framefields-compose

framefields-compose

Setup

Import everything from the framefields npm package. If the project doesn't have framefields set up yet, follow the framefields skill first (install, project scaffold, first render).

Overview

framefields is a code-first, deterministic video composition SDK executing natively on WebGPU (Dawn) in Node.js. It replaces browser-based rendering (Puppeteer/Chromium) with high-performance GPU pipelines, Yoga flexbox layout, Slug vector typography, 3D camera transforms, and reactive audio/signal drivers.

Every composition is defined as a tree of layout nodes compiled into a deterministic execution program: (composition, frame) → pixels.

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Architecture & Core Concepts

1. Composition Lifecycle

A Composition instance defines canvas dimensions, frame rate, background, registered fonts, global audio tracks, and the layer hierarchy:

import { Composition, Layer, FontManager } from "framefields";

// Always register fonts before assembling layers
await FontManager.register({
  family: "Inter",
  source: "assets/fonts/Inter.ttf"
});

const comp = new Composition({
  width: 1920,
  height: 1080,
  fps: 30,
  durationMs: 10000, // or computeDuration()
  backgroundColor: "#0E0D0C",
  fonts: ["assets/fonts/Inter.ttf"],
});

comp.add(Layer.box({ /* ... */ }));
comp.add(Layer.audio("assets/score.mp3", { volume: 1 }));

2. Scene Architecture (Production Pattern)

For commercials, films, and multi-shot videos, break the project into discrete, modular scene functions using scene-local frames:

• Tokens first (theme.ts): Centralize color palette, typography, easing curves, and a tempo-locked beat grid (BEAT = Math.round(fps * 60 / BPM)). Never use arbitrary margins or ad-hoc hex codes.
• Scene modules (scenes/hero.ts): Each scene returns a full-frame root box with startFrame and durationFrames. Children position themselves relative to scene start (frame: 0 = scene onset).
• Master assembly (film.ts): Imports scenes, mounts them to comp.add(scene()), attaches the master soundtrack and composition-level grading/grain.

// theme.ts
export const W = 1920;
export const H = 1080;
export const FPS = 30;
export const BEAT = 18; // 100 BPM at 30 fps
export const beats = (n: number) => Math.round(n * BEAT);

export const INK = "#0E0D0C";
export const PAPER = "#EEE9E0";
export const EMBER = "#FF5A1F";

export function scene(options: { id: string; from: number; to: number; children: unknown[] }) {
  return Layer.box({
    id: options.id,
    position: "absolute",
    x: 0,
    y: 0,
    width: W,
    height: H,
    startFrame: options.from,
    durationFrames: options.to - options.from,
    children: options.children as never,
  });
}

---

Layer Primitives (Layer.*)

All Layer.* builders return AnimatableNode<T> instances with chainable .animate(), .apply(), and effect methods.

