Webworker OffscreenCanvas 绘制常规图像

Question

网络工作者可以通过XMLHttpRequest获取常规图像，对吗？工作人员如何将这些图像绘制到 OffscreenCanvas？可能想用 XMLHttp.responseType = 'blob'?

另一种方法是设置图像元素的src，然后将元素传输给工作人员，但我的工作人员总是拒绝这样的图像。

谢谢

Answer 1

ImageBitmap API 就是为了这个目的（以及其他目的）。

注意：此演示目前运行仅在 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>

Webworker OffscreenCanvas 绘制常规图像

Webworker OffscreenCanvas draw regular image

javascript

canvas

web-worker