From 46e7070b6b0cb6c55312011a65209607516b621c Mon Sep 17 00:00:00 2001
From: Errilaz <errilaz@errilaz.org>
Date: Wed, 5 Aug 2026 09:24:13 +0200
Subject: [PATCH] feat: threads

---
 AGENTS.md                     | 119 +++++++++++---
 app/level.ts                  |  20 ++-
 app/main.ts                   | 285 ++++++++++++++++++++++++++--------
 app/render-worker.ts          |  61 ++++++++
 app/renderScene.ts            | 102 ++++++++++++
 app/renderer.ts               | 196 +++++++++++++++++++++++
 bun.lock                      |   9 +-
 engine/render/Framebuffer.ts  |   6 +-
 engine/render/Rasterizer.ts   |  12 +-
 engine/render/RenderConfig.ts |  10 +-
 engine/render/Sky.ts          |   9 +-
 engine/scene/Boulder.ts       |   7 +-
 engine/scene/Tree.ts          |  35 +++--
 index.html                    |   9 +-
 oxlint.config.ts              |   2 +
 package.json                  |  10 +-
 scripts/bench-browser.ts      |  93 +++++++++++
 vite.config.ts                |  18 ++-
 18 files changed, 878 insertions(+), 125 deletions(-)
 create mode 100644 app/render-worker.ts
 create mode 100644 app/renderScene.ts
 create mode 100644 app/renderer.ts
 create mode 100644 scripts/bench-browser.ts

diff --git a/AGENTS.md b/AGENTS.md
index fbe377c..65fbdd6 100644
--- a/AGENTS.md
+++ b/AGENTS.md
@@ -32,7 +32,10 @@ rules live in `.agents/rules/*.md`.
 ## Stack & tooling
 
 - **Bun** runtime + `bun test`. **TypeScript 7** (native `tsc`), strict,
-  `moduleResolution: bundler`. **Vite 8** serves/builds the client.
+  `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.
@@ -42,6 +45,9 @@ rules live in `.agents/rules/*.md`.
 - `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.
@@ -73,7 +79,19 @@ rules live in `.agents/rules/*.md`.
     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, preset switching, canvas blit, FPS meter.
+  - `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
@@ -84,7 +102,10 @@ rules live in `.agents/rules/*.md`.
     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). `Aabb` colliders (walls, crate, grown trunks, big
+    (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
@@ -104,16 +125,36 @@ rules live in `.agents/rules/*.md`.
   filenames are the contract.
 - `server/` — Bun server stub. `shared/` — isomorphic slot.
 
-## Frame pipeline (`app/main.ts` `frame`)
+## Frame pipeline (`app/main.ts` `tick`)
 
-`Player.update` → build `Camera` → `Camera.viewProjection` →
-`Sky.render` at 1/`SKY_STEP` res (fills color + resets depth, replaces a clear) →
-`Rasterizer.draw` floor, walls, crate (room, always) → `Frustum.fromViewProj`,
-then for each `Chunk` that `Frustum.intersectsAabb` passes: draw its grass, rock,
-bark, leaf, needle (backface-culled) + flowers (double-sided) →
-`Sprite.billboard(npc)` (double-sided) →
-`Framebuffer.quantize` → `present` (integer-scale, letterboxed blit;
-`imageSmoothingEnabled` follows `upscaleFilter`).
+`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,
@@ -130,8 +171,13 @@ front-out — a culled mesh that renders inside-out has its index order flipped
   `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`. (Texturing is always
-  perspective-correct — the affine-swim dial was removed.)
+  `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.
@@ -150,18 +196,45 @@ off-screen or fogged each frame, so several things keep it cheap:
   (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). This buys frame *consistency* (no GC-pause spikes; worst/mean ~1.3x)
-  and makes geometry shareable for Web-Worker rasterization later. Note it did
-  **not** raise mean fps — allocation was never the bottleneck (JSC collects the
-  churn ~free); the mean is the transform+fill **compute**.
+  (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, growing with content since culled
-chunks cost ~nothing. The remaining bottleneck is raw compute on visible tris, so
-the mean-fps levers left are to **do less** (LOD / impostors for far trees —
-`TREE_COUNT`/`BOULDER_COUNT` are the blunt content dials) or **use more cores**
-(Web-Worker banded rasterization, now unblocked by the flat geometry).
+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
 
diff --git a/app/level.ts b/app/level.ts
index 6471f3b..da651a4 100644
--- a/app/level.ts
+++ b/app/level.ts
@@ -39,6 +39,13 @@ export type Chunk = {
   rock: Mesh
   /** Flowers (color-atlas texture); drawn double-sided, so kept separate. */
   flowers: Mesh
+  /** Cheap low-poly impostors of the same trees/boulders, drawn instead of the
+   *  full meshes once the chunk is past `config.lodDistance` (renderScene). Same
+   *  textures as bark/leaf/needle/rock. Bushes/flowers have no far version. */
+  barkFar: Mesh
+  leafFar: Mesh
+  needleFar: Mesh
+  rockFar: Mesh
 }
 
 /** The playground: a flat-floored room dropped into the center of a big open
@@ -215,17 +222,26 @@ function buildChunks(trees: Tree[], boulders: Boulder[], bushes: Bush[], flowers
       const needle = mesh()
       const rock = mesh()
       const flowerMesh = mesh()
+      const barkFar = mesh()
+      const leafFar = mesh()
+      const needleFar = mesh()
+      const rockFar = mesh()
       Terrain.patch(TERRAIN, grass, x0, z0, x1, z1, TERRAIN_SUBDIV, TERRAIN_SUBDIV, GROUND_UV)
       for (const tree of trees) {
         if (inCell(tree.position, x0, z0, x1, z1)) {
-          Tree.build(tree, bark, tree.kind === "oak" ? leaf : needle)
+          const foliage = tree.kind === "oak" ? leaf : needle
+          const foliageFar = tree.kind === "oak" ? leafFar : needleFar
+          Tree.build(tree, bark, foliage)
+          Tree.build(tree, barkFar, foliageFar, "impostor")
         }
       }
       for (const boulder of boulders) {
         if (inCell(boulder.position, x0, z0, x1, z1)) {
           Boulder.build(boulder, rock)
+          Boulder.build(boulder, rockFar, "impostor")
         }
       }
+      // Bushes share the near leaf mesh; they just drop out past lodDistance.
       for (const bush of bushes) {
         if (inCell(bush.position, x0, z0, x1, z1)) {
           Bush.build(bush, leaf)
@@ -240,7 +256,7 @@ function buildChunks(trees: Tree[], boulders: Boulder[], bushes: Bush[], flowers
       if (b === null) {
         continue
       }
-      chunks.push({ ...b, grass, bark, leaf, needle, rock, flowers: flowerMesh })
+      chunks.push({ ...b, grass, bark, leaf, needle, rock, flowers: flowerMesh, barkFar, leafFar, needleFar, rockFar })
     }
   }
   return chunks
diff --git a/app/main.ts b/app/main.ts
index 8a9e195..0597c89 100644
--- a/app/main.ts
+++ b/app/main.ts
@@ -1,59 +1,109 @@
-import { Framebuffer } from "../engine/render/Framebuffer"
-import { Frustum } from "../engine/render/Frustum"
-import { Rasterizer } from "../engine/render/Rasterizer"
 import { RenderConfig } from "../engine/render/RenderConfig"
-import { Sky } from "../engine/render/Sky"
 import { Camera } from "../engine/scene/Camera"
-import { Sprite } from "../engine/scene/Sprite"
 import { loadTextures } from "./assets"
 import { buildLevel } from "./level"
 import { EYE_HEIGHT, Player } from "./player"
+import { createRenderer } from "./renderer"
+import { chunkFar, visibleChunks, type Scene } from "./renderScene"
 
 const FOV = Math.PI / 3
-/** Sky is drawn at 1/SKY_STEP resolution (the cloud fbm is the costly part). */
-const SKY_STEP = 2
 
 const screen = document.querySelector<HTMLCanvasElement>("#screen")!
 const ctx = screen.getContext("2d")!
-const back = document.createElement("canvas")
-const backCtx = back.getContext("2d")!
 const fpsEl = document.querySelector<HTMLDivElement>("#fps")!
 
