Запись видео и экрана вместе и наложение с помощью Javascript

let localCamStream,
  localScreenStream,
  localOverlayStream,
  rafId,
  cam,
  screen,
  mediaRecorder,
  audioContext,
  audioDestination;
let mediaWrapperDiv = document.getElementById("mediaWrapper");
let startWebcamBtn = document.getElementById("startWebcam");
let startScreenShareBtn = document.getElementById("startScreenShare");
let mergeStreamsBtn = document.getElementById("mergeStreams");
let startRecordingBtn = document.getElementById("startRecording");
let stopRecordingBtn = document.getElementById("stopRecording");
let stopAllStreamsBtn = document.getElementById("stopAllStreams");
let canvasElement = document.createElement("canvas");
let canvasCtx = canvasElement.getContext("2d");
let encoderOptions = {
  mimeType: "video/webm; codecs=vp9"
};
let recordedChunks = [];
let audioTracks = [];

/**
 * Internal Polyfill to simulate
 * window.requestAnimationFrame
 * since the browser will kill canvas
 * drawing when tab is inactive
 */
const requestVideoFrame = function(callback) {
  return window.setTimeout(function() {
    callback(Date.now());
  }, 1000 / 60); // 60 fps - just like requestAnimationFrame
};

/**
 * Internal polyfill to simulate
 * window.cancelAnimationFrame
 */
const cancelVideoFrame = function(id) {
  clearTimeout(id);
};

async function startWebcamFn() {
  localCamStream = await navigator.mediaDevices.getUserMedia({
    video: true,
    audio: {
      deviceId: {
        exact: "communications"
      }
    }
  });
  if (localCamStream) {
    cam = await attachToDOM("justWebcam", localCamStream);
  }
}

async function startScreenShareFn() {
  localScreenStream = await navigator.mediaDevices.getDisplayMedia({
    video: true,
    audio: true
  });
  if (localScreenStream) {
    screen = await attachToDOM("justScreenShare", localScreenStream);
  }
}

async function stopAllStreamsFn() {
  [
    ...(localCamStream ? localCamStream.getTracks() : []),
    ...(localScreenStream ? localScreenStream.getTracks() : []),
    ...(localOverlayStream ? localOverlayStream.getTracks() : [])
  ].map((track) => track.stop());
  localCamStream = null;
  localScreenStream = null;
  localOverlayStream = null;
  cancelVideoFrame(rafId);
  mediaWrapperDiv.innerHTML = "";
  document.getElementById("recordingState").innerHTML = "";
}

async function makeComposite() {
  if (cam && screen) {
    canvasCtx.save();
    canvasElement.setAttribute("width", `${screen.videoWidth}px`);
    canvasElement.setAttribute("height", `${screen.videoHeight}px`);
    canvasCtx.clearRect(0, 0, screen.videoWidth, screen.videoHeight);
    canvasCtx.drawImage(screen, 0, 0, screen.videoWidth, screen.videoHeight);
    canvasCtx.drawImage(
      cam,
      0,
      Math.floor(screen.videoHeight - screen.videoHeight / 4),
      Math.floor(screen.videoWidth / 4),
      Math.floor(screen.videoHeight / 4)
    ); // this is just a rough calculation to offset the webcam stream to bottom left
    let imageData = canvasCtx.getImageData(
      0,
      0,
      screen.videoWidth,
      screen.videoHeight
    ); // this makes it work
    canvasCtx.putImageData(imageData, 0, 0); // properly on safari/webkit browsers too
    canvasCtx.restore();
    rafId = requestVideoFrame(makeComposite);
  }
}

