# How to build a visualizer that varies

`HOWTO-visualizers.md` covers making a scene that *works*. This one covers making
a scene that looks different from one song to the next — a separate skill, and
the one the library is currently worst at.

Everything here is a measurement rather than an opinion, and the numbers are
quoted so a future change can contradict them. Several already contradict things
believed earlier in the same week. Living document: add what you learn, and
delete what stops being true.

---

## The one-line version

> A scene varies when the SONG can change what is on screen. It does not vary
> because its parameters moved.

Parameters mostly change the same picture. Content changes the picture.

---

## How to know if you succeeded

```
gallery.html          your scene, six times, on six songs — with a score
checks.html?scene=X   the per-scene gate, including `consumes:` lines
```

The gallery score is the mean structural distance between a scene's own six
frames, on the same descriptor the variety harness uses. High means the song
changes this scene a lot; low means it looks like itself wherever it is cast.

**There is no bar.** There used to be one — 0.1, drawn across the gallery as a
red line — and it was removed when composition arrived. A section is no longer
one scene: it is a ground, a shot standing on it, and sometimes a pass over
that, so what a viewer sees is a COMBINATION. A scene that is reliably itself is
a perfectly good ingredient in one, and the old line failed it for being
consistent.

What replaced it is a level up. The interesting quantity is how unalike the
scenes in one stack are, and the generator now weights its choices by exactly
that — see `structuralDistance` in `scenes/surface.js`. Your scene does not have
to be varied on its own. It has to be unlike the things it will be stacked with,
and that is measured for you.

Some reference points, measured across six songs:

```
0.324  Droste Feedback      varies a lot
0.082  Plasma Bloom
0.062  Voronoi Shatter      after a focus
0.062  Apollonian Gasket
0.047  Moiré Grid           after a subject; 0.031 before
```

### The measured metadata

`src/scenes/metadata.json` is generated, tracked in git, and holds every
measured fact about every scene: coverage, variety, per-block scores and the
structural profile the distance above is computed from. Nothing in it is
hand-written, and two decisions read it:

* **`surface`** — canvas at 50% coverage or more, composable below. It used to
  be declared per scene and it drifted; nine scenes claimed `canvas` while
  painting under a third of the frame. Declaring `surface:` in a scene file is
  now an error.
* **which scenes may be a GROUND** — a canvas that does not call `prev()`, fills the frame
  when sampled as a bed, and does not vanish under any of the six identities. Coverage is
  measured twice, once as a shot and once as a bed, because a scene paints wildly different
  amounts at different parameters: Metaballs is 69% as a drop's shot and 6% as an intro's
  ground.

Refresh it from **gallery.html → refresh metadata** after changing a shader or a
metric. The file carries a fingerprint of the scenes, the shader contract, the
identities and palettes, and the descriptor definitions; when that stops
matching, phase 12 fails and tells you to re-measure. The write goes through a
dev-only endpoint in `vite.config.js`, so it only works under `npm run dev`.

---

## What the descriptor can and cannot see

Design against this, because half of "why is my score low" is here.

**It is blind to:**

- **Brightness and contrast.** Frames are standardised to zero mean and unit
  variance before anything is measured. A scene that only gets brighter has not
  changed.
- **Colour.** Measured, reported, and excluded from the score. Six palettes
  cannot disguise one image, which was the entire point.
- **Rotation**, on the scale, orientation and texture blocks.
- **Quality.** It measures change between songs, nothing else. Apollonian Gasket
  scores 0.039 and looks great. Both facts are true and neither implies the
  other.

**It sees:**

| block | what it measures | how to move it |
|---|---|---|
| `scale` | feature size — fine grain vs large forms | let the song set element size (`stageScale()`) |
| `orient` | grid vs radial vs stripes, rotation-blind | break or vary a regular lattice |
| `layout` | where in the frame structure sits | move the subject, change what is empty |
| `texture` | element count, sparsity, mirror and radial symmetry | vary how many things there are |
| `region` | how differently the parts of the frame behave from each other | give the frame a subject, so one region is unlike its neighbours |
| `motion` | what moves and where, not how much | vary the KIND of movement, not the rate |

**`region` is the block that rewards having something to look at.** Layout says
where the energy is, normalised, so a uniform field and a field with a subject in
it can normalise to nearly the same answer. Region characterises each part of the
frame in its own right — detail, direction, element count — and reports how far
each deviates from the frame's average. A pattern spread evenly deviates by
nothing everywhere, which is exactly why there is nothing to watch.

