# How to build a visualizer (scene)

> **Start here:** `npm run new:scene -- "My Scene" --family=glitch --traits=camera,style`
>
> That writes the module, registers it, and leaves a skeleton that already passes
> every gate. Then write the `scene()` body, `npm run lint:scenes`, and open
> `checks.html?scene=My%20Scene`. The rest of this file is the reference.
>
> There is also a repo skill — `.claude/skills/build-visualizer/` — which is the
> same procedure in the form an agent will follow.

A scene is **a fragment shader plus a params block** — nothing else. Uniform
binding, the generated UI sliders, seeded per-track sampling, arc drift and the
phase-gate checks are all derived from the schema. There is no per-scene wiring.

Reference: `src/scenes/shader/kaleido-tunnel.js` (simple), `block-mosh.js`
(feedback/glitch), `scan-tear.js` (beat-quantised), `metaballs.js` (loops).
Read these before writing anything.

## The shape of a scene

```js
export const myScene = {
    name: 'My Scene',
    family: 'glitch',              // flow | organic | minimal | structural | geometric | glitch
    kind: 'fragment',
    traits: ['camera', 'style'],   // which personality traits it honours (see below)

    params: {
        density: { type: 'float',  range: [0, 1],  default: 0.5, uniform: 'u_density', bias: 'density' },
        speed:   { type: 'float',  range: [0.1, 2], default: 0.5, uniform: 'u_speed', rate: true },
        palette: { type: 'palette', count: 4 },
    },

    reactive: {
        density: { feature: 'bandLow', amount: 0.3, response: 'smooth' },
    },

    shader: `
vec4 scene(vec2 uv, vec2 p) {
    float t = u_time * u_speed + u_seed;
    return vec4(palRamp(fbm(p * 4.0 + t, 4)), 1.0);
}
`,
};
```

The scaffolder writes all of that for you, including the registry import and
entry, and bakes name-derived constants into the skeleton field so two freshly
scaffolded scenes are not identical to each other. To do it by hand: write the
file and add the import + `MODULES` entry in `src/scenes/registry.js`.

## The shader contract

You write one function: `vec4 scene(vec2 uv, vec2 p)`.

- `uv` is 0..1 across the frame; `p` is centred, aspect-corrected, ~-1..1 on the
  short axis. **Work in these, never pixels** — scale pixel-sized things by
  `u_pixelScale` so a 720p preview matches a 4K export.
- The preamble is injected for you. Never declare `main()`, `u_resolution`,
  `vUv`, or any contract uniform yourself.

**Uniforms available** (see `engine/shader-contract.js`):

- Frame: `u_time`, `u_frame`, `u_progress`, `u_seed`, `u_resolution`, `u_aspect`,
  `u_pixelScale`, `u_opacity`.
- Audio, filled per frame from the FeatureTrack: `u_loudness`, `u_rms`,
  `u_bandSub/low/mid/high/air`, `u_flux`, `u_centroid`, `u_flatness`, `u_width`,
  `u_beat`, `u_beatPhase`, `u_barPhase`, `u_phrasePhase`, `u_sectionProgress`,
  `u_sectionEnergy`, `u_buildSlope`.
- Personality (`u_sigSides`, `u_sigDrift`, `u_sigLine`, …) — see below.
- Feedback: `u_prev` sampler, read with `prev(vec2 uv)` (returns the previous
  frame's colour; black if none). This is what makes trails/smear/datamosh work
  for free.

**Helpers** (already in the preamble):

- Palette: `pal(int)`, `palRamp(float)` — always use these, or the look can't
  recolour the scene.
- Noise: `hash11/12/22`, `vnoise`, `fbm`, `curl`, `rot`, `kaleido`.
- Personality: `sigShape`/`sigForm`, `sigCamera`, `sigFolded`, `sigEdge`,
  `sigGrain`, `sigHorizonY`, `sigAir`.
- `sat(x)` = `clamp(x, 0, 1)`.

## Param schema fields

- `type`: `float` | `int` | `bool` | `vec2` | `palette`.
- `range` `[min, max]` required for numerics; `default` required unless you want
  range[0] (prefer explicit defaults).
- `uniform`: the GLSL name. Must be `u_*` and must not collide with the contract
  (see "traps").
- `bias`: which track-character axis nudges sampling — `energy`, `density`,
  `motion`. This is how a loud drop gets denser scenes without the scene knowing
  about audio.
- `rate: true`: **mandatory if the shader multiplies `u_time` by this param.**
- `reactive`: `{ feature, amount, response }`. `response`: `linear` (default) |
  `spike` | `smooth` | `inverse`.

