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.

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 repeat count from the visible duration: repeat: Math.ceil(duration / cycleDuration) - 1.

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.
• 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.

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.