Measured when it was added: Apollonian Gasket 0.039 to 0.062 and Plasma Bloom to
0.082, both on region alone, because both have a subject the old blocks were not
crediting. It is the strongest single block in the descriptor.

**A known blind spot.** `layout` is nearly useless for full-frame fields. Voronoi
Shatter measures 0.004 there no matter what changes, because edge-to-edge cells
occupy the same frame however they fall. That is honest — there genuinely is no
arrangement — but it means a whole family is judged on four blocks instead of
five. If your scene fills the frame corner to corner, expect to earn your score
on `orient` and `texture`.

---

## What has actually worked

Ordered by measured effect.

### 1. Draw the song's content instead of your own

The largest single lever. Take `consumes: ['cast', 'ink', 'staging']` and draw
`castMain`/`castChorus` where you would have drawn your own primitive. Recipe in
`MIGRATION.md`.

Measured: identity is worth 117% of what the container is worth
(`checks.html?decompose=1`) — swapping the song's cast under a fixed scene moves
the picture more than swapping the scene under a fixed cast. A migrated library
scene also carries the identity better than a stage written from scratch to
carry it, which was a surprise and is why the whole library was migrated rather
than replaced.

### 1b. Give the subject a third dimension — payoff still unmeasured

`castSolid` / `castChorusSolid` / `castMarch`, and `consumes: ['form']`. The
song's protagonist as an assembly of solids instead of an outline, so its
silhouette CHANGES as the shot moves rather than merely rotating. Recipe in
`MIGRATION.md`, including the two guards that keep many instances affordable.

Three scenes carry it — Effigy, Floating Geometry, Swarm — which is 34.8% of
videos containing at least one, against 11.9% when only Effigy had it. That is
reach, not payoff.

The payoff is still unmeasured: the variety report has not been re-run against
a library with these in it, so nothing here says the videos are more varied.
What is measured is narrower — across four seeds, the lit area of Effigy's
subject varies 14-113% over one turn against Soloist's 3-51% for the same
rotation, which says the outline genuinely changes rather than merely spinning.
Whether that reaches the variety blocks is the open question, and it is the next
thing to run. Do not migrate a field scene onto it hoping for a win.

### 2. Let the song decide element SIZE

`stageScale()`. Not the size of your features relative to each other — the size
of the whole vocabulary. A song of six huge forms and a song of four hundred
tiny ones are different videos before anything else is decided.

Measured: adding `elementScale` to the identity moved stages from
+0.0111 ±0.0043 to +0.0143 ±0.0008, and the `scale` block from 0.033 to 0.041.

### 3. Displace a regular lattice

If your scene is a grid, let the song push things off it. Bounded — past about
half a cell the structure the grid was providing stops reading.

Measured: Isometric Blocks 0.034 → 0.049, with `orient` nearly doubling from
0.093 to 0.164 as the rigid lattice softened.

### 4. Give a full-frame field somewhere to be about

`focusWarp(p, amount)` and `focusField(p)`. The song picks one to three focal
points; tile in the warped coordinate so cells crowd toward them or pull away.

Measured: Voronoi Shatter 0.0463 → 0.0588. **Note what moved:** `orient` 0.140 →
0.194, while `layout` went 0.004 → 0.007, i.e. nowhere. Warping where cells sit
changes what they look like without changing where the frame's energy is. The
gain is real; the stated reason for it was wrong.

---

## What has measurably NOT worked

Recorded because the failures were more informative than the wins, and because
each of these looked obviously right beforehand.

### Effects as parameters

Forty-nine scenes had their own `glow`; every one had its own grain. An effect
knob sampled per song makes a scene look varied across parameter draws while its
structure never moves — it inflates the score without changing the picture, and
it fights the grade, which already does bloom, grain, chroma and vignette with
an envelope the scene cannot see.

**Do not add glow, bloom, grain, haze, chroma or trails to a scene.** The post
chain owns them. `npm run lint:scenes` does not catch this yet; reviewers should.

### Concentrating disturbance instead of adding it

Isometric Blocks, twice. Spending the scatter budget near the focal points and
calming the rest measured 0.049 → 0.0374 — most of the field went back onto the
rigid lattice and took the orientation variety with it. Making the focus
additive instead recovered nothing: 0.0365.

Two plausible diagnoses, both wrong. The lesson is narrow and worth keeping:
**a focal point has to be something the field gains, not something the rest of it
pays for** — and even that framing did not rescue it, so the real cause is still
unknown.