Builder	Node kind	Description & Key Options
Layer.text(content, options)	"text"	Slug GPU vector text. Options: fontSize, fontFamily, fill, letterSpacing, lineHeight, align, direction, animators, spans. Arabic, Hebrew and Persian are shaped and laid out right to left automatically (direction: "auto"); set "rtl"/"ltr" to force it. align: "start"/"end" follow the direction.
Layer.box(options)	"box"	Rectangular container / card. Options: background, borderRadius, borderColor, borderWidth, overflow: "hidden", padding, children.
Layer.flex(options)	"flex"	Auto-layout container (Yoga). Options: dir: "row" | "column", gap, justify, align, padding, wrap, children.
Layer.video(src, options)	"media"	Video layer decoded via mediabunny. Options: fit: "cover" | "contain", trimStartSec, muted, volume.
Layer.image(src, options)	"media"	Static raster image (.png, .jpg, .webp). Options: fit: "cover" | "contain". For vector animations (.json, .lottie), use Layer.lottie.
Layer.svg(src, options)	"media"	Vector SVG asset. Options: fit: "contain".
Layer.lottie(src, options)	"media"	Lottie vector animation file (.json, .lottie). Decoded and rendered via ThorVG. Options: fit: "contain" | "cover" | "fill", width, height.
Layer.audio(src, options)	"media"	Audio track layer. Options: volume, startFrame, durationFrames.
Layer.shape(type, options)	"shape"	Parametric vector graphics ("rect", "circle", "ellipse", "polygon", "star", "arrow", "path"). Options: fill (a color or CSS gradient), d (SVG path data with arcs; keyframe it to morph shapes), strokeWidth, strokeColor, trimStart, trimEnd.
Layer.particles(options)	"media"	GPU particle system (see GPU Particles).
Layer.vfx(options)	"media"	Procedural full-frame VFX: smoke, fire, rain, snow, aurora… (see Procedural VFX).
Layer.caption(src, options)	"media"	SRT-driven synchronized subtitle engine. Bottom-anchored auto-wrapping cues.
Layer.chart(options, props)	"box"	Charts built from native nodes (d3 scales and geometry). type: "line" | "area" | "bar" | "scatter" | "candlestick" | "pie" | "donut". Options: width, height, series: [{ name, data, color, type, curve, pointRadius }] or data, categories, stacked, xAxis/yAxis (min, max, ticks, grid, format: a d3-format string like "$,.0f"), legend, valueLabels, fontFamily, fontSize, colors, animate: { start, duration, stagger, ease } | false. props positions the chart box. Children get ids <id>-bar-<series>-<i>, <id>-line-<series>, <id>-slice-<i>. Keep scatter charts to a few hundred points: each point is a node.
Layer.camera(options)	"camera"	3D scene camera with FOV, lens rack focus, and orbital coordinates.
Layer.light(options)	"light"	3D lighting primitive (ambient, directional, point, spot). Directional and spot take castShadow: true (one per scene), shadowSoftness, shadowDarkness.
Layer.environment(preset, options)	"light"	The 3D scene's world: studio image-based light, exposure, tone mapping, fog, bloom. Adding one switches the scene to physically based shading. See 3D Look below.
Layer.sequence(entries, options)	"box"	Scenes back to back with Transitions between them; returns a box whose totalFrames is the length. See Scenes & Transitions.
Layer.model(options)	"model3d"	3D mesh loader (.gltf, .glb, .obj, .fbx, .stl, .ply). Supports skinning and audio deformation compute shaders.
Layer3D.roundedBox / sphere / cylinder(options)	"model3d"	Procedural solid meshes, centered on x/y/z (y is down, so "top" is -y). roundedBox: width, height, depth, radius, segments, taper (top face scale, <1 for a keycap). sphere: radius. cylinder: radius or radiusTop/radiusBottom, height, openEnded. color is baked in; tint multiplies it and animates as the color prop. Surface: roughness, metallic, emissive, emissiveIntensity, castShadow. Equal shapes share one GPU buffer, so hundreds of copies are cheap.

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Animation System (LayerAnimation)

Create animations using LayerAnimation.create() or fluent factory methods. Animations are composed of keyframed tracks or procedural drivers.

1. Keyframe Transitions (fromTo)

import { LayerAnimation } from "framefields";

// Fluent chain
const anim = LayerAnimation.create()
  .fadeIn(0, 20, "power2.out")
  .fromTo("y", 60, 0, { start: 0, end: 24, ease: "expo.out" })
  .fromTo("scale", 0.95, 1.0, { start: 0, end: 30, ease: "power3.out" });

// Easing bases: none, power1, power2, power3, sine, circ, expo, back, elastic, bounce, spring, cubic, hold
// Directions: in, out, inOut (e.g. "expo.out", "back.out(1.7)", "power2.inOut")

For more than two keyframes on one prop, list them with keys: each tuple is [frame, value, ease?], where the ease shapes the motion into that keyframe. Frames are rounded, so beat math (at + BEAT / 2) can go straight in:

const pop = LayerAnimation.create()
  .keys("scale", [[0, 0], [10, 1.12, "back.out(1.6)"], [16, 1, "power2.out"]])
  .keys("opacity", [[0, 0], [6, 1]]);
// Static form: LayerAnimation.keys("rotation", [[0, -8], [20, 0, "expo.out"]])

Keyframes past the layer's durationFrames never play; the render warns (animation_truncated) and renderVideo({ qa: true }) reports it.