-let config: RenderConfig = RenderConfig.standard
-let fb = Framebuffer.create(1, 1)
-let image = new ImageData(1, 1)
-
-function useConfig(next: RenderConfig): void {
-  config = next
-  fb = Framebuffer.create(config.internalWidth, config.internalHeight)
-  back.width = fb.width
-  back.height = fb.height
-  image = new ImageData(new Uint8ClampedArray(fb.color.buffer as ArrayBuffer), fb.width, fb.height)
+/** Deterministic flythrough (virtual time from frame index), so the st and mt
+ *  bench runs render the exact same work. Deliberately stands *inside* the dense
+ *  tree ring (radius ~40–100) and sweeps a full 360° yaw so the frame is filled
+ *  with forest -- the heavy case that quantizes to 30fps, not the empty clearing. */
+function benchCamera(tv: number): Camera {
+  const drift = tv * 0.12
+  const radius = 65 + 30 * Math.sin(tv * 0.25)
+  return {
+    position: { x: Math.cos(drift) * radius, y: 3, z: Math.sin(drift) * radius },
+    yaw: tv * 0.7,
+    pitch: 0.05 * Math.sin(tv * 0.5),
+    fov: FOV,
+  }
 }
 
-function resize(): void {
-  screen.width = globalThis.innerWidth
-  screen.height = globalThis.innerHeight
+function round2(n: number): number {
+  return Math.round(n * 100) / 100
 }
 
-function present(): void {
-  backCtx.putImageData(image, 0, 0)
-  const scale = Math.max(1, Math.floor(Math.min(screen.width / fb.width, screen.height / fb.height)))
-  const w = fb.width * scale
-  const h = fb.height * scale
-  const x = (screen.width - w) >> 1
-  const y = (screen.height - h) >> 1
-  ctx.imageSmoothingEnabled = config.upscaleFilter === "linear"
-  ctx.clearRect(0, 0, screen.width, screen.height)
-  ctx.drawImage(back, x, y, w, h)
+function benchStats(a: number[]): { median: number; p95: number; max: number; mean: number } {
+  const s = a.toSorted((x, y) => x - y)
+  const q = (p: number): number => s[Math.min(s.length - 1, Math.floor(p * s.length))]
+  return { median: round2(q(0.5)), p95: round2(q(0.95)), max: round2(s[s.length - 1]), mean: round2(a.reduce((x, y) => x + y, 0) / a.length) }
 }
 
 async function main(): Promise<void> {
   const textures = await loadTextures()
   const level = buildLevel()
-  const npc: Sprite = { position: level.npcPosition, size: { x: 1.1, y: 1.5 }, texture: textures.npc }
-  const player: Player = { position: { x: 0, y: 0, z: 8 }, yaw: 0, pitch: 0, velocityY: 0, onGround: true }
+  const scene: Scene = {
+    chunks: level.chunks,
+    floor: level.floor,
+    walls: level.walls,
+    crate: level.crate,
+    npc: { position: level.npcPosition, size: { x: 1.1, y: 1.5 } },
+    sky: level.sky,
+    textures,
+  }
+  // `?bench=st` / `?bench=mt` runs a scripted flythrough and reports timings.
+  const benchMode = new URLSearchParams(globalThis.location.search).get("bench")
+  const forceWorkers = benchMode === "mt" ? true : benchMode === "st" ? false : undefined
 
+  let config: RenderConfig = RenderConfig.standard
+  const renderer = createRenderer(scene, config, forceWorkers)
+  let image = new ImageData(renderer.fb.width, renderer.fb.height)
+  let colorBytes = new Uint8ClampedArray(renderer.fb.color.buffer)
+
+  // The canvas backing store IS the internal resolution; the browser/compositor
+  // scales the element up (see `layout`). So `present` is one internal-res
+  // putImageData with no per-frame window-sized blit on the main thread.
+  function retarget(): void {
+    const fb = renderer.fb
+    screen.width = fb.width
+    screen.height = fb.height
+    image = new ImageData(fb.width, fb.height)
+    colorBytes = new Uint8ClampedArray(fb.color.buffer)
+    layout()
+  }
+
+  // Size + center the canvas to an integer multiple of the internal res (crisp
+  // letterboxed upscale, done by the GPU). Recomputed only on resize/config.
+  function layout(): void {
+    const fb = renderer.fb
+    const scale = Math.max(1, Math.floor(Math.min(globalThis.innerWidth / fb.width, globalThis.innerHeight / fb.height)))
+    const w = fb.width * scale
+    const h = fb.height * scale
+    screen.style.width = `${w}px`
+    screen.style.height = `${h}px`
+    screen.style.left = `${(globalThis.innerWidth - w) >> 1}px`
+    screen.style.top = `${(globalThis.innerHeight - h) >> 1}px`
+    screen.style.imageRendering = config.upscaleFilter === "linear" ? "auto" : "pixelated"
+  }
+  retarget()
+
+  function useConfig(next: RenderConfig): void {
+    config = next
+    renderer.reconfigure(next)
+    retarget()
+  }
+
+  function present(): void {
+    image.data.set(colorBytes)
+    ctx.putImageData(image, 0, 0)
+  }
+
+  globalThis.addEventListener("resize", layout)
+
+  if (benchMode) {
+    runBench(renderer, level.chunks, present, benchMode)
+    return
+  }
+
+  const player: Player = { position: { x: 0, y: 0, z: 8 }, yaw: 0, pitch: 0, velocityY: 0, onGround: true }
   const keys = new Set<string>()
   globalThis.addEventListener("keydown", (e) => {
     keys.add(e.code)
@@ -86,56 +136,161 @@ async function main(): Promise<void> {
     player.pitch = Math.max(-1.4, Math.min(1.4, player.pitch - e.movementY * 0.0025))
   })
 
-  useConfig(config)
-  globalThis.addEventListener("resize", resize)
-  resize()
+  // Triangles drawn this frame (room + each visible chunk, LOD-aware) for the HUD.
+  function frameTris(visible: number[], cam: Camera): number {
+    let t = level.floor.indices.length + level.walls.indices.length + level.crate.indices.length
+    for (const i of visible) {
+      const c = level.chunks[i]
+      t += c.grass.indices.length + c.flowers.indices.length
+      const far = chunkFar(c, cam.position, config.lodDistance)
+      t += far
+        ? c.barkFar.indices.length + c.leafFar.indices.length + c.needleFar.indices.length + c.rockFar.indices.length
+        : c.bark.indices.length + c.leaf.indices.length + c.needle.indices.length + c.rock.indices.length
+    }
+    return (t / 3) | 0
+  }
 
