Paper Drift In

<script src="https://cdn.jsdelivr.net/npm/[email protected]/dist/bundles/anime.umd.min.js"></script>
<script>
  (function () {
    // Paper Drift In: the element drifts down into place like a falling sheet
    // of paper, swinging side to side, tipping up at the end of each swing
    // and settling flat as it lands.
    // Usage: <div data-paper-drift-in>…</div>
    // Optional: data-distance (how far above its place it starts, in px,
    // default 140), data-sway (how far it swings to each side, in px, default
    // 46), data-swings (how many swings it makes, default 3), data-tilt (how
    // far it tips at the end of a swing, in degrees, default 14),
    // data-duration (ms, default 2200), data-delay (ms before the first
    // entrance), data-loop (hold, fade away and drift in again, starting from
    // the other side), data-hold (ms to stay when looping, default 1400).
    var DEFAULTS = { distance: 140, sway: 46, swings: 3, tilt: 14, duration: 2200, delay: 0, hold: 1400 };

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

    function num(v, def) {
      return v === undefined || v === '' || isNaN(+v) ? def : +v;
    }

    // Where it is at time p (0 to 1): a pendulum swing that dies away while
    // the sheet sinks, lifting a little and tipping at the end of each swing.
    function pose(o, p) {
      var phase = p * o.swings * Math.PI;
      var fade = Math.pow(1 - p, 1.3);
      var s = Math.sin(phase) * o.side;
      return {
        x: o.sway * s * fade,
        y: -o.distance * Math.pow(1 - p, 1.4) - o.sway * 0.22 * s * s * fade,
        rotate: -o.tilt * s * fade
      };
    }

    function enter(el, side, done) {
      var d = el.dataset;
      var t = num(d.duration, DEFAULTS.duration) || DEFAULTS.duration;
      var o = {
        distance: num(d.distance, DEFAULTS.distance),
        sway: num(d.sway, DEFAULTS.sway),
        swings: Math.max(0.5, num(d.swings, DEFAULTS.swings)),
        tilt: num(d.tilt, DEFAULTS.tilt),
        side: side
      };
      var clock = { p: 0 };
      var start = pose(o, 0);
      anime.utils.set(el, { x: start.x, y: start.y, rotate: start.rotate, opacity: 0 });
      anime.animate(clock, {
        p: 1,
        duration: t,
        ease: 'linear',
        onUpdate: function () {
          var q = pose(o, clock.p);
          anime.utils.set(el, { x: q.x, y: q.y, rotate: q.rotate, opacity: Math.min(1, clock.p / 0.22) });
        },
        onComplete: function () {
          anime.utils.set(el, { x: 0, y: 0, rotate: 0, opacity: 1 });
          done();
        }
      });
    }

    function cycle(el) {
      var d = el.dataset;
      var s = el._pdi;
      enter(el, s.n++ % 2 ? -1 : 1, function () {
        if (d.loop === undefined || d.loop === 'false') return;
        setTimeout(function () {
          anime.animate(el, {
            opacity: 0,
            duration: 400,
            ease: 'out(2)',
            onComplete: function () {
              // Wait until it's back on screen before the next entrance.
              setTimeout(function () {
                if (s.visible) cycle(el);
                else s.waiting = true;
              }, 500);
            }
          });
        }, num(d.hold, DEFAULTS.hold));
      });
    }

    function init() {
      var io = new IntersectionObserver(function (entries) {
        entries.forEach(function (e) {
          var s = e.target._pdi;
          s.visible = e.isIntersecting;
          if (!e.isIntersecting) return;
          if (!s.started) {
            s.started = true;
            setTimeout(function () { cycle(e.target); }, num(e.target.dataset.delay, DEFAULTS.delay));
          } else if (s.waiting) {
            s.waiting = false;
            cycle(e.target);
          }
        });
      }, { threshold: 0.2 });
      document.querySelectorAll('[data-paper-drift-in]').forEach(function (el) {
        if (el._pdi) return;
        el._pdi = { n: 0 };
        if (reduced()) return;
        anime.utils.set(el, { opacity: 0 });
        io.observe(el);
      });
    }

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

<div class="card" data-paper-drift-in>Letter received</div>

<!-- or a longer, lazier fall: -->
<div class="card" data-paper-drift-in data-distance="240" data-swings="4" data-duration="3200">Letter received</div>