meat/AGENTS.md

# meat

A from-scratch browser **first-person-shooter game engine** with a configurable
**PS1 aesthetic**. Everything is hand-written: a **software rasterizer** draws
textured triangles into a low-res CPU framebuffer, then Canvas2D blits it
upscaled. No WebGL/WebGPU, no game-engine dependencies.

This file is the durable project brief (checked in, auto-loaded). The per-topic
rules live in `.agents/rules/*.md`.

## Guiding decisions (the "why", not derivable from code)

- **PS1 look is a set of live knobs**, dial-able from full-PS1 to clean. See
  `RenderConfig`. Nothing about the era is hardcoded into the renderer.
- **Software rasterizer on purpose** — the PS1 look leans on rasterizer traits
  (vertex snap, low-res + dither/banding, no mipmaps). Raytracing was considered
  and cut (it removes the very artifacts we want). Note: **affine texture "swim"
  was dropped** — texturing is always perspective-correct now (it warped badly on
  the big outdoor terrain); the other era knobs stay.
- **Minimal architecture.** Scene-graph-lite / plain data + functions.
  Deliberately **not** ECS or any "Big Game Architecture." Prefer the smallest
  clear structure; add knobs to experiment rather than abstractions.
- **Culling beats batching here.** A "draw call" is just a JS loop (no GPU state),
  so merging the world into big meshes only defeats visibility skipping. Instead
  the outdoor world is stored as spatial **chunks** that are frustum-culled per
  frame; on-screen solids also **backface-cull**. This is what keeps a dense world
  (thousands of trees/rocks) affordable — off-screen content costs ~nothing.
- **2D assets only.** Sprites/billboards (PS1-style), **no 3D model loading**.
- **Engine is headless.** `engine/` has no DOM types and could run server-side;
  all browser glue (canvas, input, image decode) lives in `app/`.

## Stack & tooling

- **Bun** runtime + `bun test`. **TypeScript 7** (native `tsc`), strict,
  `moduleResolution: bundler`. **Vite 8** serves/builds the client (ES-module
  workers; dev/preview serve COOP/COEP headers so the multi-threaded renderer's
  `SharedArrayBuffer` works — a static prod host must send them too, or the app
  falls back to single-threaded).
- **oxc** toolchain: `oxlint` + `oxfmt` (no eslint/prettier).
- commitlint + husky (Conventional Commits), OpenSpec change workflow,
  forge-sync (Forgejo). Templates generated by `regime` from a `sigitex:` source.

## Commands

- `bun start` — Vite dev server; open the printed URL. Edits hot-reload.
- `bun run build` — production build to `dist/`.
- `bun run assets` — regenerate the placeholder PNGs in `/assets`.
- `bun run bench:browser` — Playwright: drive headless Chromium through the
  `?bench=st`/`?bench=mt` flythrough, print single-thread vs worker frame timings
  (median/p95/max). The real-browser profiler; run it on the target machine.
- `bunx tsc --build tsconfig.app.json` — **typecheck the app+engine graph. Use
  this**, not `bun run check` (see Caveats).
- `bunx oxlint engine app` — lint.
- `bun test` — tests (none yet).
- `bun run serve` — Bun server (`server/server.ts`, a stub for now).

