# Ink Theater

A deterministic, seek-safe engine for hand-drawn **"moving art"** — a minimalist black-ink-on-white world where a deadpan mascot physically *performs* an abstract idea by operating absurd low-tech contraptions. Built for OpenMontage's **atelier** path and rendered through **HyperFrames** (HTML/SVG/CSS + one paused GSAP timeline → MP4).

Inspired by Ian's `小黑 / Xiaohei` illustration skill (MIT — credit Ian for the technique); this is an original, generic, English, motion-first engine, not a copy.

## Why it exists

The illustration style is simple enough that **the illustration IS the animation** — no diffusion model needed. Vector shapes + math give you the whole thing: free, deterministic, infinitely editable, and the character genuinely acts out the concept. This engine turns the research findings (`memory: project_ink_atelier_animation`, deep-research on vector/physics/metaphor foundations) into reusable primitives.

## The five capabilities (`ink-theater.js`, global `InkTheater`)

| Module | What it does | Key API |
|---|---|---|
| **ink strokes** | Confident hand-drawn lines — variable-width brush ribbons + wobbled centerlines | `inkPath(pts, opt)`, `inkRibbon(pts, {width,taper,seed})` |
| **boil** | Seek-safe hand-drawn line "boil" — steps a `feTurbulence` seed off the timeline (~9fps), NOT SMIL | `boil(turbEl, tl, {duration,fps})` |
| **spring physics** | Closed-form damped-spring eases (anticipation/overshoot/settle) — pure functions of progress, seek-safe | `springEase({stiffness,damping,mass})`, `ease.{settle,overshoot,bouncy,soft}` |
| **rig / IK** | 2D FABRIK inverse kinematics + a riggable mascot whose arms reach a target | `fabrik(lengths,origin,target)`, `mascot({x,y,scale})` → `.reachL/.reachR([x,y])` |
| **contraption grammar** | Parametric composable machine parts | `parts.{crank,gauge,hopper,slot,lever,box}` |

## Determinism (HyperFrames render contract)

Every frame must be reproducible from time alone. This engine obeys that:

- **Closed-form springs** — `springEase` evaluates an analytic damped-oscillator step response, so any progress `p` maps deterministically (no numeric integration, no accumulated state).
- **Seek-safe boil** — driven by a GSAP stepped-seed tween on the timeline, never SMIL / render-time clocks.
- **IK-follow via `onUpdate`** — pose the arm from a target whose position is set by the timeline; GSAP fires `onUpdate` on seek, so it's pure-function-of-time.
- Seeded PRNG (`rng`) for all "random-looking" wobble — no runtime `Math.random`.
- No `repeat:-1` (finite counts only), animate only transforms/opacity/attrs.

## ⚠ The font gotcha (the REAL root cause)

Custom handwriting rendered as **serif** in every render for a long time. The cause was **not** SVG-vs-HTML — it was a **font-subset trap**: grabbing one woff2 from the Google Fonts `css2` API (`grep … | head -1`) returns a single *unicode-range subset* (often cyrillic / vietnamese / latin-ext) that is **missing basic-latin (ASCII)**. So every English word silently falls back to serif — while the renderer still logs `Fonts: 1 loaded`. (This means earlier demos whose captions "looked handwritten" were actually serif.)

**Fix (verified):** embed the **full font file** — the TrueType, or a woff2 that actually covers basic-latin:

```html
@font-face { font-family: "InkHand"; src: url("assets/patrickhand.ttf") format("truetype"); font-display: block; }
```

A working Patrick Hand TTF ships at **`ink-theater/assets/patrickhand.ttf`** (SIL Open Font License — the license ships beside it as `assets/OFL.txt`; see `THIRD_PARTY_NOTICES.md`) — copy it into your project's `assets/` and use `font-family: "InkHand"`. It renders real handwriting on normal **HTML overlay `<div>`s** (verified — put caption divs over the SVG scene). Don't hot-link Google Fonts (a render-time network fetch breaks determinism); a local `@font-face` file is auto-inlined by the compiler at build time.