+  // Poll-based pump: present the finished frame, dispatch the next; if workers
+  // aren't done we skip this vsync (no async/rAF desync). The HUD reports the
+  // critical-path budget (work + present) so the real bottleneck is visible.
+  let inFlight = false
   let last = performance.now()
   let fpsLast = last
   let fpsFrames = 0
-  function frame(now: number): void {
+  let workMax = 0
+  let presentMax = 0
+  let vsyncMax = 0
+  let lastPresent = performance.now()
+  let lastVisible: number[] = []
+  let lastCamera: Camera = { position: { x: 0, y: 0, z: 0 }, yaw: 0, pitch: 0, fov: FOV }
+
+  function show(): void {
+    const p0 = performance.now()
+    present()
+    const p1 = performance.now()
+    presentMax = Math.max(presentMax, p1 - p0)
+    vsyncMax = Math.max(vsyncMax, p1 - lastPresent)
+    lastPresent = p1
+    workMax = Math.max(workMax, renderer.workMs())
+    fpsFrames++
+  }
+
+  function tick(): void {
+    requestAnimationFrame(tick)
+    if (inFlight) {
+      if (!renderer.done()) {
+        return
+      }
+      show()
+      inFlight = false
+    }
+    const now = performance.now()
     const dt = Math.min(0.05, (now - last) / 1000)
     last = now
-    fpsFrames++
     if (now - fpsLast >= 250) {
-      fpsEl.textContent = `${Math.round((fpsFrames * 1000) / (now - fpsLast))} fps`
+      const fps = Math.round((fpsFrames * 1000) / (now - fpsLast))
+      const tag = renderer.parallel ? "" : " 1core"
+      fpsEl.textContent =
+        `${fps} fps${tag}\n` +
+        `work ${round2(workMax)} + present ${round2(presentMax)} = ${round2(workMax + presentMax)}ms\n` +
+        `vsync ${round2(vsyncMax)}ms · ${lastVisible.length} ch · ${frameTris(lastVisible, lastCamera)} tris`
       fpsLast = now
       fpsFrames = 0
+      workMax = 0
+      presentMax = 0
+      vsyncMax = 0
     }
     Player.update(player, keys, dt, level)
-
     const camera: Camera = {
       position: { x: player.position.x, y: player.position.y + EYE_HEIGHT, z: player.position.z },
       yaw: player.yaw,
       pitch: player.pitch,
       fov: FOV,
     }
-    const viewProj = Camera.viewProjection(camera, fb.width / fb.height)
-
-    Sky.render(fb, camera, level.sky, now / 1000, SKY_STEP)
-    // Room is small and always near where you play; draw it unconditionally.
-    Rasterizer.draw(fb, level.floor, textures.floor, viewProj, config)
-    Rasterizer.draw(fb, level.walls, textures.wall, viewProj, config)
-    Rasterizer.draw(fb, level.crate, textures.crate, viewProj, config)
-    // Outdoor world: skip whole chunks that fall outside the view frustum.
-    const frustum = Frustum.fromViewProj(viewProj)
-    for (const c of level.chunks) {
-      if (!Frustum.intersectsAabb(frustum, c.minX, c.minY, c.minZ, c.maxX, c.maxY, c.maxZ)) {
-        continue
-      }
-      Rasterizer.draw(fb, c.grass, textures.grass, viewProj, config, true)
-      Rasterizer.draw(fb, c.rock, textures.rock, viewProj, config, true)
-      Rasterizer.draw(fb, c.bark, textures.bark, viewProj, config, true)
-      Rasterizer.draw(fb, c.leaf, textures.leaf, viewProj, config, true)
-      Rasterizer.draw(fb, c.needle, textures.needle, viewProj, config, true)
-      Rasterizer.draw(fb, c.flowers, textures.flower, viewProj, config)
+    const viewProj = Camera.viewProjection(camera, renderer.fb.width / renderer.fb.height)
+    const visible = visibleChunks(level.chunks, viewProj)
+    lastVisible = visible
+    lastCamera = camera
+    renderer.dispatch(camera, viewProj, visible, now / 1000)
+    inFlight = true
+    if (renderer.done()) {
+      show()
+      inFlight = false
     }
-    Rasterizer.draw(fb, Sprite.billboard(npc, camera), npc.texture, viewProj, config)
-    Framebuffer.quantize(fb, config)
-    present()
-    requestAnimationFrame(frame)
   }
-  requestAnimationFrame(frame)
+  requestAnimationFrame(tick)
+}
+
+/** Scripted flythrough that records critical-path work time and present-to-present
+ *  interval, then reports the distributions (exposed on `window.__BENCH__`). */
+function runBench(
+  renderer: ReturnType<typeof createRenderer>,
+  chunks: Scene["chunks"],
+  present: () => void,
+  mode: string,
+): void {
+  const WARM = 60
+  const MEASURE = 300
+  const work: number[] = []
+  const frame: number[] = []
+  let i = 0
+  let inFlight = false
+  let prev = performance.now()
+  let finished = false
+
+  function record(): boolean {
+    present()
+    const now = performance.now()
+    if (i >= WARM) {
+      work.push(renderer.workMs())
+      frame.push(now - prev)
+    }
+    prev = now
+    inFlight = false
+    i++
+    return i >= WARM + MEASURE
+  }
+
+  function report(): void {
+    finished = true
+    const result = {
+      mode,
+      parallel: renderer.parallel,
+      cores: (globalThis.navigator as Navigator).hardwareConcurrency,
+      coi: (globalThis as { crossOriginIsolated?: boolean }).crossOriginIsolated === true,
+      res: `${renderer.fb.width}x${renderer.fb.height}`,
+      workMs: benchStats(work),
+      frameMs: benchStats(frame),
+    }
+    ;(globalThis as { __BENCH__?: unknown }).__BENCH__ = result
+    console.log(`BENCH ${JSON.stringify(result)}`)
+    fpsEl.textContent = `bench ${mode}: work ${result.workMs.median}ms (p95 ${result.workMs.p95})`
+  }
+
+  function tick(): void {
+    if (finished) {
+      return
+    }
+    requestAnimationFrame(tick)
+    if (inFlight) {
+      if (!renderer.done()) {
+        return
+      }
+      if (record()) {
+        report()
+        return
+      }
+    }
+    const camera = benchCamera(i / 60)
+    const viewProj = Camera.viewProjection(camera, renderer.fb.width / renderer.fb.height)
+    const visible = visibleChunks(chunks, viewProj)
+    renderer.dispatch(camera, viewProj, visible, i / 60)
+    inFlight = true
+    if (renderer.done() && record()) {
+      report()
+    }
+  }
+  requestAnimationFrame(tick)
 }
 
