Webworker OffscreenCanvas draw regular image

Question

Webworkers can fetch regular images through XMLHttpRequest, right? How can the workers then draw these images to OffscreenCanvas? Probably want to use XMLHttp.responseType = 'blob'?

Another way would be to set the src of an image element and then transfer the element to worker, but my workers always reject such images.

Thanks

Answer 1

The ImageBitmap API is here for this purpose (among others).

Note: this demo will currently run only on Chrome...

const offcanvas = canvas.transferControlToOffscreen();
const worker = new Worker(getWorkerURL());
worker.onmessage = e => console.log(e.data);
worker.postMessage(offcanvas, [offcanvas]);


function getWorkerURL() {
  return URL.createObjectURL(
    new Blob([
      worker_script.textContent
    ])
  );
}

<canvas id="canvas" height="450"></canvas>

<script id="worker_script" type="ws">
onmessage = async (evt) => {
  try {
    const canvas = evt.data;
    const ctx = canvas.getContext('2d');
    if(!ctx) {
      postMessage('unsupported browser');
      return;
    }
    const imgblob = await fetch('https://upload.wikimedia.org/wikipedia/commons/5/55/John_William_Waterhouse_A_Mermaid.jpg')
      .then(r => r.blob());
    const img = await createImageBitmap(imgblob);
    ctx.drawImage(img, 0,0, canvas.width, canvas.height);
 }
 catch(e) {
  postMessage('unsupported browser');
  throw e;
  }
};
</script>