2. Kinetic Typography (TextAnimator)

Text animators operate at the GPU glyph instancing level, cascading offsets across characters, words, or lines:

import { Layer, LayerAnimation, TextAnimator } from "framefields";

// Letter-by-letter wave rise with perspective tilt and blur
const headlineNode = Layer.text("PRECISION MOTION", {
  fontSize: 96,
  fontFamily: "Inter",
  fill: "#FFFFFF",
  animators: [
    TextAnimator.waveRise({
      y: 40,
      rotationX: 45,
      opacity: 0,
      blur: 10,
      easing: "expo.in"
    })
  ]
}).animate(
  // Sweep the selection offset across the text string
  LayerAnimation.create().kineticSweep(-1, 1, 0, 24, "power2.inOut")
);

Presets available: TextAnimator.waveRise(), TextAnimator.wordPop(), TextAnimator.blurIn(), TextAnimator.flip3D(), TextAnimator.colorSweep().

3. 3D Spatial Transforms & Camera Motion

Layers can be transformed in 3D perspective space:

const card = Layer.box({
  width: 600,
  height: 400,
  background: "#181715",
  borderRadius: 24,
  transformStyle: "preserve-3d",
  perspective: 1200,
}).animate(
  LayerAnimation.create()
    .fromTo("rotateX", 25, 0, { start: 0, end: 30, ease: "expo.out" })
    .fromTo("rotateY", -35, 0, { start: 0, end: 30, ease: "expo.out" })
    .fromTo("translateZ", -200, 0, { start: 0, end: 30, ease: "expo.out" })
);

For full 3D camera staging, use CameraAnimation:

import { CameraAnimation } from "framefields";

const camAnim = CameraAnimation.camera()
  .orbit({
    azimuth: { from: -45, to: 15 },
    elevation: { from: 10, to: 25 },
    radius: { from: 2000, to: 1400 },
    start: 0,
    end: 90,
    ease: "sine.inOut"
  })
  .rackFocus({
    fromDistance: 2000,
    toDistance: 1400,
    fromFStop: 1.8,
    toFStop: 4.0,
    start: 20,
    end: 60
  });

4. Procedural & Audio-Reactive Drivers

Properties can be bound to Perlin wiggles, harmonic springs, or audio-extracted signals:

// Organic floating drift
const floatAnim = LayerAnimation.create()
  .wiggle("x", { frequency: 1.5, amplitude: 25 })
  .wiggle("y", { frequency: 1.2, amplitude: 20 });

// Physics spring overshoot
const springAnim = LayerAnimation.create()
  .spring("scale", { damping: 14, stiffness: 190, mass: 1 });

// Audio reactivity: pass the signal object straight in…
const kick = Signal.fromArray(kickEnvelope, fps); // or Signal.builder({ type: "custom", fn })
const beatAnim = LayerAnimation.create()
  .signal("scale", kick, { multiplier: 0.12, offset: 1.0, smoothing: 2 });

// …or register it once by name and bind the name (handy when many layers share it)
comp.addSignal("drum_stem_signal", kick);
const beatAnim2 = LayerAnimation.create()
  .signal("opacity", "drum_stem_signal", { multiplier: 0.5, offset: 0.5 });

.signal() / .colorSignal() drive the prop with value × multiplier + offset every frame. A signal object is carried on its track and registered with the program automatically (under a generated inline_signal_N id, so the spec stays plain JSON); a string is looked up in comp.addSignal names. Anything else (a number, undefined) throws. Signals are sampled at the composition frame (ctx.frame), not the layer's local frame.

---

Color, Gradients & Text Metrics

Every color prop accepts hex, rgb(), hsl(), oklch(), oklab() and CSS names. Color keyframes blend perceptually (OKLab), so red → green passes through yellow, not mud. Color builds colors without hand-rolled math; every helper returns a hex string:

import { Color, Gradient } from "framefields";

Color.oklch(0.72, 0.16, 250);          // equal L = equal perceived brightness: balanced palettes
Color.mix("#ff5a1f", "#1f6bff", 0.5);  // perceptual mix
const wave = Color.ramp(["#3b82f6", "#8b5cf6", "#ec4899"]); wave(0.3);
Color.steps(["#0E0D0C", "#FF5A1F"], 5); // 5 swatches
Color.lighten(EMBER, 0.1); Color.alpha("#000", 0.4); Color.readableOn(bg);
Color.contrast(fg, bg);                 // WCAG ratio; ≥ 4.5 body text, ≥ 3 large

Gradients are CSS strings, usable in any background (boxes, flex, cards; rounded corners kept) and any shape fill. Linear, radial and conic, up to 8 stops, hard stops (red 0 50%, blue 0), transparent fades keep their hue:

