MediaStream 同时捕获 Canvas 和音频

Question

我正在从事一个项目，我想：

加载一个视频js并显示在canvas上。
使用滤镜改变canvas（以及视频）的外观。
使用MediaStream captureStream() 方法和一个MediaRecorder 对象来记录canvas 的表面和原始视频的音频。
同时播放 canvas 和 HTML 视频元素中的音频流。

我已经能够通过调整此 WebRTC 演示代码在视频元素中显示 canvas 录制：https://webrtc.github.io/samples/src/content/capture/canvas-record/

就是说，我不知道如何在 canvas 旁边录制视频的音频。是否可以创建一个包含来自两个不同 sources/elements 的 MediaStreamTrack 实例的 MediaStream？

根据 MediaStream API 的规范，理论上应该有一些方法可以实现这一点： https://w3c.github.io/mediacapture-main/#introduction

"The two main components in the MediaStream API are the MediaStreamTrack and MediaStream interfaces. The MediaStreamTrack object represents media of a single type that originates from one media source in the User Agent, e.g. video produced by a web camera. A MediaStream is used to group several MediaStreamTrack objects into one unit that can be recorded or rendered in a media element."

Answer 1

Is it possible to create a MediaStream containing MediaStreamTrack instances from two different sources/elements?

是的，您可以使用 MediaStream.addTrack() 方法。

但 Firefox 只会在 this bug 修复之前将初始流的轨道用于记录器。

OP 已经知道如何获取所有这些，但这里提醒未来的读者：

要从canvas获取videoStream轨道，可以调用canvas.captureStream(framerate)方法。
要从视频元素获取音频 streamTrack，您可以使用 WebAudio API 和它的 createMediaStreamDestination 方法。这将 return 包含我们的音频流的 MediaStreamDestination 节点 (dest)。然后，您必须将根据您的视频元素创建的 MediaElementSource 连接到此 dest。如果您需要向此流添加更多音轨，您应该将所有这些源连接到 dest.

现在我们有两个流，一个用于 canvas 视频，一个用于音频，我们可以在初始化 new MediaRecorder(canvasStream).[=31 之前使用 canvasStream.addTrack(audioStream.getAudioTracks()[0]) =]

这是一个完整的例子，现在只能在 chrome 中使用，可能很快会在 Firefox 中使用，届时他们将修复错误：

var cStream,
  aStream,
  vid,
  recorder,
  analyser,
  dataArray,
  bufferLength,
  chunks = [];

function clickHandler() {

  this.textContent = 'stop recording';
  cStream = canvas.captureStream(30);
  cStream.addTrack(aStream.getAudioTracks()[0]);

  recorder = new MediaRecorder(cStream);
  recorder.start();

  recorder.ondataavailable = saveChunks;

  recorder.onstop = exportStream;

  this.onclick = stopRecording;

};

function exportStream(e) {

  if (chunks.length) {

    var blob = new Blob(chunks)
    var vidURL = URL.createObjectURL(blob);
    var vid = document.createElement('video');
    vid.controls = true;
    vid.src = vidURL;
    vid.onend = function() {
      URL.revokeObjectURL(vidURL);
    }
    document.body.insertBefore(vid, canvas);

  } else {

    document.body.insertBefore(document.createTextNode('no data saved'), canvas);

  }
}

function saveChunks(e) {

  e.data.size && chunks.push(e.data);

}

function stopRecording() {

  vid.pause();
  this.parentNode.removeChild(this);
  recorder.stop();

}

function initAudioStream(evt) {

  var audioCtx = new AudioContext();
  // create a stream from our AudioContext
  var dest = audioCtx.createMediaStreamDestination();
  aStream = dest.stream;
  // connect our video element's output to the stream
  var sourceNode = audioCtx.createMediaElementSource(this);
  sourceNode.connect(dest)
    // start the video
  this.play();

  // just for the fancy canvas drawings
  analyser = audioCtx.createAnalyser();
  sourceNode.connect(analyser);

  analyser.fftSize = 2048;
  bufferLength = analyser.frequencyBinCount;
  dataArray = new Uint8Array(bufferLength);
  analyser.getByteTimeDomainData(dataArray);

  // output to our headphones
  sourceNode.connect(audioCtx.destination)

  startCanvasAnim();

  rec.onclick = clickHandler;
  rec.disabled = false;
};

var loadVideo = function() {

  vid = document.createElement('video');
  vid.crossOrigin = 'anonymous';
  vid.oncanplay = initAudioStream;
  vid.src = 'https://dl.dropboxusercontent.com/s/bch2j17v6ny4ako/movie720p.mp4';

  
}

function startCanvasAnim() {
  // from MDN https://developer.mozilla.org/en/docs/Web/API/AnalyserNode#Examples
  var canvasCtx = canvas.getContext('2d');

  canvasCtx.fillStyle = 'rgb(200, 200, 200)';
  canvasCtx.lineWidth = 2;
  canvasCtx.strokeStyle = 'rgb(0, 0, 0)';

  var draw = function() {

    var drawVisual = requestAnimationFrame(draw);

    analyser.getByteTimeDomainData(dataArray);

    canvasCtx.fillRect(0, 0, canvas.width, canvas.height);
    canvasCtx.beginPath();

    var sliceWidth = canvas.width * 1.0 / bufferLength;
    var x = 0;

    for (var i = 0; i < bufferLength; i++) {

      var v = dataArray[i] / 128.0;
      var y = v * canvas.height / 2;

      if (i === 0) {
        canvasCtx.moveTo(x, y);
      } else {
        canvasCtx.lineTo(x, y);
      }

      x += sliceWidth;
    }

    canvasCtx.lineTo(canvas.width, canvas.height / 2);
    canvasCtx.stroke();

  };

  draw();

}

loadVideo();

<canvas id="canvas" width="500" height="200"></canvas>
<button id="rec" disabled>record</button>

Ps : 由于 FF 团队似乎需要一些时间来修复这个错误，这里有一个快速修复，可以让它在 FF 上也能正常工作。

您还可以使用 new MediaStream([track1, track2]).
混合两个曲目然而，chrome 目前是这个构造函数的前缀，但由于它确实支持 addTrack，所以它并不是真正需要的，我们可以提供像

这样丑陋的东西

var mixedStream = 'MediaStream' in window ? 
  new MediaStream([cStream.getVideoTracks()[0], aStream.getAudioTracks()[0]]) :
  cStream;
recorder = new MediaRecorder(mixedStream);

Working fiddle for both FF and chrome.

Answer 2

Kaiido 的演示很棒。对于那些只是寻找 tl;dr 代码以将音频流添加到他们现有的 canvas 流的人：

let videoOrAudioElement = /* your audio source element */;
// get the audio track:
let ctx = new AudioContext();
let dest = ctx.createMediaStreamDestination();
let sourceNode = ctx.createMediaElementSource(videoOrAudioElement);
sourceNode.connect(dest);
sourceNode.connect(ctx.destination);
let audioTrack = dest.stream.getAudioTracks()[0];
// add it to your canvas stream:
canvasStream.addTrack(audioTrack);
// use your canvas stream like you would normally:
let recorder = new MediaRecorder(canvasStream);
// ...

MediaStream 同时捕获 Canvas 和音频

MediaStream Capture Canvas and Audio Simultaneously

javascript

canvas

mediarecorder

mediastream