# RE backlog

Open items that are *not* being worked right now. Each entry says what is wrong or
unknown, what evidence exists, and what the first step would be. Move an item into
`INDEX.md` (with a `structures/…md` or a parser + test) once it is actually settled.

---

## Capital ships assemble wrong in the viewer

**Reported:** 2026-07-30, by the user. **Status:** 🔎 **diagnosed 2026-08-10 — the
format layer is exonerated.** Runtime captures of three classes (`f105`, `e105`,
`e106`) at controlled range reproduce `assemble_ship` to ≤0.43 units in translation
and to 0.000 in rotation for every part that does not move; see
[`ship-placement-capture-generalisation.md`](ship-placement-capture-generalisation.md)
§4. So look at **the viewer**: first that it passes `include_external = true`
(`iso_loader.rs:4012` — with `false` an e106 loses its bridge and both nacelles,
5 parts instead of 11), then its own transform stack.

One real format-side bug was found on the way and is **fixed**: index-less parts
(`e105_brg`) never matched their `GN_Bridge_01` hardpoint, so 34 (stage, ship) entries
— `e102`, `e104`, `e105` across Stages 02–29 — assembled without a bridge. The other
apparent exception (`e105_eng_01` rotation) was an aggregation artefact and is 0.000.

The original report and its reasoning follow.

The reborn viewer builds capital ships from the split XBG7 parts via
`sylpheed-formats::ship::assemble_ship`, and they come out **wrong** — parts in the
wrong place / wrong orientation.

**Why this is a real finding and not a known limitation:** the RE write-up
[`ship-placement-runtime-capture.md`](ship-placement-runtime-capture.md) declares
static assembly ✅ **exact** as of 2026-07-26 — 9-channel joint tables
`[TX TY TZ RY RX RZ SX SY SZ]`, Euler `Ry·Rx·Rz`, with
`ship::tests::static_assembly_matches_runtime_capture` asserting static == runtime
capture (T < 1.0, R < 0.02). So either the viewer is not using that path, or the
claim generalises worse than the test suggests.

**The likely gap:** that test is **one ship** — the `e106` destroyer, 8 parts plus
two nacelles, two turrets and the hull mirror. Nothing pins the other classes.
Rules that were derived from `e106` and could easily be `e106`-specific:

- the engine cluster rig mounted at `GN_Engine_01` (two mirrored nacelles + centre);
- "X-reflect the shared-geometry twin whose lateral offset opposes the geometry's
  dominant side" — a heuristic, not a decoded flag;
- cross-id turret instancing (×2).

**First step (the oracle already exists):** re-run the runtime capture on a *different*
capital ship and diff static vs captured, exactly as `e106` was done — F10 in the
`capture-ship-placement` build of `xenia-canary-native` dumps the ship shader's
`c0..c2` WorldViewProjection rows per part; `WV_ref⁻¹ · WV_p` is the ship-space rigid
transform, which is ground truth. Pick a class whose rig differs from `e106`
(different engine count, a ship with no `sld`, a carrier). Then extend
`static_assembly_matches_runtime_capture` into a per-ship table so a regression in one
class cannot hide behind `e106` passing.

**Also worth ruling out first, cheaply:** that the viewer's own transform stack (scale,
handedness, node-instance recursion) is not re-breaking a correct assembly — compare
the viewer's placement against `assemble_ship`'s output directly before blaming the
format layer.

---

## Viewer: `include_external` is already on — that hypothesis is dead

**Checked 2026-08-11.** The item above names "first that it passes
`include_external = true` (`iso_loader.rs:4012`)" as the cheap first step. It
does: `ShipBrowser::show_external` defaults to `true`
(`iso_loader.rs:643`), the checkbox reads it (`ui.rs:1593`) and it is threaded
through `RequestShipRender` → `build_ship_model` → `assemble_ship` unchanged
(`ui.rs:1689`, `iso_loader.rs:4012`). So a ship rendered by the viewer is the
full external assembly, not the bare hull.

