Fade Through

<!--
  Fade Through - scene transition primitive.

  Put class="hf-fade-through-out" on the outgoing layer and
  class="hf-fade-through-in" on the incoming layer. Add the optional wash
  overlay below when you want a brighter mid-transition frame.

  Variables. The wash is the only surface this primitive paints, so all three
  describe it. The script reads each one, falls back to the declared default on
  anything missing or unrecognised, and writes the result as a custom property
  the CSS above consumes. The timeline recipe below is untouched by them.

    - wash (paper | white | ink | accent, default paper): the colour of the
      mid-transition frame. paper still resolves through
      --hf-fade-through-wash, so an existing override keeps working; accent
      rides --accent.
    - strength (number 0.1 to 1, default 1): how far the wash is allowed to
      cover at its peak. 1 is a full flash, 0.3 a light veil over the cut.
    - shape (flat | bloom, default flat): flat fills the frame evenly, bloom
      concentrates the wash in the centre and falls off to the edges.

  On every default the result is identical to the original: a flat, fully
  opaque #f8f5ec wash.
-->

<div
  class="hf-fade-through-wash"
  aria-hidden="true"
  data-composition-variables='[
    { "id": "wash", "type": "enum", "role": "style", "label": "Wash", "description": "Colour of the mid-transition frame. Paper resolves through --hf-fade-through-wash, accent rides --accent.", "default": "paper", "options": [{ "value": "paper", "label": "Paper" }, { "value": "white", "label": "White" }, { "value": "ink", "label": "Ink" }, { "value": "accent", "label": "Accent" }] },
    { "id": "strength", "type": "number", "role": "style", "label": "Strength", "description": "How far the wash covers at its peak. 1 is a full flash, 0.3 a light veil.", "default": 1, "min": 0.1, "max": 1, "step": 0.05 },
    { "id": "shape", "type": "enum", "role": "style", "label": "Shape", "description": "Flat fills the frame evenly, bloom concentrates the wash in the centre.", "default": "flat", "options": [{ "value": "flat", "label": "Flat" }, { "value": "bloom", "label": "Bloom" }] }
  ]'
></div>

<style>
  .hf-fade-through-in {
    opacity: 0;
  }
  .hf-fade-through-wash {
    position: absolute;
    inset: 0;
    opacity: 0;
    pointer-events: none;
    background: var(--hf-fade-through-paint, var(--hf-fade-through-wash, #f8f5ec));
    filter: opacity(var(--hf-fade-through-strength, 1));
    z-index: 900;
  }
</style>

<script>
  (function () {
    "use strict";

    var vars =
      window.__hyperframes && window.__hyperframes.getVariables
        ? window.__hyperframes.getVariables()
        : {};

    // Unrecognised overrides return to their declared defaults.
    var washes = {
      paper: "var(--hf-fade-through-wash, #f8f5ec)",
      white: "#ffffff",
      ink: "#0b0b0f",
      accent: "var(--accent, #38bdf8)",
    };

    var wash = Object.prototype.hasOwnProperty.call(washes, vars.wash) ? vars.wash : "paper";
    var shape = vars.shape === "bloom" ? "bloom" : "flat";

    var strength = parseFloat(vars.strength);
    if (!isFinite(strength)) strength = 1;
    strength = Math.min(1, Math.max(0.1, strength));

    var colour = washes[wash];
    var paint =
      shape === "bloom"
        ? "radial-gradient(circle at 50% 50%, " + colour + " 0%, transparent 72%)"
        : colour;

    var overlays = document.querySelectorAll(".hf-fade-through-wash");
    for (var i = 0; i < overlays.length; i += 1) {
      overlays[i].style.setProperty("--hf-fade-through-paint", paint);
      overlays[i].style.setProperty("--hf-fade-through-strength", String(strength));
    }
  })();
</script>

<!--
  Timeline integration:

  tl.to(".hf-fade-through-out", { opacity: 0, duration: 0.45, ease: "power2.inOut" }, startTime);
  tl.to(".hf-fade-through-wash", { opacity: 1, duration: 0.35, ease: "power2.inOut" }, startTime);
  tl.to(".hf-fade-through-wash", { opacity: 0, duration: 0.45, ease: "power2.inOut" }, startTime + 0.35);
  tl.to(".hf-fade-through-in", { opacity: 1, duration: 0.55, ease: "power2.out" }, startTime + 0.35);
-->