javascript - Webworker OffscreenCanvas draw regular image - Stack Overflow

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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

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

• javascript
• canvas
• web-worker

Share Improve this question Follow asked Jun 12, 2019 at 0:42 ituyituy 24144 silver badges1111 bronze badges

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1 Answer 1

Sorted by: Reset to default 7

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/wikipedia/mons/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>

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