diff --git a/config.json b/config.json
index e945d1c4..d1c461a4 100644
--- a/config.json
+++ b/config.json
@@ -17,5 +17,9 @@
   "hashRouter": {
     "enabled": false,
     "basename": "/"
+  },
+
+  "backgroundScene": {
+    "tier": "off"
   }
 }
diff --git a/harness/scene/README.md b/harness/scene/README.md
new file mode 100644
index 00000000..382819ec
--- /dev/null
+++ b/harness/scene/README.md
@@ -0,0 +1,19 @@
+# Scene harness
+
+Compiles, renders and times the background scene (`src/app/features/scene/`) in headless
+Chromium. Not shipped, not run by CI.
+
+    node bundle.cjs <outdir>                  # esbuild bundle of entry.ts plus an index.html
+    node drive.cjs <outdir> compile           # compile and link each stage, JSON report
+    node drive.cjs <outdir> shots <pngdir>    # rest, mid pull-back and pulled-back screenshots
+    node drive.cjs <outdir> measure           # GPU time per pass by timer query, two viewports, two levels
+    node drive.cjs <outdir> variants          # the same with source patches (no erosion, no tree, value-only fbm)
+    node drive.cjs <outdir> phacelle          # the Phacelle gradient probe behind world.ts `phacelleGradBound`
+
+Needs `esbuild` and `playwright@1.48.2` on `NODE_PATH` and a Chromium. The first run was done on a
+host with no GPU, so `drive.cjs` launches with `--use-angle=swiftshader` (software Vulkan). On a
+machine with a GPU, remove those flags to measure the real device. `bundle.cjs` must run from a
+directory whose ancestors hold no empty `package.json` (esbuild reads them).
+
+Timings from SwiftShader are CPU time: use them for the split between passes and for before and
+after of one change, never as a phone frame time.
diff --git a/harness/scene/bundle.cjs b/harness/scene/bundle.cjs
new file mode 100644
index 00000000..62a47078
--- /dev/null
+++ b/harness/scene/bundle.cjs
@@ -0,0 +1,35 @@
+// Bundles entry.ts with the scene's ?raw GLSL imports into $OUT/bundle.js plus an index.html.
+// usage: node bundle.cjs <outdir>     (esbuild from NODE_PATH)
+const esbuild = require('esbuild');
+const fs = require('fs');
+const path = require('path');
+const out = process.argv[2];
+fs.mkdirSync(out, { recursive: true });
+const rawPlugin = {
+  name: 'raw',
+  setup(b) {
+    b.onResolve({ filter: /\?raw$/ }, (a) => ({
+      path: path.resolve(a.resolveDir, a.path.slice(0, -4)),
+      namespace: 'raw',
+    }));
+    b.onLoad({ filter: /.*/, namespace: 'raw' }, (a) => ({
+      contents: fs.readFileSync(a.path, 'utf8'),
+      loader: 'text',
+    }));
+  },
+};
+esbuild
+  .build({
+    entryPoints: [path.join(__dirname, 'entry.ts')],
+    bundle: true,
+    outfile: path.join(out, 'bundle.js'),
+    plugins: [rawPlugin],
+    format: 'iife',
+    target: 'es2020',
+  })
+  .then(() => {
+    fs.writeFileSync(
+      path.join(out, 'index.html'),
+      '<!doctype html><meta charset=utf-8><body style="margin:0"><canvas id=c style="width:100vw;height:100vh;display:block"></canvas><script src=bundle.js></script>'
+    );
+  });
diff --git a/harness/scene/drive.cjs b/harness/scene/drive.cjs
new file mode 100644
index 00000000..74c4a8b0
--- /dev/null
+++ b/harness/scene/drive.cjs
@@ -0,0 +1,164 @@
+// Drives the scene in headless Chromium (software GL unless the host has a GPU).
+// usage: node drive.cjs <bundledir> <mode> [outdir]     modes: compile | shots | measure
+// Needs playwright from NODE_PATH and PLAYWRIGHT_BROWSERS_PATH. Prints one JSON report.
+const { chromium } = require('playwright');
+const path = require('path');
+const [, , dir, mode, outdir = '.'] = process.argv;
+
+(async () => {
+  const b = await chromium.launch({
+    args: [
+      '--use-gl=angle',
+      '--use-angle=swiftshader',
+      '--enable-unsafe-swiftshader',
+      '--ignore-gpu-blocklist',
+    ],
+  });
+  const page = await b.newPage({ viewport: { width: 960, height: 540 } });
+  page.on('console', (m) => process.stderr.write(`[page] ${m.text()}\n`));
+  await page.goto('file://' + path.resolve(dir, 'index.html'));
+  const report = { mode };
+  report.renderer = await page.evaluate(() => {
+    const g = document.createElement('canvas').getContext('webgl2');
+    const e = g.getExtension('WEBGL_debug_renderer_info');
+    return g.getParameter(e ? e.UNMASKED_RENDERER_WEBGL : g.RENDERER);
+  });
+
+  if (mode === 'compile') {
+    report.stages = await page.evaluate(() => {
+      const gl = document.createElement('canvas').getContext('webgl2');
+      const S = window.__scene;
+      return ['terrain', 'atmosphere', 'resolve', 'composite'].map((stage) => {
+        const t0 = performance.now();
+        try {
+          const p = S.linkStage(gl, stage);
+          return {
+            stage,
+            ok: true,
+            ms: +(performance.now() - t0).toFixed(1),
+            sourceBytes: S.fragmentSource(stage).length,
+            log: gl.getProgramInfoLog(p),
+          };
+        } catch (e) {
+          return {
+            stage,
+            ok: false,
+            ms: +(performance.now() - t0).toFixed(1),
+            log: String(e.log || e),
+          };
+        }
+      });
+    });
+  }
+
+  if (mode === 'phacelle') {
+    report.probe = await page.evaluate(() =>
+      window.__probePhacelle(
+        window.__scene.WORLD.erodeSide.slice(),
+        window.__scene.WORLD.erodeNorm,
+        4
+      )
+    );
+  }
+
+  if (mode === 'shots') {
+    await page.evaluate(() => {
+      const c = document.getElementById('c');
+      window.__s = window.__scene.createScene({
+        canvas: c,
+        unthrottled: true,
+        fixedLevel: window.__scene.LEVELS[0],
+      });
+    });
+    const wait = (ms) => page.waitForTimeout(ms);
+    await wait(4000);
+    await page.screenshot({ path: path.join(outdir, 'rest.png') });
+    await page.evaluate(() => window.__s.login());
+    await wait(3500);
+    await page.screenshot({ path: path.join(outdir, 'mid-pullback.png') });
+    await wait(9000);
+    await page.screenshot({ path: path.join(outdir, 'pulled-back.png') });
+    report.shots = ['rest.png', 'mid-pullback.png', 'pulled-back.png'];
+  }
+
+  if (mode === 'measure' || mode === 'variants') {
+    const VARIANTS = {
+      baseline: null,
+      // What a public value-only fbm would cost: the same octaves without the gradient work.
+      valueOnlyFbm: [
+        'float terrainFbm(vec2 xz) { return cultmath_fbm_grad(vec3(xz * W_FREQ, 0.0), W_OCTAVES, W_LACUNARITY, W_GAIN).w; }',
+        'float terrainFbm(vec2 xz) { float a = 1.0, fr = 1.0, v = 0.0; for (int i = 0; i < W_OCTAVES; i++) { v += a * cultmath_snoise(vec3(xz * W_FREQ * fr, 0.0)); fr *= W_LACUNARITY; a *= W_GAIN; } return v; }',
+      ],
+      noErosion: ['#define W_ERODE_ENABLED 1', '#define W_ERODE_ENABLED 0'],
+      // Upper bound on what skipping sky rays would save: every upward ray misses at once.
+      noSkyMarch: ['    bool hit = false;\n    bool onTree = false;', '    if (rd.y > 0.0) { o_color = vec4(0.0); o_depth = W_FAR; return; }\n    bool hit = false;\n    bool onTree = false;'],
+      noTree: ['float dr = treeDistance(p, leafness);', 'float dr = 1e9; leafness = 0.0;'],
+    };
+    const wanted = mode === "variants" ? (process.env.ONLY ? process.env.ONLY.split(",") : Object.keys(VARIANTS)) : ['baseline'];
+    const sizes = [
+      [960, 540],
+      [540, 1200],
+    ];
+    report.runs = [];
+    for (const [w, h] of sizes) {
+      for (const [li, variant] of wanted.flatMap((v) =>
+        mode === 'variants'
+          ? [[0, v]]
+          : [
+              [0, v],
+              [3, v],
+            ]
+      )) {
+        await page.setViewportSize({ width: w, height: h });
+        const r = await page.evaluate(
+          ({ li, rep }) =>
+            new Promise((res) => {
+              const rows = [];
+              const c = document.getElementById('c');
+              const s = window.__scene.createScene({
+                canvas: c,
+                unthrottled: true,
+                fixedLevel: window.__scene.LEVELS[li],
+                patchSource: rep
+                  ? (st, src) =>
+                      st === 'terrain'
+                        ? rep[0].startsWith('#define')
+                          ? src.replace(/#define W_ERODE_ENABLED \d/, rep[1])
+                          : src.split(rep[0]).join(rep[1])
+                        : src
+                  : undefined,
+                onStats: (st) => {
+                  rows.push(st);
+                  if (rows.length === 40) {
+                    s.dispose();
+                    const keep = rows.slice(10);
+                    const med = (k) =>
+                      keep.map((x) => x.passMs[k]).sort((a, b) => a - b)[keep.length >> 1];
+                    res({
+                      solid: keep[0].solidSize,
+                      atm: keep[0].atmosphereSize,
+                      medianMs: {
+                        terrain: med('terrain'),
+                        atmosphere: med('atmosphere'),
+                        resolve: med('resolve'),
+                        composite: med('composite'),
+                      },
+                    });
+                  }
+                },
+              });
+              if (!s) res({ error: 'createScene returned null' });
+            }),
+          { li, rep: VARIANTS[variant] }
+        );
+        report.runs.push({ viewport: [w, h], level: li, variant, ...r });
+      }
+    }
+  }
+
+  console.log(JSON.stringify(report, null, 1));
+  await b.close();
+})().catch((e) => {
+  console.log('ERR', String(e).slice(0, 2000));
+  process.exit(1);
+});
diff --git a/harness/scene/entry.ts b/harness/scene/entry.ts
new file mode 100644
index 00000000..d43a5453
--- /dev/null
+++ b/harness/scene/entry.ts
@@ -0,0 +1,87 @@
+// Browser entry for the harness: exposes the scene's seams to the driver. Not shipped.
+import { createScene, linkStage, ShaderCompileError } from '../../src/app/features/scene/Scene';
+import { LEVELS } from '../../src/app/features/scene/budget';
+import { fragmentSource } from '../../src/app/features/scene/shaderSource';
+import { WORLD } from '../../src/app/features/scene/world';
+
+(window as unknown as Record<string, unknown>).__scene = {
+  createScene,
+  linkStage,
+  ShaderCompileError,
+  LEVELS,
+  fragmentSource,
+  WORLD,
+};
+
+// ---- Phacelle gradient probe -----------------------------------------------------------------
+import cultmath from '../../src/app/features/scene/shaders/CultMath.glsl?raw';
+import phacelle from '../../src/app/features/scene/shaders/CultMath.Phacelle.glsl?raw';
+import { vertexSource } from '../../src/app/features/scene/shaderSource';
+
+const PROBE_FRAG = `#version 300 es
+precision highp float; precision highp int;
+${cultmath}
+${phacelle}
+uniform float u_seed; uniform vec3 u_side; uniform float u_norm; uniform float u_span;
+in vec2 v_uv; layout(location=0) out vec4 o;
+void main() {
+  // The scene samples the z = 0 slice: random points in a u_span square of it.
+  vec3 h = vec3(cultmath_pcg3d(vec3(gl_FragCoord.xy, u_seed))) ;
+  vec2 xy = vec2(float(uint(int(h.x)) >> 8), float(uint(int(h.y)) >> 8)) * (1.0 / 16777216.0) * u_span;
+  CultPhasor ph = cultmath_phacelle(vec3(xy, 0.0), u_side, 0.0, u_norm);
+  o = vec4(length(ph.cos.xyz), length(ph.sin.xyz), ph.cos.w, ph.sin.w);
+}`;
+
+/** Max |grad| of the phasor over `draws * 1024^2` random points; min/max of the cos component. */
+(window as unknown as Record<string, unknown>).__probePhacelle = (
+  side: number[],
+  norm: number,
+  draws: number
+) => {
+  const c = document.createElement('canvas');
+  const gl = c.getContext('webgl2')!;
+  gl.getExtension('EXT_color_buffer_float');
+  const mk = (t: number, s: string) => {
+    const x = gl.createShader(t)!;
+    gl.shaderSource(x, s);
+    gl.compileShader(x);
+    if (!gl.getShaderParameter(x, gl.COMPILE_STATUS)) throw new Error(gl.getShaderInfoLog(x)!);
+    return x;
+  };
+  const p = gl.createProgram()!;
+  gl.attachShader(p, mk(gl.VERTEX_SHADER, vertexSource));
+  gl.attachShader(p, mk(gl.FRAGMENT_SHADER, PROBE_FRAG));
+  gl.linkProgram(p);
+  gl.useProgram(p);
+  const N = 1024;
+  const tex = gl.createTexture();
+  gl.bindTexture(gl.TEXTURE_2D, tex);
+  gl.texStorage2D(gl.TEXTURE_2D, 1, gl.RGBA32F, N, N);
+  const fbo = gl.createFramebuffer();
+  gl.bindFramebuffer(gl.FRAMEBUFFER, fbo);
+  gl.framebufferTexture2D(gl.FRAMEBUFFER, gl.COLOR_ATTACHMENT0, gl.TEXTURE_2D, tex, 0);
+  gl.viewport(0, 0, N, N);
+  gl.bindVertexArray(gl.createVertexArray());
+  gl.uniform3fv(gl.getUniformLocation(p, 'u_side'), side);
+  gl.uniform1f(gl.getUniformLocation(p, 'u_norm'), norm);
+  gl.uniform1f(gl.getUniformLocation(p, 'u_span'), 400);
+  const buf = new Float32Array(N * N * 4);
+  let gradCos = 0,
+    gradSin = 0,
+    vMin = 1e9,
+    vMax = -1e9,
+    n = 0;
+  for (let d = 0; d < draws; d++) {
+    gl.uniform1f(gl.getUniformLocation(p, 'u_seed'), d + 1);
+    gl.drawArrays(gl.TRIANGLES, 0, 3);
+    gl.readPixels(0, 0, N, N, gl.RGBA, gl.FLOAT, buf);
+    for (let i = 0; i < buf.length; i += 4) {
+      if (buf[i] > gradCos) gradCos = buf[i];
+      if (buf[i + 1] > gradSin) gradSin = buf[i + 1];
+      if (buf[i + 2] < vMin) vMin = buf[i + 2];
+      if (buf[i + 2] > vMax) vMax = buf[i + 2];
+      n++;
+    }
+  }
+  return { samples: n, maxGradCos: gradCos, maxGradSin: gradSin, minCos: vMin, maxCos: vMax };
+};
diff --git a/src/app/features/scene/README.md b/src/app/features/scene/README.md
new file mode 100644
index 00000000..0d49d57d
--- /dev/null
+++ b/src/app/features/scene/README.md
@@ -0,0 +1,67 @@
+# Background scene
+
+A two-pass WebGL2 scene for the client's background canvas: an infinite terrain of CultMath
+noise with a world tree at its centre, dusk atmosphere and clouds, and a camera that pulls back
+from the tree on login. It is a self-contained module; nothing in the client imports it yet.
+Integration (a canvas under the app, `startBackgroundScene`, `login()` when the session starts,
+`dispose()`) is Void's cut `terrain-background`; the personas (`orbiting-personas`) and the squirrel
+are later cuts. This PR is the render core plus the contract the host needs: flag, tier, failure
+callbacks and the creature sizing rule. Nothing has drawn a creature.
+
+## Entry points
+
+- `index.ts` `startBackgroundScene(config.backgroundScene, capabilities, options)` is the host's
+  door. The client config's `backgroundScene.tier` (`off` default, `still`, `low`, `full`) is the
+  feature flag and the ceiling; `tier.ts` lowers it for `saveData`, low memory or few cores. At
+  `off` nothing is imported and no canvas context is requested; otherwise `Scene.ts` loads on
+  demand, so it stays out of the main chunk.
+- `Scene.ts` `createScene({ canvas, tier, onCompileFail, onContextLost, ... })` returns
+  `{ login, dispose }`, or `null` (keep the CSS ground then) when the tier is `off`, WebGL2 or
+  `EXT_color_buffer_float` is missing, or a shader fails to compile (`onCompileFail` gets the
+  `ShaderCompileError`; programs already linked are freed). `still` draws one frame at the top
+  level and stops; `low` starts the ladder at level 2; `full` at level 0. A lost WebGL context
+  stops the scene for good and calls `onContextLost`; nothing rebuilds it, the host decides what to
+  show. It caps at 30 fps, pauses when
+  the tab is hidden, renders 32 frames and stops under `prefers-reduced-motion`, and steps down a
+  quality ladder (`budget.ts`) against a 4 ms GPU budget when timer queries exist.
+- `creatures.ts` the creature record and `creatureScale`: a neutral creature is one fixed size and
+  overrides `sizeBucket`. The bucket names and scales are placeholders until the persona cut owns them.
+- `camera.ts` the pull-back path as pure functions (`pullProgress`, `cameraPose`).
+- `world.ts` every world number, and the Lipschitz bounds the sphere trace steps by. The GLSL
+  `#define`s are generated from it; no number is written twice.
+- `shaders/` pass 1 `terrain.frag.glsl`, pass 2 `atmosphere.frag.glsl`, `resolve.frag.glsl` (TAA),
+  `composite.frag.glsl`, plus CultLib's vendored GLSL (`shaders/README.md`).
+
+## How it renders
+
+1. **Solid pass** (`terrain.frag.glsl`, 0.4-0.75 of canvas resolution): sphere trace of
+   `min(terrain, tree)`. Terrain height is `mask(r) * (A * fbm + E * phacelle)`, with
+   `cultmath_fbm_grad(...).w` and `cultmath_phacelle(...).cos.w`. It writes lit colour with
+   coverage and the hit distance (second render target).
+2. **Atmosphere pass** (0.25-0.5 of canvas): closed-form exponential height fog plus a cloud
+   slab marched with a few jittered steps, both only up to the hit distance from pass 1.
+3. **Resolve**: TAA on the atmosphere (Halton jitter, history reprojected by the pass-1 depth,
+   3x3 neighbourhood clamp, 1/8 blend; reset on resize, level change and wake).
+4. **Composite** at canvas resolution: solid over analytic sky, then the atmosphere, ACES
+   tonemap, dither.
+
+## The step bound, and what is not proven
+
+The trace steps by `(y - h(x, z)) / sqrt(1 + G^2)`, `G` the Lipschitz bound of the height field,
+computed from `SNOISE_LIPSCHITZ` (CultMath's constant for `snoise`) in `world.ts`. Above the
+envelope (fbm plus the erosion amplitude) the step uses the envelope's bound alone and Phacelle is
+not evaluated.
+
+- The Phacelle gradient bound `phacelleGradBound` (7.0) is Libby's measured sup (6.63 at
+  |side| = 1.5, n = 0, CultLib PR #1 head 990521b3) plus margin: a measurement, not a proof.
+  The shader fixes roughness n = 0; a nonzero n needs the bound divided by (1 - n).
+- The bound makes grazing rays crawl. A ray that exhausts its steps while still below the highest
+  terrain is ended as a distant surface (fog hides it). That is a visible approximation, not a
+  hit proven by the march.
+- The low-resolution atmosphere samples pass-1 depth nearest, so silhouettes of near objects
+  carry a thin fog halo.
