WebRTC本地选择codec（web本地模拟）

视频编码后，再进行发送。WebRTC建立视频连接前，可以选择codec。一般来说支持多种codec，以VP8和H264为代表。

Codec： 编码译码器，编解码器

示例代码

写一个示例，用户可以在发送视频流之前选择codec。把支持的codec类型列出来，用户自行选择。

html

放上2个video控件显示收发视频。按钮控制开始，呼叫（发起连接）和挂断。

<div id="container">
    <h1><a href="https://an.rustfisher.com/webrtc/peerconnection/change-codec/" title="WebRTC示例，修改codec">WebRTC示例，修改codec</a>
    </h1>

    <video id="localVideo" playsinline autoplay muted></video>
    <video id="remoteVideo" playsinline autoplay></video>

    <div class="box">
        <button id="startBtn">开始</button>
        <button id="callBtn">呼叫</button>
        <button id="hangupBtn">挂断</button>
    </div>

    <div class="box">
        <span>选择Codec:</span>
        <select id="codecPreferences" disabled>
            <option selected value="">Default</option>
        </select>
        <div id="actualCodec"></div>
    </div>
    <p>可以在控制台观察 <code>MediaStream</code>， <code>localStream</code>， 和 <code>RTCPeerConnection</code></p>
</div>

<script src="../../src/js/adapter-2021.js"></script>
<script src="js/main.js" async></script>

select用来选择codec。获取支持的codec信息，放到下拉栏里让用户选择。

adapter-2021.js是存放在本地的文件。要使用最新的adapter，按以下地址引入

<script src="https://webrtc.github.io/adapter/adapter-latest.js"></script>

js

main.js控制主要逻辑。从开启摄像头开始。建立连接前可以选择codec。

建立连接的流程与WebRTC模拟传输视频流，video通过本地节点peer传输视频流相同

main.js完整代码如下

'use strict';

console.log('WebRTC示例，选择codec');

// --------- ui准备 ---------
const startBtn = document.getElementById('startBtn');
const callBtn = document.getElementById('callBtn');
const hangupBtn = document.getElementById('hangupBtn');
const localVideo = document.getElementById('localVideo');
const remoteVideo = document.getElementById('remoteVideo');

callBtn.disabled = true;
hangupBtn.disabled = true;
startBtn.addEventListener('click', start);
callBtn.addEventListener('click', call);
hangupBtn.addEventListener('click', hangup);
// ---------------------------

// -------- codec 的配置 --------
const codecPreferences = document.querySelector('#codecPreferences');
const supportsSetCodecPreferences = window.RTCRtpTransceiver &&
  'setCodecPreferences' in window.RTCRtpTransceiver.prototype;
// -----------------------------

let startTime;

remoteVideo.addEventListener('resize', () => {
  console.log(`Remote video size changed to ${remoteVideo.videoWidth}x${remoteVideo.videoHeight}`);
  if (startTime) {
    const elapsedTime = window.performance.now() - startTime;
    console.log('视频流连接耗时: ' + elapsedTime.toFixed(3) + 'ms');
    startTime = null;
  }
});

let localStream;
let pc1;
let pc2;
const offerOptions = {
  offerToReceiveAudio: 1,
  offerToReceiveVideo: 1
};

function getName(pc) {
  return (pc === pc1) ? 'pc1' : 'pc2';
}

function getOtherPc(pc) {
  return (pc === pc1) ? pc2 : pc1;
}

// 启动本地视频
async function start() {
  console.log('启动本地视频');
  startBtn.disabled = true;
  try {
    const stream = await navigator.mediaDevices.getUserMedia({ audio: true, video: true });
    console.log('获取到本地视频');
    localVideo.srcObject = stream;
    localStream = stream;
    callBtn.disabled = false;
  } catch (e) {
    alert(`getUserMedia() error: ${e.name}`);
  }
  if (supportsSetCodecPreferences) {
    const { codecs } = RTCRtpSender.getCapabilities('video');
    console.log('RTCRtpSender.getCapabilities(video):\n', codecs);
    codecs.forEach(codec => {
      if (['video/red', 'video/ulpfec', 'video/rtx'].includes(codec.mimeType)) {
        return;
      }
      const option = document.createElement('option');
      option.value = (codec.mimeType + ' ' + (codec.sdpFmtpLine || '')).trim();
      option.innerText = option.value;
      codecPreferences.appendChild(option);
    });
    codecPreferences.disabled = false;
  } else {
    console.warn('当前不支持更换codec');
  }
}

