Zoom Through Transition

<script src="https://cdn.jsdelivr.net/npm/[email protected]/dist/bundles/anime.umd.min.js"></script>
<script>
  (function () {
    // Zoom Through Transition: the view zooms in towards you and out into the
    // next. As a page transition, the page rushes past the viewer and fades when
    // a link is clicked, and the next page flies in from the distance.
    // Usage: <body data-zoom-through-transition>   (add the snippet to every page)
    // Optional: data-zoom-through-transition="#hex" (the colour seen behind the
    // page mid-zoom, default #0f0b1a), data-duration (ms for each half,
    // default 600 on a page, 1000 for a whole slideshow change).
    // Contained mode: put data-zoom-through-transition on any other element with
    // two or more children. Every data-interval ms (default 2200) the current
    // child zooms past and the next one arrives from far away.
    var DEFAULTS = { color: '#0f0b1a', duration: 600, slideDuration: 1000, interval: 2200 };
    var KEY = 'animacious-zoom-through';

    var css =
      'html.zth-pre body{opacity:0}' +
      '.zth-contained{position:relative;overflow:hidden;display:grid}' +
      '.zth-contained>*{grid-area:1/1;will-change:transform,opacity}';
    var style = document.createElement('style');
    style.textContent = css;
    document.head.appendChild(style);

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

    // Arriving from a link that used the transition: hide the page before it
    // first paints, so the zoom in starts cleanly. Never for more than 3 s.
    var arriving = false;
    try { arriving = sessionStorage.getItem(KEY) === '1'; sessionStorage.removeItem(KEY); } catch (e) {}
    function unhide() { document.documentElement.classList.remove('zth-pre'); }
    if (arriving && !reduced()) {
      document.documentElement.classList.add('zth-pre');
      setTimeout(unhide, 3000);
    }

    // Tweens t from 0 to 1, calling paint(t) on every frame.
    function tween(duration, ease, paint, done) {
      var s = { t: 0 };
      paint(0);
      return anime.animate(s, {
        t: 1, duration: duration, ease: ease,
        onUpdate: function () { paint(s.t); },
        onComplete: function () { paint(1); if (done) done(); }
      });
    }

    // Same-site links that should get the transition: not new tabs, downloads,
    // other sites or #anchors on this page.
    function linkFor(e) {
      var a = e.target.closest && e.target.closest('a[href]');
      if (!a || e.defaultPrevented || e.button !== 0 || e.metaKey || e.ctrlKey || e.shiftKey || e.altKey) return null;
      if ((a.target && a.target !== '_self') || a.hasAttribute('download')) return null;
      var url = new URL(a.href, location.href);
      if (url.origin !== location.origin) return null;
      if (url.pathname === location.pathname && url.search === location.search && url.hash) return null;
      return url;
    }

    function pageMode(body) {
      var duration = +body.dataset.duration || DEFAULTS.duration;
      var color = body.getAttribute('data-zoom-through-transition') || DEFAULTS.color;
      var html = document.documentElement, saved = null, anim, trip = 0;

      // Leaving: the page grows past you and fades. Arriving: it flies in from
      // far away. The page's backdrop shows the chosen colour meanwhile.
      function run(out, done) {
        if (anim) anim.pause();
        if (saved === null) saved = { bg: html.style.background, ox: html.style.overflowX };
        html.style.background = color;
        html.style.overflowX = 'hidden';
        body.style.transformOrigin = '50% ' + (window.scrollY + innerHeight / 2) + 'px';
        anim = tween(duration, out ? 'in(3)' : 'out(3)', function (t) {
          var k = out ? t : 1 - t;
          body.style.transform = 'scale(' + (out ? 1 + 1.6 * k : 1 - 0.45 * k) + ')';
          body.style.opacity = 1 - k;
        }, function () {
          if (!out) {
            body.style.transform = body.style.opacity = body.style.transformOrigin = '';
            html.style.background = saved.bg;
            html.style.overflowX = saved.ox;
            saved = null;
          }
          if (done) done();
        });
      }
      run(false);

      document.addEventListener('click', function (e) {
        var url = linkFor(e);
        if (!url) return;
        e.preventDefault();
        var t = ++trip, gone = false;
        var go = function () { if (gone || t !== trip) return; gone = true; location.href = url.href; };
        try { sessionStorage.setItem(KEY, '1'); } catch (err) {}
        // Never hold up the visit: go anyway if the zoom stalls or fails.
        setTimeout(go, duration + 400);
        try { run(true, go); } catch (err) { go(); }
      });
      // Back from the next page: the page may be restored mid-zoom.
      window.addEventListener('pageshow', function (e) {
        if (!e.persisted) return;
        trip++;
        try { sessionStorage.removeItem(KEY); } catch (err) {}
        run(false);
      });
    }

    function containedMode(el, still) {
      el.classList.add('zth-contained');
      el.style.background = el.getAttribute('data-zoom-through-transition') || DEFAULTS.color;
      var slides = Array.from(el.children);
      var duration = +el.dataset.duration || DEFAULTS.slideDuration;
      var i = 0;
      slides.forEach(function (s, n) { s.style.visibility = n ? 'hidden' : 'visible'; });
      if (still) return;

      var timer = anime.createTimer({
        duration: (+el.dataset.interval || DEFAULTS.interval) + duration,
        loop: true,
        autoplay: false,
        onLoop: function () {
          if (el.closest('[hidden]')) return;
          var from = slides[i], to = slides[(i + 1) % slides.length];
          from.style.zIndex = 1;
          to.style.zIndex = 0;
          to.style.visibility = 'visible';
          // The two overlap in the middle: the old view is still rushing past
          // as the new one comes into focus.
          tween(duration, 'linear', function (t) {
            var a = Math.min(1, t / 0.6), b = Math.max(0, (t - 0.35) / 0.65);
            a = a * a * a;                       // in(3): speeds up towards you
            b = 1 - Math.pow(1 - b, 3);          // out(3): settles into place
            from.style.transform = 'scale(' + (1 + 1.5 * a) + ')';
            from.style.opacity = 1 - a;
            from.style.filter = 'blur(' + (a * 8).toFixed(2) + 'px)';
            to.style.transform = 'scale(' + (0.45 + 0.55 * b) + ')';
            to.style.opacity = b;
            to.style.filter = b < 1 ? 'blur(' + ((1 - b) * 8).toFixed(2) + 'px)' : '';
          }, function () {
            from.style.visibility = 'hidden';
            from.style.transform = from.style.opacity = from.style.filter = from.style.zIndex = '';
            to.style.transform = to.style.opacity = to.style.filter = to.style.zIndex = '';
            i = (i + 1) % slides.length;
          });
        }
      });
      new IntersectionObserver(function (entries) {
        if (entries[0].isIntersecting) timer.play(); else timer.pause();
      }).observe(el);
    }

    function init() {
      var still = reduced();
      document.querySelectorAll('[data-zoom-through-transition]').forEach(function (el) {
        if (el._zth) return;
        el._zth = true;
        if (el === document.body) { if (!still) pageMode(el); }
        else if (el.children.length > 1) containedMode(el, still);
      });
      unhide();
    }

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

<body data-zoom-through-transition>
  …
</body>