+
+## Measured and not measured
+
+See the pull request or `harness/scene/README.md` for the numbers and how they were taken. They
+come from a software rasterizer, not a GPU, and nothing has been run on a phone.
diff --git a/src/app/features/scene/Scene.test.ts b/src/app/features/scene/Scene.test.ts
new file mode 100644
index 00000000..97988687
--- /dev/null
+++ b/src/app/features/scene/Scene.test.ts
@@ -0,0 +1,218 @@
+import { afterEach, beforeEach, describe, expect, it, Mock, vi } from 'vitest';
+import { createScene, ShaderCompileError } from './Scene';
+import { LEVELS } from './budget';
+import { startBackgroundScene } from './index';
+
+type Fn = Mock;
+
+/** A GL context where every call is a spy and every constant is 1. Compiles succeed until `failFrom`. */
+function fakeGl(failFrom = Infinity) {
+  let compiles = 0;
+  const calls: Record<string, Fn> = {};
+  const spy = (name: string, impl: () => unknown = () => ({})): Fn => {
+    if (!calls[name]) calls[name] = vi.fn(impl);
+    return calls[name];
+  };
+  const gl = new Proxy({} as Record<string, unknown>, {
+    get(_, prop: string) {
+      if (/^[A-Z][A-Z0-9_]*$/.test(prop)) return 1;
+      if (prop === 'compileShader') {
+        return spy(prop, () => {
+          compiles += 1;
+        });
+      }
+      if (prop === 'getShaderParameter') return spy(prop, () => compiles < failFrom);
+      if (prop === 'getShaderInfoLog') return spy(prop, () => 'ERROR: 0:1: bad');
+      if (prop === 'getProgramParameter') return spy(prop, () => true);
+      return spy(prop);
+    },
+  });
+  return { gl: gl as unknown as WebGL2RenderingContext, calls };
+}
+
+function fakeCanvas(gl: WebGL2RenderingContext | null) {
+  const canvas = new EventTarget() as EventTarget & {
+    clientWidth: number;
+    clientHeight: number;
+    width: number;
+    height: number;
+    getContext: Fn;
+  };
+  canvas.clientWidth = 400;
+  canvas.clientHeight = 300;
+  canvas.width = 0;
+  canvas.height = 0;
+  canvas.getContext = vi.fn(() => gl);
+  return canvas;
+}
+
+let rafQueue: Array<(t: number) => void>;
+let cancelled: number;
+let win: EventTarget;
+let doc: EventTarget & { visibilityState: string };
+
+beforeEach(() => {
+  rafQueue = [];
+  cancelled = 0;
+  win = Object.assign(new EventTarget(), {
+    devicePixelRatio: 1,
+    matchMedia: () => ({ matches: false }),
+  });
+  doc = Object.assign(new EventTarget(), { visibilityState: 'visible' });
+  vi.stubGlobal('window', win);
+  vi.stubGlobal('document', doc);
+  vi.stubGlobal('requestAnimationFrame', (cb: (t: number) => void) => rafQueue.push(cb));
+  vi.stubGlobal('cancelAnimationFrame', () => {
+    cancelled += 1;
+  });
+});
+
+afterEach(() => {
+  vi.unstubAllGlobals();
+});
+
+const runFrame = (now: number) => {
+  const cb = rafQueue.shift();
+  if (!cb) throw new Error('no frame was scheduled');
+  cb(now);
+};
+const asCanvas = (c: ReturnType<typeof fakeCanvas>) => c as unknown as HTMLCanvasElement;
+
+describe('createScene tiers', () => {
+  it("'off' returns null without asking the canvas for a context", () => {
+    const { gl } = fakeGl();
+    const canvas = fakeCanvas(gl);
+    expect(createScene({ canvas: asCanvas(canvas), tier: 'off' })).toBeNull();
+    expect(canvas.getContext).not.toHaveBeenCalled();
+    expect(rafQueue).toHaveLength(0);
+  });
+
+  it("'still' draws one frame and schedules no more", () => {
+    const { gl, calls } = fakeGl();
+    const scene = createScene({ canvas: asCanvas(fakeCanvas(gl)), tier: 'still' });
+    expect(scene).not.toBeNull();
+    runFrame(1000);
+    expect(calls.drawArrays).toHaveBeenCalledTimes(4); // four passes, one frame
+    expect(rafQueue).toHaveLength(0);
+  });
+
+  it("'still' ignores login(): the pull-back is for a moving scene", () => {
+    const { gl } = fakeGl();
+    const scene = createScene({ canvas: asCanvas(fakeCanvas(gl)), tier: 'still' });
+    runFrame(1000);
+    scene?.login();
+    expect(rafQueue).toHaveLength(0);
+  });
+
+  it("'full' keeps scheduling frames", () => {
+    const { gl } = fakeGl();
+    createScene({ canvas: asCanvas(fakeCanvas(gl)), tier: 'full' });
+    runFrame(1000);
+    expect(rafQueue).toHaveLength(1);
+  });
+
+  it("'low' traces with a cheaper level than 'full'", () => {
+    const traceSteps = (tier: 'low' | 'full') => {
+      rafQueue.length = 0; // each call is a fresh scene
+      const { gl, calls } = fakeGl();
+      createScene({ canvas: asCanvas(fakeCanvas(gl)), tier });
+      runFrame(1000);
+      // u_maxSteps is the only uniform1i whose value is a trace-step count
+      return calls.uniform1i.mock.calls.map((c) => c[1]).filter((v) => v >= 40);
+    };
+    expect(traceSteps('full')).toContain(LEVELS[0].traceSteps);
+    expect(traceSteps('low')).toContain(LEVELS[2].traceSteps);
+    expect(traceSteps('low')).not.toContain(LEVELS[0].traceSteps);
+  });
+
+  it('returns null when WebGL2 is missing, so the host keeps its ground', () => {
+    expect(createScene({ canvas: asCanvas(fakeCanvas(null)) })).toBeNull();
+  });
+});
+
+describe('shader compile failure', () => {
+  it('reports through onCompileFail, returns null, and frees the programs already linked', () => {
+    const { gl, calls } = fakeGl(4); // terrain compiles both shaders; atmosphere's fragment is the 4th and fails
+    const onCompileFail = vi.fn();
+    const scene = createScene({ canvas: asCanvas(fakeCanvas(gl)), onCompileFail });
+    expect(scene).toBeNull();
+    expect(onCompileFail).toHaveBeenCalledTimes(1);
+    const err = onCompileFail.mock.calls[0][0] as ShaderCompileError;
+    expect(err).toBeInstanceOf(ShaderCompileError);
+    expect(err.stage).toBe('atmosphere');
+    expect(err.log).toContain('bad');
+    expect(calls.deleteProgram).toHaveBeenCalled(); // the terrain program that did link
+    expect(rafQueue).toHaveLength(0);
+  });
+
+  it('does not throw out of createScene when no callback is given', () => {
+    const { gl } = fakeGl(0);
+    expect(() => createScene({ canvas: asCanvas(fakeCanvas(gl)) })).not.toThrow();
+  });
+});
+
+describe('context loss', () => {
+  it('stops the loop, tells the host, and detaches every listener', () => {
+    const { gl } = fakeGl();
+    const canvas = fakeCanvas(gl);
+    const onContextLost = vi.fn();
+    createScene({ canvas: asCanvas(canvas), onContextLost });
+    const lost = new Event('webglcontextlost', { cancelable: true });
+    canvas.dispatchEvent(lost);
+    expect(lost.defaultPrevented).toBe(true);
+    expect(onContextLost).toHaveBeenCalledTimes(1);
+    expect(cancelled).toBe(1);
+    // listeners are gone: a second loss, a resize and a visibility change do nothing
+    canvas.dispatchEvent(new Event('webglcontextlost', { cancelable: true }));
+    win.dispatchEvent(new Event('resize'));
+    doc.dispatchEvent(new Event('visibilitychange'));
+    expect(onContextLost).toHaveBeenCalledTimes(1);
+    expect(rafQueue.length).toBe(1); // only the frame queued before the loss, never run
+  });
+
+  it('does not render a queued frame after the loss', () => {
+    const { gl, calls } = fakeGl();
+    const canvas = fakeCanvas(gl);
+    createScene({ canvas: asCanvas(canvas) });
+    canvas.dispatchEvent(new Event('webglcontextlost', { cancelable: true }));
+    runFrame(1000);
+    expect(calls.drawArrays).toBeUndefined();
+  });
+});
+
+describe('startBackgroundScene', () => {
+  it("does not request a context when the config leaves the tier 'off'", async () => {
+    const { gl } = fakeGl();
+    const canvas = fakeCanvas(gl);
+    expect(await startBackgroundScene(undefined, {}, { canvas: asCanvas(canvas) })).toBeNull();
+    expect(
+      await startBackgroundScene({ tier: 'off' }, {}, { canvas: asCanvas(canvas) })
+    ).toBeNull();
+    expect(canvas.getContext).not.toHaveBeenCalled();
+  });
+
+  it('applies the device ceiling: saveData runs a still, one frame', async () => {
+    const { gl, calls } = fakeGl();
+    const scene = await startBackgroundScene(
+      { tier: 'full' },
+      { saveData: true },
+      { canvas: asCanvas(fakeCanvas(gl)) }
+    );
+    expect(scene).not.toBeNull();
+    runFrame(1000);
+    expect(calls.drawArrays).toHaveBeenCalledTimes(4);
+    expect(rafQueue).toHaveLength(0);
+  });
+
+  it('hands compile failure to the host and returns null, the fall to the CSS ground', async () => {
+    const { gl } = fakeGl(0);
+    const onCompileFail = vi.fn();
+    const scene = await startBackgroundScene(
+      { tier: 'full' },
+      {},
+      { canvas: asCanvas(fakeCanvas(gl)), onCompileFail }
+    );
+    expect(scene).toBeNull();
+    expect(onCompileFail).toHaveBeenCalledTimes(1);
+  });
+});
diff --git a/src/app/features/scene/Scene.ts b/src/app/features/scene/Scene.ts
new file mode 100644
index 00000000..10dacc58
--- /dev/null
+++ b/src/app/features/scene/Scene.ts
@@ -0,0 +1,469 @@
+import { fragmentSource, vertexSource, Stage } from './shaderSource';
+import { cameraPose, pullProgress, CameraPose } from './camera';
+import { BudgetLadder, Level, LEVELS } from './budget';
+import type { SceneTier } from './tier';
+import { PALETTE } from './palette';
+
+/** Phone GPU budget for the whole scene, from the ground-cut doc's budget table. */
+export const GPU_BUDGET_MS = 4.0;
+const FRAME_INTERVAL_MS = 1000 / 30;
+const REDUCED_MOTION_FRAMES = 32;
+const MAX_DPR = 1.5;
+
+export type PassName = 'terrain' | 'atmosphere' | 'resolve' | 'composite';
+const PASSES: PassName[] = ['terrain', 'atmosphere', 'resolve', 'composite'];
+
+export interface SceneStats {
+  /** GPU milliseconds per pass for one finished frame (timer queries; absent without the extension). */
+  passMs: Record<PassName, number>;
+  level: number;
+  solidSize: [number, number];
+  atmosphereSize: [number, number];
+}
+
+/** Measurement seam: the harness rewrites a stage's source to time a variant. Never set in the client. */
+export type SourcePatch = (stage: Stage, source: string) => string;
+
+export class ShaderCompileError extends Error {
+  constructor(readonly stage: Stage | 'vertex', readonly log: string) {
+    super(`${stage} shader failed to compile:\n${log}`);
+  }
+}
+
+/** Ladder level a 'low' scene starts at: the cost of a weak device's first 45 frames is never paid at full. */
+const LOW_START_LEVEL = 2;
+/** The 'still' tier draws one frame at this level and stops. */
+const STILL_LEVEL = LEVELS[0];
+
+export interface SceneOptions {
+  canvas: HTMLCanvasElement;
+  /**
+   * 'off' returns null without touching the canvas. 'still' draws one frame and stops. 'low' starts
+   * the adaptive ladder low. 'full' starts it at the top. Default 'full'.
+   */
+  tier?: SceneTier;
+  /** A shader failed to compile or link. createScene frees what it built and returns null. */
+  onCompileFail?: (e: ShaderCompileError) => void;
+  /** The WebGL context was lost. The scene stops for good; the host keeps its ground. */
+  onContextLost?: () => void;
+  /** Called per finished frame when timer queries exist. */
+  onStats?: (s: SceneStats) => void;
+  /** Weather knob, 0 clear .. 1 overcast. */
+  coverage?: number;
+  /** Fixed level instead of the adaptive ladder (measurement). */
+  fixedLevel?: Level;
+  /** Ignore the 30 fps cap and the visibility pause (measurement). */
+  unthrottled?: boolean;
+  patchSource?: SourcePatch;
+}
+
+export interface Scene {
+  /** Start the camera pull-back from the tree. */
+  login(): void;
+  dispose(): void;
+}
+
+function compile(
+  gl: WebGL2RenderingContext,
+  type: number,
+  source: string,
+  stage: Stage | 'vertex'
+): WebGLShader {
+  const sh = gl.createShader(type)!;
+  gl.shaderSource(sh, source);
+  gl.compileShader(sh);
+  if (!gl.getShaderParameter(sh, gl.COMPILE_STATUS)) {
+    const log = gl.getShaderInfoLog(sh) ?? '';
+    gl.deleteShader(sh);
+    throw new ShaderCompileError(stage, log);
+  }
+  return sh;
+}
+
+export function linkStage(
+  gl: WebGL2RenderingContext,
+  stage: Stage,
+  patch?: SourcePatch
+): WebGLProgram {
+  const vs = compile(gl, gl.VERTEX_SHADER, vertexSource, 'vertex');
+  const fs = compile(
+    gl,
+    gl.FRAGMENT_SHADER,
+    patch ? patch(stage, fragmentSource(stage)) : fragmentSource(stage),
+    stage
+  );
+  const prog = gl.createProgram()!;
+  gl.attachShader(prog, vs);
+  gl.attachShader(prog, fs);
+  gl.linkProgram(prog);
+  gl.deleteShader(vs);
+  gl.deleteShader(fs);
+  if (!gl.getProgramParameter(prog, gl.LINK_STATUS)) {
+    const log = gl.getProgramInfoLog(prog) ?? '';
+    gl.deleteProgram(prog);
+    throw new ShaderCompileError(stage, log);
+  }
+  return prog;
+}
+
+interface Target {
+  tex: WebGLTexture;
+  w: number;
+  h: number;
+}
+
+function halton(i: number, base: number): number {
+  let f = 1;
+  let r = 0;
+  let n = i;
+  while (n > 0) {
+    f /= base;
+    r += f * (n % base);
+    n = Math.floor(n / base);
+  }
+  return r;
+}
+
+/** Null when WebGL2 or float colour buffers are missing; the host keeps its CSS ground. */
+export function createScene(opts: SceneOptions): Scene | null {
+  const { canvas } = opts;
+  const tier = opts.tier ?? 'full';
+  if (tier === 'off') return null;
+  const gl = canvas.getContext('webgl2', {
+    antialias: false,
+    alpha: false,
+    depth: false,
+    stencil: false,
+    powerPreference: 'low-power',
+  });
+  if (!gl || !gl.getExtension('EXT_color_buffer_float')) return null;
+  gl.getExtension('OES_texture_float_linear');
+  const timer = gl.getExtension('EXT_disjoint_timer_query_webgl2') as {
+    TIME_ELAPSED_EXT: number;
+    GPU_DISJOINT_EXT: number;
+  } | null;
+
+  const programs = {} as Record<PassName, WebGLProgram>;
+  try {
+    PASSES.forEach((p) => {
+      programs[p] = linkStage(gl, p, opts.patchSource);
+    });
+  } catch (e) {
+    PASSES.forEach((p) => {
+      if (programs[p]) gl.deleteProgram(programs[p]);
+    });
+    if (!(e instanceof ShaderCompileError)) throw e;
+    opts.onCompileFail?.(e);
+    return null;
+  }
+  const uniforms = new Map<WebGLProgram, Map<string, WebGLUniformLocation | null>>();
+  const loc = (p: WebGLProgram, name: string) => {
+    let m = uniforms.get(p);
+    if (!m) uniforms.set(p, (m = new Map()));
+    if (!m.has(name)) m.set(name, gl.getUniformLocation(p, name));
+    return m.get(name) ?? null;
+  };
+
+  const vao = gl.createVertexArray();
+  gl.bindVertexArray(vao);
+
+  const makeTarget = (w: number, h: number, format: number, filter: number): Target => {
+    const tex = gl.createTexture()!;
+    gl.bindTexture(gl.TEXTURE_2D, tex);
+    gl.texStorage2D(gl.TEXTURE_2D, 1, format, w, h);
+    gl.texParameteri(gl.TEXTURE_2D, gl.TEXTURE_MIN_FILTER, filter);
+    gl.texParameteri(gl.TEXTURE_2D, gl.TEXTURE_MAG_FILTER, filter);
+    gl.texParameteri(gl.TEXTURE_2D, gl.TEXTURE_WRAP_S, gl.CLAMP_TO_EDGE);
+    gl.texParameteri(gl.TEXTURE_2D, gl.TEXTURE_WRAP_T, gl.CLAMP_TO_EDGE);
+    return { tex, w, h };
+  };
+
+  const fbos = {
+    solid: gl.createFramebuffer()!,
+    atm: gl.createFramebuffer()!,
+    hist: [gl.createFramebuffer()!, gl.createFramebuffer()!],
+  };
+  let solid: Target;
+  let depth: Target;
+  let atm: Target;
+  let hist: [Target, Target];
+  const freeTargets = () => {
+    [solid, depth, atm, hist?.[0], hist?.[1]].forEach((t) => {
+      if (t) gl.deleteTexture(t.tex);
+    });
+  };
+
+  const attach = (fbo: WebGLFramebuffer, targets: Target[]) => {
+    gl.bindFramebuffer(gl.FRAMEBUFFER, fbo);
+    targets.forEach((t, i) =>
+      gl.framebufferTexture2D(gl.FRAMEBUFFER, gl.COLOR_ATTACHMENT0 + i, gl.TEXTURE_2D, t.tex, 0)
+    );
+    gl.drawBuffers(targets.map((_, i) => gl.COLOR_ATTACHMENT0 + i));
+  };
+
+  const still = tier === 'still';
+  const ladder = new BudgetLadder(GPU_BUDGET_MS, 45, tier === 'low' ? LOW_START_LEVEL : 0);
+  const level = (): Level => opts.fixedLevel ?? (still ? STILL_LEVEL : ladder.current);
+
+  let canvasW = 0;
+  let canvasH = 0;
+  let sizedFor: Level | undefined;
+  let resetHistory = true;
+
+  const ensureSize = () => {
+    const dpr = Math.min(window.devicePixelRatio || 1, MAX_DPR);
+    const w = Math.max(2, Math.round(canvas.clientWidth * dpr));
+    const h = Math.max(2, Math.round(canvas.clientHeight * dpr));
+    const lv = level();
+    if (w === canvasW && h === canvasH && lv === sizedFor) return;
+    canvasW = w;
+    canvasH = h;
+    sizedFor = lv;
+    canvas.width = w;
+    canvas.height = h;
+    freeTargets();
+    const sw = Math.max(2, Math.round(w * lv.solidScale));
+    const sh = Math.max(2, Math.round(h * lv.solidScale));
+    const aw = Math.max(2, Math.round(w * lv.atmosphereScale));
+    const ah = Math.max(2, Math.round(h * lv.atmosphereScale));
+    solid = makeTarget(sw, sh, gl.RGBA16F, gl.LINEAR);
+    depth = makeTarget(sw, sh, gl.R16F, gl.NEAREST);
+    atm = makeTarget(aw, ah, gl.RGBA16F, gl.LINEAR);
+    hist = [makeTarget(aw, ah, gl.RGBA16F, gl.LINEAR), makeTarget(aw, ah, gl.RGBA16F, gl.LINEAR)];
+    attach(fbos.solid, [solid, depth]);
+    attach(fbos.atm, [atm]);
+    attach(fbos.hist[0], [hist[0]]);
+    attach(fbos.hist[1], [hist[1]]);
+    resetHistory = true;
+  };
+
+  const reducedMotion = window.matchMedia?.('(prefers-reduced-motion: reduce)');
+  const start = performance.now();
+  let loginAt: number | undefined;
+  let frame = 0;
+  let histIndex = 0;
+  let prev: CameraPose | undefined;
+  let raf = 0;
+  let lastRendered = -Infinity;
+  let framesLeft = still ? 1 : Infinity; // a still frame, or reduced motion, bounds this
+  let disposed = false;
+
+  // Timer queries in flight: each holds one pass's query.