## Layout

- `engine/` — headless engine, consumed by `app/` via tsconfig project ref.
  - `math/` — `Vec2`, `Vec3`, `Mat4` (column-major, OpenGL-style; verified).
  - `render/` — `Color` (packed RGBA, little-endian = canvas ImageData order),
    `Framebuffer` (Uint32 color + Float32 1/w depth; `quantize` = color-depth +
    Bayer dither), `RenderConfig` (the look dials + presets), `Rasterizer`
    (optional backface cull per draw), `Frustum` (6 planes from the viewProj +
    AABB test, for chunk culling), `Texture` (nearest/bilinear, wrapping, no
    mipmaps), `Sky` (gradient + sun + procedural clouds; renders at 1/`step` res).
  - `scene/` — `Camera` (fps yaw/pitch; far plane reaches the outdoor peaks),
    `Mesh` (indexed tris; verts stored flat: `STRIDE` floats x,y,z,u,v per vertex,
    no per-vertex objects — cache-friendly + alloc-free to draw), `Sprite`
    (Y-axis billboard), `Terrain` (procedural
    heightfield around the room: flat clearing in the center, rolling hills, tall
    edge peaks. `Terrain.patch` builds one ground patch over a rectangle -- called
    per chunk, aligned so patches weld crack-free, with a hole for the room;
    `Terrain.height` is the shared ground-height sampler for the player), `Tree`
    (procedural low-poly oak/spruce geometry, sapling..full via a `growth` knob;
    `Tree.build` appends into shared trunk + foliage meshes), `Boulder`
    (procedural low-poly rock: a squashed, jittered, part-buried sphere;
    `Boulder.build` appends into a shared mesh), `Bush` (cluster of small leaf
    blobs, shares the oak leaf texture/mesh), `Flower` (thin stem + colored bloom;
    samples a 2x2 color-atlas texture, drawn double-sided).
- `app/` — browser glue.
  - `main.ts` — game loop: input, sim, preset switching, per-frame culling, then
    the non-blocking pump (`renderer.dispatch`/`done`) + `present` (GPU/CSS
    upscale) + a multi-line frame HUD (`work + present` critical-path ms, vsync
    interval, visible chunks / LOD-aware tri count — the real profiler in play).
  - `renderer.ts` — the render driver. When the page is cross-origin-isolated it
    runs a pool of `render-worker.ts` threads (`MAX_WORKERS`) over a
    `SharedArrayBuffer` framebuffer, each owning a disjoint row band, synced by a
    lock-free `Atomics` barrier; otherwise it renders inline. `dispatch`/`done`
    are non-blocking so the caller paces on rAF. `?bench=st|mt` A/Bs the paths.
  - `renderScene.ts` — `renderBand(fb, scene, …, y0, y1)`: the single source of
    render truth (sky + room + culled chunks + sprite + quantize for a row band).
    Used full-height by the inline path, per-band by each worker. `Scene` bundles
    the meshes/textures so it clones to a worker whole.
  - `assets.ts` — load `/assets/*.png` → `Texture` (zero-copy; ImageData bytes
    are already the `Color` layout).
  - `level.ts` — builds the playground: a flat stone-floored room (three thick
    walls via `slab`, north side open) always drawn, in the center of a big grassy
    `Terrain` world (~20x across). Props are placed first (`placeTrees` /
    `placeBoulders` / `placeBushes` / `placeFlowers` → instance lists + colliders;
    `TREE_/BOULDER_/BUSH_/FLOWER_COUNT`/`_SEED`/`_REACH`), then `buildChunks` bakes
    terrain + props into a `CHUNK_GRID` x `CHUNK_GRID` grid of `Chunk`s (each =
    per-texture meshes grass/bark/leaf/needle/rock/flowers + a tight AABB) that
    `main` frustum-culls; bushes fold into the leaf mesh, flowers get their own
    (drawn double-sided). Trees + boulders are baked **twice** — full geometry and
    a low-poly impostor (`barkFar/leafFar/needleFar/rockFar`, via the builders'
    `lod` arg) — so a far chunk can swap to the cheap set with no per-frame work
    (see `chunkFar` / `RenderConfig.lodDistance`). `Aabb` colliders (walls, crate, grown trunks, big
    boulders), NPC position, `TERRAIN`/`TERRAIN_SUBDIV`/`GROUND_UV`, sky/cloud config. The stone floor is lifted by `FLOOR_LIFT` (a z-bias) so it
    stays clean over the terrain skirt that laps under the room edges. Room surfaces
    are single flat quads -- no subdivision needed since texturing is
    perspective-correct.
  - `player.ts` — feet-cylinder player: gravity/jump + Shift-run
    (`RUN_MULTIPLIER`) + circle-vs-AABB/-circle collision, substepped so fast
    running can't tunnel walls; ground height from `Terrain.height` (plus
    standable AABBs).
- `index.html` — Vite entry at repo root; holds the `#screen` canvas and the
  `#fps` meter div (styled inline).
- `scripts/gen-assets.ts` — procedurally draws the placeholder textures and
  writes PNGs (hand-rolled encoder via `node:zlib`). Run via `bun run assets`.
- `assets/` — generated `floor/grass/bark/leaf/needle/rock/flower/wall/crate/npc`
  PNGs (`floor` = room stone, `grass` = outdoor ground, `bark`/`leaf`/`needle` =
  tree trunk/oak/spruce, `rock` = boulders, `flower` = 2x2 bloom-color atlas).
  Swap for real art anytime;
  filenames are the contract.
- `server/` — Bun server stub. `shared/` — isomorphic slot.

