<script>
  if ('paintWorklet' in CSS && 'registerProperty' in CSS && 'CSSUnitValue' in window) {
    CSS.registerProperty({
      name: '--dot-spacing',
      syntax: '<length>',
      initialValue: '20px'
    });
    CSS.registerProperty({
      name: '--dot-fade-offset',
      syntax: '<percentage>',
      initialValue: '0%'
    });
    CSS.registerProperty({
      name: '--dot-color',
      syntax: '<color>',
      initialValue: '#fff'
    });

    // load this CodePen's JS file as the paint worklet
    CSS.paintWorklet.addModule('/lonekorean/pen/aYoJPv.js');
  } else {
    document.querySelector('html').classList.add('no-support');
  }
</script>

<div class="polka-dot"></div>

<div class="warning">
  <i class="fas fa-exclamation-triangle"></i><br>
  This Houdini demo requires the <strong>CSS Paint API</strong>, <strong>CSS Properties and Values API</strong>, and <strong>Typed OM</strong>.<br>
  Use <strong>Chrome</strong> and go to <strong>chrome://flags</strong> and enable <strong>Experimental Web Platform features</strong>.<br>
  Also make sure you're serving over <strong>https</strong> or <strong>localhost</strong>.
</div>

*, *::before, *::after {
  box-sizing: border-box;
}

html, body {
  height: 100%;
}

body {
  margin: 0;
  background: #000;
  font: 1rem/1.25 'Mukta Mahee', sans-serif;
}

.polka-dot {
  --dot-spacing: 20px;
  --dot-fade-offset: 0%;
  --dot-color: #40e0d0;
  background-image: paint(polka-dot-fade);

  position: absolute;
  top: 40px;
  right: 40px;
  bottom: 40px;
  left: 40px;
}

.warning {
  display: none;
  position: absolute;
  top: 10px;
  left: 10px;
  right: 10px;
  padding: 20px;
  border-top: 4px dashed #e8590c;
  background-color: #ffe8cc;
  color: #e8590c;
  text-align: center;
}

.no-support .warning {
  display: block;
}

.warning .fas {
  margin-bottom: 1rem;
  font-size: 1.5rem;
}

/*
This CodePen's JS file has been "hijacked" to serve as the paint worklet file. The if check below ensures the code only runs when referenced as such. JavaScript for the page is at the top of the HTML. I'm only doing this to squeeze everything into one CodePen.

IMPORTANT: If you fork, update the CodePen JS file path in the HTML.

ALSO IMPORTANT: If you edit code here, disable cache and reload. Live preview won't pick up changes in the paint worklet.
*/

if (typeof registerPaint !== 'undefined') {
  class PolkaDotFadePainter {
    static get inputProperties() {
      return ['--dot-spacing', '--dot-fade-offset', '--dot-color'];
    }

    paint(ctx, size, props) {
      let spacing = props.get('--dot-spacing').value;
      let fadeOffset = props.get('--dot-fade-offset').value;
      let color = props.get('--dot-color').toString();

      ctx.fillStyle = color;
      for (let y = 0; y < size.height + spacing; y += spacing) {
        for (let x = 0; x < size.width + spacing; x += spacing * 2) {
          // every other row shifts x to create staggered dots
          let staggerX = x + ((y / spacing) % 2 === 1 ? spacing : 0);

          // calculate dot radius based on horizontal position and fade offset
          let fadeRelativeX = staggerX - size.width * fadeOffset / 100;
          let radius = spacing * Math.max(Math.min(1 - fadeRelativeX / size.width, 1), 0);

          // draw dot
          ctx.beginPath();
          ctx.arc(staggerX, y, radius, 0, 2 * Math.PI);
          ctx.fill();
        }
      }
    }
  }

  registerPaint('polka-dot-fade', PolkaDotFadePainter);
}