Layer.box({ background: "linear-gradient(135deg, #ff5a1f, #ffb800 60%, #7c3aed)" });
Layer.shape("circle", { fill: Gradient.radial(["#fff", "transparent"]) });
Layer.box({ background: Gradient.conic(["red", "yellow", "lime", "aqua", "blue", "magenta", "red"]) });

Measure text with the real font instead of guessing advances (place carets, underline a word, size a pill):

const m = FontManager.measure("Hello, world", { fontFamily: "Inter", fontSize: 64 });
m.width; m.height; m.lines; m.caretX(5); m.glyphs[0].x; // px, relative to the text box

---

Composing Animations

.animate() layers onto what a node already has: call it several times, or pass several animations. Keyframe tracks on the same prop merge into one sorted track; a later key wins on a shared frame. Author behaviours separately and combine them:

const rainIn = LayerAnimation.keys("y", [[0, -300], [d, -300], [d + 12, 0, "back.out(1.5)"]]);
const press  = LayerAnimation.keys("y", [[40, 0], [42, 6, "power2.in"], [47, 0, "power2.out"]]);
const glow   = LayerAnimation.keys("color", [[42, GLOW], [60, REST, "sine.inOut"]]);
key.animate(rainIn).animate(press, glow);   // or LayerAnimation.merge(rainIn, press, glow)

Stepped timing ("on twos")

LayerAnimation.stepped(n) holds every animated value of the layer for n frames: 2 is "on twos", the choppy, hand-made feel of cel animation, stop motion and paper cut-outs. It applies to all of the layer's tracks (keyframes keep their eases; signals and wiggles step too). Mix stepped characters over a smooth camera or background for the classic look.

puppet.animate(walkCycle, bob, LayerAnimation.stepped(2));

Shape morphing

A path shape's d is animatable: keyframe two or more SVG paths and they morph. Any paths work (different point counts, curves, arcs, several subpaths); they are resampled and lined up so the morph does not twist. Eases apply as usual.

Layer.shape("path", { position: "absolute", x: 0, y: 0, width: 400, height: 400, d: BLOB_A, fillColor: "#3B82F6" })
  .animate(LayerAnimation.keys("d", [[0, BLOB_A], [24, STAR, "back.out(1.4)"], [48, HEART, "expo.inOut"]]));

For liquid merges between separate shapes, draw them into one SVG or particle layer and add Gooey (blur + alpha threshold = metaballs).

---

Blend Modes

Any layer takes blendMode, which sets how it mixes with what is drawn beneath it. Use it on glows, textures, light leaks, grain, color washes and type over footage.

Mode	Use it for
"normal" (default)	Plain over.
"screen", "lighter" (add), "color-dodge", "lighten"	Light: glows, flares, light leaks, fire/sparks footage shot on black.
"multiply", "color-burn", "darken"	Shade: paper and grunge textures, shadows, ink on paper.
"overlay", "soft-light", "hard-light"	Contrast textures, film grain, color washes that keep the image's shading.
"difference", "exclusion"	Inverting type over video, glitchy flashes.
"hue", "saturation", "color", "luminosity"	Recolour footage with a flat color layer ("color" = tint, keeps the light).
"mask-in" / "mask-out", "luma-in" / "luma-out", "source-*", "destination-*", "xor"	Compositing operators: keep the base where this layer is opaque / transparent / bright / dark. Prefer withMatte for reveals.

Layer.video("assets/light-leak.mp4", { position: "absolute", x: 0, y: 0, width: W, height: H, fit: "cover", blendMode: "screen", opacity: 0.8 })
Layer.box({ position: "absolute", x: 0, y: 0, width: W, height: H, background: "#ff3d7f", blendMode: "color", opacity: 0.6 })   // duotone-ish tint

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Track Mattes (masks, wipes, reveals)

node.withMatte(matteLayer, mode) shows node only where the matte is opaque ("alpha", default), transparent ("alpha-inverted"), bright ("luma") or dark ("luma-inverted"). The matte is laid out like a sibling (same parent; absolute), keyed from the node's start, and covers the node's box when it has no x/y/width/height. It is never drawn by itself. On a 3D layer (or model) the matte works in screen space inside the 3D pass, so the layer still sorts by depth and its hidden part does not block what is behind it; a matte with its own z is projected like any 3D layer.

