Media stream video frame I420A to I420 converter

mediastream-I420A-to-I420-converter.js

// Based on  https://web.dev/mediastreamtrack-insertable-media-processing/
// Uses Webcodecs API, which is supported only in Chrome as of November 2021

function convertI420AFrameToI420Frame(frame) {
  const { width, height } = frame.codedRect;
  // Y, U, V, Alpha values are stored sequentially. Take only YUV values
  const buffer = new Uint8Array(width * height * 3);
  frame.copyTo(buffer, { rect: frame.codedRect });
  const init = {
    timestamp: 0,
    codedWidth: width,
    codedHeight: height,
    format: "I420"
  };
  // eslint-disable-next-line no-undef
  return new VideoFrame(buffer, init);
}

function transform(stream) {
  const videoTrack = stream.getVideoTracks()[0];

  const trackProcessor = new MediaStreamTrackProcessor({
    track: videoTrack
  });
  const trackGenerator = new MediaStreamTrackGenerator({ kind: "video" });

  const transformer = new TransformStream({
    async transform(videoFrame, controller) {
      const newFrame = convertI420AFrameToI420Frame(videoFrame);
      videoFrame.close();
      controller.enqueue(newFrame);
    }
  });

  trackProcessor.readable
    .pipeThrough(transformer)
    .pipeTo(trackGenerator.writable);

  const processedStream = new MediaStream();
  processedStream.addTrack(trackGenerator);
  return processedStream;
}