+  const pending: Array<{ q: WebGLQuery; pass: PassName; frame: number }> = [];
+  const finished = new Map<number, Partial<Record<PassName, number>>>();
+
+  const timed = (pass: PassName, f: number, body: () => void) => {
+    if (!timer || !opts.onStats) {
+      body();
+      return;
+    }
+    const q = gl.createQuery()!;
+    gl.beginQuery(timer.TIME_ELAPSED_EXT, q);
+    body();
+    gl.endQuery(timer.TIME_ELAPSED_EXT);
+    pending.push({ q, pass, frame: f });
+  };
+
+  const collect = () => {
+    if (!timer) return;
+    while (pending.length && gl.getQueryParameter(pending[0].q, gl.QUERY_RESULT_AVAILABLE)) {
+      const { q, pass, frame: f } = pending.shift()!;
+      const disjoint = gl.getParameter(timer.GPU_DISJOINT_EXT);
+      const ns = gl.getQueryParameter(q, gl.QUERY_RESULT) as number;
+      gl.deleteQuery(q);
+      const row = finished.get(f) ?? {};
+      if (!disjoint) row[pass] = ns / 1e6;
+      finished.set(f, row);
+      if (Object.keys(row).length === PASSES.length) {
+        finished.delete(f);
+        const passMs = row as Record<PassName, number>;
+        const total = PASSES.reduce((s, p) => s + passMs[p], 0);
+        if (!opts.fixedLevel && ladder.push(total)) sizedFor = undefined;
+        opts.onStats?.({
+          passMs,
+          level: ladder.index,
+          solidSize: [solid.w, solid.h],
+          atmosphereSize: [atm.w, atm.h],
+        });
+      }
+    }
+  };
+
+  const setCommon = (p: WebGLProgram, pose: CameraPose, time: number) => {
+    const aspect = canvasW / canvasH;
+    const th = Math.tan(pose.fov / 2);
+    gl.uniform3fv(loc(p, 'u_camPos'), pose.position as unknown as number[]);
+    gl.uniformMatrix3fv(loc(p, 'u_camBasis'), false, pose.basis as unknown as number[]);
+    gl.uniform2f(loc(p, 'u_tanHalf'), th * aspect, th);
+    gl.uniform1f(loc(p, 'u_time'), time);
+    gl.uniform3fv(loc(p, 'u_sunDir'), PALETTE.sunDir as unknown as number[]);
+    gl.uniform3fv(loc(p, 'u_sunColor'), PALETTE.sunColor as unknown as number[]);
+    gl.uniform3fv(loc(p, 'u_skyTop'), PALETTE.skyTop as unknown as number[]);
+    gl.uniform3fv(loc(p, 'u_skyHorizon'), PALETTE.skyHorizon as unknown as number[]);
+    gl.uniform3fv(loc(p, 'u_fogColor'), PALETTE.fogColor as unknown as number[]);
+  };
+
+  const bindTex = (unit: number, t: Target, p: WebGLProgram, name: string) => {
+    gl.activeTexture(gl.TEXTURE0 + unit);
+    gl.bindTexture(gl.TEXTURE_2D, t.tex);
+    gl.uniform1i(loc(p, name), unit);
+  };
+
+  const draw = () => gl.drawArrays(gl.TRIANGLES, 0, 3);
+
+  const render = (now: number) => {
+    ensureSize();
+    const lv = level();
+    const frozen = reducedMotion?.matches ?? false;
+    const time = frozen ? 0 : (now - start) / 1000;
+    let elapsed: number | undefined;
+    if (loginAt !== undefined) elapsed = frozen ? 1e9 : (now - loginAt) / 1000;
+    const pose = cameraPose(pullProgress(elapsed), time);
+    const f = frame;
+    frame += 1;
+    const jx = halton((f % 16) + 1, 2) - 0.5;
+    const jy = halton((f % 16) + 1, 3) - 0.5;
+
+    // Pass 1: solid surfaces, writes colour and depth.
+    timed('terrain', f, () => {
+      gl.bindFramebuffer(gl.FRAMEBUFFER, fbos.solid);
+      gl.viewport(0, 0, solid.w, solid.h);
+      const p = programs.terrain;
+      gl.useProgram(p);
+      setCommon(p, pose, time);
+      gl.uniform3fv(loc(p, 'u_grass'), PALETTE.grass as unknown as number[]);
+      gl.uniform3fv(loc(p, 'u_rock'), PALETTE.rock as unknown as number[]);
+      gl.uniform3fv(loc(p, 'u_bark'), PALETTE.bark as unknown as number[]);
+      gl.uniform3fv(loc(p, 'u_leaf'), PALETTE.leaf as unknown as number[]);
+      gl.uniform1i(loc(p, 'u_maxSteps'), lv.traceSteps);
+      draw();
+    });
+
+    // Pass 2: atmosphere up to pass 1's depth, low-res.
+    timed('atmosphere', f, () => {
+      gl.bindFramebuffer(gl.FRAMEBUFFER, fbos.atm);
+      gl.viewport(0, 0, atm.w, atm.h);
+      const p = programs.atmosphere;
+      gl.useProgram(p);
+      setCommon(p, pose, time);
+      bindTex(0, depth, p, 'u_depth');
+      gl.uniform1i(loc(p, 'u_steps'), lv.cloudSteps);
+      gl.uniform1f(loc(p, 'u_frame'), f);
+      gl.uniform2f(loc(p, 'u_jitter'), jx, jy);
+      gl.uniform2f(loc(p, 'u_size'), atm.w, atm.h);
+      gl.uniform1f(loc(p, 'u_coverage'), opts.coverage ?? 0.45);
+      draw();
+    });
+
+    // TAA resolve into the history that is not being read.
+    const write = histIndex === 0 ? 1 : 0;
+    timed('resolve', f, () => {
+      gl.bindFramebuffer(gl.FRAMEBUFFER, fbos.hist[write]);
+      gl.viewport(0, 0, atm.w, atm.h);
+      const p = programs.resolve;
+      gl.useProgram(p);
+      setCommon(p, pose, time);
+      bindTex(0, atm, p, 'u_cur');
+      bindTex(1, hist[histIndex], p, 'u_history');
+      bindTex(2, depth, p, 'u_depth');
+      const pp = prev ?? pose;
+      const th = Math.tan(pp.fov / 2);
+      gl.uniform3fv(loc(p, 'u_prevPos'), pp.position as unknown as number[]);
+      gl.uniformMatrix3fv(loc(p, 'u_prevBasis'), false, pp.basis as unknown as number[]);
+      gl.uniform2f(loc(p, 'u_prevTanHalf'), th * (canvasW / canvasH), th);
+      gl.uniform1f(loc(p, 'u_blend'), resetHistory ? 1 : 0.125);
+      gl.uniform2f(loc(p, 'u_curSize'), atm.w, atm.h);
+      draw();
+    });
+
+    // Full-resolution composite to the canvas.
+    timed('composite', f, () => {
+      gl.bindFramebuffer(gl.FRAMEBUFFER, null);
+      gl.viewport(0, 0, canvasW, canvasH);
+      const p = programs.composite;
+      gl.useProgram(p);
+      setCommon(p, pose, time);
+      bindTex(0, solid, p, 'u_solid');
+      bindTex(1, hist[write], p, 'u_atm');
+      gl.uniform1f(loc(p, 'u_frame'), f);
+      draw();
+    });
+
+    histIndex = write;
+    prev = pose;
+    resetHistory = false;
+    collect();
+  };
+
+  const tick = (now: number) => {
+    raf = 0;
+    if (disposed) return;
+    const due = opts.unthrottled || now - lastRendered >= FRAME_INTERVAL_MS - 2;
+    if (due) {
+      if (
+        !opts.fixedLevel &&
+        !timer &&
+        lastRendered > 0 &&
+        now - lastRendered > FRAME_INTERVAL_MS * 1.5
+      ) {
+        // no timer: a missed cap counts as over budget
+        if (ladder.push(GPU_BUDGET_MS * 2)) sizedFor = undefined;
+      }
+      lastRendered = now;
+      render(now);
+      framesLeft -= 1;
+    }
+    if (framesLeft > 0 && (opts.unthrottled || document.visibilityState === 'visible'))
+      raf = requestAnimationFrame(tick);
+  };
+
+  const wake = () => {
+    if (disposed || raf) return;
+    if (reducedMotion?.matches && framesLeft <= 0) return;
+    resetHistory = true; // the first frame after a pause has a real elapsed dt
+    raf = requestAnimationFrame(tick);
+  };
+  const undo: Array<() => void> = [];
+  const listen = (target: EventTarget, type: string, handler: (e: Event) => void) => {
+    target.addEventListener(type, handler);
+    undo.push(() => target.removeEventListener(type, handler));
+  };
+  const stop = () => {
+    disposed = true;
+    if (raf) cancelAnimationFrame(raf);
+    undo.forEach((f) => f());
+  };
+  if (!opts.unthrottled) {
+    listen(document, 'visibilitychange', () => {
+      if (document.visibilityState === 'visible') wake();
+    });
+  }
+  listen(window, 'resize', () => wake());
+  // A lost context takes every texture and program with it and nothing here rebuilds them, so the
+  // scene ends and the host decides what to show. preventDefault: otherwise the browser never
+  // restores the context, which is what we want, and a restore would find a dead scene.
+  listen(canvas, 'webglcontextlost', (e) => {
+    e.preventDefault();
+    stop();
+    opts.onContextLost?.();
+  });
+  if (reducedMotion?.matches && !still) framesLeft = REDUCED_MOTION_FRAMES;
+  wake();
+
+  return {
+    login() {
+      if (still || disposed) return; // a still has no pull-back
+      loginAt = performance.now();
+      if (reducedMotion?.matches) framesLeft = REDUCED_MOTION_FRAMES;
+      wake();
+    },
+    dispose() {
+      if (disposed) return;
+      stop();
+      freeTargets();
+      PASSES.forEach((p) => gl.deleteProgram(programs[p]));
+      gl.deleteVertexArray(vao);
+      [fbos.solid, fbos.atm, ...fbos.hist].forEach((fb) => gl.deleteFramebuffer(fb));
+    },
+  };
+}
diff --git a/src/app/features/scene/budget.test.ts b/src/app/features/scene/budget.test.ts
new file mode 100644
index 00000000..9ac8f0af
--- /dev/null
+++ b/src/app/features/scene/budget.test.ts
@@ -0,0 +1,41 @@
+import { describe, expect, it } from 'vitest';
+import { BudgetLadder, LEVELS } from './budget';
+
+describe('budget ladder', () => {
+  it('steps down after a whole window over budget and never back up', () => {
+    const ladder = new BudgetLadder(4, 5);
+    Array.from({ length: 4 }).forEach(() => expect(ladder.push(9)).toBe(false));
+    expect(ladder.push(9)).toBe(true);
+    expect(ladder.index).toBe(1);
+    Array.from({ length: 100 }).forEach(() => ladder.push(0.1)); // fast frames never raise the level
+    expect(ladder.index).toBe(1);
+  });
+
+  it('judges the median, so one slow frame does not step down', () => {
+    const ladder = new BudgetLadder(4, 5);
+    [1, 1, 50, 1, 1].forEach((ms) => ladder.push(ms));
+    expect(ladder.index).toBe(0);
+  });
+
+  it('stops at the last level', () => {
+    const ladder = new BudgetLadder(4, 1);
+    Array.from({ length: 20 }).forEach(() => ladder.push(99));
+    expect(ladder.index).toBe(LEVELS.length - 1);
+  });
+
+  it('every level is cheaper than the one before in each cost axis', () => {
+    LEVELS.slice(1).forEach((lv, i) => {
+      const before = LEVELS[i];
+      expect(lv.solidScale).toBeLessThan(before.solidScale);
+      expect(lv.atmosphereScale).toBeLessThan(before.atmosphereScale);
+      expect(lv.cloudSteps).toBeLessThan(before.cloudSteps);
+      expect(lv.traceSteps).toBeLessThan(before.traceSteps);
+    });
+  });
+
+  it('starts at the level it is given, clamped into the ladder', () => {
+    expect(new BudgetLadder(4, 5, 2).index).toBe(2);
+    expect(new BudgetLadder(4, 5, 99).index).toBe(LEVELS.length - 1);
+    expect(new BudgetLadder(4, 5, -3).index).toBe(0);
+  });
+});
diff --git a/src/app/features/scene/budget.ts b/src/app/features/scene/budget.ts
new file mode 100644
index 00000000..3b373d86
--- /dev/null
+++ b/src/app/features/scene/budget.ts
@@ -0,0 +1,59 @@
+// The adaptive step-down. The ladder only descends within a session; nothing here ever steps up.
+
+export interface Level {
+  /** Pass 1 resolution as a fraction of the canvas. */
+  solidScale: number;
+  /** Pass 2 resolution as a fraction of the canvas. */
+  atmosphereScale: number;
+  /** Cloud march steps. */
+  cloudSteps: number;
+  /** Sphere-trace step cap. */
+  traceSteps: number;
+}
+
+export const LEVELS: readonly Level[] = [
+  { solidScale: 0.75, atmosphereScale: 0.5, cloudSteps: 12, traceSteps: 96 },
+  { solidScale: 0.6, atmosphereScale: 0.4, cloudSteps: 8, traceSteps: 80 },
+  { solidScale: 0.5, atmosphereScale: 0.33, cloudSteps: 6, traceSteps: 64 },
+  { solidScale: 0.4, atmosphereScale: 0.25, cloudSteps: 4, traceSteps: 48 },
+];
+
+/** Median of the last `window` frame times above `budgetMs` for a whole window steps down one level. */
+export class BudgetLadder {
+  private level: number;
+
+  private recent: number[] = [];
+
+  private readonly budgetMs: number;
+
+  private readonly window: number;
+
+  /** `startLevel` lets a weak device begin below level 0; it is clamped into the ladder. */
+  constructor(budgetMs: number, window = 45, startLevel = 0) {
+    this.budgetMs = budgetMs;
+    this.window = window;
+    this.level = Math.min(Math.max(startLevel, 0), LEVELS.length - 1);
+  }
+
+  get current(): Level {
+    return LEVELS[this.level];
+  }
+
+  get index(): number {
+    return this.level;
+  }
+
+  /** Feed one frame's GPU (or cadence) time. Returns true when the level changed. */
+  push(ms: number): boolean {
+    this.recent.push(ms);
+    if (this.recent.length < this.window) return false;
+    const sorted = [...this.recent].sort((a, b) => a - b);
+    const median = sorted[Math.floor(sorted.length / 2)];
+    this.recent = [];
+    if (median > this.budgetMs && this.level < LEVELS.length - 1) {
+      this.level += 1;
+      return true;
+    }
+    return false;
+  }
+}
diff --git a/src/app/features/scene/camera.test.ts b/src/app/features/scene/camera.test.ts
new file mode 100644
index 00000000..f7199a65
--- /dev/null
+++ b/src/app/features/scene/camera.test.ts
@@ -0,0 +1,42 @@
+import { describe, expect, it } from 'vitest';
+import { cameraPose, pullProgress, PULL_BACK_SECONDS } from './camera';
+import { WORLD } from './world';
+
+const dist = (p: readonly [number, number, number]) => Math.hypot(p[0], p[2]);
+
+describe('login pull-back', () => {
+  it('is at rest before login and complete after PULL_BACK_SECONDS', () => {
+    expect(pullProgress(undefined)).toBe(0);
+    expect(pullProgress(0)).toBe(0);
+    expect(pullProgress(PULL_BACK_SECONDS)).toBe(1);
+    expect(pullProgress(PULL_BACK_SECONDS * 10)).toBe(1);
+  });
+
+  it('only moves away from the tree and never reverses', () => {
+    let last = -1;
+    for (let s = 0; s <= PULL_BACK_SECONDS; s += 0.1) {
+      const d = dist(cameraPose(pullProgress(s), 0).position);
+      expect(d).toBeGreaterThanOrEqual(last);
+      last = d;
+    }
+  });
+
+  it('ends over the flat ground so it cannot be inside the terrain, whatever the orbit angle', () => {
+    for (let t = 0; t < 2000; t += 37) {
+      expect(dist(cameraPose(1, t).position)).toBeLessThan(WORLD.plateauInner);
+    }
+  });
+
+  it('looks at the tree: forward points at the axis and the basis is orthonormal', () => {
+    const pose = cameraPose(0.5, 12);
+    const [rx, ry, rz, ux, uy, uz, fx, fy, fz] = pose.basis;
+    const dot = (a: number[], b: number[]) => a[0] * b[0] + a[1] * b[1] + a[2] * b[2];
+    expect(dot([rx, ry, rz], [ux, uy, uz])).toBeCloseTo(0, 6);
+    expect(dot([rx, ry, rz], [fx, fy, fz])).toBeCloseTo(0, 6);
+    expect(dot([ux, uy, uz], [fx, fy, fz])).toBeCloseTo(0, 6);
+    expect(Math.hypot(fx, fy, fz)).toBeCloseTo(1, 6);
+    // Forward points back across the tree axis, not away from it.
+    const [px, , pz] = pose.position;
+    expect(fx * px + fz * pz).toBeLessThan(0);
+  });
+});
diff --git a/src/app/features/scene/camera.ts b/src/app/features/scene/camera.ts
new file mode 100644
index 00000000..2b08f0d1
--- /dev/null
+++ b/src/app/features/scene/camera.ts
@@ -0,0 +1,56 @@
+// The scene's camera: close under the tree at rest, pulled back on login, drifting after.
+// Pure functions of time so the pull-back is testable without a GL context.
+
+export type Vec3 = readonly [number, number, number];
+
+export interface CameraPose {
+  position: Vec3;
+  /** Column-major right, up, forward: the layout of the shader's mat3 uniform. */
+  basis: readonly [number, number, number, number, number, number, number, number, number];
+  /** Vertical field of view in radians. */
+  fov: number;
+}
+
+export const PULL_BACK_SECONDS = 6;
+
+// Keep the end distance inside WORLD.plateauInner so the camera is always over flat ground.
+const NEAR = { radius: 20, camY: 5, targetY: 34, fov: 1.15 };
+const FAR = { radius: 110, camY: 50, targetY: 36, fov: 0.82 };
+const DRIFT_RAD_PER_SEC = 0.025;
+
+const smoother = (x: number) => x * x * x * (x * (x * 6 - 15) + 10);
+const lerp = (a: number, b: number, t: number) => a + (b - a) * t;
+
+/** 0 before login, easing to 1 over PULL_BACK_SECONDS. `elapsed` is seconds since login. */
+export function pullProgress(elapsed: number | undefined): number {
+  if (elapsed === undefined || elapsed <= 0) return 0;
+  return smoother(Math.min(1, elapsed / PULL_BACK_SECONDS));
+}
+
+function norm(v: Vec3): Vec3 {
+  const l = Math.hypot(v[0], v[1], v[2]) || 1;
+  return [v[0] / l, v[1] / l, v[2] / l];
+}
+function cross(a: Vec3, b: Vec3): Vec3 {
+  return [a[1] * b[2] - a[2] * b[1], a[2] * b[0] - a[0] * b[2], a[0] * b[1] - a[1] * b[0]];
+}
+
+/**
+ * @param pull 0..1 from pullProgress
+ * @param time seconds since the scene started, for the slow orbit
+ */
+export function cameraPose(pull: number, time: number): CameraPose {
+  const radius = lerp(NEAR.radius, FAR.radius, pull);
+  const camY = lerp(NEAR.camY, FAR.camY, pull);
+  const targetY = lerp(NEAR.targetY, FAR.targetY, pull);
+  const azimuth = 0.9 + time * DRIFT_RAD_PER_SEC;
+  const position: Vec3 = [radius * Math.cos(azimuth), camY, radius * Math.sin(azimuth)];
+  const forward = norm([-position[0], targetY - position[1], -position[2]]);
+  const right = norm(cross([0, 1, 0], forward));
+  const up = cross(forward, right);
+  return {
+    position,
+    basis: [...right, ...up, ...forward] as unknown as CameraPose['basis'],
+    fov: lerp(NEAR.fov, FAR.fov, pull),
+  };
+}
diff --git a/src/app/features/scene/creatures.test.ts b/src/app/features/scene/creatures.test.ts
new file mode 100644
index 00000000..8e59175b
--- /dev/null
+++ b/src/app/features/scene/creatures.test.ts
@@ -0,0 +1,23 @@
+import { describe, expect, it } from 'vitest';
+import { BUCKET_SCALE, creatureScale, NEUTRAL_SCALE, SizeBucket } from './creatures';
+
+const buckets: SizeBucket[] = ['small', 'medium', 'large'];
+
+describe('creature scale', () => {
+  it('follows sizeBucket for a creature with an estate', () => {
+    buckets.forEach((sizeBucket) =>
+      expect(creatureScale({ sizeBucket, neutral: false })).toBe(BUCKET_SCALE[sizeBucket])
+    );
+  });
+
+  it('is one fixed size for every neutral creature, whatever its sizeBucket', () => {
+    buckets.forEach((sizeBucket) =>
+      expect(creatureScale({ sizeBucket, neutral: true })).toBe(NEUTRAL_SCALE)
+    );
+  });
+
+  it('is only visible as an override where the bucket would have said something else', () => {
+    expect(BUCKET_SCALE.small).not.toBe(NEUTRAL_SCALE);
+    expect(BUCKET_SCALE.large).not.toBe(NEUTRAL_SCALE);
+  });
+});
diff --git a/src/app/features/scene/creatures.ts b/src/app/features/scene/creatures.ts
new file mode 100644
index 00000000..5026d2e2
--- /dev/null
+++ b/src/app/features/scene/creatures.ts
@@ -0,0 +1,37 @@
+// Creature sizing for the orbiting personas. Only the rule lives here; drawing them is a later cut.
+//
+// Props contract: heimdall-body:follow_up:ratatoskr-creature-projection-v2 (against GameCult/Bifrost; supersedes heimdall-body/ratatoskr-creature-projection).
+// Nothing here is binding until Bifrost's steward cuts that contract.
+//
+// Shape: {id, estate, sizeBucket, neutral, activity}.
+// Invariants:
+//   - Exact vote weights stay in Bifrost; Ratatoskr only ever sees a bucket.
+//   - neutral=true overrides any bucket, and no size is drawn.
+//   - activity comes from the Heimdall session.
+// Consent: humans get a coarse bucket by default plus a typed opt-out; Minds and Personas may show finer buckets.
+// Humans have three coarse buckets. The scales below (0.7 / 1 / 1.4) are placeholders, not fixed.