 main().catch((error) => {
diff --git a/app/render-worker.ts b/app/render-worker.ts
new file mode 100644
index 0000000..8e7b824
--- /dev/null
+++ b/app/render-worker.ts
@@ -0,0 +1,61 @@
+import type { Framebuffer } from "../engine/render/Framebuffer"
+import type { RenderConfig } from "../engine/render/RenderConfig"
+import { renderBand, type Scene } from "./renderScene"
+
+/** One-time setup: shared framebuffer + control/param buffers, the (cloned)
+ *  scene, this worker's row band, and its index into the per-worker times array. */
+type Init = {
+  colorSAB: SharedArrayBuffer
+  depthSAB: SharedArrayBuffer
+  width: number
+  height: number
+  scene: Scene
+  band: [number, number]
+  config: RenderConfig
+  skyStep: number
+  ctrlSAB: SharedArrayBuffer
+  camSAB: SharedArrayBuffer
+  vpSAB: SharedArrayBuffer
+  visSAB: SharedArrayBuffer
+  timesSAB: SharedArrayBuffer
+  index: number
+}
+
+const FRAME = 0
+const DONE = 1
+const VIS = 2
+
+const ctx = globalThis as unknown as {
+  addEventListener: (type: "message", handler: (e: { data: Init }) => void) => void
+}
+
+ctx.addEventListener("message", (e) => {
+  const m = e.data
+  const fb: Framebuffer = {
+    width: m.width,
+    height: m.height,
+    color: new Uint32Array(m.colorSAB),
+    depth: new Float32Array(m.depthSAB),
+  }
+  const ctrl = new Int32Array(m.ctrlSAB)
+  const cam = new Float64Array(m.camSAB)
+  const vp = new Float32Array(m.vpSAB)
+  const vis = new Int32Array(m.visSAB)
+  const times = new Float64Array(m.timesSAB)
+  const { scene, band, config, skyStep, index } = m
+
+  // Lock-free frame loop: block until main bumps the frame counter, render this
+  // band, record the band time, and signal done. No messages per frame.
+  let last = 0
+  for (;;) {
+    Atomics.wait(ctrl, FRAME, last)
+    last = Atomics.load(ctrl, FRAME)
+    const t0 = performance.now()
+    const camera = { position: { x: cam[0], y: cam[1], z: cam[2] }, yaw: cam[3], pitch: cam[4], fov: cam[5] }
+    const count = Atomics.load(ctrl, VIS)
+    const visible = [...vis.subarray(0, count)]
+    renderBand(fb, scene, camera, vp, visible, config, skyStep, cam[6], band[0], band[1])
+    times[index] = performance.now() - t0
+    Atomics.add(ctrl, DONE, 1)
+  }
+})
diff --git a/app/renderScene.ts b/app/renderScene.ts
new file mode 100644
index 0000000..d4cebdc
--- /dev/null
+++ b/app/renderScene.ts
@@ -0,0 +1,102 @@
+import { Framebuffer } from "../engine/render/Framebuffer"
+import { Frustum } from "../engine/render/Frustum"
+import { Rasterizer } from "../engine/render/Rasterizer"
+import type { RenderConfig } from "../engine/render/RenderConfig"
+import { Sky, type SkyConfig } from "../engine/render/Sky"
+import type { Camera } from "../engine/scene/Camera"
+import type { Mat4 } from "../engine/math/Mat4"
+import type { Mesh } from "../engine/scene/Mesh"
+import { Sprite } from "../engine/scene/Sprite"
+import type { Vec2 } from "../engine/math/Vec2"
+import type { Vec3 } from "../engine/math/Vec3"
+import type { Textures } from "./assets"
+import type { Chunk } from "./level"
+
+/** Everything needed to render the world: the room, the cullable chunks, the NPC
+ *  billboard source, sky, and textures. Bundled so it can be handed to a worker
+ *  whole (it is plain data + typed arrays, structured-clone friendly). */
+export type Scene = {
+  chunks: Chunk[]
+  floor: Mesh
+  walls: Mesh
+  crate: Mesh
+  npc: { position: Vec3; size: Vec2 }
+  sky: SkyConfig
+  textures: Textures
+}
+
+/** Chunk indices whose bounding box is inside the view frustum. Computed once on
+ *  the main thread and shared with every worker (so they don't each re-cull). */
+export function visibleChunks(chunks: Chunk[], viewProj: Mat4): number[] {
+  const frustum = Frustum.fromViewProj(viewProj)
+  const out: number[] = []
+  for (let i = 0; i < chunks.length; i++) {
+    const c = chunks[i]
+    if (Frustum.intersectsAabb(frustum, c.minX, c.minY, c.minZ, c.maxX, c.maxY, c.maxZ)) {
+      out.push(i)
+    }
+  }
+  return out
+}
+
+/**
+ * Render rows [y0, y1) of one frame into `fb`. This is the single source of
+ * render truth: the single-thread path calls it with the full height, and each
+ * worker calls it with its own disjoint band -- same output either way, and no
+ * two bands touch the same pixel (so the shared framebuffer needs no locking).
+ */
+export function renderBand(
+  fb: Framebuffer,
+  scene: Scene,
+  camera: Camera,
+  viewProj: Mat4,
+  visible: number[],
+  config: RenderConfig,
+  skyStep: number,
+  time: number,
+  y0: number,
+  y1: number,
+): void {
+  const tx = scene.textures
+  Sky.render(fb, camera, scene.sky, time, skyStep, y0, y1)
+  // Room: small and always near, drawn unconditionally (double-sided).
+  Rasterizer.draw(fb, scene.floor, tx.floor, viewProj, config, false, y0, y1)
+  Rasterizer.draw(fb, scene.walls, tx.wall, viewProj, config, false, y0, y1)
+  Rasterizer.draw(fb, scene.crate, tx.crate, viewProj, config, false, y0, y1)
+  for (const i of visible) {
+    const c = scene.chunks[i]
+    Rasterizer.draw(fb, c.grass, tx.grass, viewProj, config, true, y0, y1)
+    // Past lodDistance, swap full tree/boulder geometry for cheap impostors.
+    // `chunkFar` is pure (camera + chunk bounds + config), so every worker band
+    // makes the identical choice -- no full/impostor seam across bands.
+    if (chunkFar(c, camera.position, config.lodDistance)) {
+      Rasterizer.draw(fb, c.rockFar, tx.rock, viewProj, config, true, y0, y1)
+      Rasterizer.draw(fb, c.barkFar, tx.bark, viewProj, config, true, y0, y1)
+      Rasterizer.draw(fb, c.leafFar, tx.leaf, viewProj, config, true, y0, y1)
+      Rasterizer.draw(fb, c.needleFar, tx.needle, viewProj, config, true, y0, y1)
+    } else {
+      Rasterizer.draw(fb, c.rock, tx.rock, viewProj, config, true, y0, y1)
+      Rasterizer.draw(fb, c.bark, tx.bark, viewProj, config, true, y0, y1)
+      Rasterizer.draw(fb, c.leaf, tx.leaf, viewProj, config, true, y0, y1)
+      Rasterizer.draw(fb, c.needle, tx.needle, viewProj, config, true, y0, y1)
+      Rasterizer.draw(fb, c.flowers, tx.flower, viewProj, config, false, y0, y1)
+    }
+  }
+  const sprite: Sprite = { position: scene.npc.position, size: scene.npc.size, texture: tx.npc }
+  Rasterizer.draw(fb, Sprite.billboard(sprite, camera), tx.npc, viewProj, config, false, y0, y1)
+  Framebuffer.quantize(fb, config, y0, y1)
+}
+
+/** Whether a chunk is far enough to draw its impostor meshes: squared distance
+ *  from the camera to the chunk's AABB vs `lodDistance²`. Pure -- depends only on
+ *  camera, the chunk's baked bounds, and the config constant, all of which every
+ *  worker already holds, so the choice is identical across bands. */
+export function chunkFar(chunk: Chunk, eye: Vec3, lodDistance: number): boolean {
+  if (!(lodDistance < Infinity)) {
+    return false
+  }
+  const dx = eye.x - Math.max(chunk.minX, Math.min(chunk.maxX, eye.x))
+  const dy = eye.y - Math.max(chunk.minY, Math.min(chunk.maxY, eye.y))
+  const dz = eye.z - Math.max(chunk.minZ, Math.min(chunk.maxZ, eye.z))
+  return dx * dx + dy * dy + dz * dz > lodDistance * lodDistance
+}
diff --git a/app/renderer.ts b/app/renderer.ts
new file mode 100644
index 0000000..1f4e9a7
--- /dev/null
+++ b/app/renderer.ts
@@ -0,0 +1,196 @@
+import { Framebuffer } from "../engine/render/Framebuffer"
+import type { RenderConfig } from "../engine/render/RenderConfig"
+import type { Mat4 } from "../engine/math/Mat4"
+import type { Camera } from "../engine/scene/Camera"
+import { renderBand, type Scene } from "./renderScene"
+