// Text revealed by a sliding bar
Layer.text("LAUNCH", { id: "title", position: "absolute", x: 560, y: 480, width: 800, height: 120, fontSize: 120 })
  .withMatte(
    Layer.box({ position: "absolute", x: 560, y: 480, width: 800, height: 120, background: "#fff" })
      .animate(LayerAnimation.keys("x", [[0, -240], [24, 560, "expo.out"]])),
  );

// Video inside type: the text is the matte
Layer.video("assets/surf.mp4", { position: "absolute", x: 0, y: 0, width: W, height: H, fit: "cover" })
  .withMatte(Layer.text("OCEAN", { position: "absolute", x: 0, y: 340, width: W, fontSize: 380, align: "center", fill: "#fff" }));

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Scenes & Transitions

Layer.sequence places full-frame scenes end to end, overlapping each pair by its transition, and writes the scenes' startFrame / durationFrames for you (each scene keys from its own frame 0). Transitions are ordinary keyframes and mattes merged into the scenes:

import { Layer, Transition } from "framefields";

const film = Layer.sequence([
  { scene: hero(), duration: beats(8) },
  { scene: features(), duration: beats(12), transition: Transition.push("left", 18) },
  { scene: proof(), duration: beats(8), transition: Transition.wipe("right", 16) },
  { scene: outro(), duration: beats(6), transition: Transition.iris(20) },
], { width: W, height: H });
comp.add(film);                       // film.totalFrames → composition length

Transition	Look
cut()	hard cut
crossfade(frames)	new scene fades in over the old
fadeThrough(frames)	dip to the background color and back
push(dir, frames, ease)	new scene pushes the old one out
slide(dir, frames)	new scene slides over the old (which dims)
zoom(frames)	old scene grows and fades, new one settles in
whip(dir, frames)	fast motion-blurred push
wipe(dir, frames)	hard edge sweeps across (matte)
iris(frames, { at })	circle opens from a point (matte)

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3D Look: Environment, Materials, Shadows, Bloom

A scene without Layer.environment keeps the classic Blinn-Phong look. Add one and the whole 3D pass becomes physically based: linear-light GGX shading where roughness and metallic behave like in Blender, a procedural studio environment that lights and reflects off everything, exposure, tone mapping, and optional fog and bloom.

Fastest path to a great product shot: a lighting rig. It returns an environment plus placed key/rim(/fill) lights, the key casting a soft shadow:

import { Lighting } from "framefields";

comp.add(Lighting.studio({ target: [W / 2, H / 2, 0] }));   // also: soft, sunset, dramatic
comp.add(Lighting.dramatic({ target: [W / 2, H / 2, 0], bloom: true, fog: { color: BG, start: 2500, end: 6000 } }));
// knobs: intensity, exposure (stops), rotation (turns the rig), shadowSoftness, shadows: false

Or by hand:

comp.add([
  Layer.environment("studio", {        // presets: studio | soft | sunset | night | neutral
    exposure: 0.3,                     // stops
    toneMapping: "neutral",            // default; "agx" filmic, "aces" punchy, "none"
    rotation: 30,                      // turn the reflections
    fog: { color: BG, start: 2000, end: 5000 },   // surfaces fade into the background color
    bloom: true,                       // or { intensity: 0.3, threshold: 1.6, radius: 6 }
  }),
  Layer.directionalLight({ x: 400, y: -1200, z: -600, targetX: 960, targetY: 540, castShadow: true }),
]);

Layer3D.sphere({ radius: 90, color: "#e8e8e8", metallic: 1, roughness: 0.15 });  // chrome
Layer3D.roundedBox({ ..., color: "#ff5a1f", roughness: 0.45 });                  // satin plastic
Layer3D.cylinder({ ..., color: "#111", emissive: "#4da3ff", emissiveIntensity: 3 }); // neon tube

