Ken Burns

<script src="https://cdn.jsdelivr.net/npm/[email protected]/dist/bundles/anime.umd.min.js"></script>
<script>
  (function () {
    // Ken Burns: a slow, cinematic pan and zoom over a still image, the way
    // documentaries bring photographs to life. Each move drifts towards a new
    // spot, zooming in and then back out, for as long as it's on screen.
    // Usage:
    //   <div data-ken-burns style="aspect-ratio: 16 / 9">
    //     <img src="photo.jpg" alt="…">
    //   </div>
    // The first img, svg, picture or video moves; anything else (a caption, a
    // heading) stays still on top. Optional: data-zoom (how far it zooms,
    // default 1.25), data-duration (ms for each move, default 9000),
    // data-from and data-to (focus points such as "30% 40%" to drift between,
    // instead of choosing new ones each time), data-once (one move, then stop).
    var DEFAULTS = { zoom: 1.25, duration: 9000 };

    var css =
      '[data-ken-burns]{position:relative;overflow:hidden}' +
      '.kbn-media{display:block;width:100%;height:100%;object-fit:cover;transform-origin:50% 50%;will-change:transform}';
    var style = document.createElement('style');
    style.textContent = css;
    document.head.appendChild(style);

    // "30% 40%" -> [0.3, 0.4]
    function point(s) {
      if (!s) return null;
      var p = s.trim().split(/\s+/).map(function (v) { return parseFloat(v) / 100; });
      return p.length === 2 && !isNaN(p[0]) && !isNaN(p[1]) ? p : null;
    }

    function setup(el) {
      var media = el.querySelector('img,svg,picture,video');
      if (!media) return;
      if (media.tagName === 'PICTURE') media = media.querySelector('img') || media;
      media.classList.add('kbn-media');
      var reduced = window.matchMedia('(prefers-reduced-motion: reduce)').matches;
      if (reduced) return;
      var d = el.dataset;
      var zoom = Math.max(1.02, +d.zoom || DEFAULTS.zoom);
      var duration = +d.duration || DEFAULTS.duration;
      var fixed = [point(d.from), point(d.to)];
      var zoomIn = true, n = 0, anim = null, visible = false;

      // Where the frame looks: a focus point (0–1 across and down) and a scale.
      // Keeping the translate within (scale - 1) / 2 means no edge ever shows.
      function frame(f, s) {
        return { x: (0.5 - f[0]) * (s - 1) * 100 + '%', y: (0.5 - f[1]) * (s - 1) * 100 + '%', scale: s };
      }

      function pick(prev) {
        if (fixed[0] && fixed[1]) return fixed[n % 2];
        // A new spot, well away from the last one, so it always visibly drifts.
        var f;
        do { f = [0.15 + Math.random() * 0.7, 0.15 + Math.random() * 0.7]; }
        while (prev && Math.hypot(f[0] - prev[0], f[1] - prev[1]) < 0.35);
        return f;
      }

      var at = fixed[0] || [0.5, 0.5];
      // Start a little zoomed, so even the first move has room to pan.
      var scaleAt = 1 + (zoom - 1) * 0.15;
      anime.utils.set(media, frame(at, scaleAt));

      function next() {
        n++;
        var to = pick(at);
        var s = zoomIn ? zoom : 1 + (zoom - 1) * 0.15;
        var from = frame(at, scaleAt), dest = frame(to, s);
        anim = anime.animate(media, {
          x: [from.x, dest.x], y: [from.y, dest.y], scale: [from.scale, dest.scale],
          duration: duration, ease: 'inOut(1.6)', autoplay: visible,
          onComplete: function () {
            at = to;
            scaleAt = s;
            zoomIn = !zoomIn;
            if (d.once === undefined) next();
          }
        });
      }

      new IntersectionObserver(function (entries) {
        visible = entries[0].isIntersecting;
        if (!anim) { if (visible) next(); return; }
        if (visible) anim.play(); else anim.pause();
      }).observe(el);
    }

    function init() {
      document.querySelectorAll('[data-ken-burns]').forEach(function (el) {
        if (el._kbn) return;
        el._kbn = true;
        setup(el);
      });
    }

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

<div data-ken-burns style="aspect-ratio: 16 / 9">
  <img src="photo.jpg" alt="…">
</div>