Liquid Wave Transition

<script src="https://cdn.jsdelivr.net/npm/[email protected]/dist/bundles/anime.umd.min.js"></script>
<script>
  (function () {
    // Liquid Wave Transition: a page transition. When a link is clicked a wave
    // of colour rolls up from the bottom of the screen until it covers it; on
    // the next page the wave carries on up and over the top, uncovering it.
    // Usage: <body data-liquid-wave-transition>   (add the snippet to every page)
    // Optional: data-liquid-wave-transition="#hex" (wave colour, default
    // #6a0dad), data-duration (ms for each half, default 900).
    // Contained mode: put data-liquid-wave-transition on any other element with
    // two or more children and a wave rolls over between them every
    // data-interval ms (default 2200).
    var DEFAULTS = { color: '#6a0dad', duration: 900, interval: 2200 };

    var css =
      '.lwt-layer{position:fixed;inset:0;width:100%;height:100%;z-index:2147483000;pointer-events:none;visibility:hidden;display:block}' +
      '.lwt-contained{position:relative;overflow:hidden;display:grid}' +
      '.lwt-contained>*:not(.lwt-layer){grid-area:1/1}' +
      '.lwt-contained>.lwt-layer{position:absolute;z-index:2}';
    var style = document.createElement('style');
    style.textContent = css;
    document.head.appendChild(style);

    function layer(parent, color) {
      var c = document.createElement('canvas');
      c.className = 'lwt-layer';
      c.setAttribute('aria-hidden', 'true');
      parent.appendChild(c);
      var L = { c: c, ctx: c.getContext('2d'), color: color, w: 0, h: 0 };
      fit(L);
      return L;
    }

    function fit(L) {
      var w = L.c.clientWidth, h = L.c.clientHeight, dpr = Math.min(2, window.devicePixelRatio || 1);
      if (L.c.width !== Math.round(w * dpr) || L.c.height !== Math.round(h * dpr)) {
        L.c.width = Math.round(w * dpr);
        L.c.height = Math.round(h * dpr);
      }
      L.ctx.setTransform(dpr, 0, 0, dpr, 0, 0);
      L.w = w;
      L.h = h;
    }

    // Height of a rolling wave edge at x: two swells moving opposite ways.
    function edge(x, w, base, amp, time, shift) {
      return base + amp * Math.sin(x / w * 7 + time * 3 + shift) + amp * 0.45 * Math.sin(x / w * 13 - time * 4.2 + shift * 2);
    }

    // Fill between the top of the box (fromTop) or its bottom and a wavy edge.
    function fill(ctx, w, h, base, amp, time, shift, fromTop) {
      ctx.beginPath();
      ctx.moveTo(0, fromTop ? 0 : h);
      for (var x = 0; x <= w + 6; x += 6) ctx.lineTo(x, edge(x, w, base, amp, time, shift));
      ctx.lineTo(w, fromTop ? 0 : h);
      ctx.closePath();
      ctx.fill();
    }

    function ease(t) { return t < 0.5 ? 2 * t * t : 1 - Math.pow(-2 * t + 2, 2) / 2; }

    // Rising: a paler crest leads the wave up. Leaving: the wave's tail rises
    // after it, the paler wash trailing behind.
    function paint(L, rising, t) {
      var ctx = L.ctx, w = L.w, h = L.h;
      var amp = Math.min(36, Math.max(8, h * 0.045)), gap = Math.max(18, h * 0.08);
      var time = performance.now() / 1000;
      var travel = h + amp * 3 + gap;
      var base = h + amp * 1.5 + gap - ease(t) * travel;
      ctx.clearRect(0, 0, w, h);
      ctx.fillStyle = L.color;
      if (rising) {
        ctx.globalAlpha = 0.4;
        fill(ctx, w, h, base - gap, amp, time, 1.3, false);
        ctx.globalAlpha = 1;
        fill(ctx, w, h, base, amp, time, 0, false);
      } else {
        ctx.globalAlpha = 0.4;
        fill(ctx, w, h, base, amp, time, 1.3, true);
        ctx.globalAlpha = 1;
        fill(ctx, w, h, base - gap, amp, time, 0, true);
      }
    }

    function run(L, rising, duration, done) {
      if (L.anim) L.anim.pause();
      fit(L);
      var s = { t: 0 };
      L.c.style.visibility = 'visible';
      paint(L, rising, 0);
      L.anim = anime.animate(s, {
        t: 1,
        duration: duration,
        ease: 'linear',
        onUpdate: function () { paint(L, rising, s.t); },
        onComplete: function () {
          paint(L, rising, 1);
          if (!rising) L.c.style.visibility = 'hidden';
          if (done) done();
        }
      });
    }

    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 L = layer(body, body.getAttribute('data-liquid-wave-transition') || DEFAULTS.color);
      var trip = 0;
      run(L, false, duration);

      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 {
          run(L, true, duration, go);
          setTimeout(go, duration + 400);
        } catch (err) { go(); }
      });
      window.addEventListener('pageshow', function (e) {
        if (!e.persisted) return;
        trip++;
        run(L, false, duration);
      });
    }

    function containedMode(el, reduced) {
      el.classList.add('lwt-contained');
      var slides = Array.from(el.children);
      var duration = +el.dataset.duration || DEFAULTS.duration;
      var L = layer(el, el.getAttribute('data-liquid-wave-transition') || DEFAULTS.color);
      var i = 0;
      slides.forEach(function (s, n) { s.style.visibility = n ? 'hidden' : 'visible'; });
      if (reduced) return;
      var timer = anime.createTimer({
        duration: (+el.dataset.interval || DEFAULTS.interval) + duration * 2 + 80,
        loop: true,
        autoplay: false,
        onLoop: function () {
          if (el.closest('[hidden]')) return;
          run(L, true, duration, function () {
            slides[i].style.visibility = 'hidden';
            i = (i + 1) % slides.length;
            slides[i].style.visibility = 'visible';
            setTimeout(function () { run(L, false, duration); }, 80);
          });
        }
      });
      new IntersectionObserver(function (entries) {
        if (entries[0].isIntersecting) timer.play(); else timer.pause();
      }).observe(el);
    }

    function init() {
      var reduced = window.matchMedia('(prefers-reduced-motion: reduce)').matches;
      document.querySelectorAll('[data-liquid-wave-transition]').forEach(function (el) {
        if (el._lwt) return;
        el._lwt = true;
        if (el === document.body) { if (!reduced) pageMode(el); }
        else if (el.children.length > 1) containedMode(el, reduced);
      });
    }

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

<body data-liquid-wave-transition>
  …
</body>