+
+export type SizeBucket = 'small' | 'medium' | 'large';
+
+export interface Creature {
+  id: string;
+  estate: string;
+  sizeBucket: SizeBucket;
+  /** A neutral creature (no estate allegiance) is drawn at one fixed size whatever its bucket. */
+  neutral: boolean;
+  activity: 'idle' | 'active';
+}
+
+export const BUCKET_SCALE: Readonly<Record<SizeBucket, number>> = {
+  small: 0.7,
+  medium: 1,
+  large: 1.4,
+};
+
+/** Neutral creatures are medium sized, not necessarily their bucket's size. */
+export const NEUTRAL_SCALE = BUCKET_SCALE.medium;
+
+/** Neutral overrides sizeBucket. */
+export function creatureScale(c: Pick<Creature, 'sizeBucket' | 'neutral'>): number {
+  return c.neutral ? NEUTRAL_SCALE : BUCKET_SCALE[c.sizeBucket];
+}
diff --git a/src/app/features/scene/index.ts b/src/app/features/scene/index.ts
new file mode 100644
index 00000000..f4109796
--- /dev/null
+++ b/src/app/features/scene/index.ts
@@ -0,0 +1,23 @@
+import type { Scene, SceneOptions } from './Scene';
+import { BackgroundSceneConfig, Capabilities, resolveTier } from './tier';
+
+export type { Scene, SceneStats } from './Scene';
+export type { SceneTier } from './tier';
+
+export type StartOptions = Omit<SceneOptions, 'tier' | 'patchSource'>;
+
+/**
+ * The client's one door to the scene. When the tier resolves to 'off' nothing is imported and no
+ * canvas context is requested; otherwise the scene code loads on demand, so it stays out of the
+ * main chunk. Null means "keep the CSS ground".
+ */
+export async function startBackgroundScene(
+  config: BackgroundSceneConfig | undefined,
+  capabilities: Capabilities,
+  options: StartOptions
+): Promise<Scene | null> {
+  const tier = resolveTier(config, capabilities);
+  if (tier === 'off') return null;
+  const { createScene } = await import('./Scene');
+  return createScene({ ...options, tier });
+}
diff --git a/src/app/features/scene/palette.ts b/src/app/features/scene/palette.ts
new file mode 100644
index 00000000..54a84ad3
--- /dev/null
+++ b/src/app/features/scene/palette.ts
@@ -0,0 +1,27 @@
+// The scene's colours, as linear RGB uniforms. The sky tints are GameCult's three nebula inputs
+// (orange 255 138 42, violet 109 96 255, sky 89 183 255) over the brand base #07111a, so the
+// background reads as the same dusk as gamecult.org. The shaders hold no colour literal.
+
+export type Rgb = readonly [number, number, number];
+
+const srgb = (r: number, g: number, b: number): Rgb => [
+  (r / 255) ** 2.2,
+  (g / 255) ** 2.2,
+  (b / 255) ** 2.2,
+];
+
+export const PALETTE = {
+  sunColor: srgb(255, 150, 70).map((c) => c * 3.2) as unknown as Rgb,
+  skyTop: srgb(30, 36, 92),
+  skyHorizon: srgb(255, 138, 42).map((c) => c * 0.55) as unknown as Rgb,
+  fogColor: srgb(109, 96, 255).map((c) => c * 0.12) as unknown as Rgb,
+  grass: srgb(72, 112, 86),
+  rock: srgb(112, 104, 122),
+  bark: srgb(62, 46, 40),
+  leaf: srgb(89, 183, 255).map((c) => c * 0.7) as unknown as Rgb,
+  sunDir: (() => {
+    const v = [0.55, 0.16, -0.82];
+    const l = Math.hypot(v[0], v[1], v[2]);
+    return [v[0] / l, v[1] / l, v[2] / l] as Rgb;
+  })(),
+} as const;
diff --git a/src/app/features/scene/shaderSource.ts b/src/app/features/scene/shaderSource.ts
new file mode 100644
index 00000000..aa44d9b2
--- /dev/null
+++ b/src/app/features/scene/shaderSource.ts
@@ -0,0 +1,29 @@
+import cultmath from './shaders/CultMath.glsl?raw';
+import phacelle from './shaders/CultMath.Phacelle.glsl?raw';
+import common from './shaders/common.glsl?raw';
+import fullscreenVert from './shaders/fullscreen.vert.glsl?raw';
+import terrainFrag from './shaders/terrain.frag.glsl?raw';
+import atmosphereFrag from './shaders/atmosphere.frag.glsl?raw';
+import resolveFrag from './shaders/resolve.frag.glsl?raw';
+import compositeFrag from './shaders/composite.frag.glsl?raw';
+import { WORLD, worldDefines } from './world';
+
+export type Stage = 'terrain' | 'atmosphere' | 'resolve' | 'composite';
+
+const HEADER =
+  '#version 300 es\nprecision highp float;\nprecision highp int;\nprecision highp sampler2D;\n';
+
+export const vertexSource = HEADER + fullscreenVert;
+
+const stages: Record<Stage, string> = {
+  terrain: terrainFrag,
+  atmosphere: atmosphereFrag,
+  resolve: resolveFrag,
+  composite: compositeFrag,
+};
+
+/** Fragment source for a stage: header, world defines, CultMath, the shared block, the stage. */
+export function fragmentSource(stage: Stage): string {
+  const erosion = stage === 'terrain' && WORLD.phacelleGradBound > 0 ? phacelle : '';
+  return `${HEADER}${worldDefines()}\n${cultmath}\n${erosion}\n${common}\n${stages[stage]}`;
+}
diff --git a/src/app/features/scene/shaders/CultMath.Phacelle.glsl b/src/app/features/scene/shaders/CultMath.Phacelle.glsl
new file mode 100644
index 00000000..11405fa2
--- /dev/null
+++ b/src/app/features/scene/shaders/CultMath.Phacelle.glsl
@@ -0,0 +1,99 @@
+// Generated from shaders/CultMath.Phacelle.hlsl by GlslLowering (packages/cultmath/tests/CultMath.Tests/GlslLowering.cs). Do not edit; regenerate with CULTMATH_WRITE_GLSL=1 dotnet test --filter GlslMirrorTests. MPL-2.0; requires CultMath.glsl before it.
+#ifndef CULTMATH_PHACELLE_GLSL
+#define CULTMATH_PHACELLE_GLSL
+// This Source Code Form is subject to the terms of the Mozilla Public License, v. 2.0. If a copy of
+// the MPL was not distributed with this file, You can obtain one at https://mozilla.org/MPL/2.0/.
+//
+// Phacelle noise. Copyright (c) 2025 Rune Skovbo Johansen (Shadertoy t3dyWl; MPL-2.0), read from the
+// C# port PhacelleNoise in lpmitchell/AdvancedTerrainErosion, converted to burstable C# by Luke
+// Mitchell (2026). Modified by GameCult: 3D cells, a caller-supplied stripe wave vector, an exact
+// gradient and exact pruning. This file is an MPL-2.0 file; CultMath.hlsl, which includes it after
+// its own definitions of CULTMATH_TAU and cultmath_pcg3d, is MIT (see THIRD-PARTY-NOTICES.md).
+
+// Invariant 8's struct return shape for cultmath_phacelle, compared field by field and bit for bit
+// against CultMath.CultPhasor, same field order.
+struct CultPhasor
+{
+    vec4 cos;
+    vec4 sin;
+};
+
+// Johansen's Phacelle noise on 3D cells. See math.phacelle's comment (math.Phacelle.cs) for the stripe wave
+// vector, the weight, the normalization, the exact gradient and why the prune is exact. Mirrors
+// the C# body operation for operation; the C#-only internal overload that switches the prune off
+// has no shader twin.
+CultPhasor cultmath_phacelle(vec3 p, vec3 side, float offset, float normalization)
+{
+    vec3 cell = floor(p);
+    vec3 local = p - cell;
+    float phaseOffset = offset * CULTMATH_TAU;
+    float sumW = 0.0;
+    float sumCos = 0.0;
+    float sumSin = 0.0;
+    vec3 gradW = vec3(0.0, 0.0, 0.0);
+    vec3 gradCos = vec3(0.0, 0.0, 0.0);
+    vec3 gradSin = vec3(0.0, 0.0, 0.0);
+
+    for (int dz = -1; dz <= 2; dz++)
+    {
+        float gz = max(0.0, abs(local.z - float(dz)) - 0.5);
+        for (int dy = -1; dy <= 2; dy++)
+        {
+            float gy = max(0.0, abs(local.y - float(dy)) - 0.5);
+            for (int dx = -1; dx <= 2; dx++)
+            {
+                float gx = max(0.0, abs(local.x - float(dx)) - 0.5);
+                if (gx * gx + gy * gy + gz * gz >= 2.25)
+                    continue;
+
+                vec3 gridStep = vec3(dx, dy, dz);
+                ivec3 hash = cultmath_pcg3d(ivec3(cell + gridStep));
+                vec3 jitter = vec3(
+                    float(uint(hash.x) >> 8) * (1.0 / 16777216.0) - 0.5,
+                    float(uint(hash.y) >> 8) * (1.0 / 16777216.0) - 0.5,
+                    float(uint(hash.z) >> 8) * (1.0 / 16777216.0) - 0.5);
+                vec3 v = local - gridStep - jitter;
+                float falloff = exp(-2.0 * dot(v, v));
+                float w = max(0.0, falloff - 0.01111);
+                vec3 dw = w > 0.0 ? (-4.0 * falloff) * v : vec3(0.0, 0.0, 0.0);
+                float phase = dot(v, side) + phaseOffset;
+                float c = cos(phase);
+                float s = sin(phase);
+
+                sumW += w;
+                sumCos += w * c;
+                sumSin += w * s;
+                gradW += dw;
+                gradCos += dw * c - (w * s) * side;
+                gradSin += dw * s + (w * c) * side;
+            }
+        }
+    }
+
+    float invW = 1.0 / sumW;
+    float rawCos = sumCos * invW;
+    float rawSin = sumSin * invW;
+    vec3 gradRawCos = (gradCos - rawCos * gradW) * invW;
+    vec3 gradRawSin = (gradSin - rawSin * gradW) * invW;
+    float rawLength = sqrt(rawCos * rawCos + rawSin * rawSin);
+    float floorLength = 1.0 - normalization;
+
+    CultPhasor result;
+    if (rawLength > floorLength)
+    {
+        float outCos = rawCos / rawLength;
+        float outSin = rawSin / rawLength;
+        vec3 gradLength = (rawCos * gradRawCos + rawSin * gradRawSin) / rawLength;
+        result.cos = vec4((gradRawCos - outCos * gradLength) / rawLength, outCos);
+        result.sin = vec4((gradRawSin - outSin * gradLength) / rawLength, outSin);
+    }
+    else
+    {
+        result.cos = vec4(gradRawCos / floorLength, rawCos / floorLength);
+        result.sin = vec4(gradRawSin / floorLength, rawSin / floorLength);
+    }
+
+    return result;
+}
+
+#endif
diff --git a/src/app/features/scene/shaders/CultMath.glsl b/src/app/features/scene/shaders/CultMath.glsl
new file mode 100644
index 00000000..8a380a96
--- /dev/null
+++ b/src/app/features/scene/shaders/CultMath.glsl
@@ -0,0 +1,1035 @@
+// Generated from shaders/CultMath.hlsl and CultMath.Interval.hlsl and CultMath.Affine.hlsl by GlslLowering (packages/cultmath/tests/CultMath.Tests/GlslLowering.cs). Do not edit; regenerate with CULTMATH_WRITE_GLSL=1 dotnet test --filter GlslMirrorTests. MIT. cultmath_phacelle is in CultMath.Phacelle.glsl, to be concatenated after this file by consumers that call it. cultmath_cycles_divide, cultmath_cycles_power, cultmath_cycles_logarithm, cultmath_cycles_sqrt, cultmath_cycles_inverse_sqrt, cultmath_cycles_modulo, cultmath_cycles_floored_modulo, cultmath_cycles_wrap, cultmath_cycles_snap, cultmath_cycles_pingpong, cultmath_cycles_smooth_min, cultmath_cycles_smooth_max, cultmath_cycles_compare, cultmath_cycles_round, cultmath_cycles_trunc, cultmath_cycles_fract, cultmath_cycles_sign, cultmath_cycles_arcsine, cultmath_cycles_arccosine, cultmath_cycles_arctan2, cultmath_cycles_map_range_linear, cultmath_cycles_map_range_stepped, cultmath_cycles_map_range_smoothstep, cultmath_cycles_map_range_smootherstep, cultmath_cycles_clamp_minmax, cultmath_cycles_clamp_range, cultmath_cycles_mix_float, cultmath_cycles_mix_vector, cultmath_cycles_mix_blend, cultmath_cycles_mix_add, cultmath_cycles_mix_mul, cultmath_cycles_mix_screen, cultmath_cycles_mix_overlay, cultmath_cycles_mix_sub, cultmath_cycles_mix_div, cultmath_cycles_mix_diff, cultmath_cycles_mix_exclusion, cultmath_cycles_mix_dark, cultmath_cycles_mix_light, cultmath_cycles_mix_dodge, cultmath_cycles_mix_burn, cultmath_cycles_rgb_to_hsv, cultmath_cycles_hsv_to_rgb, cultmath_cycles_mix_hue, cultmath_cycles_mix_sat, cultmath_cycles_mix_val, cultmath_cycles_mix_color, cultmath_cycles_mix_soft, cultmath_cycles_mix_linear, cultmath_cycles_rgb_to_bw, cultmath_cycles_ramp_index is in CultMath.Cycles.glsl, to be concatenated after this file by consumers that call it.
+#ifndef CULTMATH_GLSL
+#define CULTMATH_GLSL
+// CultMath HLSL mirror.
+//
+// HLSL already owns the vec2/vec3/vec4 type names and most primitive
+// intrinsics. This include names the CultMath semantic surface explicitly so
+// shader code, C# wrappers, and Rust/native parity fixtures can share the same
+// small vocabulary without local helper drift.
+
+const float CULTMATH_PI = 3.14159265358979323846;
+const float CULTMATH_TAU = 6.28318530717958647692;
+const float CULTMATH_HALF_PI = 1.57079632679489661923;
+
+float cultmath_radians(float degrees_value) { return degrees_value * (CULTMATH_PI / 180.0); }
+float cultmath_degrees(float radians_value) { return radians_value * (180.0 / CULTMATH_PI); }
+
+float cultmath_frac(float value) { return value - floor(value); }
+vec2 cultmath_frac(vec2 value) { return value - floor(value); }
+vec3 cultmath_frac(vec3 value) { return value - floor(value); }
+vec4 cultmath_frac(vec4 value) { return value - floor(value); }
+
+float cultmath_clamp(float value, float minimum, float maximum) { return min(max(value, minimum), maximum); }
+vec2 cultmath_clamp(vec2 value, vec2 minimum, vec2 maximum) { return min(max(value, minimum), maximum); }
+vec3 cultmath_clamp(vec3 value, vec3 minimum, vec3 maximum) { return min(max(value, minimum), maximum); }
+vec4 cultmath_clamp(vec4 value, vec4 minimum, vec4 maximum) { return min(max(value, minimum), maximum); }
+
+// DXIL Saturate is FMin(1, FMax(0, x)); dxc lowers min/max to the same FMin/FMax, NaN rules included.
+float cultmath_saturate(float value) { return min(1.0, max(0.0, value)); }
+vec2 cultmath_saturate(vec2 value) { return min(vec2(1.0, 1.0), max(vec2(0.0, 0.0), value)); }
+vec3 cultmath_saturate(vec3 value) { return min(vec3(1.0, 1.0, 1.0), max(vec3(0.0, 0.0, 0.0), value)); }
+vec4 cultmath_saturate(vec4 value) { return min(vec4(1.0, 1.0, 1.0, 1.0), max(vec4(0.0, 0.0, 0.0, 0.0), value)); }
+
+float cultmath_lerp(float start, float end, float amount) { return start + (end - start) * amount; }
+vec2 cultmath_lerp(vec2 start, vec2 end, vec2 amount) { return start + (end - start) * amount; }
+vec3 cultmath_lerp(vec3 start, vec3 end, vec3 amount) { return start + (end - start) * amount; }
+vec4 cultmath_lerp(vec4 start, vec4 end, vec4 amount) { return start + (end - start) * amount; }
+
+float cultmath_step(float edge, float value) { return value < edge ? 0.0 : 1.0; }
+vec2 cultmath_step(vec2 edge, vec2 value) { return vec2(cultmath_step(edge.x, value.x), cultmath_step(edge.y, value.y)); }
+vec3 cultmath_step(vec3 edge, vec3 value) { return vec3(cultmath_step(edge.x, value.x), cultmath_step(edge.y, value.y), cultmath_step(edge.z, value.z)); }
+vec4 cultmath_step(vec4 edge, vec4 value) { return vec4(cultmath_step(edge.x, value.x), cultmath_step(edge.y, value.y), cultmath_step(edge.z, value.z), cultmath_step(edge.w, value.w)); }
+
+float cultmath_smoothstep(float minimum, float maximum, float value)
+{
+    float t = cultmath_saturate((value - minimum) / (maximum - minimum));
+    return t * t * (3.0 - 2.0 * t);
+}
+
+vec2 cultmath_smoothstep(vec2 minimum, vec2 maximum, vec2 value)
+{
+    vec2 t = cultmath_saturate((value - minimum) / (maximum - minimum));
+    return t * t * (vec2(3.0, 3.0) - vec2(2.0, 2.0) * t);
+}
+
+vec3 cultmath_smoothstep(vec3 minimum, vec3 maximum, vec3 value)
+{
+    vec3 t = cultmath_saturate((value - minimum) / (maximum - minimum));
+    return t * t * (vec3(3.0, 3.0, 3.0) - vec3(2.0, 2.0, 2.0) * t);
+}
+
+vec4 cultmath_smoothstep(vec4 minimum, vec4 maximum, vec4 value)
+{
+    vec4 t = cultmath_saturate((value - minimum) / (maximum - minimum));
+    return t * t * (vec4(3.0, 3.0, 3.0, 3.0) - vec4(2.0, 2.0, 2.0, 2.0) * t);
+}
+
+float cultmath_smoothstep01(float value) { return cultmath_smoothstep(0.0, 1.0, value); }
+
+float cultmath_smootherstep(float value)
+{
+    value = cultmath_saturate(value);
+    return value * value * value * (value * (value * 6.0 - 15.0) + 10.0);
+}
+
+vec2 cultmath_snoise_mod289(vec2 value) { return value - floor(value * (1.0 / 289.0)) * 289.0; }
+vec3 cultmath_snoise_mod289(vec3 value) { return value - floor(value * (1.0 / 289.0)) * 289.0; }
+vec4 cultmath_snoise_mod289(vec4 value) { return value - floor(value * (1.0 / 289.0)) * 289.0; }
+vec3 cultmath_snoise_permute(vec3 value) { return cultmath_snoise_mod289(((value * 34.0) + 10.0) * value); }
+vec4 cultmath_snoise_permute(vec4 value) { return cultmath_snoise_mod289(((value * 34.0) + 10.0) * value); }
+
+// Ashima Arts / Ian McEwan 3D simplex noise (MIT), same float32 evaluation
+// order as the C# math.snoise(vec3) mirror. Ties resolve in simplexnoise1234's
+// total order; see math.snoise.
