Article WebRTC two-way transceiver replaceTrack

HTML

<b>Original canvas</b><br>
<canvas id="canvas" width="100" height="100"></canvas>
<script>
var canvas = document.getElementById("canvas");
var ctx = canvas.getContext("2d");
var radius = canvas.height / 2;
ctx.translate(radius, radius);
radius = radius * 0.90
setInterval(drawClock, 1000);

function drawClock() {
  drawFace(ctx, radius);
  drawNumbers(ctx, radius);
  drawTime(ctx, radius);
}

function drawFace(ctx, radius) {
  var grad;
  ctx.beginPath();
  ctx.arc(0, 0, radius, 0, 2*Math.PI);
  ctx.fillStyle = 'white';
  ctx.fill();
  grad = ctx.createRadialGradient(0,0,radius*0.95, 0,0,radius*1.05);
  grad.addColorStop(0, '#333');
  grad.addColorStop(0.5, 'white');
  grad.addColorStop(1, '#333');
  ctx.strokeStyle = grad;
  ctx.lineWidth = radius*0.1;
  ctx.stroke();
  ctx.beginPath();
  ctx.arc(0, 0, radius*0.1, 0, 2*Math.PI);
  ctx.fillStyle = '#333';
  ctx.fill();
}

function drawNumbers(ctx, radius) {
  var ang;
  var num;
  ctx.font = radius*0.15 + "px arial";
  ctx.textBaseline="middle";
  ctx.textAlign="center";
  for(num = 1; num < 13; num++){
    ang = num * Math.PI / 6;
    ctx.rotate(ang);
    ctx.translate(0, -radius*0.85);
    ctx.rotate(-ang);
    ctx.fillText(num.toString(), 0, 0);
    ctx.rotate(ang);
    ctx.translate(0, radius*0.85);
    ctx.rotate(-ang);
  }
}

function drawTime(ctx, radius){
    var now = new Date();
    var hour = now.getHours();
    var minute = now.getMinutes();
    var second = now.getSeconds();
    //hour
    hour=hour%12;
    hour=(hour*Math.PI/6)+
    (minute*Math.PI/(6*60))+
    (second*Math.PI/(360*60));
    drawHand(ctx, hour, radius*0.5, radius*0.07);
    //minute
    minute=(minute*Math.PI/30)+(second*Math.PI/(30*60));
    drawHand(ctx, minute, radius*0.8, radius*0.07);
    // second
    second=(second*Math.PI/30);
    drawHand(ctx, second, radius*0.9, radius*0.02);
}

function drawHand(ctx, pos, length, width) {
    ctx.beginPath();
    ctx.lineWidth = width;
    ctx.lineCap = "round";
    ctx.moveTo(0,0);
   ...

CSS

canvas, video {
  background: #000;
}

JavaScript

const [pc1, pc2] = localPeerConnectionLoop()
const canvasStream = canvas.captureStream(200)

videoA.srcObject = canvasStream
videoA.play()

// Note: using two MediaRecorder at the same time seem problematic
// stream2mediaSorce(canvasStream, videoB)
// setTimeout(videoB.play.bind(videoB), 5000)

pc1.addTransceiver(canvasStream.getTracks()[0], {
  streams: [ canvasStream ]
})

pc2.onaddstream = (evt) => {
  videoC.srcObject = evt.stream
  videoC.play()

  // Note: using two MediaRecorder at the same time seem problematic
  stream2mediaSorce(evt.stream, videoD)
  setTimeout(() => videoD.play(), 2000)
}

/**
 * Turn a MediaStream into a SourceBuffer
 * 
 * @param  {MediaStream}      stream   Live Stream to record
 * @param  {HTMLVideoElement} videoElm Video element to play the recorded video in
 * @return {undefined}
 */
function stream2mediaSorce (stream, videoElm) {
  const RECORDER_MIME_TYPE = 'video/webm;codecs=vp9'
  const recorder = new MediaRecorder(stream, { mimeType : RECORDER_MIME_TYPE })

  const mediaSource = new MediaSource()
  videoElm.src = URL.createObjectURL(mediaSource)
  mediaSource.onsourceopen = (e) => {
    sourceBuffer = mediaSource.addSourceBuffer(RECORDER_MIME_TYPE);

    const fr = new FileReader()
    fr.onerror = console.log
    fr.onload = ({ target }) => {
      console.log(target.result)
      sourceBuffer.appendBuffer(target.result)
    }
    recorder.ondataavailable = ({ data }) => {
      console.log(data)
      fr.readAsArrayBuffer(data)
    }
    setInterval(recorder.requestData.bind(recorder), 1000)
  }

  console.log('Recorder created')
  recorder.start() 
}