+/** Sky is drawn at 1/SKY_STEP resolution; band splits align to it. */
+const SKY_STEP = 2
+/** Use worker threads when the page can share memory (else single-thread). */
+const ENABLE_WORKERS = true
+/** Cap on render worker threads. Deliberately low: a browser game shares the
+ *  machine with the browser itself, the compositor, and whatever else is open, so
+ *  grabbing every core backfires -- the frame-time *tail* blows up even while the
+ *  median improves. Measured (`bun run bench:browser`) on a busy 16-core desktop:
+ *  3 workers beat single-thread on both median (~1.5x) and p95 (~1.2x); 6 were far
+ *  worse. Raise only if the target is a dedicated/idle machine; retune via the
+ *  bench. */
+const MAX_WORKERS = 3
+
+// Indices into the shared control Int32Array.
+const FRAME = 0 // bumped by main to dispatch a frame
+const DONE = 1 // workers add 1 when their band is finished
+const VIS = 2 // number of visible chunk indices this frame
+
+/**
+ * Render driver. When the page is cross-origin-isolated it runs a pool of worker
+ * threads, each owning a disjoint row band of a `SharedArrayBuffer` framebuffer;
+ * otherwise it renders inline on the main thread. Same output either way.
+ *
+ * The worker barrier is lock-free: workers `Atomics.wait` on a frame counter, so
+ * there are **no per-frame messages** (the old postMessage barrier was the jitter
+ * source). Per-frame inputs (camera, matrix, visible list) live in shared arrays.
+ * `dispatch` starts a frame without blocking; the caller polls `done()` and
+ * presents when ready, so pacing stays on the caller's `requestAnimationFrame`.
+ */
+export type Renderer = {
+  readonly fb: Framebuffer
+  readonly parallel: boolean
+  reconfigure: (config: RenderConfig) => void
+  /** Start rendering one frame (non-blocking in the worker path). */
+  dispatch: (camera: Camera, viewProj: Mat4, visible: number[], time: number) => void
+  /** Has the dispatched frame finished? (always true single-threaded.) */
+  done: () => boolean
+  /** Critical-path render time of the last frame, ms (max band time / inline time). */
+  workMs: () => number
+}
+
+export function createRenderer(scene: Scene, initial: RenderConfig, forceWorkers?: boolean): Renderer {
+  const hw = (globalThis.navigator as Navigator | undefined)?.hardwareConcurrency ?? 4
+  const workerCount = Math.max(1, Math.min(MAX_WORKERS, hw - 1))
+  const maxVis = Math.max(1, scene.chunks.length)
+  let config = initial
+  const want = forceWorkers ?? ENABLE_WORKERS
+  let parallel = want && canShare()
+
+  let fb = Framebuffer.create(1, 1)
+  let workers: Worker[] = []
+  let ctrl: Int32Array<ArrayBufferLike> = new Int32Array(0)
+  let cam: Float64Array<ArrayBufferLike> = new Float64Array(0) // pos x/y/z, yaw, pitch, fov, time
+  let vp: Float32Array<ArrayBufferLike> = new Float32Array(0) // the view-projection matrix
+  let vis: Int32Array<ArrayBufferLike> = new Int32Array(0) // visible chunk indices
+  let times: Float64Array<ArrayBufferLike> = new Float64Array(0) // per-worker band render ms
+  let lastWork = 0
+
+  function setup(): void {
+    for (const w of workers) {
+      w.terminate()
+    }
+    workers = []
+    const width = config.internalWidth
+    const height = config.internalHeight
+    if (parallel) {
+      const n = width * height
+      fb = { width, height, color: new Uint32Array(new SharedArrayBuffer(n * 4)), depth: new Float32Array(new SharedArrayBuffer(n * 4)) }
+      const bands = splitBands(height, workerCount, SKY_STEP)
+      ctrl = new Int32Array(new SharedArrayBuffer(4 * 4))
+      cam = new Float64Array(new SharedArrayBuffer(7 * 8))
+      vp = new Float32Array(new SharedArrayBuffer(16 * 4))
+      vis = new Int32Array(new SharedArrayBuffer(maxVis * 4))
+      times = new Float64Array(new SharedArrayBuffer(bands.length * 8))
+      try {
+        bands.forEach((band, index) => {
+          const worker = new Worker(new URL("./render-worker.ts", import.meta.url), { type: "module" })
+          worker.addEventListener("error", () => {
+            parallel = false
+          })
+          worker.postMessage({
+            colorSAB: fb.color.buffer,
+            depthSAB: fb.depth.buffer,
+            width,
+            height,
+            scene,
+            band,
+            config,
+            skyStep: SKY_STEP,
+            ctrlSAB: ctrl.buffer,
+            camSAB: cam.buffer,
+            vpSAB: vp.buffer,
+            visSAB: vis.buffer,
+            timesSAB: times.buffer,
+            index,
+          })
+          workers.push(worker)
+        })
+      } catch {
+        parallel = false
+        for (const w of workers) {
+          w.terminate()
+        }
+        workers = []
+      }
+    }
+    if (!parallel || workers.length === 0) {
+      fb = Framebuffer.create(width, height)
+    }
+  }
+  setup()
+
+  return {
+    get fb() {
+      return fb
+    },
+    get parallel() {
+      return parallel && workers.length > 0
+    },
+    reconfigure(next) {
+      config = next
+      setup()
+    },
+    dispatch(camera, viewProj, visible, time) {
+      if (parallel && workers.length > 0) {
+        cam[0] = camera.position.x
+        cam[1] = camera.position.y
+        cam[2] = camera.position.z
+        cam[3] = camera.yaw
+        cam[4] = camera.pitch
+        cam[5] = camera.fov
+        cam[6] = time
+        vp.set(viewProj)
+        const count = Math.min(visible.length, vis.length)
+        for (let i = 0; i < count; i++) {
+          vis[i] = visible[i]
+        }
+        Atomics.store(ctrl, VIS, count)
+        Atomics.store(ctrl, DONE, 0)
+        Atomics.add(ctrl, FRAME, 1)
+        Atomics.notify(ctrl, FRAME, workers.length)
+        return
+      }
+      const t0 = performance.now()
+      renderBand(fb, scene, camera, viewProj, visible, config, SKY_STEP, time, 0, fb.height)
+      lastWork = performance.now() - t0
+    },
+    done() {
+      return !(parallel && workers.length > 0) || Atomics.load(ctrl, DONE) >= workers.length
+    },
+    workMs() {
+      if (parallel && workers.length > 0) {
+        let m = 0
+        for (let i = 0; i < workers.length; i++) {
+          if (times[i] > m) {
+            m = times[i]
+          }
+        }
+        return m
+      }
+      return lastWork
+    },
+  }
+}
+
+/** Shared memory needs SharedArrayBuffer + a cross-origin-isolated page (the
+ *  COOP/COEP headers Vite serves). Without it, render on the main thread. */
+function canShare(): boolean {
+  return (
+    typeof SharedArrayBuffer !== "undefined" &&
+    typeof Worker !== "undefined" &&
+    (globalThis as { crossOriginIsolated?: boolean }).crossOriginIsolated === true
+  )
+}
+
+/** Split `height` rows into ~`count` bands. Interior boundaries snap up to a
+ *  multiple of `step` so the sky's block grid stays aligned (no seam), while the
+ *  bands stay disjoint so no two workers write the same pixel. */
+function splitBands(height: number, count: number, step: number): [number, number][] {
+  const bands: [number, number][] = []
+  const per = Math.ceil(height / count)
+  let y = 0
+  while (y < height) {
+    const raw = y + per
+    const y1 = raw >= height ? height : Math.min(height, Math.ceil(raw / step) * step)
+    bands.push([y, y1])
+    y = y1
+  }
+  return bands
+}
diff --git a/bun.lock b/bun.lock
index e0c0f48..8d42a78 100644
--- a/bun.lock
+++ b/bun.lock
@@ -11,6 +11,7 @@
         "husky": "^9.1.7",
         "oxfmt": "^0.47.0",
         "oxlint": "^1.62.0",
+        "playwright": "^1.62.1",
         "typescript": "^7.0.2",
         "vite": "^8.1.5",
       },
@@ -277,7 +278,7 @@
 