## Frame pipeline (`app/main.ts` `tick`)

`Player.update` → build `Camera` → `Camera.viewProjection` → `visibleChunks`
(frustum-cull, once on the main thread) → `renderer.dispatch` (non-blocking) →
next rAF: `renderer.done()` ? `present` : skip this vsync. Frame N is presented
while N+1 is dispatched; the pump never blocks or async-awaits, so it can't
desync from rAF.

**`present` is a GPU/CSS upscale, not a CPU blit.** The `#screen` canvas backing
store *is* the internal render resolution, so `present` is one internal-res
`putImageData` (viewport-independent, ~fixed cost). The browser compositor scales
the element to the display via CSS — `layout()` sets the element's pixel size to
an integer multiple (crisp letterbox, centered) once per resize/config, and
`image-rendering` follows `upscaleFilter` (`pixelated` for `nearest`, `auto` for
`linear`). This replaced a per-frame main-thread `drawImage` that scaled to the
whole window (cost grew with window size); present is now ~0.2ms.

`renderBand` runs `renderScene.renderBand` for rows [y0,y1): `Sky.render` at
1/`SKY_STEP` res (fills color + resets depth, replaces a clear) → `Rasterizer.draw`
floor/walls/crate (room, always) → for each visible `Chunk` draw grass, then —
per chunk via the pure `chunkFar` test (dist² from camera to the chunk AABB vs
`lodDistance²`) — either the full rock/bark/leaf/needle + flowers, or the cheap
`rockFar/barkFar/needleFar/leafFar` impostor meshes (foliage detail dropped).
All backface-culled except double-sided flowers → `Sprite.billboard(npc)`
→ `Framebuffer.quantize`. `chunkFar` is pure (camera + baked bounds + config
only), so every worker band picks the same LOD for a chunk → no horizontal seam.
Multi-threaded: N workers each run `renderBand` over
their band of the shared framebuffer in parallel; single-threaded: one call over
the full height. Bands are disjoint (no two workers touch a pixel) and their
interior edges snap to `SKY_STEP` so the sky's block grid stays seamless.

Rasterizer specifics: near-plane clip (Sutherland-Hodgman), **1/w z-buffer**,
perspective-correct UVs, screen-space vertex snap, flat directional lighting,
distance fog, **alpha cutout** (discard texel alpha < 128, for sprites).
**Backface culling is opt-in** (`draw(..., cull)`, default off = double-sided):
on for solid world chunks, off for sprites and the room. It relies on winding, so
generators feeding culled draws (terrain patch, tree/boulder builders) must wind
front-out — a culled mesh that renders inside-out has its index order flipped
(see `Terrain.patch`). The cull sign: back-facing == positive screen area here.

## The look — where to tune

- **`engine/render/RenderConfig.ts`** — per-frame render dials + presets:
  `standard` (384×216, the startup default), `soft`, `clean`, plus an unbound
  `ps1` (320×240). In-app keys **1/2/3** switch standard/soft/clean live.
  Knobs: `internalWidth/Height`, `upscaleFilter`, `colorDepth`, `dither`,
  `vertexSnap`, `textureFilter`, `lighting`, `fog`, `lodDistance`. (Texturing is
  always perspective-correct — the affine-swim dial was removed.)
- **`RenderConfig.lodDistance`** — beyond this many world units, a chunk's trees
  + boulders draw as cheap impostors (see Performance). ~60 for standard/soft/ps1
  (well inside `fog.far`, so far detail is already fog-dimmed at the switch),
  `Infinity` on `clean` to disable LOD. Lower it for more headroom (more pop),
  raise it for more far detail (more tris).
- **`app/level.ts` `GROUND_UV`** (0.25) — outdoor ground texture tiles per world
  unit. Lower = the stone tiles bigger and less busy = less far-distance moire
  (there are no mipmaps); higher = finer but shimmerier.
- **`app/level.ts` `CHUNK_GRID`** (12) / `TERRAIN_SUBDIV` (5) — spatial-cull
  granularity and terrain resolution. World terrain divisions = `CHUNK_GRID *
  TERRAIN_SUBDIV`. Smaller cells cull tighter (draw less off-screen) but cost more
  per-cell tests/bounds. This is the lever if a dense world still lags.

