threejs/threejs-offscreencanvas.html

<!-- Licensed under a BSD license. See license.html for license -->
<!DOCTYPE html>
<html>
  <head>
    <meta charset="utf-8">
    <meta name="viewport" content="width=device-width, initial-scale=1.0, user-scalable=yes">
    <title>Three.js - OffscreenCanvas</title>
    <style>
    html, body {
        height: 100%;
        margin: 0;
    }
    #c {
        width: 100%;
        height: 100%;
        display: block;
    }
    #noOffscreenCanvas {
        display: flex;
        width: 100%;
        height: 100%;
        align-items: center;
        justify-content: center;
        background: red;
        color: white;
    }
    </style>
  </head>
  <body>
    <canvas id="c"></canvas>
    <div id="noOffscreenCanvas" style="display:none;">
      <div>no OffscreenCanvas support</div>
    </div>
  </body>
<script type="module">
function main() {  /* eslint consistent-return: 0 */
  const canvas = document.querySelector('#c');
  if (!canvas.transferControlToOffscreen) {
    canvas.style.display = 'none';
    document.querySelector('#noOffscreenCanvas').style.display = '';
    return;
  }
  const offscreen = canvas.transferControlToOffscreen();
  const worker = new Worker('offscreencanvas-cubes.js', {type: 'module'});
  worker.postMessage({type: 'main', canvas: offscreen}, [offscreen]);

  function sendSize() {
    worker.postMessage({
      type: 'size',
      width: canvas.clientWidth,
      height: canvas.clientHeight,
    });
  }

  window.addEventListener('resize', sendSize);
  sendSize();
}
main();

</script>
</html>