     "fdir": ["fdir@6.5.0", "", { "peerDependencies": { "picomatch": "^3 || ^4" }, "optionalPeers": ["picomatch"] }, "sha512-tIbYtZbucOs0BRGqPJkshJUYdL+SDH7dVM8gjy+ERp3WAUjLEFJE+02kanyHtwjWOnwrKYBiwAmM0p4kLJAnXg=="],
 
-    "fsevents": ["fsevents@2.3.3", "", { "os": "darwin" }, "sha512-5xoDfX+fL7faATnagmWPpbFtwh/R77WmMMqqHGS65C3vvB0YHrgF+B1YmZ3441tMj5n63k0212XNoJwzlhffQw=="],
+    "fsevents": ["fsevents@2.3.2", "", { "os": "darwin" }, "sha512-xiqMQR4xAeHTuB9uWm+fFRcIOgKBMiOBP+eXiyT7jsgVCq1bkVygt00oASowB7EdtpOHaaPgKt812P9ab+DDKA=="],
 
     "get-caller-file": ["get-caller-file@2.0.5", "", {}, "sha512-DyFP3BM/3YHTQOCUL/w0OZHR0lpKeGrxotcHWcqNEdnltqFwXVfhEBQ94eIo34AfQpo0rGki4cyIiftY06h2Fg=="],
 
@@ -355,6 +356,10 @@
 
     "picomatch": ["picomatch@4.0.5", "", {}, "sha512-RvwwcruNjI1ncT5xRakeyS9Lf8lcItv34KD+aif+VH9kduAyfYBipGh12274xtenIPZ119/R9BdTBa8gAwSh0A=="],
 
+    "playwright": ["playwright@1.62.1", "", { "dependencies": { "playwright-core": "1.62.1" }, "optionalDependencies": { "fsevents": "2.3.2" }, "bin": { "playwright": "cli.js" } }, "sha512-0M+L3LAD8/nm554LOla9Ayx0j0tmFZ0FBcoQ7F1VuVHpM/XpiC8RcDzBQB8W5+hA8L22THxELzeF+2WcUzvcLg=="],
+
+    "playwright-core": ["playwright-core@1.62.1", "", { "bin": { "playwright-core": "cli.js" } }, "sha512-wPYSwEBJY9GHraISXqyqtx0na0LpO3XEX7jNDhntbex7tzUS7kLnZsOlFruFJB4Hi/rhDMjXGqHewDZ68nYZVw=="],
+
     "postcss": ["postcss@8.5.25", "", { "dependencies": { "nanoid": "^3.3.16", "picocolors": "^1.1.1", "source-map-js": "^1.2.1" } }, "sha512-DTPx3RWSSnWyzLxQnlH0rJP+EW5ekl16ZU4/psbIhA0e53kJfdgaN5vKM+xP7yJtXVu+nfdVFmlgFDEKAe4Pyw=="],
 
     "require-directory": ["require-directory@2.1.1", "", {}, "sha512-fGxEI7+wsG9xrvdjsrlmL22OMTTiHRwAMroiEeMgq8gzoLC/PQr7RsRDSTLUg/bZAZtF+TVIkHc6/4RIKrui+Q=="],
@@ -396,5 +401,7 @@
     "yargs-parser": ["yargs-parser@21.1.1", "", {}, "sha512-tVpsJW7DdjecAiFpbIB1e3qxIQsE6NoPc5/eTdrbbIC4h0LVsWhnoa3g+m2HclBIujHzsxZ4VJVA+GUuc2/LBw=="],
 
     "import-fresh/resolve-from": ["resolve-from@4.0.0", "", {}, "sha512-pb/MYmXstAkysRFx8piNI1tGFNQIFA3vkE3Gq4EuA1dF6gHp/+vgZqsCGJapvy8N3Q+4o7FwvquPJcnZ7RYy4g=="],
+
+    "vite/fsevents": ["fsevents@2.3.3", "", { "os": "darwin" }, "sha512-5xoDfX+fL7faATnagmWPpbFtwh/R77WmMMqqHGS65C3vvB0YHrgF+B1YmZ3441tMj5n63k0212XNoJwzlhffQw=="],
   }
 }
diff --git a/engine/render/Framebuffer.ts b/engine/render/Framebuffer.ts
index bd4e6d2..5b1ffed 100644
--- a/engine/render/Framebuffer.ts
+++ b/engine/render/Framebuffer.ts
@@ -40,13 +40,13 @@ export namespace Framebuffer {
    * 15-bit output. Skipped entirely when it would be a no-op (full depth, no
    * dither).
    */
-  export function quantize(fb: Framebuffer, config: RenderConfig): void {
+  export function quantize(fb: Framebuffer, config: RenderConfig, y0 = 0, y1 = fb.height): void {
     const levels = (1 << config.colorDepth) - 1
     if (levels >= 255 && config.dither === 0) {
       return
     }
-    const { width, height, color } = fb
-    for (let y = 0; y < height; y++) {
+    const { width, color } = fb
+    for (let y = y0; y < y1; y++) {
       for (let x = 0; x < width; x++) {
         // Per-pixel threshold from the tiled Bayer matrix, centered on 0 and
         // scaled by strength, nudges each channel before it snaps to a level.
diff --git a/engine/render/Rasterizer.ts b/engine/render/Rasterizer.ts
index b907e52..68da0b3 100644
--- a/engine/render/Rasterizer.ts
+++ b/engine/render/Rasterizer.ts
@@ -51,6 +51,8 @@ export namespace Rasterizer {
     viewProj: Mat4,
     config: RenderConfig,
     cull = false,
+    clipY0 = 0,
+    clipY1 = 1 << 30,
   ): void {
     const { verts, indices } = mesh
     const flat = config.lighting === "flat"
@@ -65,7 +67,7 @@ export namespace Rasterizer {
       // Near-clipping can turn one triangle into a quad; fan it back to tris.
       const n = clipNear(3)
       for (let k = 1; k + 1 < n; k++) {
-        fillTriangle(fb, 0, k, k + 1, shade, texture, config, cull)
+        fillTriangle(fb, 0, k, k + 1, shade, texture, config, cull, clipY0, clipY1)
       }
     }
   }
@@ -167,6 +169,8 @@ export namespace Rasterizer {
     texture: Texture,
     config: RenderConfig,
     cull: boolean,
+    clipY0: number,
+    clipY1: number,
   ): void {
     const oa = ia * CLIP
     const ob = ib * CLIP
@@ -210,8 +214,10 @@ export namespace Rasterizer {
     const vC = dst[oc + 4]
     const minX = Math.max(0, Math.floor(Math.min(sxA, sxB, sxC)))
     const maxX = Math.min(width - 1, Math.ceil(Math.max(sxA, sxB, sxC)))
-    const minY = Math.max(0, Math.floor(Math.min(syA, syB, syC)))
-    const maxY = Math.min(height - 1, Math.ceil(Math.max(syA, syB, syC)))
+    // Clamp to the caller's Y-band (default full frame) so worker threads can
+    // each fill a disjoint slice of rows without ever touching the same pixel.
+    const minY = Math.max(0, clipY0, Math.floor(Math.min(syA, syB, syC)))
+    const maxY = Math.min(height - 1, clipY1 - 1, Math.ceil(Math.max(syA, syB, syC)))
     // Edge deltas for the three barycentric edge functions (b->c, c->a, a->b).
     const dx0 = sxC - sxB
     const dy0 = syC - syB
diff --git a/engine/render/RenderConfig.ts b/engine/render/RenderConfig.ts
index 1d252ba..ed147f9 100644
--- a/engine/render/RenderConfig.ts
+++ b/engine/render/RenderConfig.ts
@@ -46,6 +46,11 @@ export type RenderConfig = {
    *  short draw distance and the shimmer of far geometry. It also colors pixels
    *  no triangle covers, so the frame's clear color should match `fog.color`. */
   fog: Fog | null
+  /** Beyond this distance (world units) trees + boulders draw as cheap low-poly
+   *  impostors instead of full geometry, cutting per-triangle work in dense
+   *  views. Kept inside `fog.far` so far detail is already fog-dimmed at the
+   *  switch; `Infinity` disables LOD. */
+  lodDistance: number
 }
 
