# Capital-ship part placement — runtime capture (ground truth)

**Status:** pipeline working end-to-end for one ship (`e106` ADAN Destroyer);
correlator recovers exact ship-relative transforms. Not yet baked into the viewer.

## Problem

Capital ships ship as **split XBG7 parts** (hull bodies, bridge, engines, turrets)
in `hidden/resource3d/Stage_SNN.xpr`. Reconstructing the whole ship needs each
part's placement. Static reverse-engineering of the composite scene graph
(`e_rou_<id>`, `GN_*` frames) got the **hull right but external parts only
approximately** — a `GN_*` frame is the mount *pivot*, and the real outboard /
rotational offset lives in a detail sub-rig or in runtime code not recoverable
statically. See `sylpheed-formats::ship::assemble_ship` (the static best-effort).

## Finding

The **guest vertex buffer holds LOCAL coordinates** — byte-identical to the
`.xpr` (verified: `e106_bdy_04`'s first vertex `(-20.0000,153.1539,45.9997)`
matches the decode exactly). So parts are placed **entirely in the vertex
shader**; the buffer carries no placement. (Contrast: static *stage* geometry is
pre-transformed into world space and byte-matches the `.xpr` at file offsets —
that's a different case.)

The per-part transform is in the **vertex-shader float constants**. For the ship
shader (`vs=0xC7F781F4C1D58054`), constants `c0..c2` are the **WorldViewProjection**
matrix rows:

- Their norms are `(sx, sy, 1)` = the projection x/y scales; `sy/sx ≈ 1.78 = 16:9`
  aspect. Dividing each row by its norm gives the rigid **WorldView** (verified:
  normalized rows orthonormal to 1e-4, `det = +1`).
- The camera View cancels when expressing every part relative to a reference part:
  `rel_p = WV_ref⁻¹ · WV_p` = `(Rᵀ_ref·R_p , Rᵀ_ref·(T_p − T_ref))` — a pure
  ship-space rigid transform. Applying `rel_p` to part `p`'s local geometry
  assembles the ship exactly, in the reference part's frame.

(`c4..c14` also vary per part — likely a normal matrix / a second WVP variant /
WorldView split; not needed, `c0..c2` suffice.)

## Capture instrumentation

Branch **`capture-ship-placement`** in the `xenia-canary-native` worktree (off the
`instrument-current`-descended native branch, which already has GPU `log_draws`).

- **F10** in the emulator window → `xe::gpu::RequestShipCaptureFrame()`.
- Dumps the next ≤8000 draws (de-duped by vertex-buffer address) to
  **`xenia_ship_capture.log`** in the binary's dir
  (`build/bin/Linux/Release/`). Per draw:
  ```
  DRAW vbase=0x… stride=… vcount=… indices=… prim=… vs=0x…
    pos: (x,y,z) …            up to 64 LOCAL vertex positions (== .xpr)
    vsconst base=N: c0=(x,y,z,w) c1=… …   first 48 VS float4 constants
  ```
- Files: `src/xenia/gpu/command_processor.{cc,h}` (`CaptureShipDrawForRE`,
  `RequestShipCaptureFrame`), `src/xenia/app/emulator_window.cc` (F10 key).
- Run: `run-canary-native.sh` (HW Vulkan, interactive). Play into the mission,
  frame the ship side-on, press F10.

## Correlator

`sylpheed-formats/examples/correlate_capture.rs`:
```
SYLPHEED_ISO=… cargo run --release --example correlate_capture -- \
    <capture.log> <Stage_SNN> <ship_id> [ref_part_substr]
```
Parses the log, decodes the ship's base parts, matches each part to a draw by
**vertex count** (unique per part), recovers WorldView from `c0..c2`, and prints
each part's ship-relative transform (reference-part frame) + assembled extent.

## Validated result — `e106` ADAN Destroyer (Mission 1 / Stage_S01, side view)

Vertex-count → part map (all matched exactly): bdy_01/02 = 1261, bdy_03 = 815,
bdy_04 = 1633, eng_01 = 583, eng_02 = 491, wep_02_01 = 1002, brg_01 = 202.

Ship-relative placement (reference = `bdy_04`, aft hull):

| part | T (ship-relative) | note |
|---|---|---|
| bdy_04 | (0, 0, 0) | reference, aft hull |
| eng_01 | (131, −133, −132) | **aft** (−Z), starboard | 
| eng_02 | (0, −49, −140) | aft, centre |
| bdy_01 | (**−264**, −150, 1165) | **port** hull, forward |
| bdy_02 | (**+264**, −150, 1165) | **starboard** hull, forward |
| bdy_03 | (0, −35, 1076) | forward keel/spine |
| wep_02_01 | (0, 49, 1009) | forward weapon |

Assembled extent **872 × 670 × 2181** (a coherent ~2181-long destroyer).

**Key wins vs static:** `bdy_01`/`bdy_02` are a **port/starboard pair** (X = ∓264),
which static had overlapping at one point; engines land correctly aft; the ship is
not the 1894-tall tower the static Y put out.

**Missing:** `brg_01` (bridge, vcount 202) was **culled/occluded** at that camera
angle (0 draws) — needs a second capture framing the superstructure.

## Next steps

1. **Bridge:** re-capture with the bridge visible → fills `brg_01`.
2. **Bake:** promote the correlator to a module; emit a per-ship placement table
   (`ship_id → [(part, R, T)]`) as a checked-in data file; have the viewer's
   `build_ship_model` prefer the captured table over `assemble_ship` when present.
3. **Other ships:** same capture for the cruiser (`e105`, Stage_S02), ACROPOLIS
   (`f101`), TCAF cruiser/destroyer (`f105`/`f106`). One side-on F10 each.
4. **Turrets:** the shared `e3NN`/`e4NN` gun models appear as their own draws in the
   capture too — correlate them to place turrets (static never resolved these).