75Exposure flash

When to use

Use when a screen film cuts between worlds and the cut must read as a camera over-exposing: the frame blows out for 3 frames, the shot swaps inside the peak, and the new shot starts over-bright and settles in 0.15 s. A 0.3 s white div at partial opacity is not this: it is a fade, not a flash.

The code

<div id="host"><div class="shot">...</div><div class="shot">...</div></div>
<div id="white" style="position:absolute;inset:0;background:#fff;opacity:0"></div>
<script type="module">
import '../../core/engine/page-api.js';
import { flashAt } from '../../core/motion/exposure.js';
const CUTS = [0.7, 1.3];
vawe.onFrame((t) => {
  const { white: w, brightness, shot } = flashAt(t, CUTS);   // { fps: 30, frames: 3, peak: 0.75, over: 2, settle: 0.15 }
  shots.forEach((s, i) => s.toggleAttribute('data-on', i === shot));
  host.style.filter = brightness === 1 ? 'none' : `brightness(${brightness})`;
  white.style.opacity = w;
});
</script>

Notes

• The peak is frames / fps seconds centred on the cut: white at 0.75 alpha lifts a mean luma of 60 to about 200. shot counts the cuts passed, so the swap happens in the middle of the peak and the viewer never sees it.
• After the peak the shot is at brightness(2) and eases to 1 on a squared curve over settle s.
• Safety: flashAt flashes at most 3 cuts in any one second (flashCuts). A fourth cut in that second still cuts, without the flash. Keep flashes to a few per film; they are accents.
• Use it once or twice a film, on the strongest beats only; a flash on every cut is a tell. Held for 6 frames or more it is a strobe. Give the outgoing shot a little element motion up to the flash so the last frame before it is not a still. The renderer flags an all-white frame: declare it with <meta name="blank" content="0.52-0.58">.
• A page-built variant with no helper: a white div at opacity 0 -> 1 over 0.04 of 420 ms, held about three frames, then a long fall on EASE.leave, with the shot swap (duration: 1) inside the peak and the new shot starting at brightness(2.4) and scale 1.06, both settling on EASE.land over 0.7 s. The fall is what makes it read as light and not as a white slide.
• Measured on the clip (60 fps): mean luma 27 before the cut, 203 to 208 in the peak, 67 on the first frame after it, then 38 after 0.15 s.

Looks (ground, type, radius, accent) live in demo/demo.css; this snippet keeps neutral tokens.

Sound

Sound: a recorded hit at each cut, or one soft swell that stops dead on the cut (the film’s one swell, on one flash only).