 /** Ready-made looks. The demo binds keys 1/2/3 to these, sweeping resolution,
@@ -61,6 +66,7 @@ export namespace RenderConfig {
     textureFilter: "nearest",
     lighting: "flat",
     fog: { color: Color.rgb(150, 170, 200), near: 12, far: 200 },
+    lodDistance: 60,
   }
 
   export const ps1: RenderConfig = {
@@ -73,6 +79,7 @@ export namespace RenderConfig {
     textureFilter: "nearest",
     lighting: "flat",
     fog: { color: Color.rgb(150, 170, 200), near: 12, far: 200 },
+    lodDistance: 60,
   }
 
   export const soft: RenderConfig = {
@@ -85,6 +92,7 @@ export namespace RenderConfig {
     textureFilter: "nearest",
     lighting: "flat",
     fog: { color: Color.rgb(170, 190, 215), near: 16, far: 240 },
+    lodDistance: 70,
   }
 
   export const clean: RenderConfig = {
@@ -94,9 +102,9 @@ export namespace RenderConfig {
     colorDepth: 8,
     dither: 0,
     vertexSnap: 0,
-    perspectiveCorrect: 1,
     textureFilter: "linear",
     lighting: "flat",
     fog: null,
+    lodDistance: Infinity,
   }
 }
diff --git a/engine/render/Sky.ts b/engine/render/Sky.ts
index 7f442bc..58684dd 100644
--- a/engine/render/Sky.ts
+++ b/engine/render/Sky.ts
@@ -64,8 +64,9 @@ export namespace Sky {
    * block and is copied across it. The sky is low-frequency, so 2 is nearly free
    * visually and quarters the cloud cost; 1 is full resolution.
    */
-  export function render(fb: Framebuffer, camera: Camera, sky: SkyConfig, time: number, step = 1): void {
+  export function render(fb: Framebuffer, camera: Camera, sky: SkyConfig, time: number, step = 1, y0 = 0, y1 = -1): void {
     const { width, height, color, depth } = fb
+    const bottom = y1 < 0 ? height : y1
     const forward = Camera.forward(camera)
     const right = Vec3.normalize(Vec3.cross(forward, UP))
     const up = Vec3.cross(right, forward)
@@ -76,11 +77,13 @@ export namespace Sky {
     const clouds = sky.clouds
     const cloud: CloudSample = { cover: 0, shade: 1 }
     const s = Math.max(1, step | 0)
-    for (let by = 0; by < height; by += s) {
+    // Band `y0`..`bottom` must be step-aligned (callers ensure it) so the block
+    // grid stays global and neighboring bands don't seam.
+    for (let by = y0; by < bottom; by += s) {
       // Shade at the block center, then flood the whole block with that color.
       const sampleY = Math.min(height - 1, by + (s >> 1))
       const ndcY = 1 - ((sampleY + 0.5) / height) * 2
-      const yEnd = Math.min(height, by + s)
+      const yEnd = Math.min(bottom, by + s)
       for (let bx = 0; bx < width; bx += s) {
         const sampleX = Math.min(width - 1, bx + (s >> 1))
         const ndcX = ((sampleX + 0.5) / width) * 2 - 1
diff --git a/engine/scene/Boulder.ts b/engine/scene/Boulder.ts
index 9e8b1e9..e71c869 100644
--- a/engine/scene/Boulder.ts
+++ b/engine/scene/Boulder.ts
@@ -22,10 +22,11 @@ export type Boulder = {
  * field of boulders batches into a single draw call.
  */
 export namespace Boulder {
-  export function build(boulder: Boulder, mesh: Mesh): void {
+  /** `lod` "impostor" bakes a coarser rock (fewer facets) for far chunks. */
+  export function build(boulder: Boulder, mesh: Mesh, lod: "full" | "impostor" = "full"): void {
     const rand = rng(boulder.seed)
-    const seg = 5
-    const rings = 4
+    const seg = lod === "impostor" ? 4 : 5
+    const rings = lod === "impostor" ? 2 : 4
     const r = boulder.radius
     // Squat and slightly oval, so it reads as a rock, not a ball.
     const sx = r * (0.8 + rand() * 0.5)
diff --git a/engine/scene/Tree.ts b/engine/scene/Tree.ts
index cb0a700..ad92662 100644
--- a/engine/scene/Tree.ts
+++ b/engine/scene/Tree.ts
@@ -28,24 +28,32 @@ export type Tree = {
 export namespace Tree {
   /** Append one tree into the shared `trunk` (bark) mesh and the `foliage` mesh
    *  for its kind (oak leaf vs spruce needle). */
-  export function build(tree: Tree, trunk: Mesh, foliage: Mesh): void {
+  /** `lod` "impostor" bakes a much cheaper stand-in (few tris, same textures +
+   *  faceted look, same height/position) for far chunks; "full" is up close. */
+  export function build(tree: Tree, trunk: Mesh, foliage: Mesh, lod: "full" | "impostor" = "full"): void {
     const rand = rng(tree.seed)
     if (tree.kind === "oak") {
-      oak(tree.position, tree.growth, rand, trunk, foliage)
+      oak(tree.position, tree.growth, rand, trunk, foliage, lod)
     } else {
-      spruce(tree.position, tree.growth, rand, trunk, foliage)
+      spruce(tree.position, tree.growth, rand, trunk, foliage, lod)
     }
   }
 
-  function oak(base: Vec3, g: number, rand: () => number, trunk: Mesh, leaves: Mesh): void {
+  function oak(base: Vec3, g: number, rand: () => number, trunk: Mesh, leaves: Mesh, lod: "full" | "impostor"): void {
     const h = lerp(0.8, 7, g)
     const rTrunk = lerp(0.04, 0.32, g)
     const forkY = base.y + h * 0.5
+    const canopyY = base.y + h * 0.72
+    const blobR = h * 0.3
+    if (lod === "impostor") {
+      // One low-poly blob on a stubby trunk -- reads as an oak at distance.
+      limb(trunk, base, { x: base.x, y: forkY, z: base.z }, rTrunk, rTrunk * 0.6, 3)
+      blob(leaves, { x: base.x, y: canopyY, z: base.z }, blobR * 1.15, rand, 4, 2)
+      return
+    }
     limb(trunk, base, { x: base.x, y: forkY, z: base.z }, rTrunk, rTrunk * 0.6, 5)
 
-    const blobR = h * 0.3
     const spread = h * 0.32
-    const canopyY = base.y + h * 0.72
     // Central blob plus, as it grows, a couple offset ones -> broad bushy crown.
     const blobs = 1 + Math.round(g * 2)
     for (let i = 0; i < blobs; i++) {
@@ -72,13 +80,16 @@ export namespace Tree {
     }
   }
 
-  function spruce(base: Vec3, g: number, rand: () => number, trunk: Mesh, needles: Mesh): void {
+  function spruce(base: Vec3, g: number, rand: () => number, trunk: Mesh, needles: Mesh, lod: "full" | "impostor"): void {
     const h = lerp(0.6, 9, g)
     const rTrunk = lerp(0.03, 0.2, g)
-    limb(trunk, base, { x: base.x, y: base.y + h, z: base.z }, rTrunk, rTrunk * 0.25, 5)
+    const impostor = lod === "impostor"
+    limb(trunk, base, { x: base.x, y: base.y + h, z: base.z }, rTrunk, rTrunk * 0.25, impostor ? 3 : 5)
 
     // Stacked cones: widest low, shrinking to a point up top -> conical tiers.
-    const tiers = 2 + Math.round(g * 3)
+    // The impostor keeps the first two tiers at low sides (same seed => aligned).
+    const tiers = impostor ? 2 : 2 + Math.round(g * 3)
+    const sides = impostor ? 4 : 6
     const bottom = base.y + h * 0.1
     const span = h * 0.9
     for (let i = 0; i < tiers; i++) {
@@ -86,7 +97,7 @@ export namespace Tree {
       const y = bottom + t * span * 0.82
       const radius = lerp(h * 0.3, h * 0.05, t) * (0.9 + rand() * 0.2)
       const coneH = (span / tiers) * 1.9
-      cone(needles, { x: base.x, y, z: base.z }, coneH, radius, 6)
+      cone(needles, { x: base.x, y, z: base.z }, coneH, radius, sides)
     }
   }
 