+float cultmath_snoise(vec3 value)
+{
+    vec2 c = vec2(1.0 / 6.0, 1.0 / 3.0);
+    vec3 i = floor(value + dot(value, c.yyy));
+    vec3 x0 = value - i + dot(i, c.xxx);
+    vec3 g = vec3(step(x0.y, x0.x), step(x0.z, x0.y), 1.0 - step(x0.z, x0.x));
+    vec3 l = 1.0 - g;
+    vec3 i1 = min(g.xyz, l.zxy);
+    vec3 i2 = max(g.xyz, l.zxy);
+    vec3 x1 = x0 - i1 + c.xxx;
+    vec3 x2 = x0 - i2 + c.yyy;
+    vec3 x3 = x0 - 0.5;
+
+    i = cultmath_snoise_mod289(i);
+    vec4 p = cultmath_snoise_permute(cultmath_snoise_permute(cultmath_snoise_permute(
+        i.z + vec4(0.0, i1.z, i2.z, 1.0)) + i.y + vec4(0.0, i1.y, i2.y, 1.0)) + i.x + vec4(0.0, i1.x, i2.x, 1.0));
+
+    const float n_ = 0.142857142857;
+    vec3 ns = vec3(2.0 * n_, 0.5 * n_ - 1.0, n_);
+    vec4 j = p - 49.0 * floor(p * ns.z * ns.z);
+    vec4 x_ = floor(j * ns.z);
+    vec4 y_ = floor(j - 7.0 * x_);
+    vec4 x = x_ * ns.x + ns.yyyy;
+    vec4 y = y_ * ns.x + ns.yyyy;
+    vec4 h = 1.0 - abs(x) - abs(y);
+
+    vec4 b0 = vec4(x.xy, y.xy);
+    vec4 b1 = vec4(x.zw, y.zw);
+    vec4 s0 = floor(b0) * 2.0 + 1.0;
+    vec4 s1 = floor(b1) * 2.0 + 1.0;
+    vec4 sh = -step(h, vec4(0.0, 0.0, 0.0, 0.0));
+    vec4 a0 = b0.xzyw + s0.xzyw * sh.xxyy;
+    vec4 a1 = b1.xzyw + s1.xzyw * sh.zzww;
+
+    vec3 p0 = vec3(a0.xy, h.x);
+    vec3 p1 = vec3(a0.zw, h.y);
+    vec3 p2 = vec3(a1.xy, h.z);
+    vec3 p3 = vec3(a1.zw, h.w);
+    vec4 norm = 1.79284291400159 - 0.85373472095314 * vec4(dot(p0, p0), dot(p1, p1), dot(p2, p2), dot(p3, p3));
+    p0 *= norm.x;
+    p1 *= norm.y;
+    p2 *= norm.z;
+    p3 *= norm.w;
+
+    vec4 m = max(0.5 - vec4(dot(x0, x0), dot(x1, x1), dot(x2, x2), dot(x3, x3)), 0.0);
+    m = m * m;
+    return 105.0 * dot(m * m, vec4(dot(p0, x0), dot(p1, x1), dot(p2, x2), dot(p3, x3)));
+}
+
+float cultmath_snoise(vec2 value)
+{
+    vec4 c = vec4(
+        0.211324865405187,
+        0.366025403784439,
+        -0.577350269189626,
+        0.024390243902439);
+    vec2 i = floor(value + dot(value, c.yy));
+    vec2 x0 = value - i + dot(i, c.xx);
+    vec2 i1 = x0.x > x0.y ? vec2(1.0, 0.0) : vec2(0.0, 1.0);
+    vec4 x12 = vec4(x0.x + c.x - i1.x, x0.y + c.x - i1.y, x0.x + c.z, x0.y + c.z);
+
+    i = cultmath_snoise_mod289(i);
+    vec3 p = cultmath_snoise_permute(
+        cultmath_snoise_permute(i.y + vec3(0.0, i1.y, 1.0)) + i.x + vec3(0.0, i1.x, 1.0));
+    vec3 m = max(0.5 - vec3(dot(x0, x0), dot(x12.xy, x12.xy), dot(x12.zw, x12.zw)), 0.0);
+    m *= m;
+    m *= m;
+
+    vec3 x = 2.0 * fract(p * c.w) - 1.0;
+    vec3 h = abs(x) - 0.5;
+    vec3 ox = floor(x + 0.5);
+    vec3 a0 = x - ox;
+    m *= 1.79284291400159 - 0.85373472095314 * (a0 * a0 + h * h);
+
+    vec3 g = vec3(
+        a0.x * x0.x + h.x * x0.y,
+        a0.y * x12.x + h.y * x12.y,
+        a0.z * x12.z + h.z * x12.w);
+    return 130.0 * dot(m, g);
+}
+
+// Analytic gradient of cultmath_snoise(vec3) (invariant 8), following webgl-noise
+// src/noise3Dgrad.glsl (Ashima Arts / McEwan, MIT); same kernel, permutation and scale as
+// cultmath_snoise(vec3) (stegu/webgl-noise 22434e04d7). See the C# math.snoise_grad comment for the
+// derivation. Every local up through p0..p3 and x0..x3 is cultmath_snoise(vec3)'s own derivation
+// verbatim.
+vec4 cultmath_snoise_grad(vec3 value)
+{
+    vec2 c = vec2(1.0 / 6.0, 1.0 / 3.0);
+    vec3 i = floor(value + dot(value, c.yyy));
+    vec3 x0 = value - i + dot(i, c.xxx);
+    vec3 g = vec3(step(x0.y, x0.x), step(x0.z, x0.y), 1.0 - step(x0.z, x0.x));
+    vec3 l = 1.0 - g;
+    vec3 i1 = min(g.xyz, l.zxy);
+    vec3 i2 = max(g.xyz, l.zxy);
+    vec3 x1 = x0 - i1 + c.xxx;
+    vec3 x2 = x0 - i2 + c.yyy;
+    vec3 x3 = x0 - 0.5;
+
+    i = cultmath_snoise_mod289(i);
+    vec4 p = cultmath_snoise_permute(cultmath_snoise_permute(cultmath_snoise_permute(
+        i.z + vec4(0.0, i1.z, i2.z, 1.0)) + i.y + vec4(0.0, i1.y, i2.y, 1.0)) + i.x + vec4(0.0, i1.x, i2.x, 1.0));
+
+    const float n_ = 0.142857142857;
+    vec3 ns = vec3(2.0 * n_, 0.5 * n_ - 1.0, n_);
+    vec4 j = p - 49.0 * floor(p * ns.z * ns.z);
+    vec4 x_ = floor(j * ns.z);
+    vec4 y_ = floor(j - 7.0 * x_);
+    vec4 x = x_ * ns.x + ns.yyyy;
+    vec4 y = y_ * ns.x + ns.yyyy;
+    vec4 h = 1.0 - abs(x) - abs(y);
+
+    vec4 b0 = vec4(x.xy, y.xy);
+    vec4 b1 = vec4(x.zw, y.zw);
+    vec4 s0 = floor(b0) * 2.0 + 1.0;
+    vec4 s1 = floor(b1) * 2.0 + 1.0;
+    vec4 sh = -step(h, vec4(0.0, 0.0, 0.0, 0.0));
+    vec4 a0 = b0.xzyw + s0.xzyw * sh.xxyy;
+    vec4 a1 = b1.xzyw + s1.xzyw * sh.zzww;
+
+    vec3 p0 = vec3(a0.xy, h.x);
+    vec3 p1 = vec3(a0.zw, h.y);
+    vec3 p2 = vec3(a1.xy, h.z);
+    vec3 p3 = vec3(a1.zw, h.w);
+    vec4 norm = 1.79284291400159 - 0.85373472095314 * vec4(dot(p0, p0), dot(p1, p1), dot(p2, p2), dot(p3, p3));
+    p0 *= norm.x;
+    p1 *= norm.y;
+    p2 *= norm.z;
+    p3 *= norm.w;
+
+    vec4 m0 = max(0.5 - vec4(dot(x0, x0), dot(x1, x1), dot(x2, x2), dot(x3, x3)), 0.0);
+    vec4 m2 = m0 * m0;
+    vec4 m3 = m2 * m0;
+    vec4 m4 = m2 * m2;
+
+    vec4 px = vec4(dot(p0, x0), dot(p1, x1), dot(p2, x2), dot(p3, x3));
+    float value2 = 105.0 * dot(m4, px);
+    vec3 grad = -8.0 * m3.x * x0 * px.x + m4.x * p0
+        + -8.0 * m3.y * x1 * px.y + m4.y * p1
+        + -8.0 * m3.z * x2 * px.z + m4.z * p2
+        + -8.0 * m3.w * x3 * px.w + m4.w * p3;
+    grad *= 105.0;
+
+    return vec4(grad, value2);
+}
+
+// Analytic Hessian of cultmath_snoise(vec3); see the C# math.snoise_hessian comment for the
+// derivation. Every local up through p0..p3 and x0..x3 is cultmath_snoise_grad's own. The matrix is
+// built from nine scalars, which read as rows in HLSL and as columns in GLSL; it is symmetric, so the
+// two agree.
+mat3 cultmath_snoise_hessian(vec3 value)
+{
+    vec2 c = vec2(1.0 / 6.0, 1.0 / 3.0);
+    vec3 i = floor(value + dot(value, c.yyy));
+    vec3 x0 = value - i + dot(i, c.xxx);
+    vec3 g = vec3(step(x0.y, x0.x), step(x0.z, x0.y), 1.0 - step(x0.z, x0.x));
+    vec3 l = 1.0 - g;
+    vec3 i1 = min(g.xyz, l.zxy);
+    vec3 i2 = max(g.xyz, l.zxy);
+    vec3 x1 = x0 - i1 + c.xxx;
+    vec3 x2 = x0 - i2 + c.yyy;
+    vec3 x3 = x0 - 0.5;
+
+    i = cultmath_snoise_mod289(i);
+    vec4 p = cultmath_snoise_permute(cultmath_snoise_permute(cultmath_snoise_permute(
+        i.z + vec4(0.0, i1.z, i2.z, 1.0)) + i.y + vec4(0.0, i1.y, i2.y, 1.0)) + i.x + vec4(0.0, i1.x, i2.x, 1.0));
+
+    const float n_ = 0.142857142857;
+    vec3 ns = vec3(2.0 * n_, 0.5 * n_ - 1.0, n_);
+    vec4 j = p - 49.0 * floor(p * ns.z * ns.z);
+    vec4 x_ = floor(j * ns.z);
+    vec4 y_ = floor(j - 7.0 * x_);
+    vec4 x = x_ * ns.x + ns.yyyy;
+    vec4 y = y_ * ns.x + ns.yyyy;
+    vec4 h = 1.0 - abs(x) - abs(y);
+
+    vec4 b0 = vec4(x.xy, y.xy);
+    vec4 b1 = vec4(x.zw, y.zw);
+    vec4 s0 = floor(b0) * 2.0 + 1.0;
+    vec4 s1 = floor(b1) * 2.0 + 1.0;
+    vec4 sh = -step(h, vec4(0.0, 0.0, 0.0, 0.0));
+    vec4 a0 = b0.xzyw + s0.xzyw * sh.xxyy;
+    vec4 a1 = b1.xzyw + s1.xzyw * sh.zzww;
+
+    vec3 p0 = vec3(a0.xy, h.x);
+    vec3 p1 = vec3(a0.zw, h.y);
+    vec3 p2 = vec3(a1.xy, h.z);
+    vec3 p3 = vec3(a1.zw, h.w);
+    vec4 norm = 1.79284291400159 - 0.85373472095314 * vec4(dot(p0, p0), dot(p1, p1), dot(p2, p2), dot(p3, p3));
+    p0 *= norm.x;
+    p1 *= norm.y;
+    p2 *= norm.z;
+    p3 *= norm.w;
+
+    vec4 m0 = max(0.5 - vec4(dot(x0, x0), dot(x1, x1), dot(x2, x2), dot(x3, x3)), 0.0);
+    vec4 m2 = m0 * m0;
+    vec4 m3 = m2 * m0;
+    vec4 px = vec4(dot(p0, x0), dot(p1, x1), dot(p2, x2), dot(p3, x3));
+    vec4 wide = 48.0 * m2 * px;
+    vec4 narrow = 8.0 * m3;
+
+    vec4 cxx = vec4(x0.x, x1.x, x2.x, x3.x);
+    vec4 cxy = vec4(x0.y, x1.y, x2.y, x3.y);
+    vec4 cxz = vec4(x0.z, x1.z, x2.z, x3.z);
+    vec4 cpx = vec4(p0.x, p1.x, p2.x, p3.x);
+    vec4 cpy = vec4(p0.y, p1.y, p2.y, p3.y);
+    vec4 cpz = vec4(p0.z, p1.z, p2.z, p3.z);
+
+    float hxx = 105.0 * (dot(wide * cxx, cxx) - dot(narrow, 2.0 * cpx * cxx + px));
+    float hyy = 105.0 * (dot(wide * cxy, cxy) - dot(narrow, 2.0 * cpy * cxy + px));
+    float hzz = 105.0 * (dot(wide * cxz, cxz) - dot(narrow, 2.0 * cpz * cxz + px));
+    float hxy = 105.0 * (dot(wide * cxx, cxy) - dot(narrow, cpx * cxy + cpy * cxx));
+    float hxz = 105.0 * (dot(wide * cxx, cxz) - dot(narrow, cpx * cxz + cpz * cxx));
+    float hyz = 105.0 * (dot(wide * cxy, cxz) - dot(narrow, cpy * cxz + cpz * cxy));
+    return mat3(hxx, hxy, hxz, hxy, hyy, hyz, hxz, hyz, hzz);
+}
+
+// The integral over [t0, t1] of clamp(a + b t, lo, hi); see the C# math.integral_clamp_linear comment.
+float cultmath_integral_clamp_linear(float a, float b, float lo, float hi, float t0, float t1)
+{
+    lo = min(lo, hi);
+    if (b == 0.0)
+        return t1 != t0 ? clamp(a, lo, hi) * (t1 - t0) : 0.0;
+
+    float direction = t1 < t0 ? -1.0 : 1.0;
+    float start = min(t0, t1);
+    float stop = max(t0, t1);
+    float atLo = (lo - a) / b;
+    float atHi = (hi - a) / b;
+    float enter = clamp(min(atLo, atHi), start, stop);
+    float leave = clamp(max(atLo, atHi), start, stop);
+    float before = b > 0.0 ? lo : hi;
+    float after = b > 0.0 ? hi : lo;
+    float held = enter > start ? before * (enter - start) : 0.0;
+    float kept = stop > leave ? after * (stop - leave) : 0.0;
+    float total = held + (a + b * (0.5 * (enter + leave))) * (leave - enter) + kept;
+    return direction * total;
+}
+
+// Octave sum of cultmath_snoise_grad (invariant 8); see the C# math.fbm_grad comment for the
+// per-octave frequency/amplitude derivation and why octaves is clamped.
+vec4 cultmath_fbm_grad(vec3 p, int octaves, float lacunarity, float gain)
+{
+    octaves = clamp(octaves, 0, 16);
+    float amplitude = 1.0;
+    float frequency = 1.0;
+    float value = 0.0;
+    vec3 gradient = vec3(0.0, 0.0, 0.0);
+    for (int i = 0; i < octaves; i++)
+    {
+        vec4 n = cultmath_snoise_grad(p * frequency);
+        value += amplitude * n.w;
+        gradient += amplitude * frequency * n.xyz;
+        frequency *= lacunarity;
+        amplitude *= gain;
+    }
+
+    return vec4(gradient, value);
+}
+
+// Ridged multifractal noise (Musgrave, Texturing and Modeling, ch. 16); see the C# math.ridged_grad
+// comment for the fold/sign derivation, the deliberate crease at n = 0, and why octaves is clamped.
+vec4 cultmath_ridged_grad(vec3 p, int octaves, float lacunarity, float gain)
+{
+    octaves = clamp(octaves, 0, 16);
+    float amplitude = 1.0;
+    float frequency = 1.0;
+    float value = 0.0;
+    vec3 gradient = vec3(0.0, 0.0, 0.0);
+    for (int i = 0; i < octaves; i++)
+    {
+        vec4 n = cultmath_snoise_grad(p * frequency);
+        float s = sign(n.w);
+        value += amplitude * (1.0 - abs(n.w));
+        gradient += -amplitude * frequency * s * n.xyz;
+        frequency *= lacunarity;
+        amplitude *= gain;
+    }
+
+    return vec4(gradient, value);
+}
+
+float cultmath_lengthsq(vec2 value) { return dot(value, value); }
+float cultmath_lengthsq(vec3 value) { return dot(value, value); }
+float cultmath_lengthsq(vec4 value) { return dot(value, value); }
+
+float cultmath_distance(vec2 left, vec2 right) { return length(left - right); }
+float cultmath_distance(vec3 left, vec3 right) { return length(left - right); }
+float cultmath_distance(vec4 left, vec4 right) { return length(left - right); }
+
+vec2 cultmath_reflect(vec2 incident, vec2 normal_value)
+{
+    return incident - 2.0 * dot(normal_value, incident) * normal_value;
+}
+
+vec3 cultmath_reflect(vec3 incident, vec3 normal_value)
+{
+    return incident - 2.0 * dot(normal_value, incident) * normal_value;
+}
+
+vec2 cultmath_rotate(vec2 value, float radians_value)
+{
+    float s = sin(radians_value);
+    float c = cos(radians_value);
+    return vec2(value.x * c - value.y * s, value.x * s + value.y * c);
+}
+
+vec2 cultmath_rotate_degrees(vec2 value, float degrees_value)
+{
+    return cultmath_rotate(value, cultmath_radians(degrees_value));
+}
+
+float cultmath_csum(vec2 value) { return value.x + value.y; }
+float cultmath_csum(vec3 value) { return value.x + value.y + value.z; }
+float cultmath_csum(vec4 value) { return value.x + value.y + value.z + value.w; }
+
+float cultmath_decay(float source, float lambda_value, float dt) { return source * exp(-lambda_value * dt); }
+vec2 cultmath_decay(vec2 source, float lambda_value, float dt) { return source * exp(-lambda_value * dt); }
+vec3 cultmath_decay(vec3 source, float lambda_value, float dt) { return source * exp(-lambda_value * dt); }
+vec4 cultmath_decay(vec4 source, float lambda_value, float dt) { return source * exp(-lambda_value * dt); }
+
+float cultmath_damp(float start, float end, float lambda_value, float dt)
+{
+    return cultmath_lerp(start, end, 1.0 - exp(-lambda_value * dt));
+}
+
+vec2 cultmath_damp(vec2 start, vec2 end, float lambda_value, float dt)
+{
+    float amount = 1.0 - exp(-lambda_value * dt);
+    return cultmath_lerp(start, end, vec2(amount, amount));
+}
+
+vec3 cultmath_damp(vec3 start, vec3 end, float lambda_value, float dt)
+{
+    float amount = 1.0 - exp(-lambda_value * dt);
+    return cultmath_lerp(start, end, vec3(amount, amount, amount));
+}
+
+vec4 cultmath_damp(vec4 start, vec4 end, float lambda_value, float dt)
+{
+    float amount = 1.0 - exp(-lambda_value * dt);
+    return cultmath_lerp(start, end, vec4(amount, amount, amount, amount));
+}
+
+float cultmath_catmullrom(float p0, float p1, float p2, float p3, float t)
+{
+    float t2 = t * t;
+    float t3 = t2 * t;
+    return 0.5 * ((2.0 * p1) + (p2 - p0) * t + (2.0 * p0 - 5.0 * p1 + 4.0 * p2 - p3) * t2 + (-p0 + 3.0 * p1 - 3.0 * p2 + p3) * t3);
+}
+
+vec2 cultmath_catmullrom(vec2 p0, vec2 p1, vec2 p2, vec2 p3, float t)
+{
+    float t2 = t * t;
+    float t3 = t2 * t;
+    return 0.5 * ((2.0 * p1) + (p2 - p0) * t + (2.0 * p0 - 5.0 * p1 + 4.0 * p2 - p3) * t2 + (-p0 + 3.0 * p1 - 3.0 * p2 + p3) * t3);
+}
+
+vec3 cultmath_catmullrom(vec3 p0, vec3 p1, vec3 p2, vec3 p3, float t)
+{
+    float t2 = t * t;
+    float t3 = t2 * t;
+    return 0.5 * ((2.0 * p1) + (p2 - p0) * t + (2.0 * p0 - 5.0 * p1 + 4.0 * p2 - p3) * t2 + (-p0 + 3.0 * p1 - 3.0 * p2 + p3) * t3);
+}
+
+vec4 cultmath_catmullrom(vec4 p0, vec4 p1, vec4 p2, vec4 p3, float t)
+{
+    float t2 = t * t;
+    float t3 = t2 * t;
+    return 0.5 * ((2.0 * p1) + (p2 - p0) * t + (2.0 * p0 - 5.0 * p1 + 4.0 * p2 - p3) * t2 + (-p0 + 3.0 * p1 - 3.0 * p2 + p3) * t3);
+}
+
+float cultmath_quadratic_bezier(float p0, float p1, float p2, float t)
+{
+    t = cultmath_saturate(t);
+    float inv = 1.0 - t;
+    return inv * inv * p0 + 2.0 * inv * t * p1 + t * t * p2;
+}
+
+vec3 cultmath_quadratic_bezier(vec3 p0, vec3 p1, vec3 p2, float t)
+{
+    t = cultmath_saturate(t);
+    float inv = 1.0 - t;
+    return inv * inv * p0 + 2.0 * inv * t * p1 + t * t * p2;
+}
+
+float cultmath_cubic_bezier(float p0, float p1, float p2, float p3, float t)
+{
+    t = cultmath_saturate(t);
+    float inv = 1.0 - t;
+    return inv * inv * inv * p0 + 3.0 * inv * inv * t * p1 + 3.0 * inv * t * t * p2 + t * t * t * p3;
+}
+
+vec3 cultmath_cubic_bezier(vec3 p0, vec3 p1, vec3 p2, vec3 p3, float t)
+{
+    t = cultmath_saturate(t);
+    float inv = 1.0 - t;
+    return inv * inv * inv * p0 + 3.0 * inv * inv * t * p1 + 3.0 * inv * t * t * p2 + t * t * t * p3;
+}
+
+float cultmath_hash(float value) { return cultmath_frac(sin(value) * 43758.5453); }
+float cultmath_hash(vec2 value) { return cultmath_hash(dot(value, vec2(127.1, 311.7))); }
+float cultmath_hash(vec3 value) { return cultmath_hash(dot(value, vec3(127.1, 311.7, 74.7))); }
+
+// PCG hashes (Jarzynski and Olano, JCGT 2020), written over scalar uints so CultMath's ivec3/ivec4 carry the bits.
