# Mod "Mirror mode" (`mods.mirror.MirrorMod`, id `mirror`, GAMEPLAY)

Mirrors every playable level left-right (x → W − x on the W×H tile grid), like the mirror mode of
racing games. Controls are unchanged (the world is mirrored, not the input).
Code: `mods/mirror/Mirror.java` (pure byte transform), `MirrorMod.java`; test `MirrorTest`.

## Options

| Key | Default | Meaning |
|---|---|---|
| `worlds` | `all` | `all` or world numbers, e.g. `1,3` (9 = TestLevel / Test World) |
| `readableTextures` | false | on: face flag 0x20 (mirror U) is toggled so writing reads normally, but textures that continue across faces get seams (fountain water rim in the village); off: true mirror image |
| `objects` | false | also mirror rigid STPC object models (verts, face planes, collision); skinned characters never |

`order()` = 75: after `testworld` (50, so the Test World is mirrored too) and before
`debug-wireframe` (100, so the overlay is built from the mirrored collision and lines up).
Files: `wads/t1l<w>m<c>.wad` only (front end `t0*`, map `t6l6*` stay).

## What is transformed (all chunk sizes unchanged → STPC absolute offsets stay valid)

| Data | Transform |
|---|---|
| TRAK models (all, shared by tiles) | vertex x, normal x, face plane nx negated; bbox centre x negated, corner pairs (−x/+x) swapped + x negated; collision: i8 nx of all 5 planes negated (d unchanged; −128 → 127) |
| Face winding | **kept**. groove.exe sets `D3DRENDERSTATE_CULLMODE = D3DCULL_NONE` once (decomp ~line 31587, Device3 vtable +0x58) and culls in software with the stored face plane. Reordering vertices would break the fixed UV corner patterns (pattern 0x800+0x10 has no odd permutation that keeps the UVs). |
| MAP tiles (float A) | x → W − x, rot → (4096 − rot) & 0xfff |
| MAP tiles (fixed B) | x → W·4096 − x, rot as above (A and B stay exactly consistent) |
| Region grid, tile-swap block (cell x) | columns mirrored (cell → W − 1 − cell) |
| Strats | x → W·4096 − 1 − x (keeps `floor(x)` in the mirrored cell: LevelDoor compares cells), ry and rz → −angle, rx kept |
| Waypoints / camera nodes | same as strats (x, ry, rz) |
| 90-byte "env zones" | really **teleport/respawn entries** (FUN_0054d560 / FUN_0054de00): f0..2 = trigger cell (x,y,z), f3..5 = target position (fixed), f7 bits 4..11 = facing angle (bits 0/1 = y compare flags). Mirrored: cell x, position x, angle. Default entry (pos 1,1,1) untouched. f11 (+0x2c) = sky model yaw offset, kept. |
| LGHT (FUN_0042c180) | u32 n; per light u8 type, r, g, b; type 1 directional: 3 f32 → x negated; types 2/4 point: 3 f32 + 2 f32 + u8 → x → W − x |
| STPC (option) | rigid models (nBones = 0) incl. WFPC&0x100 extra models, same as TRAK models |
| Vertex colours | unchanged (vertex order unchanged) |

Rotation rule: mirroring x conjugates rotations; about y and z they become −angle, about x
unchanged. 0 = facing +z, so facing (sin a, cos a) → (−sin a, cos a) = angle −a.

Mirroring twice gives the original WAD byte for byte (except LGHT float rounding and −128 normals) –
checked in `MirrorTest`, together with: every render vertex and collision corner of every tile of
t1l1m001 lands exactly at the mirrored world position, region per tile cell preserved.

## Not mirrored / known limits
* Strat bytecode is untouched: hard-coded offsets/directions in scripts (if any) are not mirrored.
  Parameter-pool values are not interpreted (could contain coordinates/angles for some types).
* Object models stay unmirrored by default (placed and turned mirrored, so their collision matches
  their own look). Skinned/animated characters are never mirrored (bone matrices and compressed
  vertex deltas of the anim sets would need conjugation).
* Sky/background models (STPC, drawn camera-relative) are not mirrored.
* Attract-mode demos (`*.DEM`, recorded input) desync in mirrored levels (harmless).
* Textures are mirrored by default (true mirror); `readableTextures` flips them back per face,
  which breaks textures that are laid out across several faces.
* **River intro fly-by (t1l3m001)**: the camera partly shows an empty grey view and the intro
  dialog starts earlier than in the original. No crash; after the intro the river plays normally.
  Most likely a hard-coded lateral camera offset in strat code (camera ends up in a cell whose
  visibility zone hides the river). The village fly-by is correct.

## In-game verification (apptest, display :7, 2026-11-06)
* **Village t1l1m001**: same key sequence on original and mirrored – a perfect mirror image
  (house/fountain swapped sides), llama stands on the ground, walls/fountain rim block, coins
  collect (counter matches the original run), Pacha dialog triggers, camera rotate (A) works,
  intro fly-by (not skipped) correct. With `debug-wireframe` + `objects=true`: overlay lines up
  with the mirrored geometry, no crash.
* **River t1l3m001**: loads, raft/tree floats on the mirrored river, auto-scroll and steering work,
  coins collected, wireframe lines up; fly-by see limits.
* **City t1l6m001**: loads, intro cutscene with Pacha (cutscene cameras) fine, walls and crates
  block, wireframe lines up.
* **Test World** (`testworld,mirror`, world 9): mirrored island (parkour stairs on the right),
  walking/jumping fine.
* Images: `notes/img/mods/mirror_01_village.jpg` (original vs. mirror), `mirror_02_wireframe.jpg`
  (village + city with the debug wireframe), `mirror_03_river_testworld.jpg`.