Curtain Reveal

<script src="https://cdn.jsdelivr.net/npm/[email protected]/dist/bundles/anime.umd.min.js"></script>
<script>
  (function () {
    // Curtain Reveal: a coloured block sweeps across the text, and as it pulls
    // away the text is left behind, as if it was painted on by the block.
    // Usage: <h1 data-curtain-reveal>Now showing</h1>
    // Optional: data-color (the block's colour; default #a855f7), data-direction
    // (which way the block travels: "right", the default, "left", "up" or
    // "down"), data-loop (the block sweeps back over to take the text away, then
    // reveals it again), data-hold (ms to stay before sweeping away),
    // data-duration (ms for each sweep), data-delay (ms before starting).
    var DEFAULTS = { color: '#a855f7', direction: 'right', duration: 1200, delay: 0, hold: 1800 };

    // The block grows from `from`, then shrinks towards `to`, along `axis`.
    var SIDES = {
      right: { axis: 'scaleX', from: '0% 50%', to: '100% 50%' },
      left: { axis: 'scaleX', from: '100% 50%', to: '0% 50%' },
      down: { axis: 'scaleY', from: '50% 0%', to: '50% 100%' },
      up: { axis: 'scaleY', from: '50% 100%', to: '50% 0%' }
    };

    var css =
      '.crv-box{position:relative;display:inline-block}' +
      '.crv-block{position:absolute;inset:-0.04em -0.1em;border-radius:0.06em;pointer-events:none;will-change:transform}';
    var style = document.createElement('style');
    style.textContent = css;
    document.head.appendChild(style);

    function reduced() {
      return window.matchMedia('(prefers-reduced-motion: reduce)').matches;
    }

    // Wraps the contents in an inline box, so the block hugs the text rather
    // than the full width of a heading, and adds the block on top.
    function build(el) {
      var box = document.createElement('span');
      box.className = 'crv-box';
      var text = document.createElement('span');
      while (el.firstChild) text.appendChild(el.firstChild);
      var block = document.createElement('span');
      block.className = 'crv-block';
      block.setAttribute('aria-hidden', 'true');
      block.style.background = el.dataset.color || DEFAULTS.color;
      box.appendChild(text);
      box.appendChild(block);
      el.appendChild(box);
      return { box: box, text: text, block: block };
    }

    function play(el, parts) {
      var d = el.dataset;
      var loop = d.loop !== undefined && d.loop !== 'false';
      var duration = +d.duration || DEFAULTS.duration;
      var side = SIDES[d.direction] || SIDES[DEFAULTS.direction];
      var cover = {}; cover[side.axis] = [0, 1];
      var uncover = {}; uncover[side.axis] = [1, 0];
      var half = duration / 2;

      function sweep(tl, showText) {
        tl.set(parts.block, { transformOrigin: side.from })
          .add(parts.block, Object.assign({ duration: half, ease: 'inOutQuart' }, cover))
          // Swap what's behind the block while it covers the text completely.
          .set(parts.text, { opacity: showText ? 1 : 0 })
          .set(parts.block, { transformOrigin: side.to })
          .add(parts.block, Object.assign({ duration: half, ease: 'inOutQuart' }, uncover), '+=60');
      }

      var tl = anime.createTimeline({ loop: loop, loopDelay: 400, delay: +d.delay || DEFAULTS.delay });
      sweep(tl, true);
      if (loop) {
        tl.add({ duration: +d.hold || DEFAULTS.hold });
        sweep(tl, false);
      }
    }

    function init() {
      var io = new IntersectionObserver(function (entries) {
        entries.forEach(function (e) {
          if (!e.isIntersecting) return;
          io.unobserve(e.target);
          play(e.target, e.target._crv);
        });
      }, { threshold: 0.4 });
      document.querySelectorAll('[data-curtain-reveal]').forEach(function (el) {
        if (el._crv) return;
        var parts = (el._crv = build(el));
        var axis = (SIDES[el.dataset.direction] || SIDES[DEFAULTS.direction]).axis;
        parts.block.style.transform = axis + '(0)';
        if (reduced()) return;
        parts.text.style.opacity = 0;
        io.observe(el);
      });
    }

    if (document.readyState === 'loading') document.addEventListener('DOMContentLoaded', init);
    else init();
  })();
</script>

<h1 data-curtain-reveal>Curtain up</h1>

<!-- a pink block sweeping down, looping: -->
<h1 data-curtain-reveal data-color="#ec4899" data-direction="down" data-loop>Curtain up</h1>