+uint cultmath_pcg(uint value)
+{
+    uint state = value * 747796405u + 2891336453u;
+    uint word = ((state >> int((state >> 28) + 4u)) ^ state) * 277803737u;
+    return (word >> 22) ^ word;
+}
+
+ivec3 cultmath_pcg3d(ivec3 value)
+{
+    uint x = uint(value.x) * 1664525u + 1013904223u, y = uint(value.y) * 1664525u + 1013904223u, z = uint(value.z) * 1664525u + 1013904223u;
+    x += y * z; y += z * x; z += x * y;
+    x ^= x >> 16; y ^= y >> 16; z ^= z >> 16;
+    x += y * z; y += z * x; z += x * y;
+    return ivec3(int(x), int(y), int(z));
+}
+
+ivec3 cultmath_pcg3d(vec3 value) { return cultmath_pcg3d(ivec3(int(floatBitsToUint(value.x)), int(floatBitsToUint(value.y)), int(floatBitsToUint(value.z)))); }
+ivec3 cultmath_pcg3d(vec2 value) { return cultmath_pcg3d(vec3(value, 0.0)); }
+
+ivec4 cultmath_pcg4d(ivec4 value)
+{
+    uint x = uint(value.x) * 1664525u + 1013904223u, y = uint(value.y) * 1664525u + 1013904223u;
+    uint z = uint(value.z) * 1664525u + 1013904223u, w = uint(value.w) * 1664525u + 1013904223u;
+    x += y * w; y += z * x; z += x * y; w += y * z;
+    x ^= x >> 16; y ^= y >> 16; z ^= z >> 16; w ^= w >> 16;
+    x += y * w; y += z * x; z += x * y; w += y * z;
+    return ivec4(int(x), int(y), int(z), int(w));
+}
+
+ivec4 cultmath_pcg4d(vec4 value) { return cultmath_pcg4d(ivec4(int(floatBitsToUint(value.x)), int(floatBitsToUint(value.y)), int(floatBitsToUint(value.z)), int(floatBitsToUint(value.w)))); }
+
+// Inigo Quilez, "smooth minimum": quadratic-polynomial smin carried to value-and-gradient form. See
+// math.smin_grad's comment (math.cs) for why the gradient blend is exact, not an approximation.
+vec4 cultmath_smin_grad(vec4 a, vec4 b, float k)
+{
+    float h = clamp(0.5 + 0.5 * (b.w - a.w) / k, 0.0, 1.0);
+    float value = mix(b.w, a.w, h) - k * h * (1.0 - h);
+    vec3 gradient = mix(b.xyz, a.xyz, h);
+    return vec4(gradient, value);
+}
+
+// Invariant 8's one struct return shape (design.md, "HLSL Target: Source Transformations"):
+// compared field by field and bit for bit against CultMath.CultCellular, same field order.
+struct CultCellular
+{
+    vec4 nearest;
+    vec4 edge;
+    float id;
+};
+
+// Worley, "A Cellular Texture Basis Function" (SIGGRAPH 1996). See math.cellular's comment
+// (math.cs) for the hashing, gradient, search-radius exactness proof, id source and
+// degenerate-point conventions this mirrors exactly, including the 5x5x5 lower-bound prune.
+CultCellular cultmath_cellular(vec3 p)
+{
+    vec3 cell = floor(p);
+    float f1 = 1.0e30;
+    float f2 = 1.0e30;
+    vec3 c1 = vec3(0.0, 0.0, 0.0);
+    vec3 c2 = vec3(0.0, 0.0, 0.0);
+    vec3 cellId = vec3(0.0, 0.0, 0.0);
+
+    for (int dz = -2; dz <= 2; dz++)
+    {
+        float lz = max(0.0, abs(float(dz)) - 1.0);
+        for (int dy = -2; dy <= 2; dy++)
+        {
+            float ly = max(0.0, abs(float(dy)) - 1.0);
+            for (int dx = -2; dx <= 2; dx++)
+            {
+                float lx = max(0.0, abs(float(dx)) - 1.0);
+                float lowerBound = sqrt(lx * lx + ly * ly + lz * lz);
+                if (lowerBound >= f2)
+                    continue;
+
+                vec3 neighbor = cell + vec3(dx, dy, dz);
+                ivec3 hash = cultmath_pcg3d(ivec3(neighbor));
+                vec3 jitter = vec3(
+                    float(uint(hash.x) >> 8) * (1.0 / 16777216.0),
+                    float(uint(hash.y) >> 8) * (1.0 / 16777216.0),
+                    float(uint(hash.z) >> 8) * (1.0 / 16777216.0));
+                vec3 feature = neighbor + jitter;
+                float d = cultmath_distance(p, feature);
+
+                if (d < f1)
+                {
+                    f2 = f1; c2 = c1;
+                    f1 = d; c1 = feature; cellId = neighbor;
+                }
+                else if (d < f2)
+                {
+                    f2 = d; c2 = feature;
+                }
+            }
+        }
+    }
+
+    // F1 = 0 and F2 = 0 share the same degenerate-gradient guard; see math.cellular's comment
+    // (math.cs) for why F2 = 0 is reachable past roughly |p| = 2^23.
+    vec3 grad1 = f1 > 0.0 ? (p - c1) / f1 : vec3(0.0, 0.0, 0.0);
+    vec3 grad2 = f2 > 0.0 ? (p - c2) / f2 : vec3(0.0, 0.0, 0.0);
+
+    // id comes from pcg4d over the winning cell's integer coordinate alone (same int-hashing rule
+    // as the jitter above), never from the pcg3d hash that produced its jitter (F4).
+    ivec4 idHash = cultmath_pcg4d(ivec4(ivec3(cellId), 0));
+
+    CultCellular result;
+    result.nearest = vec4(grad1, f1);
+    result.edge = vec4(grad2 - grad1, f2 - f1);
+    result.id = float(uint(idHash.w) >> 8) * (1.0 / 16777216.0);
+    return result;
+}
+
+
+
+
+// Interval arithmetic over vec2(lo, hi), lo <= hi, both finite. Every cultmath_iv_* function returns
+// an interval containing f(x) for every x in its input intervals, mirroring math.Interval.cs bit for
+// bit; see that file and docs/design.md ("Intervals") for each op's enclosure and its ulp widening.
+// Included by CultMath.hlsl after its own definitions of cultmath_smoothstep and cultmath_snoise.
+
+// Lipschitz constant of cultmath_snoise(vec3); provenance in math.Interval.cs (SNOISE_LIPSCHITZ).
+const float CULTMATH_SNOISE_LIPSCHITZ = 7.9640074;
+
+// Lipschitz constant of cultmath_snoise(vec2); provenance in math.Interval.cs (SNOISE2_LIPSCHITZ).
+const float CULTMATH_SNOISE2_LIPSCHITZ = 8.117002;
+
+vec2 cultmath_iv_point(float value) { return vec2(value, value); }
+vec2 cultmath_iv_add(vec2 a, vec2 b) { return vec2(a.x + b.x, a.y + b.y); }
+vec2 cultmath_iv_sub(vec2 a, vec2 b) { return vec2(a.x - b.y, a.y - b.x); }
+vec2 cultmath_iv_neg(vec2 a) { return vec2(-a.y, -a.x); }
+
+vec2 cultmath_iv_mul(vec2 a, vec2 b)
+{
+    float p0 = a.x * b.x;
+    float p1 = a.x * b.y;
+    float p2 = a.y * b.x;
+    float p3 = a.y * b.y;
+    return vec2(min(min(p0, p1), min(p2, p3)), max(max(p0, p1), max(p2, p3)));
+}
+
+vec2 cultmath_iv_scale(vec2 a, float s) { return s >= 0.0 ? vec2(a.x * s, a.y * s) : vec2(a.y * s, a.x * s); }
+
+vec2 cultmath_iv_abs(vec2 a)
+{
+    if (a.x >= 0.0)
+        return a;
+    if (a.y <= 0.0)
+        return vec2(-a.y, -a.x);
+    return vec2(0.0, max(-a.x, a.y));
+}
+
+vec2 cultmath_iv_min(vec2 a, vec2 b) { return vec2(min(a.x, b.x), min(a.y, b.y)); }
+vec2 cultmath_iv_max(vec2 a, vec2 b) { return vec2(max(a.x, b.x), max(a.y, b.y)); }
+
+vec2 cultmath_iv_sqr(vec2 a)
+{
+    float l = a.x * a.x;
+    float h = a.y * a.y;
+    if (a.x >= 0.0)
+        return vec2(l, h);
+    if (a.y <= 0.0)
+        return vec2(h, l);
+    return vec2(0.0, max(l, h));
+}
+
+vec2 cultmath_iv_sqrt(vec2 a) { return vec2(sqrt(max(a.x, 0.0)), sqrt(max(a.y, 0.0))); }
+
+// Widened by one ulp on each side: exp is not required to be correctly rounded.
+vec2 cultmath_iv_exp(vec2 a)
+{
+    float lo = exp(a.x);
+    float hi = exp(a.y);
+    return vec2(lo > 0.0 ? uintBitsToFloat(floatBitsToUint(lo) - 1u) : 0.0, uintBitsToFloat(floatBitsToUint(hi) + 1u));
+}
+
+vec2 cultmath_iv_clamp(vec2 a, float minimum, float maximum)
+{
+    return vec2(cultmath_clamp(a.x, minimum, maximum), cultmath_clamp(a.y, minimum, maximum));
+}
+
+vec2 cultmath_iv_saturate(vec2 a) { return vec2(cultmath_saturate(a.x), cultmath_saturate(a.y)); }
+
+// The endpoints, ordered, widened by one ulp inside [0, 1]: the float32 evaluation of smoothstep is
+// monotone only to within one ulp.
+vec2 cultmath_iv_smoothstep(float minimum, float maximum, vec2 a)
+{
+    float s0 = cultmath_smoothstep(minimum, maximum, a.x);
+    float s1 = cultmath_smoothstep(minimum, maximum, a.y);
+    float lo = min(s0, s1);
+    float hi = max(s0, s1);
+    return vec2(lo > 0.0 ? uintBitsToFloat(floatBitsToUint(lo) - 1u) : 0.0, hi < 1.0 ? uintBitsToFloat(floatBitsToUint(hi) + 1u) : 1.0);
+}
+
+// The natural interval extension of x + (y - x) * amount, which is how cultmath_lerp evaluates.
+vec2 cultmath_iv_lerp(vec2 a, vec2 b, float amount) { return cultmath_iv_add(a, cultmath_iv_scale(cultmath_iv_sub(b, a), amount)); }
+
+// [n - L r, n + L r] intersected with [-1, 1], n = cultmath_snoise(centre). One snoise, value only.
+vec2 cultmath_iv_snoise_ball(vec3 centre, float radius)
+{
+    float n = cultmath_snoise(centre);
+    float e = CULTMATH_SNOISE_LIPSCHITZ * radius;
+    return vec2(max(n - e, -1.0), min(n + e, 1.0));
+}
+
+// The 2D overload: [n - L2 r, n + L2 r] intersected with [-1, 1], n = cultmath_snoise(centre). One snoise, value only.
+vec2 cultmath_iv_snoise_ball(vec2 centre, float radius)
+{
+    float n = cultmath_snoise(centre);
+    float e = CULTMATH_SNOISE2_LIPSCHITZ * radius;
+    return vec2(max(n - e, -1.0), min(n + e, 1.0));
+}
+
+// Octave sum of cultmath_iv_snoise_ball, compounding frequency and amplitude as the value-and-gradient fBm does.
+vec2 cultmath_iv_fbm_ball(vec3 centre, float radius, int octaves, float lacunarity, float gain)
+{
+    octaves = clamp(octaves, 0, 16);
+    float amplitude = 1.0;
+    float frequency = 1.0;
+    vec2 sum = vec2(0.0, 0.0);
+    for (int i = 0; i < octaves; i++)
+    {
+        sum = cultmath_iv_add(sum, cultmath_iv_scale(cultmath_iv_snoise_ball(centre * frequency, radius * abs(frequency)), amplitude));
+        frequency *= lacunarity;
+        amplitude *= gain;
+    }
+
+    return sum;
+}
+
+// A ball (xyz centre, w radius) enclosing every point of every ray of a screen tile over the depth segment
+// [z0, z1], warped by at most warp. Camera frame: pinhole at the origin looking down +z, ray (m z, z).
+// footprintPerDepth = N / (sqrt(2) f) for an N x N tile at focal length f. The radius is widened by
+// (|c|_1 + radius) 2^-20, which covers the ball's own float32 rounding. Derivation in math.Interval.cs.
+vec4 cultmath_iv_frustum_ball(vec2 centreSlope, float z0, float z1, float footprintPerDepth, float warp)
+{
+    float zm = (z0 + z1) * 0.5;
+    float axis = sqrt(centreSlope.x * centreSlope.x + centreSlope.y * centreSlope.y + 1.0);
+    float cx = centreSlope.x * zm;
+    float cy = centreSlope.y * zm;
+    float radius = (z1 - z0) * 0.5 * axis + z1 * footprintPerDepth + warp;
+    radius += (abs(cx) + abs(cy) + abs(zm) + radius) * 9.5367431640625e-7;
+    return vec4(cx, cy, zm, radius);
+}
+
+
+
+
+
+
+// Reduced affine forms with one shared symbol over vec3(x0, a, e): the set { x0 + a eps + e delta :
+// delta in [-1, 1] } at the region's shared eps in [-1, 1], e >= 0. Every cultmath_af_* function mirrors
+// math.Affine.cs bit for bit; see that file and docs/design.md ("Affine forms") for each op's enclosure
+// and the float32 rounding allowance it folds into e. Included by CultMath.hlsl after
+// CultMath.Interval.hlsl, whose CULTMATH_SNOISE_LIPSCHITZ it shares, and after its own
+// definition of cultmath_snoise_grad.
+
+// An upper bound on the spectral norm of the Hessian of cultmath_snoise(vec3); provenance in
+// math.Affine.cs (SNOISE_HESSIAN).
+const float CULTMATH_SNOISE_HESSIAN = 55.642143;
+
+// The absolute rounding of one float32 read of cultmath_snoise_grad, value and gradient; the owner is
+// math.Affine.cs (SNOISE_READ_VALUE, SNOISE_READ_GRADIENT), read here by cultmath_af_snoise and in
+// CultMath.hlsl by cultmath_span_slack.
+const float CULTMATH_SNOISE_READ_VALUE = 6.103515625e-5;
+const float CULTMATH_SNOISE_READ_GRADIENT = 2.44140625e-4;
+
+// An upper bound on the operator norm of the third-derivative tensor of cultmath_snoise(vec3), the
+// next order after CULTMATH_SNOISE_HESSIAN; provenance in math.Span.cs (SNOISE_THIRD).
+const float CULTMATH_SNOISE_THIRD = 535.8009;
+
+vec3 cultmath_af_point(float value) { return vec3(value, 0.0, 0.0); }
+vec3 cultmath_af_symbol(float x0, float a) { return vec3(x0, a, 0.0); }
+
+vec3 cultmath_af_from_iv(vec2 interval)
+{
+    float x0 = interval.x * 0.5 + interval.y * 0.5;
+    float e = interval.y * 0.5 - interval.x * 0.5;
+    return vec3(x0, 0.0, e + (abs(x0) + e) * 9.5367431640625e-7 + 1.17549435e-38);
+}
+
+// An endpoint that would be NaN is the infinity on its side. NaN is found by its bit pattern: dxc marks
+// compares fast, so a test that needs NaN to make `lo == lo` false is deleted under its default flags.
+vec2 cultmath_af_range(vec3 x)
+{
+    float r = abs(x.y) + x.z;
+    float w = abs(x.x) * 9.5367431640625e-7 + r * 9.5367431640625e-7;
+    float lo = x.x - r - w;
+    float hi = x.x + r + w;
+    return vec2((floatBitsToUint(lo) & 0x7fffffffu) > 0x7f800000u ? uintBitsToFloat(0xff800000u) : lo,
+        (floatBitsToUint(hi) & 0x7fffffffu) > 0x7f800000u ? uintBitsToFloat(0x7f800000u) : hi);
+}
+
+vec3 cultmath_af_add(vec3 x, vec3 y)
+{
+    float x0 = x.x + y.x;
+    float a = x.y + y.y;
+    float e = x.z + y.z;
+    return vec3(x0, a, e + (abs(x0) + abs(a) + e) * 9.5367431640625e-7);
+}
+
+vec3 cultmath_af_sub(vec3 x, vec3 y)
+{
+    float x0 = x.x - y.x;
+    float a = x.y - y.y;
+    float e = x.z + y.z;
+    return vec3(x0, a, e + (abs(x0) + abs(a) + e) * 9.5367431640625e-7);
+}
+
+vec3 cultmath_af_neg(vec3 x) { return vec3(-x.x, -x.y, x.z); }
+
+vec3 cultmath_af_scale(vec3 x, float s)
+{
+    float x0 = x.x * s;
+    float a = x.y * s;
+    float e = x.z * abs(s);
+    return vec3(x0, a, e + (abs(x0) + abs(a) + e) * 9.5367431640625e-7 + 1.17549435e-38);
+}
+
+vec3 cultmath_af_add_iv(vec3 x, vec2 interval) { return cultmath_af_add(x, cultmath_af_from_iv(interval)); }
+
+vec3 cultmath_af_mul(vec3 x, vec3 y)
+{
+    float x0 = x.x * y.x;
+    float p = x.x * y.y;
+    float q = y.x * x.y;
+    float a = p + q;
+    float e = abs(x.x) * y.z + abs(y.x) * x.z + (abs(x.y) + x.z) * (abs(y.y) + y.z);
+    return vec3(x0, a, e + (abs(x0) + abs(p) + abs(q) + e) * 9.5367431640625e-7 + 1.17549435e-38);
+}
+
+// The narrower of the Lipschitz form and the centred (Taylor) form over every p = centre + axis eps + w,
+// |w| <= radius, from one cultmath_snoise_grad at the centre; the allowance for the float32 evaluation is
+// in math.Affine.cs.
+vec3 cultmath_af_snoise(vec3 centre, vec3 axis, float radius)
+{
+    vec4 g = cultmath_snoise_grad(centre);
+    vec3 gradient = vec3(g.x, g.y, g.z);
+    float reach = length(axis) + radius;
+    float lipschitz = CULTMATH_SNOISE_LIPSCHITZ * reach;
+    float a = dot(gradient, axis);
+    float remainder = length(gradient) * radius + 0.5 * CULTMATH_SNOISE_HESSIAN * reach * reach;
+    float allowance = CULTMATH_SNOISE_READ_VALUE + CULTMATH_SNOISE_READ_GRADIENT * reach
+        + CULTMATH_SNOISE_LIPSCHITZ * 9.5367431640625e-7 * (abs(centre.x) + abs(centre.y) + abs(centre.z) + reach);
+    bool centred = abs(a) + remainder < lipschitz;
+    float slope = centred ? a : 0.0;
+    float e = (centred ? remainder : lipschitz) + allowance;
+    return vec3(g.w, slope, e + (abs(g.w) + abs(slope) + e) * 9.5367431640625e-7);
+}
+
+// Octave sum of cultmath_af_snoise, compounding frequency and amplitude as the value-and-gradient fBm does.
+vec3 cultmath_af_fbm(vec3 centre, vec3 axis, float radius, int octaves, float lacunarity, float gain)
+{
+    octaves = clamp(octaves, 0, 16);
+    float amplitude = 1.0;
+    float frequency = 1.0;
+    vec3 sum = vec3(0.0, 0.0, 0.0);
+    for (int i = 0; i < octaves; i++)
+    {
+        sum = cultmath_af_add(sum, cultmath_af_scale(cultmath_af_snoise(centre * frequency, axis * frequency, radius * abs(frequency)), amplitude));
+        frequency *= lacunarity;
+        amplitude *= gain;
+    }
+
+    return sum;
+}
+
+// The axis of a screen tile's slice over the depth segment [z0, z1]; camera frame of
+// cultmath_iv_frustum_ball.
+vec3 cultmath_af_frustum_axis(vec2 centreSlope, float z0, float z1)
+{
+    float h = (z1 - z0) * 0.5;
+    return vec3(centreSlope.x * h, centreSlope.y * h, h);
+}
+
+// The affine slice of a screen tile over [z0, z1]: centre xyz, radius w. Derivation in math.Affine.cs.
+vec4 cultmath_af_frustum_ball(vec2 centreSlope, float z0, float z1, float footprintPerDepth, float warpVariation)
+{
+    float zm = (z0 + z1) * 0.5;
+    float h = (z1 - z0) * 0.5;
+    float cx = centreSlope.x * zm;
+    float cy = centreSlope.y * zm;
+    float spread = (abs(centreSlope.x) + abs(centreSlope.y) + 1.0) * h;
+    float radius = z1 * footprintPerDepth + warpVariation;
+    radius += (abs(cx) + abs(cy) + abs(zm) + spread + radius) * 9.5367431640625e-7;
+    return vec4(cx, cy, zm, radius);
+}
+
+
+
+
+// Span stages (invariant 8): the SpanPath, SpanStage and SpanNoise of math.Span.cs, field for field in
+// C#'s order, and cultmath_span_stage, its octave stage, statement for statement so the text is a bit-for-bit
+// mirror. They follow the includes because the bounds read CULTMATH_SNOISE_LIPSCHITZ, _HESSIAN and _THIRD.
+// A shader caller builds a SpanStage itself (SpanStage(1.0, 1.0, vec3(0.0), vec3(0.0), vec3(0.0), vec3(0.0))
+// is the identity of math.SpanStage.Identity, which has no shader form); see math.span_stage's comments for
+// the span, the transform, the bounds and the allowances. cultmath_span_rounding and cultmath_span_slack
+// are math.span_rounding and math.span_slack, the owners of S_o and of the three allowances.