• Tone mapping "neutral" (default) keeps brand colors as authored: orange stays orange. Use "agx" for a filmic roll-off.
• Bloom only takes light well past white (emissive ≳ 3, hot speculars), so ordinary bright surfaces don't glow. Raise emissiveIntensity to make something glow.
• Shadows: one castShadow directional or spot light per scene. Meshes cast by default (castShadow: false opts out); flat layers (planes, cards) cast only with castShadow: true. Everything receives.
• Fog fades to fog.color exactly, so set it to the composition background for a seamless horizon.
• Models: glTF materials bring base color, normal, metallic-roughness, occlusion and emissive maps; OBJ (map_Kd, norm in the .mtl) and FBX bring diffuse and normal maps, looked up next to the model. Textures must be PNG, JPEG, WebP, GIF, TIFF or AVIF: TGA, PSD, DDS and KTX2 are skipped with a warning (convert them). Draco / meshopt-compressed glTF files fail to load: re-export without mesh compression. Vertex colors (VOX, PLY, OFF, glTF COLOR_0) multiply the base color.
• Model orientation: the world is y-down, so a y-up model (most glTF, OBJ and FBX files) needs rotateX: 180 to stand upright.
• emissive / emissiveIntensity / exposure animate like any prop (.keys("emissiveIntensity", …)).
• Glass: material: "glass" | "acrylic" | "frosted" on any Layer3D primitive, extruded text (Layer3D.extrudedText) or model refracts whatever is behind it on screen — the video, 2D layers, particles — with a rainbow fringe (dispersion, default 0.04), bend strength thickness (px, default 80) and ior (1.5). color tints the glass (deeper toward the edges); frosted blurs by roughness. Glass needs no environment: it draws its own studio reflections, and against a dark grade it reads best — skip Lighting.* rigs for glass-only scenes (their environment washes glass out). Glass shows only 2D content and opaque 3D drawn before it, not other glass.

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Physics (Physics.world2d / Physics.world3d)

Use for anything that should fall, bounce, stack, swing or collide: gravity drops of words or cards, UI pieces tumbling into a pile, a "jiggle" on a spring, balls rattling in a box, dominoes, physics-driven 3D products. Use keyframes instead for motion that should hit an exact designed pose at an exact frame.

A world runs once while the composition is built (Rapier, deterministic build: identical on every machine) and replays as per-frame signals, so frames render in any order. simulate() binds each body's layer (x, y, rotation; in 3D x/y/z, rotateX/Y/Z):

import { Layer, LayerAnimation, Physics } from "framefields";

const world = await Physics.world2d({ fps: FPS, frames: 150 }); // gravity 2400 px/s², y down
world.walls({ width: W, height: H - 160 });                       // floor + sides (ceiling: true to close)

const cards = words.map((word, i) => {
  const card = Layer.box({ position: "absolute", x: 140, y: -300 - i * 260, width: 560, height: 170, rotation: -8, borderRadius: 28, background: EMBER /* … */ });
  world.body(card, { restitution: 0.35, startFrame: i * 6 });   // staggered drop
  return card;
});
world.simulate();

// Every body exposes signals; bind them to anything (props, effect uniforms):
glow.animate(LayerAnimation.create().signal("opacity", world.impact, { multiplier: 1.5 })); // loudest hit
stage.animate(LayerAnimation.create().signal("y", world.impact, { multiplier: 14 }));       // camera shake