async function mergeStreamsFn() {
  document.getElementById("mutingStreams").style.display = "block";
  await makeComposite();
  audioContext = new AudioContext();
  audioDestination = audioContext.createMediaStreamDestination();
  let fullVideoStream = canvasElement.captureStream();
  let existingAudioStreams = [
    ...(localCamStream ? localCamStream.getAudioTracks() : []),
    ...(localScreenStream ? localScreenStream.getAudioTracks() : [])
  ];
  audioTracks.push(
    audioContext.createMediaStreamSource(
      new MediaStream([existingAudioStreams[0]])
    )
  );
  if (existingAudioStreams.length > 1) {
    audioTracks.push(
      audioContext.createMediaStreamSource(
        new MediaStream([existingAudioStreams[1]])
      )
    );
  }
  audioTracks.map((track) => track.connect(audioDestination));
  console.info(audioDestination.stream);
  localOverlayStream = new MediaStream([...fullVideoStream.getVideoTracks()]);
  let fullOverlayStream = new MediaStream([
    ...fullVideoStream.getVideoTracks(),
    ...audioDestination.stream.getTracks()
  ]);
  console.info(localOverlayStream, existingAudioStreams);
  if (localOverlayStream) {
    overlay = await attachToDOM("pipOverlayStream", localOverlayStream);
    mediaRecorder = new MediaRecorder(fullOverlayStream, encoderOptions);
    mediaRecorder.ondataavailable = handleDataAvailable;
    overlay.volume = 0;
    cam.volume = 0;
    screen.volume = 0;
    cam.style.display = "none";
    // localCamStream.getAudioTracks().map(track => { track.enabled = false });
    screen.style.display = "none";
    // localScreenStream.getAudioTracks().map(track => { track.enabled = false });
  }
}

async function startRecordingFn() {
  mediaRecorder.start();
  console.info(mediaRecorder.state);
  console.info("recorder started");
  document.getElementById("pipOverlayStream").style.border = "10px solid red";
  document.getElementById(
    "recordingState"
  ).innerHTML = `${mediaRecorder.state}...`;
}

async function attachToDOM(id, stream) {
  let videoElem = document.createElement("video");
  videoElem.id = id;
  videoElem.width = 640;
  videoElem.height = 360;
  videoElem.autoplay = true;
  videoElem.setAttribute("playsinline", true);
  videoElem.srcObject = new MediaStream(stream.getTracks());
  mediaWrapperDiv.appendChild(videoElem);
  return videoElem;
}

function handleDataAvailable(event) {
  console.info("data-available");
  if (event.data.size > 0) {
    recordedChunks.push(event.data);
    console.info(recordedChunks);
    download();
  } else {}
}

function download() {
  var blob = new Blob(recordedChunks, {
    type: "video/webm"
  });
  var url = URL.createObjectURL(blob);
  var a = document.createElement("a");
  document.body.appendChild(a);
  a.style = "display: none";
  a.href = url;
  a.download = "result.webm";
  a.click();
  window.URL.revokeObjectURL(url);
}

function stopRecordingFn() {
  mediaRecorder.stop();
  document.getElementById(
    "recordingState"
  ).innerHTML = `${mediaRecorder.state}...`;
}

startWebcamBtn.addEventListener("click", startWebcamFn);
startScreenShareBtn.addEventListener("click", startScreenShareFn);
mergeStreamsBtn.addEventListener("click", mergeStreamsFn);
stopAllStreamsBtn.addEventListener("click", stopAllStreamsFn);
startRecordingBtn.addEventListener("click", startRecordingFn);
stopRecordingBtn.addEventListener("click", stopRecordingFn);

div#mediaWrapper,
div#buttonWrapper {
  display: flex;
  flex: 1 1 100%;
  flex-basis: row nowrap;
}

div#mediaWrapper video {
  border: 1px solid black;
  margin: 1px;
  max-width: 33%;
  height: auto;
}

div#mediaWrapper video#pipOverlayStream {
  max-width: 100% !important;
}

div#buttonWrapper button {
  border-radius: 0.25rem;
  color: #ffffff;
  display: inline-block;
  font-size: 1rem;
  font-weight: bold;
  line-height: 1.6;
  padding: 0.375rem 0.75rem;
  text-align: center;
  -webkit-user-select: none;
  -moz-user-select: none;
  -ms-user-select: none;
  user-select: none;
  vertical-align: middle;
  margin: 5px;
  cursor: pointer;
}