## Personality traits (`traits`)

The look generator builds each track on a signature of 1-2 traits, and a scene
that doesn't honour **all** of them is never cast in that track — so the library
intentionally shrinks per track. Only declare what you genuinely use:

| trait | what to call | lint evidence |
|---|---|---|
| `shape` | `sigShape` / `sigForm` | a `sigShape`/`sigForm(` call |
| `camera` | `sigCamera(p)` | a `sigCamera(` call |
| `space` | `sigHorizonY` / `sigAir` | those calls or `u_sigHorizon/Depth/Wash` |
| `style` | `sigEdge` / `sigGrain` / `sigFolded`, or `u_sigLine/Soft/Texture/Fold` | the calls / those uniforms |

The lint greps your shader and fails a declared trait with no evidence. Declaring
`[]` (none) is valid.

### `texture` — how much surface grain your scene accepts

Optional module field, `0..2`, default `1`. It scales `u_sigTexture` — and so
every `sigGrain(uv)` in your shader — for this scene only. Set it to `0` if your
scene is crisp line work that grain only furs up, or to something under 1 if it
should be dusted rather than dirty. It has nothing to do with the *grade's*
grain, which is a per-track treatment (see `look/grain.js`) and is off entirely
for most tracks.

## Traps that have actually bitten here

1. **Anything multiplying `u_time` must be `rate: true`.** Phase is
   `elapsed × rate`; modulating a rate jumps the phase by `elapsed × Δrate` — a
   minute in a small wobble throws the image several units between frames, which
   measured as strobing at 2× the accessibility limit. So don't react a rate
   param, and add a *bounded* term instead: `u_time * u_speed + u_bandLow * 2.0`
   is fine; `u_time * (u_speed + u_bandLow)` is not.
2. **Never reuse a contract uniform name** (`u_width`, `u_time`, `u_seed`, …).
   It's a GLSL redefinition error; the only symptom is a black frame.
3. **Don't modulate whole-frame luminance on the beat.** A per-kick min→max→min
   cycle is exactly what the WCAG 2.3.1 / Harding 3-flashes-per-second ceiling
   bans. Pulse a *small local* term (per-block tint) instead, and use `smooth`
   responses on loud things. If it's glitchy, **quantise it** — `floor(u_barPhase
   * n) + floor(t * k) * n` makes corruption step on the grid instead of
   crawling, which both reads better and stays below the flash rate (see
   `scan-tear.js`, `block-mosh.js`).
4. **Determinism is absolute.** No `Math.random()`, `performance.now()`,
   `Date.now()`, `new Date()` — use `hash*`/`fbm` for variation, and let time flow
   through `u_time`/`u_frame` only. The lint greps for these.
5. **Set a base image.** A scene that only reads `prev()` is black for the first
   frames and fragile under seek. Generate your own field underneath the effect.
6. Don't hardcode saturated `vec3(r,g,b)` literals when you declared a palette —
   the lint flags more than two.

## Families

Chosen by section kind in the arc driver — a breakdown never lands on a strobing
glitch scene. The library is at **seven per family** (42 scenes). Check the
current spread before adding another:

```bash
node -e "import('./src/scenes/registry.js').then(({scenes})=>{const b={};for(const m of scenes)(b[m.family]??=[]).push(m.name);console.log(b)})"
```

Depth matters more than it looks: the casting rule in `look/Personality.js`
disqualifies scenes that do not honour the track's signature traits, so the pool
a given track draws from is smaller than the library. Thin traits (`space`,
`shape`) are worth more than thin families.

## Verify

Three rungs, each about ten times cheaper than the next. Climb them in order.

```bash
npm run lint:scenes   # ~1s, no browser
```

Static gates: schema and shader agreeing both ways, determinism grep, rate flags,
a declared trait with no evidence in the source, a **dead camera**
(`p = sigCamera(p)` and then nothing reads `p`), `prev()` with no base image, and
loops with a large bound and no early break. A loop whose cost is genuinely fixed
— sampling a curve at a set resolution — can say so with a `// lint: fixed-cost`
comment just above it.

```
http://localhost:5180/checks.html?scene=My%20Scene
```

The per-scene acceptance battery for one scene: schema, renders, animates,
deterministic, distinct from every other scene, param sweep, flash rate, and one
line per declared trait proving the image actually responds to it. Ten lines and
a verdict — this is the loop to stay in while writing.

```
http://localhost:5180/checks.html?slow=1
```

Everything. Phases 2, 5 and 7 iterate the registry so a new scene is covered
automatically; Phases 8-10 cover how the look generator uses it. Run this once
before committing.

---

## Making it VARY

This document gets a scene working. Making it look different from one song to
the next is a separate skill with its own measurements, its own failures worth
not repeating, and its own gate — the gallery, with a minimum score of 0.04.

See `HOWTO-variety.md`.
