有没有办法将视频数据从视频标签/媒体流发送到 OffscreenCanvas？

Question

基本上我希望能够有效地执行相同的代码：

const video = document.getElementById('video');
const canvas = document.getElementById('canvas');
const context = canvas.getContext('2d');

const draw = () => {
  context.drawImage(video, 0, 0);
  requestAnimationFrame(draw);
}

video.onplay = () => {
  requestAnimationFrame(draw);
}

仅使用屏幕外画布。我可以通过消息向屏幕外画布所在的工作人员发送图像，但不能发送视频，因为它直接绑定到HTMLElement. 目前有没有办法以某种方式仍然MediaStream在屏幕外画布中渲染视频数据或 a ？

Answer 1

您可以通过使用以下更改修改脚本，将视频帧发送到 Web Worker 中的 OffscreenCanvas：

const worker = new Worker('my-worker.js');
const video = document.getElementById('video');
const stream = video.captureStream();
const [track] = stream.getVideoTracks();
const imageCapture = new ImageCapture(track);
const canvas = document.getElementById('canvas');
const offscreen = canvas.transferControlToOffscreen();

worker.postMessage({ offscreen }, [offscreen]);

const draw = () => {
  imageCapture.grabFrame().then(imageBitmap => {
    worker.postMessage({ imageBitmap }, [imageBitmap]);
  });

  requestAnimationFrame(draw);
};

video.onplay = () => {
  requestAnimationFrame(draw);
};

my-worker.js

let canvas;
let context;

addEventListener('message', event => {
  if (event.data.offscreen) {
    canvas = event.data.offscreen;
    context = canvas.getContext('2d');
  } else if (event.data.imageBitmap && context) {
    context.drawImage(event.data.imageBitmap, 0, 0);
    // do something with frame
  }
});

参考

• HTMLMediaElement.prototype.captureStream()
• MediaStream.prototype.getVideoTracks()
• new ImageCapture()
• ImageCapture.prototype.grabFrame()