div#buttonWrapper button#startWebcam {
  background-color: #007bff;
  border: 1px solid #007bff;
}

div#buttonWrapper button#startScreenShare {
  background-color: #17a2b8;
  border: 1px solid #17a2b8;
}

div#buttonWrapper button#mergeStreams {
  background-color: #28a745;
  border: 1px solid #28a745;
}

div#buttonWrapper button#startRecording {
  background-color: #17a2b8;
  border: 1px solid #17a2b8;
}

div#buttonWrapper button#stopRecording {
  background-color: #000000;
  border: 1px solid #000000;
}

div#buttonWrapper button#stopAllStreams {
  background-color: #dc3545;
  border: 1px solid #dc3545;
}

<main>
  <p>This demo is a proof-of-concept solution for this <a href = "https://stackoverflow.com/questions/71557879" target = "_blank" rel = "noopener noreferrer">StackOverflow question</a> and <a href = "https://stackoverflow.com/questions/37404860" target = "_blank"
      rel = "noopener noreferrer">also this one</a> - as long as you make the required changes<br>i.e. <b>mimeType: "video/mp4; codecs=h264"</b> instead of <b>mimeType: "video/webm; codecs=vp9"</b><br>AND<br><b>type: "video/mp4"</b> instead of <b>type: "video/webm"</b><br>AND<br><b>result.mp4</b>    instead of <b>result.webm</b></p>
  <h2>Click on "Start Webcam" to get started. </h2>
  <h3>
    Core Idea:<br>
    <ol>
      <li>Fetch Webcam Stream via getUserMedia()</li>
      <li>Fetch Screen Share Stream via getDisplayMedia()</li>
      <li>Merge Both Stream using some math & canvas operations</li>
      <li>Use canvas.captureStream() to generate the composite video stream.</li>
      <li>Use AudioContext to merge audio clips (especially needed if using both microphone & system audio).</li>
      <li>Use MediaStream constructor to create a new stream using - the video from the new stream + audio from audioContext Destination Node as follows -<br><br>
        <code>new MediaStream([...newStream.getVideoTracks(), ...audioDestination.stream.getTracks()]);</code>
      </li><br>
      <li>Use newly generated MediaStream as required (i.e. replace in RTCPeerConnection, etc.).</li>
      <li>In this example - MediaRecorder API is used to record the resulting composite/picture-in-picture video. Recording begins when the "Record Resulting Stream" button is clicked. The final result can be downloaded upon clicking the "Stop Recording and
        Download Resulting Stream" button</li>
    </ol>
  </h3>
  <div id = "mediaWrapper"></div>
  <div id = "buttonWrapper">
    <button id = "startWebcam" title = "Start Webcam">Start Webcam</button>
    <button id = "startScreenShare" title = "Start Screen Share">Start Screen Share</button>
    <button id = "mergeStreams" title = "Merge Streams">Merge Streams</button>
    <button id = "startRecording" title = "Record Resulting Stream">Record Resulting Stream</button>
    <button id = "stopRecording" title = "Stop Recording and Download Resulting Stream">Stop Recording and Download Resulting Stream</button>
    <button id = "stopAllStreams" title = "Stop All Streams">Stop All Streams</button>
  </div>

  <div id = "helpText">
    <h1 id = "recordingState"></h1><br>
    <h2 id = "mutingStreams">
      Note: In a WebRTC setting, you wouldn't be hearing your own voice or the screen share audio via the "video" tag. The same has been simulated here by ensuring that all video tags have a "volume = 0". Removing this will create a loopback hell which you
      do not want.<br><br> Another way to avoid this issue is to ensure that the video tags are created with ONLY video stream tracks using <em style = "color: blue;">new MediaStream([ source.getVideoTracks() ])</em> during the srcObject assignment.
    </h2>
    <h1>
      Remember to send the correct stream (with both audio and video) to the rest of the peers though.
    </h1>
  </div>
</main>