+struct SpanPath
+{
+    vec3 midpoint;
+    vec3 velocity;
+    float speed;
+    float acceleration;
+    float halfLength;
+};
+
+struct SpanStage
+{
+    float frequency;
+    float stretch;
+    vec3 axis;
+    vec3 shift;
+    vec3 offset;
+    vec3 octaveStep;
+};
+
+struct SpanNoise
+{
+    vec4 field;
+    float curvature;
+    float fold;
+};
+
+float cultmath_span_rounding(float scale, vec3 mq, SpanStage stage, vec3 octaveShift, float nu, float halfLength)
+{
+    return 9.5367431640625e-7 * (abs(scale) * (cultmath_csum(abs(mq)) + cultmath_csum(abs(stage.shift))) + cultmath_csum(abs(stage.offset)) + cultmath_csum(abs(octaveShift)) + 1.7320508 * nu * halfLength);
+}
+
+vec3 cultmath_span_slack(float rounding)
+{
+    return vec3(CULTMATH_SNOISE_READ_VALUE + CULTMATH_SNOISE_LIPSCHITZ * rounding,
+        CULTMATH_SNOISE_READ_GRADIENT + CULTMATH_SNOISE_HESSIAN * rounding,
+        9.765625e-4 + CULTMATH_SNOISE_THIRD * rounding);
+}
+
+// The octave stage behind cultmath_span_fbm and cultmath_span_ridged, as math.span_stage is behind span_fbm
+// and span_ridged. The Frobenius norm of the Hessian is written out in C#'s order (GLSL has no _mRC member
+// access); the C# local named step is octaveShift here (it would shadow GLSL's step()).
+SpanNoise cultmath_span_stage(bool ridged, SpanPath path, SpanStage stage, int octaves, float lacunarity, float gain)
+{
+    vec3 q = path.midpoint;
+    float halfLength = path.halfLength;
+    octaves = clamp(octaves, 0, 16);
+    bool stretched = dot(stage.axis, stage.axis) > 0.0 && stage.stretch != 1.0;
+    vec3 direction = stretched ? normalize(stage.axis) : vec3(0.0, 0.0, 0.0);
+    float squeeze = stretched ? 1.0 / stage.stretch - 1.0 : 0.0;
+    float norm = abs(stage.frequency) * (stretched ? max(1.0, 1.0 / stage.stretch) : 1.0);
+    vec3 mq = stretched ? stage.frequency * (q + squeeze * dot(direction, q) * direction) : stage.frequency * q;
+    vec3 mv = stretched ? stage.frequency * (path.velocity + squeeze * dot(direction, path.velocity) * direction) : stage.frequency * path.velocity;
+    float amplitude = 1.0;
+    float scale = 1.0;
+    float value = 0.0;
+    vec3 gradient = vec3(0.0, 0.0, 0.0);
+    float curvature = 0.0;
+    float fold = 0.0;
+    for (int i = 0; i < octaves; i++)
+    {
+        vec3 octaveShift = float(i) * stage.octaveStep;
+        vec3 x = scale * (mq + stage.shift) + (stage.offset + octaveShift);
+        vec4 n = cultmath_snoise_grad(x);
+        vec3 g = vec3(n.x, n.y, n.z);
+        vec3 mg = stretched ? stage.frequency * (g + squeeze * dot(direction, g) * direction) : stage.frequency * g;
+        if (ridged)
+        {
+            float s = sign(n.w);
+            value += amplitude * (1.0 - abs(n.w));
+            gradient += -amplitude * scale * s * mg;
+        }
+        else
+        {
+            value += amplitude * n.w;
+            gradient += amplitude * scale * mg;
+        }
+
+        float weight = abs(amplitude);
+        float factor = abs(scale) * norm;
+        float nu = factor * path.speed;
+        float rounding = cultmath_span_rounding(scale, mq, stage, octaveShift, nu, halfLength);
+        vec3 slack = cultmath_span_slack(rounding);
+        float valueSlack = slack.x;
+        float gradientSlack = slack.y;
+        float hessianSlack = slack.z;
+        mat3 hm = cultmath_snoise_hessian(x);
+        float frobenius = sqrt(hm[0][0] * hm[0][0] + hm[0][1] * hm[0][1] + hm[0][2] * hm[0][2] + hm[1][0] * hm[1][0] + hm[1][1] * hm[1][1] + hm[1][2] * hm[1][2]
+            + hm[2][0] * hm[2][0] + hm[2][1] * hm[2][1] + hm[2][2] * hm[2][2]);
+        float hessianBound = min(CULTMATH_SNOISE_HESSIAN, frobenius + hessianSlack + CULTMATH_SNOISE_THIRD * nu * halfLength);
+        curvature += weight * factor * factor * hessianBound;
+        if (ridged)
+        {
+            vec3 w = scale * mv;
+            float reach = hessianBound * nu * halfLength;
+            float gl = min(CULTMATH_SNOISE_LIPSCHITZ, length(g) + gradientSlack + reach);
+            float travel = (abs(dot(g, w)) + gradientSlack * length(w)) * halfLength + (hessianBound * nu * length(w) + gl * factor * path.acceleration) * (0.5 * halfLength * halfLength) + 2.0 * valueSlack;
+            fold += 2.0 * weight * max(travel - abs(n.w), 0.0);
+        }
+
+        scale *= lacunarity;
+        amplitude *= gain;
+    }
+
+    SpanNoise result;
+    result.field = vec4(gradient, value);
+    result.curvature = curvature;
+    result.fold = fold;
+    return result;
+}
+
+SpanNoise cultmath_span_fbm(SpanPath path, SpanStage stage, int octaves, float lacunarity, float gain)
+{
+    return cultmath_span_stage(false, path, stage, octaves, lacunarity, gain);
+}
+
+SpanNoise cultmath_span_ridged(SpanPath path, SpanStage stage, int octaves, float lacunarity, float gain)
+{
+    return cultmath_span_stage(true, path, stage, octaves, lacunarity, gain);
+}
+
+float cultmath_value_noise(vec2 position)
+{
+    vec2 cell = floor(position);
+    vec2 local = cultmath_frac(position);
+    vec2 u = local * local * (vec2(3.0, 3.0) - vec2(2.0, 2.0) * local);
+
+    float a = cultmath_hash(cell);
+    float b = cultmath_hash(cell + vec2(1.0, 0.0));
+    float c = cultmath_hash(cell + vec2(0.0, 1.0));
+    float d = cultmath_hash(cell + vec2(1.0, 1.0));
+
+    return cultmath_lerp(cultmath_lerp(a, b, u.x), cultmath_lerp(c, d, u.x), u.y);
+}
+
+float cultmath_value_noise_bicubic(vec2 position)
+{
+    vec2 cell = floor(position);
+    vec2 local = cultmath_frac(position);
+
+    float y0 = cultmath_catmullrom(cultmath_hash(cell + vec2(-1.0, -1.0)), cultmath_hash(cell + vec2(0.0, -1.0)), cultmath_hash(cell + vec2(1.0, -1.0)), cultmath_hash(cell + vec2(2.0, -1.0)), local.x);
+    float y1 = cultmath_catmullrom(cultmath_hash(cell + vec2(-1.0, 0.0)), cultmath_hash(cell + vec2(0.0, 0.0)), cultmath_hash(cell + vec2(1.0, 0.0)), cultmath_hash(cell + vec2(2.0, 0.0)), local.x);
+    float y2 = cultmath_catmullrom(cultmath_hash(cell + vec2(-1.0, 1.0)), cultmath_hash(cell + vec2(0.0, 1.0)), cultmath_hash(cell + vec2(1.0, 1.0)), cultmath_hash(cell + vec2(2.0, 1.0)), local.x);
+    float y3 = cultmath_catmullrom(cultmath_hash(cell + vec2(-1.0, 2.0)), cultmath_hash(cell + vec2(0.0, 2.0)), cultmath_hash(cell + vec2(1.0, 2.0)), cultmath_hash(cell + vec2(2.0, 2.0)), local.x);
+
+    return cultmath_catmullrom(y0, y1, y2, y3, local.y);
+}
+
+#endif
diff --git a/src/app/features/scene/shaders/README.md b/src/app/features/scene/shaders/README.md
new file mode 100644
index 00000000..617644fb
--- /dev/null
+++ b/src/app/features/scene/shaders/README.md
@@ -0,0 +1,15 @@
+# Vendored CultMath GLSL
+
+`CultMath.glsl` and `CultMath.Phacelle.glsl` are CultLib's generated WebGL2
+lowering of `packages/cultmath/shaders/CultMath.hlsl` and
+`CultMath.Phacelle.hlsl`, copied unedited from CultLib branch
+`eureka/cultmath-tapes-interval-ops-r5` at `658c09e` (the branch that carries
+the interval ops and `CULTMATH_SNOISE2_LIPSCHITZ`; the repo default branch
+does not carry the GLSL yet). Never edit them here: regenerate in CultLib and
+re-copy. `CultMath.glsl` is MIT; `CultMath.Phacelle.glsl` is MPL-2.0 (Rune
+Skovbo Johansen's Phacelle noise, see its header), so that file keeps its
+header and any change to it is published under MPL-2.0.
+
+The other `*.glsl` files here are the scene's own stages. They are
+concatenated by `../shaderSource.ts` after a prelude; they use no colour
+literal (the palette is uniforms) and define no noise.
diff --git a/src/app/features/scene/shaders/atmosphere.frag.glsl b/src/app/features/scene/shaders/atmosphere.frag.glsl
new file mode 100644
index 00000000..adbf1b5b
--- /dev/null
+++ b/src/app/features/scene/shaders/atmosphere.frag.glsl
@@ -0,0 +1,90 @@
+// Pass 2: atmosphere, marched only up to pass 1's depth. Height fog is closed-form; the cloud
+// slab is marched with a few jittered steps. Output: rgb in-scattered light, a transmittance.
+uniform sampler2D u_depth;     // pass 1 hit distance, nearest
+uniform int u_steps;           // cloud march steps
+uniform float u_frame;
+uniform vec2 u_jitter;         // sub-pixel jitter in pixels of this target
+uniform vec2 u_size;           // this target's size in pixels
+uniform float u_coverage;      // weather knob: 0 clear .. 1 overcast
+
+in vec2 v_uv;
+layout(location = 0) out vec4 o_atm;
+
+float hg(float cosTheta, float g)
+{
+    float g2 = g * g;
+    return (1.0 - g2) / (12.566370614 * pow(1.0 + g2 - 2.0 * g * cosTheta, 1.5));
+}
+
+float cloudDensity(vec3 p)
+{
+    float wind = u_time * 2.5;
+    float f = cultmath_fbm_grad(vec3(p.x * 0.0042 + wind * 0.0042, p.y * 0.0075, p.z * 0.0042), 3, 2.0, 0.5).w;
+    float h = (p.y - W_CLOUD_Y0) / (W_CLOUD_Y1 - W_CLOUD_Y0);
+    float profile = smoothstep(0.0, 0.2, h) * smoothstep(1.0, 0.55, h);
+    return clamp((f + 0.55 * u_coverage - 0.25) * 2.4, 0.0, 1.0) * profile;
+}
+
+void main()
+{
+    vec2 jittered = (gl_FragCoord.xy + u_jitter) / u_size;
+    vec3 rd = camRay(jittered);
+    vec3 ro = u_camPos;
+    float depth = texture(u_depth, v_uv).r;
+    float tEnd = min(depth, W_ATM_FAR);
+
+    // Exponential height fog, closed form: optical depth of density d0 * exp(-k y) along the ray.
+    float k = W_FOG_FALLOFF;
+    float dy = rd.y * tEnd;
+    float tau = W_FOG_DENSITY * exp(-k * ro.y) * (abs(dy) > 1e-3 ? (1.0 - exp(-k * dy)) / (k * rd.y) : tEnd);
+    float fogT = exp(-max(tau, 0.0));
+    float cosSun = dot(rd, u_sunDir);
+    vec3 fogLight = u_fogColor + u_sunColor * hg(cosSun, 0.55) * 0.9;
+    vec3 inscatter = fogLight * (1.0 - fogT);
+    float trans = fogT;
+
+    // Clouds: only the part of the ray inside the slab and in front of the surface.
+    float ta = 0.0;
+    float tb = tEnd;
+    if (abs(rd.y) > 1e-4)
+    {
+        float t0 = (W_CLOUD_Y0 - ro.y) / rd.y;
+        float t1 = (W_CLOUD_Y1 - ro.y) / rd.y;
+        ta = max(0.0, min(t0, t1));
+        tb = min(tEnd, max(t0, t1));
+    }
+    else if (ro.y < W_CLOUD_Y0 || ro.y > W_CLOUD_Y1)
+        tb = 0.0;
+
+    if (tb > ta && u_steps > 0)
+    {
+        float dt = (tb - ta) / float(u_steps);
+        float t = ta + dt * ign(gl_FragCoord.xy, u_frame);
+        float T = 1.0;
+        vec3 L = vec3(0.0);
+        float phase = hg(cosSun, 0.6) + 0.35 * hg(cosSun, -0.2);
+        for (int i = 0; i < u_steps; i++)
+        {
+            vec3 p = ro + rd * t;
+            float d = cloudDensity(p);
+            if (d > 0.01)
+            {
+                // One sample toward the sun stands in for self-shadowing.
+                float shade = exp(-cloudDensity(p + u_sunDir * 38.0) * 2.2);
+                vec3 light = u_sunColor * (phase * 6.0) * shade + mix(u_skyHorizon, u_skyTop, 0.6) * 0.35;
+                float ext = d * dt * 0.035;
+                float stepT = exp(-ext);
+                L += T * (1.0 - stepT) * light;
+                T *= stepT;
+                if (T < 0.03)
+                    break;
+            }
+            t += dt;
+        }
+        // Fog in front of the slab is already in `inscatter`; clouds sit behind it.
+        inscatter += trans * L;
+        trans *= T;
+    }
+
+    o_atm = vec4(inscatter, trans);
+}
diff --git a/src/app/features/scene/shaders/common.glsl b/src/app/features/scene/shaders/common.glsl
new file mode 100644
index 00000000..6378a2e6
--- /dev/null
+++ b/src/app/features/scene/shaders/common.glsl
@@ -0,0 +1,31 @@
+// Shared by every fragment stage; concatenated after the prelude, CultMath and the world defines.
+uniform vec3 u_camPos;
+uniform mat3 u_camBasis;   // columns: right, up, forward
+uniform vec2 u_tanHalf;    // x: tan(fov/2) * aspect, y: tan(fov/2)
+uniform float u_time;
+uniform vec3 u_sunDir;     // unit, towards the sun
+uniform vec3 u_sunColor;
+uniform vec3 u_skyTop;
+uniform vec3 u_skyHorizon;
+uniform vec3 u_fogColor;
+
+vec3 camRay(vec2 uv)
+{
+    vec2 ndc = uv * 2.0 - 1.0;
+    return normalize(u_camBasis * vec3(ndc * u_tanHalf, 1.0));
+}
+
+float ign(vec2 p, float frame)
+{
+    p += 5.588238 * mod(frame, 64.0);
+    return fract(52.9829189 * fract(dot(p, vec2(0.06711056, 0.00583715))));
+}
+
+vec3 skyColor(vec3 rd)
+{
+    float h = clamp(rd.y, 0.0, 1.0);
+    vec3 c = mix(u_skyHorizon, u_skyTop, pow(h, 0.45));
+    float s = max(dot(rd, u_sunDir), 0.0);
+    c += u_sunColor * (1.5 * pow(s, 256.0));
+    return c;
+}
diff --git a/src/app/features/scene/shaders/composite.frag.glsl b/src/app/features/scene/shaders/composite.frag.glsl
new file mode 100644
index 00000000..9dce31f1
--- /dev/null
+++ b/src/app/features/scene/shaders/composite.frag.glsl
@@ -0,0 +1,26 @@
+// Full-resolution composite to the canvas: solid over sky, then the accumulated atmosphere.
+uniform sampler2D u_solid;   // rgb lit, a coverage (bilinear)
+uniform sampler2D u_atm;     // TAA'd atmosphere (bilinear)
+uniform float u_frame;
+
+in vec2 v_uv;
+out vec4 o_frag;
+
+vec3 tonemap(vec3 x)
+{
+    // Narkowicz ACES fit.
+    return clamp((x * (2.51 * x + 0.03)) / (x * (2.43 * x + 0.59) + 0.14), 0.0, 1.0);
+}
+
+void main()
+{
+    vec3 rd = camRay(v_uv);
+    vec4 solid = texture(u_solid, v_uv);
+    vec4 atm = texture(u_atm, v_uv);
+    vec3 base = mix(skyColor(rd), solid.rgb, solid.a);
+    vec3 col = base * atm.a + atm.rgb;
+    col = tonemap(col);
+    col = pow(col, vec3(1.0 / 2.2));
+    col += (ign(gl_FragCoord.xy, u_frame) - 0.5) / 255.0;
+    o_frag = vec4(col, 1.0);
+}
diff --git a/src/app/features/scene/shaders/fullscreen.vert.glsl b/src/app/features/scene/shaders/fullscreen.vert.glsl
new file mode 100644
index 00000000..cab0e527
--- /dev/null
+++ b/src/app/features/scene/shaders/fullscreen.vert.glsl
@@ -0,0 +1,8 @@
+out vec2 v_uv;
+void main()
+{
+    // One oversized triangle; uv spans 0..1 over the visible part.
+    vec2 p = vec2(float((gl_VertexID << 1) & 2), float(gl_VertexID & 2));
+    v_uv = p;
+    gl_Position = vec4(p * 2.0 - 1.0, 0.0, 1.0);
+}
diff --git a/src/app/features/scene/shaders/resolve.frag.glsl b/src/app/features/scene/shaders/resolve.frag.glsl
new file mode 100644
index 00000000..cc6dd2e0
--- /dev/null
+++ b/src/app/features/scene/shaders/resolve.frag.glsl
@@ -0,0 +1,41 @@
+// TAA resolve for the atmosphere target: reproject history by the pass-1 depth, clamp it to the
+// current frame's 3x3 neighbourhood, blend.
+uniform sampler2D u_cur;
+uniform sampler2D u_history;
+uniform sampler2D u_depth;
+uniform vec3 u_prevPos;
+uniform mat3 u_prevBasis;
+uniform vec2 u_prevTanHalf;
+uniform float u_blend;        // weight of the current frame; 1 resets
+uniform vec2 u_curSize;
+
+in vec2 v_uv;
+layout(location = 0) out vec4 o_hist;
+
+void main()
+{
+    vec2 px = 1.0 / u_curSize;
+    vec4 cur = texture(u_cur, v_uv);
+    vec4 lo = cur;
+    vec4 hi = cur;
+    for (int y = -1; y <= 1; y++)
+        for (int x = -1; x <= 1; x++)
+        {
+            vec4 s = texture(u_cur, v_uv + vec2(float(x), float(y)) * px);
+            lo = min(lo, s);
+            hi = max(hi, s);
+        }
+
+    // Where this pixel's ray ended, reprojected into last frame's camera.
+    vec3 rd = camRay(v_uv);
+    float t = min(texture(u_depth, v_uv).r, 600.0);
+    vec3 v = u_camPos + rd * t - u_prevPos;
+    vec3 local = transpose(u_prevBasis) * v;
+    vec2 prevNdc = local.z > 0.0 ? local.xy / (local.z * u_prevTanHalf) : vec2(2.0);
+    vec2 prevUv = prevNdc * 0.5 + 0.5;
+    bool inside = all(greaterThanEqual(prevUv, vec2(0.0))) && all(lessThanEqual(prevUv, vec2(1.0)));
+
+    float a = (inside && u_blend < 1.0) ? u_blend : 1.0;
+    vec4 hist = clamp(texture(u_history, prevUv), lo, hi);
+    o_hist = mix(hist, cur, a);
+}
diff --git a/src/app/features/scene/shaders/terrain.frag.glsl b/src/app/features/scene/shaders/terrain.frag.glsl
new file mode 100644
index 00000000..b8f2f718
--- /dev/null
+++ b/src/app/features/scene/shaders/terrain.frag.glsl
@@ -0,0 +1,184 @@
+// Pass 1: solid surfaces (terrain + world tree), sphere-traced. Writes lit colour with coverage
+// and, separately, the hit distance that bounds pass 2.
+uniform vec3 u_grass;
+uniform vec3 u_rock;
+uniform vec3 u_bark;
+uniform vec3 u_leaf;
+uniform int u_maxSteps;
+
+in vec2 v_uv;
+layout(location = 0) out vec4 o_color;   // rgb lit, a coverage
+layout(location = 1) out float o_depth;  // hit distance along the ray, W_FAR on a miss
+
+// ---- terrain -------------------------------------------------------------------------------
+
+float terrainMask(vec2 xz) { return smoothstep(W_PLATEAU_IN, W_PLATEAU_OUT, length(xz)); }
+
+// Value only; cultmath_fbm_grad is the shipped fbm, a public value-only fbm is Libby's gap.
