can not render alpha video with canvas

[gecaitao]

muxer = new WebMMuxer.Muxer({
target: new WebMMuxer.ArrayBufferTarget(),

video: {
  codec: 'V_VP9',
  width: canvas.width,
  height: canvas.height,
  alpha: true
},
audio: undefined,
fastStart: 'in-memory',
firstTimestampBehavior: 'offset'

});

videoEncoder = new VideoEncoder({
output: (chunk, meta) => muxer.addVideoChunk(chunk, meta),
error: e => console.error(e)
});
videoEncoder.configure({
codec: 'vp09.00.10.08',
width: canvas.width,
height: canvas.height,
bitrate: 1e6,
});
encodeVideoFrame();
intervalId = setInterval(encodeVideoFrame, 1000 / 30);

const encodeVideoFrame = () => {
const canvas = document.querySelector('canvas');
let elapsedTime = document.timeline.currentTime - startTime;
let frame = new VideoFrame(canvas, {
timestamp: framesGenerated * 1e6 / 30 // Ensure equally-spaced frames every 1/30th of a second
});
framesGenerated++;
// Ensure a video key frame at least every 10 seconds for good scrubbing
let needsKeyFrame = elapsedTime - lastKeyFrame >= 10000;
if (needsKeyFrame) lastKeyFrame = elapsedTime;

videoEncoder.encode(frame, { keyFrame: needsKeyFrame });
frame.close();

};

[Vanilagy]

Yes, not supported by VideoEncoder, unfortunately.