heygen-com--hyperframes/skills/hyperframes-core/references/determinism-rules.md

Determinism, Animation Runtime, and Layout

HyperFrames seeks compositions frame-by-frame. Every frame must be reproducible from its time value alone — same input time → same pixels. Three contracts enforce this: the animation runtime contract, the determinism rules, and the layout contract.

Animation Runtime Contract

GSAP is the primary runtime. The core requirement is generic: animation state must be seekable from HyperFrames time.

For GSAP:

• Create the timeline synchronously during page initialization.
• Use gsap.timeline({ paused: true }).
• Register it on window.__timelines["<composition-id>"].
• The key must match data-composition-id on the composition root.
• Do not call tl.play() for render-critical motion.
• Do not build timelines inside async, Promise, setTimeout, or event handlers — the renderer can sample before they finish.
• Do not create empty tweens only to set duration; use data-duration on the clip instead.
• Do not gsap.set() clip elements from later scenes — they are not in the DOM at page load. Use tl.set(selector, vars, time) inside the timeline at or after the clip's data-start.

Use the hyperframes-animation skill for tween syntax, position parameters, eases, and performance rules.

Duration Contract For Non-GSAP Runtimes

The render engine needs a positive total duration before it will capture a single frame — without one, capture fails outright with "Composition has zero duration." A GSAP timeline supplies this automatically. CSS, WAAPI, and Lottie compositions have no timeline object, so the runtime infers duration itself:

• CSS: longest animation-delay + animation-duration × finite animation-iteration-count across animated elements (offset by each element's data-start). animation-iteration-count: infinite cannot be inferred.
• WAAPI: longest element.animate() effect's getComputedTiming().endTime. Infinite iterations cannot be inferred.
• Lottie: the registered animation's native length (totalFrames / frameRate, or the dotLottie player's own duration) — always finite regardless of loop.
• Three.js: not inferable. The three adapter only forwards time via hf-seek — it has no AnimationClip/AnimationMixer inspection.

data-duration on the root [data-composition-id] element is therefore optional whenever every non-GSAP animation on the page is finite (CSS/WAAPI with finite iteration counts, or Lottie). It is required when: the composition has an infinite/unbounded CSS or WAAPI animation, the composition uses Three.js, or there is no GSAP timeline and no animation signal at all for any adapter to discover. npx hyperframes lint enforces exactly this (root_composition_missing_duration_source) — see the runtime/adapter-specific docs under hyperframes-animation/adapters/ for the full contract per runtime.

Determinism Rules

Rendered frames must be reproducible from the requested time. Do not use any of the following for visual state:

• Date.now(), performance.now(), or any render-time clock.
• Unseeded Math.random(). Use a seeded PRNG if random-looking placement is needed.
• Render-time network fetches for required assets. Inline or pre-bundle them.
• Hover, scroll, pointer, or focus state. The renderer has no input events.
• Infinite loops such as repeat: -1. Compute a finite count: repeat: Math.max(0, Math.floor(duration / cycleDuration) - 1) — floor, not ceil (ceil overshoots data-duration and trips the gsap_repeat_ceil_overshoot lint; max(0, …) avoids a negative repeat = infinite).

Also avoid:

• Animating anything outside the visual-property allowlist: opacity, x, y, scale, rotation, color, backgroundColor, borderRadius, transforms. Never animate display or visibility — use opacity/transforms and timed clip visibility instead.
• Animating the same property on the same element from multiple timelines at the same time — GSAP's overwrite behavior is order-dependent and can flip between renders.

Layout Contract

Build the visible end-state in static HTML and CSS first, then animate from/to that state.

• The composition root has fixed pixel frame dimensions.
• The root composition's total duration (render length / frame count) is fixed at compile time, read once from the static root data-duration before scripts run, like data-width / data-height. A script or --variables value that rewrites the root data-duration afterward is ignored. To vary render length per output, author the root data-duration directly. (A clip's own data-duration is re-read from the live DOM, so scripts/variables can still drive clip lengths. Only when the root omits data-duration does the renderer probe the live DOM / timeline for total length.)
• Scene containers should fill the scene with width: 100%; height: 100%; box-sizing: border-box.
• Use padding, flex, grid, and max-width for layout. Avoid positioning main content with hardcoded top/left offsets when a layout container can do it.
• Use position: absolute for layers and decorative elements, not as the default content-layout strategy.
• Prefer transforms and opacity for animation.
• Keep text inside its intended container. For dynamic text, use max-width, wrapping, or window.__hyperframes.fitTextFontSize(text, { maxWidth, fontFamily, fontWeight }).
• For text measurement without DOM reflow, use window.__hyperframes.pretext: pretext.prepare(text, font) then pretext.layout(prepared, maxWidth, lineHeight). Pure arithmetic, ~0.0002 ms per call — safe for per-frame text reflow, shrinkwrap containers, and computing layout before render. fitTextFontSize is built on it.
• Do not use <br> in body text. Forced breaks ignore the actual rendered font width and produce an extra break when the line already wraps naturally, causing overlap. Let text wrap via max-width. Exception: short display titles where each word is deliberately on its own line.
• Transformed elements must be block-level + sized. transform/scaleX/scaleY is a no-op on an inline <span>, and scaling an auto-width (0px) element shows nothing → invisible bars/fills. Give them display: block/inline-block/flex-item and a real width/height (e.g. width: 100% inside a sized parent). (silent — lint/inspect miss it.)
• Absolutely-positioned decoratives that pulse or overshoot (yoyo scale, back.out) need clearance at their peak size and must not straddle an overflow: hidden edge — else they overlap a neighbor or get clipped. Position for the largest frame, not the resting one. (silent.)

Why This Matters

The renderer takes a time value and produces a pixel buffer. There is no notion of "playback" — every frame is a fresh seek. Any state that depends on having reached this frame through a prior frame (timers, accumulated state, event-driven animations) will desync when the renderer samples out of order or in parallel.

If you find yourself reaching for setTimeout, requestAnimationFrame, or addEventListener to drive a visual, rebuild it as a tween on the timeline instead.