray trailing marker ends the chain. Result: **all 19 previously-declined weapons
  decode** (`wep_81` 460 t missile w/ tail fins; `wep_30_hangar` 334 t). DeltaSaber unchanged (its
  body IS the largest marker → same pivot). 7/7 disc tests green, stage quality audit unchanged.
