Syplheed-Reborn/docs/re/ship-placement-runtime-capture.md

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).