The viewer also does not have a transform stack of its own to blame: it bakes
`ScenePart::apply` straight into the vertices and rotates normals by the same
`p.m` (`iso_loader.rs:4030-4062`), so its placement is `assemble_ship`'s output
by construction. What remains unexcluded, in order of cheapness: the mirror
handling (`det < 0` reverses triangle winding only — a reflected part keeps its
reflected geometry), `Xbg7Model::models_named` resolving the wrong sub-model when
a resource name repeats, and the exhaust cones. **Next step is a visual**: the
diagnosis has run out of things it can settle by reading, so the viewer needs to
be run against a known-good class (`e106`) and its render compared with
`ship_render`'s.

---

## Viewer: the duplicate-resource-name hypothesis is dead too

**Checked 2026-08-11.** The diagnosis above left three candidates for why capital
ships assemble wrong in the viewer: mirror handling, `Xbg7Model::models_named`
resolving the wrong sub-model when a resource name repeats, and the exhaust
cones. The second is now **refuted**, and comprehensively.

`build_ship_model` resolves each placement with
`base.iter().find(|m| m.name == p.resource)` (`iso_loader.rs:4041`) — first match
wins — so a repeated resource name inside a container would silently draw the
wrong geometry. It cannot happen: decoding **every** XBG7 resource in **all 22
stage containers** gives **4 603 resources and zero repeated names**.

```
Stage_S01  62/62      Stage_S07 323/323     Stage_S13 290/290     Stage_S25 351/351
Stage_S02 304/304     Stage_S08 388/388     Stage_S14  22/22      Stage_S26 318/318
Stage_S03 214/214     Stage_S09 316/316     Stage_S15 386/386     Stage_S27 321/321
Stage_S04 179/179     Stage_S10   7/7       Stage_S16  65/65      Stage_S28 118/118
Stage_S05  92/92      Stage_S11 157/157     Stage_S24 162/162     Stage_S29 386/386
Stage_S06 266/266     Stage_S12 376/376
```

Per-ship it is tighter still: `e106` wants 9 distinct names and decodes exactly
9 models for 11 placements; `e105` 9 for 9; `f105` 5 for 6. Every placement
resolves to the one model it names.

**So two of the three candidates are gone** (this one and `include_external`),
leaving **mirror handling** and **the exhaust cones** — and the still-untried
visual comparison, which remains the right next step.

---

## Viewer: mirror handling and the exhaust cones are cleared too — the static avenue is exhausted

**Checked 2026-08-11.** Both remaining candidates were tested across every ship
on the disc, and neither shows the reported signature.

**Mirror handling.** The concern was that `ScenePart::apply` bakes `R·(S·v)+T`
while the viewer takes its winding-flip decision from `det(m)` alone and rotates
normals by `m` alone — both ignoring `s`. A mirror encoded as a *negative scale*
would then reflect geometry without flipping winding, drawing the part
inside-out. It never happens: across **1 485 assembled parts** in all 22
containers there are **22 mirrored parts, every one with `det(m) < 0`**, and
**zero** parts with a negative scale or a non-uniform one. `apply_twin_mirrors`
writes the reflection into `m` (negating its X column), so the viewer's flip
always fires, and ignoring `s` for normals is harmless because `s` is always
uniform.

**Exhaust cones.** These are the one piece of geometry the viewer *invents* — a
cone at each `GN_Jet`/`GN_SJet` frame, because the real engine geometry is
recessed and the game draws FX there instead. If they landed wrongly they would
read exactly as "a part in the wrong place". Across **335 assembled ships, 192 of
which have exhaust frames, not one cone sits outside its hull's bounding box**
(tolerance 10 % of the axis span).

**Caveat, stated rather than glossed:** "inside the hull box" does not prove a
cone is *right* — orientation and size are untested, and a cone could be wrong
while still inside. What it does rule out is the reported symptom for that part.

So every mechanism this diagnosis proposed is now eliminated: `include_external`,
duplicate resource names, mirror handling, and cones-in-the-wrong-place. The
format and assembly layers pass every static test available, and **the visual
comparison is no longer merely the next step — it is the only remaining one.**
Render `e106` in the viewer beside `ship_render`'s output of the same
`assemble_ship` result; if they agree, the bug is in neither and the original
report needs re-grounding against a specific ship and a specific expectation.