• Bodies come from layers with numeric x, y and a size; the layer's pose (incl. rotation, scale, anchorX/Y) is the start pose. Colliders follow the layer: circles are balls, borderRadius rounds the box, polygons / stars / paths use their convex hull, text without a size is measured. Override with shape: "box" | "circle" | "hull" and size: [w, h]. Plain { x, y, width, height } makes an invisible obstacle.
• Body options: type: "dynamic" | "fixed" | "kinematic", restitution (0.3), friction (0.6), density (1), velocity: [vx, vy] px/s, angularVelocity °/s, linearDamping, angularDamping, gravityScale (0 floats), lockRotation, ccd (very fast bodies), sensor, startFrame (holds its pose until then), output: "signals" | "keyframes" | "none".
• Signals per body: x, y, rotation, centerX, centerY, vx, vy, speed, angularVelocity, sleeping, impact (jumps on each new contact, 1 ≈ a 1500 px/s hit, decays over impactDecay frames). world.impact is the loudest body each frame. body.collisions / world.collisions list first contacts { frame, with, speed }: place hit sounds or flashes at those frames.
• Scene: world.ground({ y }), world.walls({ x, y, width, height, ceiling }).
• Joints: world.pin(body, [x, y]) (swing around a point), world.hinge(a, b, [x, y]), world.weld(a, b), world.spring(a, bOrPoint, { frequency: 2, dampingRatio: 0.2, length }), world.rope(a, bOrPoint, { length }). Points are world px.
• Driving the simulation: world.force(body, { x, y }) (px/s², mass-independent), world.impulse(body, frame, [vx, vy]) (a kick), kinematic follow: { x, y, rotation }. Each of these takes a number, a signal (audio, Signal.fromArray) or (frame) => value: e.g. a floor that bounces on the kick.
• Force fields: world.field((s) => [ax, ay], { bodies }) is called every solver step with a body's center s.x, s.y (px), velocity s.vx, s.vy and s.frame; return px/s² or nothing. Wind, vortices, attractors, flow fields — and performers (below).
• Performers in the simulation: const dancer = comp.vision("dancer.mp4") gives the clip's pose and silhouette per frame, analysed on demand (see framefields-effects). Make the world live so it steps as frames render instead of all at once: world.live(comp) in place of simulate(). Before each frame draws, the clip's vision for that frame is computed (or read from the cache) and the world advances to it — nothing is analysed up front, the preview plays at once and fills in, and export gives the identical result (same simulation, same order). Then, with const person = dancer.person.place({ width: W, height: H, fit: "cover" }):
  • limbs as kinematic colliders that catch and bat things: world.body({ x, y, width: 2 * r, height: 2 * r }, { type: "kinematic", shape: "circle", follow: { x: (f) => person.point("leftWrist", f).x - r, y: (f) => person.point("leftWrist", f).y - r } }), a few balls along each shoulder → elbow → wrist;
  • the silhouette (dress included) as a force field: person.sample(frame, x, y) returns alpha (inside > 0.5) and a gradient gx, gy pointing inward, so world.field((s) => { const m = person.sample(s.frame, s.x, s.y); … return [-m.gx * k, -m.gy * k]; }) makes particles and props flow around a moving body.
  • Reads are synchronous and need the frame analysed: inside a live world, while rendering, or after await dancer.ready(frame) (e.g. ready(0) to lay things out around the first pose). A live world can't look ahead, so place things from early frames.
  • Layer a vision cutout of the same clip (Layer.video(src).withVision({ mode: "matte" })) above the particles so they pass behind the performer.
  • Live worlds bind signals only (no bakeKeyframes()); await world.advanceTo(frame) steps one without rendering (e.g. to read collisions).
• Particles: for clouds, dust, sparks, fluids and flow fields use Layer.particles (GPU, below), not hundreds of physics bodies. Keep physics bodies for a few dozen things that must collide and stack (cards, letters, products).
• Keyframes instead of signals: world.bakeKeyframes({ tolerance: 0.25 }) in place of simulate() writes plain, simplified linear keyframes (editable, plain JSON).
• 3D: Physics.world3d takes Layer3D.roundedBox / sphere / cylinder and models (convex hull of the mesh); same options with 3-vectors, world.hinge(a, b, at, axis), world.walls({ width, height, depth }).
• Timing: signals are on the composition clock. In a scene that starts at frame S, pass startFrame: S to the world. Before it and after frames the signals hold their first/last value. Add every body, joint and force before simulate() (or before reading any signal): the world then locks.
• Feel: gravity 2400 px/s² reads snappy on a 1080p canvas; pixelsPerMeter (100) scales how heavy things feel. Raise substeps (4) for tall stacks or thin, fast bodies.
• Install note: physics uses the optional dependency @dimforge/rapier2d-deterministic-compat / rapier3d-deterministic-compat (installed by default with framefields).

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GPU Particles (Layer.particles)

Tens of thousands of particles simulated in a compute shader and drawn in one call — sparks, embers, dust, snow, smoke, ink, flow fields, light painting, a fluid a performer moves through. A particle layer is a box in composition px; positions inside it are layer px.

comp.add(Layer.particles({
  width: W, height: H, max: 20000, seed: 7,
  emitter: { shape: "rect", x: 0, y: H * 0.6, width: W, height: 200, rate: 3000 },  // point | line | rect | circle | ring; radial
  lifetime: [4, 6], speed: [0, 40], direction: -90, spread: 360,
  gravity: 20, drag: 1.5,                                      // px/s², per second
  noise: { strength: 260, scale: 170, speed: 0.25 },          // curl-noise flow: smoke, ink, flow fields
  attractors: [{ x: W / 2, y: H / 2, strength: 900, radius: 200 }],   // negative strength repels
  push: { person, strength: 9000, sweep: 120000 },             // a performer's silhouette pushes them (comp.vision)
  floor: H - 60, bounce: 0.2, bounds: "none",                  // none | kill | bounce | wrap
  size: [10, 4], sizeJitter: 0.5, fade: [0.1, 0.3],
  shape: "circle",                                             // circle | soft (glow) | square | spark (streaks along motion)
  color: ["#ffd23f", "#ff5a1f", "#7a2e14"],                    // over life, or by speed with colorBySpeed: 600
  blend: "normal",                                             // add for glows and light painting
  prewarm: 3,                                                  // seconds simulated before frame 0: starts full
}));