### Making the roster smaller

A whole afternoon went into the theory that a track drawing on fewer scenes
would look more like itself. Swept directly across pool sizes 4, 8, 16 and 32
over twelve songs with three draws each: differences of 0.003 to 0.007 against a
run-to-run noise of ±0.003 to ±0.005. **Nothing.** The theory came from an
experiment whose arms differed in two ways at once.

### Reading a form as a metric rather than drawing it

Spectrum Sculpture used `sigShape` as a radial distance rather than as a subject.
Migrating it to `castMain` rendered pure black, because the cast carries notches
and a hollow and an annulus used as a radius turns a sculpture inside out. If
your scene consults a form's geometry rather than drawing it, it wants
`sigShape`, and the cast is not for you.

---

## Scene shapes, and how hard each is to vary

- **Element-placing** (`metaballs`, `floating-geometry`, `firefly-drift`) —
  easiest. Take the cast, place on `stageNode`, keep your motion. Nearly all the
  wins above are this shape.
- **Single figure** (`slow-orb`, `eclipse-field`) — take the cast and
  `stageScale()`. Composition is yours to vary; most such scenes centre their
  subject and never move it, which is free `layout` left on the table.
- **Full-frame field** (`voronoi-shatter`, `plasma-bloom`, `curl-flow`) — hard.
  `layout` is unavailable to you. Earn it on `orient` and `texture`, and consider
  a focus.
- **Fixed geometry** (`truchet-fold`, `quasicrystal`, `apollonian-gasket`) —
  hardest, and currently unsolved. The image *is* the maths, so the cast cannot
  be pasted on top of it. The open idea is to make the cast the repeating UNIT
  the tiling is built from, which is a real rewrite per scene rather than a
  recipe step. Nobody has tried it yet.
- **Treatment** (`analog-wow`, `scan-tear`, `halftone-misprint`) — these are
  effects wearing a scene's clothes. They score 0.031–0.040 and their whole
  migration was one posterisation. They probably belong in the identity's
  effects register rather than competing for screen time as subjects.

---

## Trusting the numbers

This harness has two classes of measurement and only one is safe to steer by.

**Direct render comparisons** — the gallery score, `checks.html?decompose=1`, the
per-scene gate — compare rendered frames to each other. Noise floor of exactly
zero. Trust these.

**Aggregate ratios** — chiefly `separation`, which divides one small difference
by another — swing wildly. Three runs of the same post-migration measurement gave
0.31, 0.56 and 0.07. **Four conclusions were drawn and withdrawn during this epic
for exactly that reason.** Anything under about 0.01 of spread needs
`&repeats=3` before it is believed, and a difference that changes sign with the
sample size is not a difference.

Two more traps, both of which have bitten:

- **A score of exactly 0.000 is a dead shader, not a boring scene.** The gallery
  now says so outright — the row goes purple and carries the luminance and
  variance — but the underlying trap is general: a broken render produces the
  most boring possible numbers rather than an error.
- **A permanently-zero block looks like a property of your scene.** The gallery
  built four of the descriptor's five blocks for weeks; `motion` came back 0.000
  for every scene and depressed every score by a fifth. If a block is identical
  across many unrelated scenes, suspect the instrument.
- **Arms that differ in more than one way cannot be read as if they differed in
  one.** This produced the roster-size result above, and it was believed for a
  day.

---

## Checklist for a new or reworked scene

1. `consumes` declared, and the `consumes:` lines pass in
   `checks.html?scene=<name>`. A declared artifact that is ignored is worse than
   one not declared, because the casting code believes it.
2. No glow, bloom, grain, haze, chroma or trail parameters.
3. Element size goes through `stageScale()`.
4. Placement goes through `stageNode()` unless the composition IS the scene.
5. Gallery score above 0.1, checked before and after.
6. The rest of the per-scene battery still passes — especially `distinct`. The
   more scenes share a cast, the easier it is to become one of your neighbours.

---

## Open questions

- **Coupling is zero and has never moved.** Whether a song *looks* different in
  proportion to how it *sounds* different measures -0.02 ±0.03. Every change in
  this epic left it there. Nobody has attacked it directly.
- **Fixed-geometry scenes** have no known route to variety.
- **The `layout` blindness** for full-frame fields would need a new descriptor
  block measuring cell statistics rather than where structure sits.
- **The bar is a target, not a description.** Most of the library does not clear
  0.1 — that is the point of it. It has moved from 0.04 to 0.05 to 0.1
  as the descriptor gained a block and lost a bug, both of which raised every
  score. Expect to move it again whenever the descriptor changes, and re-read it
  off a fresh gallery rather than carrying the old number forward.