@@ -127,9 +138,7 @@ export namespace Tree {
 
   /** A lumpy low-poly sphere (one oak canopy blob). Per-ring radius wobble keeps
    *  it organic without cracking the longitude seam. */
-  function blob(mesh: Mesh, center: Vec3, radius: number, rand: () => number): void {
-    const seg = 5
-    const rings = 3
+  function blob(mesh: Mesh, center: Vec3, radius: number, rand: () => number, seg = 5, rings = 3): void {
     const start = mesh.verts.length / STRIDE
     for (let r = 0; r <= rings; r++) {
       const phi = (r / rings) * Math.PI
diff --git a/index.html b/index.html
index 432d1d2..9f59971 100644
--- a/index.html
+++ b/index.html
@@ -13,18 +13,21 @@
         overflow: hidden;
       }
       #screen {
-        display: block;
+        position: absolute;
         image-rendering: pixelated;
       }
       #fps {
         position: fixed;
         right: 6px;
         bottom: 6px;
-        font: 16px monospace;
+        font: 13px monospace;
+        white-space: pre;
+        line-height: 1.35;
+        text-align: right;
         color: #fff;
         background: #000;
         opacity: 0.5;
-        padding: 1px 5px;
+        padding: 2px 6px;
         pointer-events: none;
       }
     </style>
diff --git a/oxlint.config.ts b/oxlint.config.ts
index e75b9b1..3a5aa6d 100644
--- a/oxlint.config.ts
+++ b/oxlint.config.ts
@@ -67,6 +67,8 @@ export default defineConfig({
     "typescript/prefer-function-type": "off",
     "unicorn/no-process-exit": "off",
     "unicorn/prefer-string-raw": "off",
+    "unicorn/relative-url-style": "off",
+    "unicorn/require-post-message-target-origin": "off",
     "unicorn/text-encoding-identifier-case": "off",
   },
   overrides: [
diff --git a/package.json b/package.json
index e3c86ee..68bc967 100644
--- a/package.json
+++ b/package.json
@@ -12,19 +12,21 @@
     "url": "git+https://github.com/sigitex/meat.git"
   },
   "devDependencies": {
-    "typescript": "^7.0.2",
-    "@types/bun": "^1.3.13",
-    "oxfmt": "^0.47.0",
-    "oxlint": "^1.62.0",
     "@commitlint/cli": "^20.5.3",
     "@commitlint/config-conventional": "^20.5.3",
+    "@types/bun": "^1.3.13",
     "husky": "^9.1.7",
+    "oxfmt": "^0.47.0",
+    "oxlint": "^1.62.0",
+    "playwright": "^1.62.1",
+    "typescript": "^7.0.2",
     "vite": "^8.1.5"
   },
   "scripts": {
     "start": "bunx --bun vite --host",
     "build": "bunx --bun vite build",
     "assets": "bun scripts/gen-assets.ts",
+    "bench:browser": "bun scripts/bench-browser.ts",
     "publish:builds": "rsync -avz builds/ sigitex.com:~/meat.errilaz.org/builds",
     "publish:current": "rsync -avz --delete --exclude builds dist/ sigitex.com:~/meat.errilaz.org",
     "publish": "bun run publish:current && bun run publish:builds",
diff --git a/scripts/bench-browser.ts b/scripts/bench-browser.ts
new file mode 100644
index 0000000..92d31de
--- /dev/null
+++ b/scripts/bench-browser.ts
@@ -0,0 +1,93 @@
+import { chromium } from "playwright"
+
+// Start the Vite dev server (it serves the COOP/COEP headers needed for
+// SharedArrayBuffer), drive real headless Chromium through both bench modes, and
+// read the timings back. Real V8 + Web Workers + SAB + rAF -- the environment my
+// headless Bun benches can't see.
+
+const PORT = 5199
+const URL = `http://localhost:${PORT}`
+
+const vite = Bun.spawn(["bunx", "--bun", "vite", "--port", String(PORT), "--strictPort"], {
+  cwd: import.meta.dir + "/..",
+  stdout: "pipe",
+  stderr: "pipe",
+  env: { ...process.env, NO_COLOR: "1" },
+})
+
+async function waitForServer(): Promise<void> {
+  for (let i = 0; i < 150; i++) {
+    try {
+      const r = await fetch(URL + "/")
+      if (r.ok) {
+        return
+      }
+    } catch {
+      // not up yet
+    }
+    await Bun.sleep(200)
+  }
+  throw new Error("vite dev server did not start")
+}
+
+type Bench = {
+  mode: string
+  parallel: boolean
+  cores: number
+  coi: boolean
+  res: string
+  workMs: { median: number; p95: number; max: number; mean: number }
+  frameMs: { median: number; p95: number; max: number; mean: number }
+}
+
+async function main(): Promise<void> {
+  await waitForServer()
+  const browser = await chromium.launch({
+    headless: true,
+    args: [
+      "--enable-features=SharedArrayBuffer",
+      "--disable-background-timer-throttling",
+      "--disable-renderer-backgrounding",
+      "--disable-backgrounding-occluded-windows",
+    ],
+  })
+
+  async function run(mode: string): Promise<Bench> {
+    const page = await browser.newPage()
+    page.on("pageerror", (e) => console.log(`  [page error] ${e.message}`))
+    page.on("console", (m) => {
+      const t = m.text()
+      if (t.startsWith("BENCH") || m.type() === "error") {
+        console.log(`  [console] ${t}`)
+      }
+    })
+    await page.setViewportSize({ width: 800, height: 600 })
+    await page.goto(`${URL}/?bench=${mode}`, { waitUntil: "load" })
+    await page.waitForFunction("window.__BENCH__ !== undefined", null, { timeout: 120000 })
+    const result = (await page.evaluate("window.__BENCH__")) as Bench
+    await page.close()
+    return result
+  }
+
+  console.log("--- single-thread ---")
+  const st = await run("st")
+  console.log("--- workers ---")
+  const mt = await run("mt")
+  await browser.close()
+
+  const line = (b: Bench) =>
+    `parallel=${b.parallel} coi=${b.coi} cores=${b.cores} res=${b.res}  work med/p95/max = ${b.workMs.median}/${b.workMs.p95}/${b.workMs.max} ms  frame med/p95 = ${b.frameMs.median}/${b.frameMs.p95} ms`
+  console.log("")
+  console.log(`single-thread : ${line(st)}`)
+  console.log(`workers       : ${line(mt)}`)
+  console.log("")
+  console.log(`work median   : ${st.workMs.median} -> ${mt.workMs.median} ms  (${(st.workMs.median / mt.workMs.median).toFixed(2)}x)`)
+  console.log(`work p95      : ${st.workMs.p95} -> ${mt.workMs.p95} ms  (${(st.workMs.p95 / mt.workMs.p95).toFixed(2)}x)`)
+  console.log(`frame p95     : ${st.frameMs.p95} -> ${mt.frameMs.p95} ms   (jitter: lower p95/median = steadier)`)
+}
+
+try {
+  await main()
+} finally {
+  vite.kill()
+}
diff --git a/vite.config.ts b/vite.config.ts
index 8153be3..cb69edb 100644
--- a/vite.config.ts
+++ b/vite.config.ts
@@ -1,8 +1,24 @@
 import { defineConfig } from "vite"
 
+// Cross-origin isolation (COOP + COEP) is required for SharedArrayBuffer, which
+// the multi-threaded renderer uses to share the framebuffer across workers.
+// Without these headers the app still runs -- it falls back to single-threaded.
+const crossOriginIsolation = {
+  "Cross-Origin-Opener-Policy": "same-origin",
+  "Cross-Origin-Embedder-Policy": "require-corp",
+}
+
 export default defineConfig({
   resolve: {
     tsconfigPaths: true,
   },
+  worker: {
+    format: "es",
+  },
+  server: {
+    headers: crossOriginIsolation,
+  },
+  preview: {
+    headers: crossOriginIsolation,
+  },
 })
-