## Performance / where the frame goes

The world is dense (hundreds of trees + boulders, ~50k tris) but most of it is
off-screen or fogged each frame, so several things keep it cheap:
- **Frustum culling** (`Frustum` + per-`Chunk` AABB test in `main`) — skips whole
  chunks that fall outside the view. Behind you + off to the sides = free.
- **Backface culling** (`draw(..., true)`) — ~halves fill on solid geometry
  (terrain, foliage, rock). See the Rasterizer note re winding.
- **Half-res sky** (`SKY_STEP` in `main`, default 2) — the cloud fbm runs per
  pixel and dominated the frame; sampling once per 2×2 block quarters it.
- **Distance LOD** (`RenderConfig.lodDistance`, `chunkFar` in `renderScene`) —
  past `lodDistance` a chunk's trees + boulders swap to pre-baked low-poly
  impostors and its bushes/flowers drop; both meshes are baked once at load, and
  the near/far pick is a pure function of camera + chunk bounds, so it costs
  nothing per frame and stays worker-safe (no seam). In a dense forest view this
  is what tips the per-frame work under the 16.67ms (60fps) vsync budget — it cut
  ~1.4x of the triangles in-forest and drops far-tree/rock detail that fog is
  already dimming anyway.
- **Flat geometry + zero-alloc raster** — `Mesh` is a flat float array and the
  whole per-triangle path uses reused scratch, so a frame allocates ~0 bytes
  (measured). Buys frame *consistency* (no GC-pause spikes; worst/mean ~1.3x) and
  makes geometry shareable across worker threads. It did **not** raise mean fps —
  allocation was never the bottleneck; the mean is the transform+fill **compute**.
- **Multi-threaded rasterization** (`renderer.ts` + `render-worker.ts`) — split the
  framebuffer into row bands, one worker each, over a `SharedArrayBuffer`. The
  barrier is **lock-free**: workers `Atomics.wait` on a frame counter (no per-frame
  messages), main writes camera/matrix/visible-list into shared arrays, `dispatch`
  is non-blocking, and `main` polls `done()` on its rAF and presents the finished
  frame (present frame N, dispatch N+1). Measured real-browser (`bun run
  bench:browser`): **~1.5x median AND ~1.2x p95** vs single-thread, frame time
  pinned near vsync. The gotcha is **oversubscription** — too many workers (main +
  browser + OS competing) wrecks the p95 tail even as the median improves (6
  workers were far worse than 3); `MAX_WORKERS` caps it, retune per machine.
  Requires a cross-origin-isolated page (COOP/COEP; Vite serves them) — else it
  falls back to single-thread, so the app never breaks.

Frustum + backface + half-res sky give ~1.5–2x, workers ~1.5x more, distance LOD
another ~1.4x of the tris in dense views. Together they get the heavy in-forest
view (the worst case) under the 60fps vsync budget on the worker path; the
single-thread fallback still lands ~30fps there. The blunt content dials if it
still lags are `TREE_COUNT`/`BOULDER_COUNT` (less world) and `lodDistance` (more
aggressive impostor swap).

**Profiling**: `bun run bench:browser` (Playwright) starts Vite, drives headless
Chromium through `?bench=st` and `?bench=mt` (a scripted flythrough with a fixed
camera path), and prints median/p95/max **work time** (critical-path band time)
and **frame time** for both. Headless absolute fps ≠ a real display, but the
single-vs-workers *relative* result and the *tail* (p95/max = jitter) are real —
that's how the worker path was actually validated instead of guessed.