+float terrainFbm(vec2 xz) { return cultmath_fbm_grad(vec3(xz * W_FREQ, 0.0), W_OCTAVES, W_LACUNARITY, W_GAIN).w; }
+
+#if W_ERODE_ENABLED
+CultPhasor terrainPhacelle(vec2 xz)
+{
+    // A fixed stripe wave vector: ridges across the sheet, not slope-aligned erosion.
+    return cultmath_phacelle(vec3(xz * W_ERODE_FREQ, 0.0), W_ERODE_SIDE, 0.0, W_ERODE_NORM);
+}
+#endif
+
+// Conservative distance to the terrain graph. Above the envelope (fbm + erosion amplitude) the
+// cheap envelope bound is exact enough to step by; only within it is the eroded field evaluated.
+float terrainDistance(vec3 p)
+{
+    float m = terrainMask(p.xz);
+    // Flat plateau: the height is exactly 0, so neither noise is evaluated. Sky rays spend most
+    // of their steps here.
+    if (m <= 0.0)
+        return p.y * W_SLOPE_ENV_INV;
+    float hb = m * W_AMP * terrainFbm(p.xz);
+    float above = p.y - (hb + m * W_ERODE_AMP);
+    if (above > 0.0)
+        return above * W_SLOPE_ENV_INV;
+#if W_ERODE_ENABLED
+    hb += m * W_ERODE_AMP * terrainPhacelle(p.xz).cos.w;
+#endif
+    return (p.y - hb) * W_SLOPE_FULL_INV;
+}
+
+vec3 terrainNormal(vec2 xz)
+{
+    float r = length(xz);
+    float m = smoothstep(W_PLATEAU_IN, W_PLATEAU_OUT, r);
+    float s = clamp((r - W_PLATEAU_IN) / (W_PLATEAU_OUT - W_PLATEAU_IN), 0.0, 1.0);
+    float dm = r > 0.0 ? 6.0 * s * (1.0 - s) / (W_PLATEAU_OUT - W_PLATEAU_IN) : 0.0;
+    vec2 dir = r > 0.0 ? xz / r : vec2(0.0);
+    vec4 fg = cultmath_fbm_grad(vec3(xz * W_FREQ, 0.0), W_OCTAVES, W_LACUNARITY, W_GAIN);
+    float f = fg.w;
+    vec2 g = m * W_AMP * W_FREQ * fg.xz + dm * W_AMP * f * dir;
+#if W_ERODE_ENABLED
+    CultPhasor ph = terrainPhacelle(xz);
+    g += m * W_ERODE_AMP * W_ERODE_FREQ * ph.cos.xz + dm * W_ERODE_AMP * ph.cos.w * dir;
+#endif
+    return normalize(vec3(-g.x, 1.0, -g.y));
+}
+
+// ---- world tree ----------------------------------------------------------------------------
+
+float smin(float a, float b, float k)
+{
+    float h = max(k - abs(a - b), 0.0) / k;
+    return min(a, b) - h * h * k * 0.25;
+}
+
+// Not an exact SDF: the flare, ellipsoid and displacement are all scaled to stay under 1-Lipschitz.
+float treeDistance(vec3 p, out float leafness)
+{
+    vec3 c = vec3(0.0, 0.5 * (W_CANOPY_Y + W_CANOPY_R.y), 0.0);
+    float bound = length(p - c) - (W_CANOPY_Y + W_CANOPY_R.y);
+    leafness = 0.0;
+    if (bound > 8.0)
+        return bound;
+
+    float ang = atan(p.z, p.x);
+    float y01 = clamp(p.y / W_TRUNK_H, 0.0, 1.0);
+    float radius = mix(W_TRUNK_R0, W_TRUNK_R1, y01) + 6.0 * exp(-p.y * 0.2) * (1.0 + 0.35 * sin(6.0 * ang + p.y * 0.15));
+    float trunk = max(length(p.xz) - radius, p.y - W_TRUNK_H * 1.2) * 0.7;
+
+    vec3 q = (p - vec3(0.0, W_CANOPY_Y, 0.0)) / W_CANOPY_R;
+    float disp = cultmath_snoise(p * 0.05 + vec3(0.0, u_time * 0.015, 0.0));
+    float canopy = (length(q) - 1.0) * min(W_CANOPY_R.x, W_CANOPY_R.y) - 3.0 * disp;
+    canopy *= 0.45; // 1 / (1 + 3 * 0.05 * L)
+
+    float d = smin(trunk, canopy, 6.0);
+    leafness = clamp(0.5 + (trunk - canopy) * 0.15, 0.0, 1.0);
+    return d;
+}
+
+// ---- trace ---------------------------------------------------------------------------------
+
+float sceneDistance(vec3 p, out float leafness, out bool onTree)
+{
+    float dt = terrainDistance(p);
+    float dr = treeDistance(p, leafness);
+    onTree = dr < dt;
+    return min(dt, dr);
+}
+
+void main()
+{
+    vec3 ro = u_camPos;
+    vec3 rd = camRay(v_uv);
+
+    float t = 0.0;
+    // Nothing is above W_MAX_HEIGHT: skip the empty air above it, or miss outright.
+    if (ro.y > W_MAX_HEIGHT)
+    {
+        if (rd.y >= 0.0)
+        {
+            o_color = vec4(0.0);
+            o_depth = W_FAR;
+            return;
+        }
+        t = (W_MAX_HEIGHT - ro.y) / rd.y;
+    }
+
+    bool hit = false;
+    bool onTree = false;
+    float leafness = 0.0;
+    int i = 0;
+    for (; i < u_maxSteps; i++)
+    {
+        vec3 p = ro + rd * t;
+        float d = sceneDistance(p, leafness, onTree);
+        if (d < 0.0006 * t + 0.01)
+        {
+            hit = true;
+            break;
+        }
+        t += d;
+        if (t > W_FAR || (p.y > W_MAX_HEIGHT && rd.y > 0.0))
+            break;
+    }
+    // The conservative bound makes grazing rays crawl: a ray that ran out of steps while still
+    // below the highest terrain is a distant surface under thick fog, so it ends there.
+    if (!hit && i == u_maxSteps && t < W_FAR)
+    {
+        hit = true;
+        sceneDistance(ro + rd * t, leafness, onTree);
+    }
+
+    if (!hit)
+    {
+        o_color = vec4(0.0);
+        o_depth = W_FAR;
+        return;
+    }
+
+    vec3 p = ro + rd * t;
+    vec3 n;
+    vec3 albedo;
+    if (onTree)
+    {
+        float e = 0.35;
+        float l;
+        n = normalize(vec3(
+            treeDistance(p + vec3(e, 0.0, 0.0), l) - treeDistance(p - vec3(e, 0.0, 0.0), l),
+            treeDistance(p + vec3(0.0, e, 0.0), l) - treeDistance(p - vec3(0.0, e, 0.0), l),
+            treeDistance(p + vec3(0.0, 0.0, e), l) - treeDistance(p - vec3(0.0, 0.0, e), l)));
+        float speckle = 0.75 + 0.25 * cultmath_snoise(p * 0.8);
+        albedo = mix(u_bark, u_leaf * speckle, leafness);
+    }
+    else
+    {
+        n = terrainNormal(p.xz);
+        float rockness = smoothstep(0.78, 0.55, n.y);
+        albedo = mix(u_grass, u_rock, rockness);
+    }
+
+    float ndl = dot(n, u_sunDir);
+    float wrap = clamp(ndl * 0.6 + 0.4, 0.0, 1.0); // half-lambert: no shadow ray in the cheap pass
+    vec3 ambient = mix(u_skyHorizon, u_skyTop, 0.5 + 0.5 * n.y) * 0.45;
+    vec3 col = albedo * (u_sunColor * wrap * wrap + ambient);
+
+    o_color = vec4(col, 1.0);
+    o_depth = t;
+}
diff --git a/src/app/features/scene/tier.test.ts b/src/app/features/scene/tier.test.ts
new file mode 100644
index 00000000..d35b6bb6
--- /dev/null
+++ b/src/app/features/scene/tier.test.ts
@@ -0,0 +1,26 @@
+import { describe, expect, it } from 'vitest';
+import { resolveTier } from './tier';
+
+describe('scene tier', () => {
+  it('is off unless the config turns it on, which is the feature flag', () => {
+    expect(resolveTier(undefined)).toBe('off');
+    expect(resolveTier({})).toBe('off');
+    expect(resolveTier({ tier: 'off' }, { deviceMemory: 16 })).toBe('off');
+    expect(resolveTier({ tier: 'full' })).toBe('full');
+  });
+
+  it('lets the device lower the configured tier and never raise it', () => {
+    expect(resolveTier({ tier: 'full' }, { saveData: true })).toBe('still');
+    expect(resolveTier({ tier: 'full' }, { deviceMemory: 2 })).toBe('low');
+    expect(resolveTier({ tier: 'full' }, { hardwareConcurrency: 4 })).toBe('low');
+    expect(resolveTier({ tier: 'low' }, { saveData: true })).toBe('still');
+    expect(resolveTier({ tier: 'still' }, { deviceMemory: 16, hardwareConcurrency: 16 })).toBe(
+      'still'
+    );
+    expect(resolveTier({ tier: 'low' }, { deviceMemory: 16, hardwareConcurrency: 16 })).toBe('low');
+  });
+
+  it('treats unknown capabilities as capable', () => {
+    expect(resolveTier({ tier: 'full' }, {})).toBe('full');
+  });
+});
diff --git a/src/app/features/scene/tier.ts b/src/app/features/scene/tier.ts
new file mode 100644
index 00000000..1c1668c7
--- /dev/null
+++ b/src/app/features/scene/tier.ts
@@ -0,0 +1,49 @@
+// Which quality the background scene runs at. The client config's `backgroundScene.tier` is both
+// the feature flag (absent or 'off' means the scene is not even imported) and the ceiling; the
+// device can only lower it.
+
+export type SceneTier = 'off' | 'still' | 'low' | 'full';
+
+/** Cheapest first. */
+export const TIERS: readonly SceneTier[] = ['off', 'still', 'low', 'full'];
+
+export interface Capabilities {
+  /** `navigator.connection.saveData`. */
+  saveData?: boolean;
+  /** `navigator.deviceMemory` in GiB (Chromium only). */
+  deviceMemory?: number;
+  hardwareConcurrency?: number;
+}
+
+export interface BackgroundSceneConfig {
+  tier?: SceneTier;
+}
+
+export function readCapabilities(nav: Navigator): Capabilities {
+  const n = nav as Navigator & {
+    deviceMemory?: number;
+    connection?: { saveData?: boolean };
+  };
+  return {
+    saveData: n.connection?.saveData,
+    deviceMemory: n.deviceMemory,
+    hardwareConcurrency: n.hardwareConcurrency,
+  };
+}
+
+/** The most the device is allowed to run, before the configured ceiling is applied. */
+function deviceCeiling(c: Capabilities): SceneTier {
+  if (c.saveData) return 'still';
+  if ((c.deviceMemory ?? Infinity) <= 2 || (c.hardwareConcurrency ?? Infinity) <= 4) return 'low';
+  return 'full';
+}
+
+/** The tier to run: the configured one (default 'off'), lowered by what the device can carry. */
+export function resolveTier(
+  config: BackgroundSceneConfig | undefined,
+  capabilities: Capabilities = {}
+): SceneTier {
+  const configured = config?.tier ?? 'off';
+  const ceiling = deviceCeiling(capabilities);
+  return TIERS.indexOf(configured) <= TIERS.indexOf(ceiling) ? configured : ceiling;
+}
diff --git a/src/app/features/scene/world.test.ts b/src/app/features/scene/world.test.ts
new file mode 100644
index 00000000..caa04fad
--- /dev/null
+++ b/src/app/features/scene/world.test.ts
@@ -0,0 +1,40 @@
+import { describe, expect, it } from 'vitest';
+import common from './shaders/common.glsl?raw';
+import terrain from './shaders/terrain.frag.glsl?raw';
+import atmosphere from './shaders/atmosphere.frag.glsl?raw';
+import resolve from './shaders/resolve.frag.glsl?raw';
+import composite from './shaders/composite.frag.glsl?raw';
+import { terrainBounds, WORLD, worldDefines } from './world';
+
+describe('world defines', () => {
+  it('define every W_ name any scene shader uses', () => {
+    const defined = new Set([...worldDefines().matchAll(/#define (W_\w+)/g)].map((m) => m[1]));
+    [common, terrain, atmosphere, resolve, composite]
+      .flatMap((src) => [...src.matchAll(/\bW_[A-Z0-9_]+\b/g)])
+      .forEach((m) => expect(defined, m[0]).toContain(m[0]));
+  });
+
+  it('emit only valid GLSL numbers', () => {
+    expect(worldDefines()).not.toMatch(/NaN|Infinity|undefined/);
+  });
+});
+
+describe('terrain step bound', () => {
+  it('is more conservative with erosion than without, and without erosion carries no phacelle term', () => {
+    const b = terrainBounds();
+    expect(b.slopeFull).toBeGreaterThan(b.slopeEnvelope);
+    const off = terrainBounds({ ...WORLD, phacelleGradBound: 0 });
+    expect(off.slopeFull).toBe(off.slopeEnvelope);
+  });
+
+  it('puts every surface under the height the tracer skips to', () => {
+    const maxHeight = Number(/#define W_MAX_HEIGHT (\S+)/.exec(worldDefines())![1]);
+    expect(maxHeight).toBeGreaterThanOrEqual(terrainBounds().maxHeight);
+    expect(maxHeight).toBeGreaterThanOrEqual(WORLD.tree.canopyY + WORLD.tree.canopyRadius[1]);
+  });
+
+  it('leaves the erosion bound turned on only when a bound exists', () => {
+    expect(worldDefines()).toContain('#define W_ERODE_ENABLED 1');
+    expect(worldDefines({ ...WORLD, phacelleGradBound: 0 })).toContain('#define W_ERODE_ENABLED 0');
+  });
+});
diff --git a/src/app/features/scene/world.ts b/src/app/features/scene/world.ts
new file mode 100644
index 00000000..ba9aec53
--- /dev/null
+++ b/src/app/features/scene/world.ts
@@ -0,0 +1,116 @@
+// The scene's world constants. One owner: the shader prelude below is generated from this
+// object, so a number lives here and nowhere else, and the Lipschitz bounds that make the
+// sphere-trace step safe are computed beside the numbers they bound.
+
+// Lipschitz constant of cultmath_snoise(vec3), CULTMATH_SNOISE_LIPSCHITZ in the vendored
+// CultMath.glsl (provenance in CultLib math.Interval.cs).
+export const SNOISE_LIPSCHITZ = 7.9640074;
+
+export const WORLD = {
+  far: 1400,
+  // Terrain: height = mask(r) * (amp * fbm(xz * freq) + erodeAmp * phacelle(xz * erodeFreq)).
+  amp: 70,
+  freq: 1 / 520,
+  octaves: 4,
+  lacunarity: 2.0,
+  gain: 0.45,
+  // Flat ground (height 0) out to plateauInner, full terrain from plateauOuter. The login
+  // camera stays inside plateauInner, so it can never end up inside the terrain.
+  plateauInner: 135,
+  plateauOuter: 380,
+  erodeAmp: 2.5,
+  erodeFreq: 1 / 38,
+  // The stripe wave vector and normalization the terrain feeds cultmath_phacelle.
+  erodeSide: [1.7, 0, 1] as const,
+  erodeNorm: 0.5,
+  // Bound on |grad| of the phasor's cos component for the side and norm above. Libby measured a
+  // sup of 6.63 at |side| = 1.5, n = 0 (CultLib PR #1 head 990521b3); 7.0 adds margin. The
+  // shader passes roughness n = 0 as a literal, so the doc envelope max(10, 1.25|side|)/(1-n)
+  // needs no 1/(1-n) factor here; if n ever becomes nonzero, divide this bound by (1 - n).
+  // |value| <= 1 holds by construction (the output is a normalized phasor).
+  // Typed number, not the literal 7, so callers can build variants such as { ...WORLD, phacelleGradBound: 0 }.
+  phacelleGradBound: 7.0 as number,
+  // World tree at the origin.
+  tree: {
+    trunkHeight: 38,
+    baseRadius: 3.4,
+    topRadius: 1.9,
+    canopyY: 52,
+    canopyRadius: [30, 17, 30] as const,
+  },
+  // Atmosphere.
+  atmosphereFar: 1400,
+  cloudBase: 130,
+  cloudTop: 230,
+  fogDensity: 0.0028,
+  fogFalloff: 0.011,
+} as const;
+
+const f = (n: number) => (Number.isInteger(n) ? `${n}.0` : `${n}`);
+
+/** Sum over octaves of (lacunarity * gain)^i: the slope bound's octave factor. */
+function slopeSum(octaves: number, lacunarity: number, gain: number): number {
+  let s = 0;
+  for (let i = 0; i < octaves; i += 1) s += (lacunarity * gain) ** i;
+  return s;
+}
+
+/** Sum over octaves of gain^i: the fbm value bound. */
+function valueSum(octaves: number, gain: number): number {
+  let s = 0;
+  for (let i = 0; i < octaves; i += 1) s += gain ** i;
+  return s;
+}
+
+export interface TerrainBounds {
+  /** Lipschitz bound of the envelope surface (no erosion). */
+  slopeEnvelope: number;
+  /** Lipschitz bound of the eroded surface. */
+  slopeFull: number;
+  /** Highest point the terrain can reach. */
+  maxHeight: number;
+}
+
+export function terrainBounds(w: typeof WORLD = WORLD): TerrainBounds {
+  const fbmMax = valueSum(w.octaves, w.gain);
+  const maskSlope = 1.5 / (w.plateauOuter - w.plateauInner); // smoothstep's max slope
+  const fbmSlope = w.amp * w.freq * SNOISE_LIPSCHITZ * slopeSum(w.octaves, w.lacunarity, w.gain);
+  const slopeEnvelope = fbmSlope + (w.amp * fbmMax + w.erodeAmp) * maskSlope;
+  const slopeFull = slopeEnvelope + w.erodeAmp * w.erodeFreq * w.phacelleGradBound;
+  return { slopeEnvelope, slopeFull, maxHeight: w.amp * fbmMax + w.erodeAmp };
+}
+
+/** The #defines every stage sees. */
+export function worldDefines(w: typeof WORLD = WORLD): string {
+  const b = terrainBounds(w);
+  const t = w.tree;
+  const lines: Array<[string, string]> = [
+    ['W_FAR', f(w.far)],
+    ['W_AMP', f(w.amp)],
+    ['W_FREQ', f(w.freq)],
+    ['W_OCTAVES', String(w.octaves)],
+    ['W_LACUNARITY', f(w.lacunarity)],
+    ['W_GAIN', f(w.gain)],
+    ['W_PLATEAU_IN', f(w.plateauInner)],
+    ['W_PLATEAU_OUT', f(w.plateauOuter)],
+    ['W_ERODE_AMP', f(w.erodeAmp)],
+    ['W_ERODE_FREQ', f(w.erodeFreq)],
+    ['W_ERODE_SIDE', `vec3(${w.erodeSide.map(f).join(', ')})`],
+    ['W_ERODE_NORM', f(w.erodeNorm)],
+    ['W_ERODE_ENABLED', w.phacelleGradBound > 0 ? '1' : '0'],
+    ['W_SLOPE_ENV_INV', f(1 / Math.sqrt(1 + b.slopeEnvelope ** 2))],
+    ['W_SLOPE_FULL_INV', f(1 / Math.sqrt(1 + b.slopeFull ** 2))],
+    ['W_MAX_HEIGHT', f(Math.max(b.maxHeight, t.canopyY + t.canopyRadius[1] + 6))],
+    ['W_TRUNK_H', f(t.trunkHeight)],
+    ['W_TRUNK_R0', f(t.baseRadius)],
+    ['W_TRUNK_R1', f(t.topRadius)],
+    ['W_CANOPY_Y', f(t.canopyY)],
+    ['W_CANOPY_R', `vec3(${t.canopyRadius.map(f).join(', ')})`],
+    ['W_ATM_FAR', f(w.atmosphereFar)],
+    ['W_CLOUD_Y0', f(w.cloudBase)],
+    ['W_CLOUD_Y1', f(w.cloudTop)],
+    ['W_FOG_DENSITY', f(w.fogDensity)],
+    ['W_FOG_FALLOFF', f(w.fogFalloff)],
+  ];
+  return lines.map(([k, v]) => `#define ${k} ${v}`).join('\n');
+}
diff --git a/src/app/hooks/useClientConfig.ts b/src/app/hooks/useClientConfig.ts
index e5fc6cc6..7a0586eb 100644
--- a/src/app/hooks/useClientConfig.ts
+++ b/src/app/hooks/useClientConfig.ts
@@ -1,4 +1,5 @@
 import { createContext, useContext } from 'react';
+import type { BackgroundSceneConfig } from '../features/scene/tier';
 
 export type HashRouterConfig = {
   enabled?: boolean;
@@ -18,6 +19,8 @@ export type ClientConfig = {
   };
 
   hashRouter?: HashRouterConfig;
+
+  backgroundScene?: BackgroundSceneConfig;
 };
 
 const ClientConfigContext = createContext<ClientConfig | null>(null);