Paint Swirl

<script src="https://cdn.jsdelivr.net/npm/[email protected]/dist/bundles/anime.umd.min.js"></script>
<script>
  (function () {
    // Paint Swirl: bands of colour swirl, stretch and fold into each other,
    // like paint stirred into water or marbled paper. Drawn small on a canvas
    // behind your content (it's all soft curves, so it scales up cleanly),
    // and paused when off screen.
    // Usage: <section data-paint-swirl>…</section>
    // Optional: data-colors (comma-separated, any number, default navy, coral,
    // cream, teal and violet), data-speed (default 1), data-scale (size of
    // the swirls, default 1).
    var DEFAULTS = { colors: ['#1e3a8a', '#fb7185', '#fef3c7', '#14b8a6', '#7c3aed'], speed: 1, scale: 1 };
    var MAX_PIXELS = 48000;

    var css =
      '[data-paint-swirl]{position:relative;isolation:isolate;overflow:hidden}' +
      '.pns-canvas{position:absolute;inset:0;width:100%;height:100%;z-index:-1;pointer-events:none}';
    var style = document.createElement('style');
    style.textContent = css;
    document.head.appendChild(style);

    var probe = document.createElement('canvas').getContext('2d');
    function rgb(c) {
      probe.fillStyle = '#000';
      probe.fillStyle = c;
      var v = probe.fillStyle;
      if (v[0] === '#') return [parseInt(v.slice(1, 3), 16), parseInt(v.slice(3, 5), 16), parseInt(v.slice(5, 7), 16)];
      return v.match(/[\d.]+/g).slice(0, 3).map(Number);
    }

    // A looping palette of 512 steps: each paint holds its colour for a
    // while, then blends quickly into the next, so the bands stay distinct.
    function palette(colors) {
      var n = colors.length, lut = new Uint8ClampedArray(512 * 3);
      for (var i = 0; i < 512; i++) {
        var p = (i / 512) * n, a = Math.floor(p), f = p - a;
        f = f < 0.55 ? 0 : (f - 0.55) / 0.45;
        f = f * f * (3 - 2 * f);
        var c1 = colors[a % n], c2 = colors[(a + 1) % n];
        for (var k = 0; k < 3; k++) lut[i * 3 + k] = c1[k] + (c2[k] - c1[k]) * f;
      }
      return lut;
    }

    function build(el) {
      var d = el.dataset;
      var colors = (d.colors ? d.colors.split(',') : DEFAULTS.colors).map(function (c) { return rgb(c.trim()); });
      var lut = palette(colors);
      var speed = d.speed !== undefined ? +d.speed : DEFAULTS.speed;
      var scale = +d.scale || DEFAULTS.scale;
      var canvas = document.createElement('canvas');
      canvas.className = 'pns-canvas';
      canvas.setAttribute('aria-hidden', 'true');
      el.insertBefore(canvas, el.firstChild);
      var ctx = canvas.getContext('2d');
      var w = 0, h = 0, step = 3, img = null, t = 5;

      function resize() {
        var cw = Math.max(1, el.clientWidth), ch = Math.max(1, el.clientHeight);
        step = Math.max(3, Math.sqrt((cw * ch) / MAX_PIXELS));
        w = Math.ceil(cw / step);
        h = Math.ceil(ch / step);
        canvas.width = w;
        canvas.height = h;
        img = ctx.createImageData(w, h);
        draw();
      }

      function draw() {
        var data = img.data;
        var S = (170 * scale) / step;   // about 170 css px per unit
        var cx = w / 2, cy = h / 2;
        for (var y = 0; y < h; y++) {
          for (var x = 0; x < w; x++) {
            var px = (x - cx) / S, py = (y - cy) / S;
            // Stir: each pass pushes the point along a wave made by the last,
            // which folds the bands over and over.
            for (var i = 1; i < 5; i++) {
              var nx = px + (0.75 / i) * Math.sin(i * py * 1.1 + t * 0.35 + i * 1.7);
              py = py + (0.75 / i) * Math.cos(i * px * 0.9 - t * 0.28 + i * 0.9);
              px = nx;
            }
            var v = 0.5 * Math.sin(px * 0.9 + py * 0.7 + t * 0.1) + 0.5 * Math.cos(py * 1.3 - px * 0.4);
            var idx = Math.floor(((v * 0.75 + 1.5 + t * 0.015) % 1) * 512) * 3;
            var o = (y * w + x) * 4;
            data[o] = lut[idx];
            data[o + 1] = lut[idx + 1];
            data[o + 2] = lut[idx + 2];
            data[o + 3] = 255;
          }
        }
        ctx.putImageData(img, 0, 0);
      }

      resize();
      if (window.ResizeObserver) new ResizeObserver(resize).observe(el);
      if (window.matchMedia('(prefers-reduced-motion: reduce)').matches) return;

      var last = 0;
      var timer = anime.createTimer({
        loop: true,
        duration: 1000,
        autoplay: false,
        onUpdate: function () {
          var now = performance.now();
          var dt = last ? Math.min(50, now - last) : 16;
          last = now;
          t += (dt / 1000) * speed;
          draw();
        }
      });
      new IntersectionObserver(function (entries) {
        if (entries[0].isIntersecting) { last = 0; timer.play(); } else timer.pause();
      }).observe(el);
    }

    function init() {
      document.querySelectorAll('[data-paint-swirl]').forEach(function (el) {
        if (el._pns) return;
        el._pns = true;
        build(el);
      });
    }

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

<section data-paint-swirl data-colors="#0f172a,#f97316,#fef3c7,#0ea5e9" style="color: #fff">
  <h1>Stir it up</h1>
</section>