## Clouds

Procedural, moving, in the sky pass (`engine/render/Sky.ts`). Two styles picked
by a `CloudLayer` discriminated union `kind`:
- **`basicCumulus`** — flat hard-thresholded white puffs, 1 noise lookup/pixel.
- **`fancyCumulus`** — domain-warped + heightfield-shaded fake volume,
  ~5 lookups/pixel (pricier; watch the FPS meter).

Both are exported presets in `app/level.ts`; the active one is set in
`buildLevel`'s `sky.clouds`. Add new cloud types by extending the union and
branching in the cloud shader. Cost scales with sky resolution — fine at
`standard`, heavy at `clean` (mitigate: fewer fbm octaves or half-res sky).

## Trees (`engine/scene/Tree.ts`)

Procedural low-poly geometry, faceted flat-shaded like everything else. Two
`kind`s carry the species read purely by silhouette:
- **`oak`** — short tapered trunk, a couple of branches, a broad cluster of
  lumpy canopy `blob`s (wider than tall, bushy).
- **`spruce`** — tall thin trunk under stacked narrowing `cone` tiers pointing
  to a tip (taller than wide, conical).

`growth` (0..1) runs **sapling → full grown**: it scales height/girth and adds
canopy blobs (oak) / tiers (spruce); `seed` gives each tree its own wobble.
`Tree.build` appends into caller meshes so a forest batches into 3 draw calls
(one bark trunk mesh, oak-leaf and spruce-needle foliage meshes). `app/level.ts`
`scatterTrees` seeds the forest; add a species by extending the union + a builder.

**Boulders** (`engine/scene/Boulder.ts`) work the same way: `Boulder.build`
appends a squashed, per-vertex-jittered low-poly sphere (seam/pole-safe so it
never cracks) into one shared rock mesh, sunk partway into the ground.
`scatterBoulders` sizes them small→big (biased small) and drops colliders on the
big ones. `Bush` (leaf-blob clusters) and `Flower` (stem + colored bloom, atlas
UVs) are the same again — ground detail scattered near the play area, no
colliders; add a new prop type by cloning the pattern (generator + scatter).

## Controls

WASD move · **Shift** run (speed ×`RUN_MULTIPLIER` in `app/player.ts`) · mouse
look (click canvas to pointer-lock) · **Space** jump · **1/2/3** switch look
presets. FPS shown bottom-right. The room's north wall is open — walk out onto
the terrain and toward the peaks.

## Code style (oxlint-enforced — match it)

No semicolons, double quotes, 2-space indent, trailing commas. **`type`, never
`interface`.** Function declarations (arrows allowed). Uppercase hex (`0xFF`).
Curly braces required on all `if`. No `void` operator. `T[]` not `Array<T>`.
Prefer `globalThis` over `window`. Domain behavior lives in a `type` +
matching `namespace` (see `Vec3`, `Color`, `Rasterizer`).

## Caveats

- No mipmaps, so distant textures shimmer (period-correct); far cloud/geometry
  is hidden by fog.

## Debugging the renderer without a browser (key workflow)

The engine is pure, so you can **render headless to a PNG and inspect it**. A
throwaway `bun` script can: `buildLevel()`, decode `assets/*.png` (they're
filter-0 RGBA — trivial to inflate), set a `Camera` pose, run
`Sky.render` + `Rasterizer.draw` into a `Framebuffer`, encode the color buffer
to PNG (same hand-rolled encoder as `scripts/gen-assets.ts`), write it, and read
it back. Sampling individual pixels this way tuned the clouds, framed the terrain
peaks, and confirmed the ground textures flat (no swim). Put temp scripts in the
job tmp dir, not the repo.

## Roadmap / not yet built

In-browser RenderConfig slider panel; mipmaps; `painter` depth mode; gouraud
lighting; more cloud types; more props / a weapon / moving enemies. `shared/`
is nearly empty. The FPS meter is static HTML + `textContent` writes only — no
DOM-built UI yet (deliberate).