• emitter.x / emitter.y, rate, gravity, drag, noise.strength, attractor positions/strengths and push strengths take a number, a signal (audio, physics impact) or (frame) => value, read every frame: bursts on the beat, wind gusts, an attractor following a tracked hand (person.point("rightWrist", f)). emitter.bursts: [{ frame, count }] adds one-off explosions.
• Recipes: light painting an additive "soft" point emitter whose x/y follow a tracked wrist ((f) => person.point("rightWrist", f).x), lifetime ~1 s, small speed, births spread along the hand's path so fast moves stay continuous; sparks/fountain shape: "spark", blend: "add", gravity, floor; smoke/ink shape: "soft", low opacity, noise, sizes growing; snow/dust rect emitter above the frame, small gravity, noise, bounds: "wrap"; swirl ring emitter + attractor + colorBySpeed; fluid around a dancer push: { person } + drag 2 + gentle noise, and a vision cutout of the clip above the particle layer.
• The system steps as frames render and snapshots itself every 24 frames, so scrubbing back restores instead of replaying from the start; the same GPU gives the same frames.

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Procedural VFX (Layer.vfx)

Full-frame procedural looks drawn by one shader, transparent where there is no effect. Stack them above or below footage, and set a blendMode (usually "screen") for light effects.

Layer.vfx({ effectType: "Aurora", width: W, height: H, colorStart: "#2bff88", colorEnd: "#7a3cff", density: 0.7, speed: 1 })

effectType: Smoke, Fire, Rain, Sparks, Snow, Dust, Lightning, Magic, LensFlare, Embers, EnergyBeam, Aurora. Shared knobs: density, scale, speed, intensity and seed (numbers or signals), plus colorStart / colorEnd. For particles that react to physics, a performer or attractors, use Layer.particles instead.

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Critical Authoring Rules & Gotchas

1. Local Keyframe Coordinates: Keyframes are strictly relative to the node's own timeline window (0 = node appearance). Never offset keyframe frames by the parent's startFrame.
2. Coordinate Overrides: Animating y overrides the node's layout y. If a node is placed at y: 400, animate fromTo("y", 440, 400), NOT fromTo("y", 40, 0).
3. No String Styles for Borders: Use borderColor and borderWidth. CSS border: "1px solid red" is silently ignored.
4. Font Registration is Mandatory: In headless WebGPU, system fonts do not exist. Every font must be registered via FontManager.register() and listed in comp.fonts. Emoji need a color emoji font too: framefields ships none, so register one with FontManager.registerEmojiFont("assets/fonts/NotoColorEmoji.ttf") (a CBDT/CBLC bitmap font such as Noto Color Emoji). Without it exports draw nothing for emoji.
5. Integer Frames Only: Keyframe frames, start frames, and durations must be integers. Decimal frames cause schema validation errors.
6. Relative vs. Absolute: Providing x or y without position: "relative" turns the node into an absolute-positioned layer removed from flex flow.
7. Audio Layer Duration: Every Layer.audio() must be assigned durationFrames to prevent audio truncation in headless rendering.
8. Premultiplied Alpha: Alpha shaders operate with premultiplied alpha. Never use "src-alpha" factor in custom blends.
9. Lint before rendering: comp.lint() lists colors/gradients that would draw transparent, low-contrast text, off-canvas layers, shadow settings that do nothing and roughness/metallic without an environment. renderVideo({ qa: true }) includes these findings.
10. Lottie vs. Image Layers: Use Layer.lottie("file.json" | "file.lottie", { width, height, fit }) for Lottie vector animations. Never pass .json or .lottie to Layer.image(), which only decodes raster formats and throws/warns if given a vector animation.
11. AI-generated media are assets, not build steps: Images, clips, music or voice made with ai (from framefields) are generated once by a separate script into assets/generated/ and loaded like any other file. Never call ai from buildFilm() or a scene function: it would spend money and change the film on every render, preview and test. The framefields skill (section 6) covers the key, the script and the model catalog.