> Note: HyperFrames also pre-bundles ~18 fonts (none are handwriting) — see `hyperframes-creative/references/typography.md`. For handwriting you must embed your own full font as above.

## Usage in a HyperFrames project

1. Copy `ink-theater.js` into the project root; `<script src="ink-theater.js">` after gsap.
2. Build the scene programmatically into a mount `<g>`, keep node refs.
3. Apply `filter="url(#boil)"` to ink groups; call `InkTheater.boil(...)` once.
4. Captions = HTML overlay divs (see gotcha).
5. Register one `gsap.timeline({paused:true})` on `window.__timelines["<id>"]`.

## Ink Puppet — real mocap on a hand-drawn figure (recommended for characters)

The right way to animate a doodle *character* (walk / dance / wave / jump) is **not** hand-tuned math — it is **real motion-capture retargeted onto a stick figure**. An agent should only choose the character and choreograph named moves; it must never hand-tune motion. Two pieces:

- **`mocap/bvh2clip.mjs`** — offline converter: a 3D BVH mocap file → a compact 2D "clip" (per-frame joint tracks, hips-relative pose + root motion, scaled to a fixed figure height). Run once per motion; bundle clips with `clips.js`.
- **`ink-puppet.js`** — runtime: builds the stick figure, plays clips, exposes a **declarative choreography API**:

```js
var p = InkPuppet.create(mount, { cx: 960, ground: 902, boil: "boil" });
p.drawIn(tl, { start: 0.4 });                         // pencil sketches the figure limb-by-limb
InkPuppet.choreograph(tl, p, [                          // then plays named mocap clips — zero hand-tuning
  { clip: "walk" }, { clip: "dance_spin" }, { clip: "kick" }, { clip: "wave" }
], { start: 3.7 });

// speak — comic balloon tethered to the mouth (HTML text = webfont works)
InkTheater.balloon(tl, { into: fxGroup, overlay: htmlOverlay, at: 5, dur: 2, text: "hello!", boil: "boil" });
```

Deterministic + seek-safe (pose is a pure function of each segment's local time).

**The action library (`mocap/catalog.json`)** ships 12 varied moves the agent picks by name — locomotion (`walk`, `run`, `climb`, `march`, `shuffle`), action (`jump`, `kick`), posture (`sit`), gesture (`wave`), dance (`dance_spin`, `dance_glide`, `twist`). Every clip is CMU-sourced (free for any use). **Read the catalog and pick moves that fit the story — don't loop one clip.**

**Extend it in one command** (self-extending, no code changes) — the converter auto-maps fair1 / CMU / Mixamo skeletons:
```
node mocap/add-motion.mjs backflip 05_20 dance "a backflip"   # CMU id, or a URL, or a local .bvh
```
Free **CMU mocap** (`una-dinosauria/cmu-mocap`) has thousands. This is what Meta's *Animated Drawings* does, but here it stays **vector, white-ink, with a draw-on reveal** (AD is raster, humanoid-only, no reveal). Provenance (all clips CMU, free for any use): `mocap/NOTE.md` · `THIRD_PARTY_NOTICES.md`.

### Speech balloons — `InkTheater.balloon(tl, opts)`
Comic balloon that grows from the mouth, with HTML overlay text (so the webfont applies). `opts`: `into` (an SVG `<g>`), `overlay` (an HTML div), `at`, `dur`, `text`, `mouth:[x,y]`, `center:[x,y]`, `w`, `size`, `boil`.

## Demos

- `examples/mocap-figure/` — the pencil figure draws itself, then walks / runs / dances / kicks / sits / waves via **real CMU mocap**. Self-contained and lintable (`npx hyperframes lint ink-theater/examples/mocap-figure`); see `examples/README.md`.