html - MediaRecorder switch video tracks

Question

I am using MediaRecorder API to record videos in web applications. The application has the option to switch between the camera and screen. I am using Canvas to augment stream recording. The logic involves capturing stream from the camera and redirecting it to the video element. This video is then rendered on canvas and the stream from canvas is passed to MediaRecorder. What I noticed is that switching from screen to video (and vice-versa) works fine as long as the user doesn't switch/minimize the chrome window. The canvas rendering uses requestAnimationFrame and it freezes after the tab loses its focus.

Is there any way to instruct chrome not to pause the execution of requestAnimationFrame? Is there any alternate way to switch streams without impacting MediaRecorder recording?

Update: After reading through the documentation, tabs which play audio or having active websocket connection are not throttled. This is something which we are not doing at this moment. This might be a workaround, but hoping for any alternative solution from community. (setTimeout or setInterval are too throttled and hence not using that, plus it impacts rendering quality)

Update 2: I could able to fix this problem using Worker. Instead of using Main UI Thread for requestAnimationFrame, the worker is invoking the API and the notification is sent to Main Thread via postMessage. Upon completion of rendering by UI Thread, a message is sent back to Worker. There is also a delta period calculation to throttle overwhelming messages from worker.

Answer 1

There is an ongoing proposal to add a .replaceTrack() method to the MediaRecorder API, but for the time being, the specs still read

If at any point, a track is added to or removed from stream’s track set, the UA MUST immediately stop gathering data, discard any data that it has gathered [...]

And that's what is implemented.

---

So we still have to rely on hacks to make this by ourselves...

The best one is probably to create a local RTC connection, and to record the receiving end.

// creates a mixable stream
async function mixableStream( initial_track ) {
  
  const source_stream = new MediaStream( [] );
  const pc1 = new RTCPeerConnection();
  const pc2 = new RTCPeerConnection();
    pc1.onicecandidate = (evt) => pc2.addIceCandidate( evt.candidate );
    pc2.onicecandidate = (evt) => pc1.addIceCandidate( evt.candidate );

  const wait_for_stream = waitForEvent( pc2, 'track')
    .then( evt => new MediaStream( [ evt.track ] ) );

    pc1.addTrack( initial_track, source_stream );
  
  await waitForEvent( pc1, 'negotiationneeded' );
  try {
    await pc1.setLocalDescription( await pc1.createOffer() );
    await pc2.setRemoteDescription( pc1.localDescription );
    await pc2.setLocalDescription( await pc2.createAnswer() );
    await pc1.setRemoteDescription( pc2.localDescription );
  } catch ( err ) {
    console.error( err );
  }
  
  return {
    stream: await wait_for_stream,
    async replaceTrack( new_track ) {
      const sender = pc1.getSenders().find( ( { track } ) => track.kind == new_track.kind );
      return sender && sender.replaceTrack( new_track ) ||
        Promise.reject( "no such track" );
    }
  }  
}


{ // remap unstable FF version
  const proto = HTMLMediaElement.prototype;
  if( !proto.captureStream ) { proto.captureStream = proto.mozCaptureStream; }
}

waitForEvent( document.getElementById( 'starter' ), 'click' )
  .then( (evt) => evt.target.parentNode.remove() )
  .then( (async() => {

  const urls = [
    "2/22/Volcano_Lava_Sample.webm",
    "/a/a4/BBH_gravitational_lensing_of_gw150914.webm"
  ].map( (suffix) => "https://upload.wikimedia.org/wikipedia/commons/" + suffix );
  
  const switcher_btn = document.getElementById( 'switcher' );
  const stop_btn =     document.getElementById( 'stopper' );
  const video_out =    document.getElementById( 'out' );
  
  let current = 0;
  
  // see below for 'recordVid'
  const video_tracks = await Promise.all( urls.map( (url, index) =>  getVideoTracks( url ) ) );
  
  const mixable_stream = await mixableStream( video_tracks[ current ].track );

  switcher_btn.onclick = async (evt) => {

    current = +!current;
    await mixable_stream.replaceTrack( video_tracks[ current ].track );
    
  };

  // final recording part below

  // only for demo, so we can see what happens now
  video_out.srcObject = mixable_stream.stream;

  const rec = new MediaRecorder( mixable_stream.stream );
  const chunks = [];

  rec.ondataavailable = (evt) => chunks.push( evt.data );
  rec.onerror = console.log;
  rec.onstop = (evt) => {

    const final_file = new Blob( chunks );
    video_tracks.forEach( (track) => track.stop() );
    // only for demo, since we did set its srcObject
    video_out.srcObject = null;
    video_out.src = URL.createObjectURL( final_file );
    switcher_btn.remove();
    stop_btn.remove();

        const anchor = document.createElement( 'a' );
    anchor.download = 'file.webm';
    anchor.textContent = 'download';
        anchor.href = video_out.src;
    document.body.prepend( anchor );
    
  };

  stop_btn.onclick = (evt) => rec.stop();

  rec.start();
      
}))
.catch( console.error )

// some helpers below



// returns a video loaded to given url
function makeVid( url ) {

  const vid = document.createElement('video');
  vid.crossOrigin = true;
  vid.loop = true;
  vid.muted = true;
  vid.src = url;
  return vid.play()
    .then( (_) => vid );
  
}

/* Records videos from given url
** @method stop() ::pauses the linked <video>
** @property track ::the video track
*/
async function getVideoTracks( url ) {
  const player = await makeVid( url );
  const track = player.captureStream().getVideoTracks()[ 0 ];
  
  return {
    track,
    stop() { player.pause(); }
  };
}

// Promisifies EventTarget.addEventListener
function waitForEvent( target, type ) {
  return new Promise( (res) => target.addEventListener( type, res, { once: true } ) );
}

video { max-height: 100vh; max-width: 100vw; vertical-align: top; }
.overlay {
  background: #ded;
  position: fixed;
  z-index: 999;
  height: 100vh;
  width: 100vw;
  top: 0;
  left: 0;
  display: flex;
  align-items: center;
  justify-content: center;
}

<div class="overlay">
  <button id="starter">start demo</button>
</div>
<button id="switcher">switch source</button>
<button id="stopper">stop recording</button> 
<video id="out" muted controls autoplay></video>