// 呼叫并建立连接
async function call() {
  callBtn.disabled = true;
  hangupBtn.disabled = false;
  console.log('开始呼叫');
  startTime = window.performance.now();
  const videoTracks = localStream.getVideoTracks();
  const audioTracks = localStream.getAudioTracks();
  if (videoTracks.length > 0) {
    console.log(`使用的摄像头: ${videoTracks[0].label}`);
  }
  if (audioTracks.length > 0) {
    console.log(`使用的麦克风: ${audioTracks[0].label}`);
  }
  const configuration = {};
  pc1 = new RTCPeerConnection(configuration);
  pc1.addEventListener('icecandidate', e => onIceCandidate(pc1, e));
  pc2 = new RTCPeerConnection(configuration);
  pc2.addEventListener('icecandidate', e => onIceCandidate(pc2, e));
  pc2.addEventListener('track', gotRemoteStream);

  localStream.getTracks().forEach(track => pc1.addTrack(track, localStream));
  if (supportsSetCodecPreferences) {
    // 获取选择的codec
    const preferredCodec = codecPreferences.options[codecPreferences.selectedIndex];
    if (preferredCodec.value !== '') {
      const [mimeType, sdpFmtpLine] = preferredCodec.value.split(' ');
      const { codecs } = RTCRtpSender.getCapabilities('video');
      const selectedCodecIndex = codecs.findIndex(c => c.mimeType === mimeType && c.sdpFmtpLine === sdpFmtpLine);
      const selectedCodec = codecs[selectedCodecIndex];
      codecs.splice(selectedCodecIndex, 1);
      codecs.unshift(selectedCodec);
      console.log(codecs);
      const transceiver = pc1.getTransceivers().find(t => t.sender && t.sender.track === localStream.getVideoTracks()[0]);
      transceiver.setCodecPreferences(codecs);
      console.log('选择的codec', selectedCodec);
    }
  }
  codecPreferences.disabled = true;

  try {
    const offer = await pc1.createOffer(offerOptions);
    await onCreateOfferSuccess(offer);
  } catch (e) {
    console.log(`Failed, pc1 createOffer: ${e.toString()}`);
  }
}

async function onCreateOfferSuccess(desc) {
  try {
    await pc1.setLocalDescription(desc);
    console.log('pc1 setLocalDescription 成功');
  } catch (e) {
    console.error('pc1 setLocalDescription 出错', e);
  }
  try {
    await pc2.setRemoteDescription(desc);
    console.log('pc2 setRemoteDescription ok');
  } catch (e) {
    console.error('pc2 setRemoteDescription fail', e);
  }
  try {
    const answer = await pc2.createAnswer();
    await onCreateAnswerSuccess(answer);
  } catch (e) {
    console.log(`pc2 create answer fail: ${e.toString()}`);
  }
}

function gotRemoteStream(e) {
  if (remoteVideo.srcObject !== e.streams[0]) {
    remoteVideo.srcObject = e.streams[0];
    console.log('pc2 received remote stream');
  }
}

// 应答（接收）成功
async function onCreateAnswerSuccess(desc) {
  console.log(`Answer from pc2:\n${desc.sdp}`);
  console.log('pc2 setLocalDescription start');
  try {
    await pc2.setLocalDescription(desc);
  } catch (e) {
    console.error('pc2 set local d fail', e);
  }
  console.log('pc1 setRemoteDescription start');
  try {
    await pc1.setRemoteDescription(desc);

    // Display the video codec that is actually used.
    setTimeout(async () => {
      const stats = await pc1.getStats();
      stats.forEach(stat => {
        if (!(stat.type === 'outbound-rtp' && stat.kind === 'video')) {
          return;
        }
        const codec = stats.get(stat.codecId);
        document.getElementById('actualCodec').innerText = 'Using ' + codec.mimeType +
          ' ' + (codec.sdpFmtpLine ? codec.sdpFmtpLine + ' ' : '') +
          ', payloadType=' + codec.payloadType + '. Encoder: ' + stat.encoderImplementation;
      });
    }, 1000);
  } catch (e) {
    console.error(e);
  }
}

async function onIceCandidate(pc, event) {
  try {
    await (getOtherPc(pc).addIceCandidate(event.candidate));
    onAddIceCandidateSuccess(pc);
  } catch (e) {
    onAddIceCandidateError(pc, e);
  }
  console.log(`${getName(pc)} ICE candidate:\n${event.candidate ? event.candidate.candidate : '(null)'}`);
}

function onAddIceCandidateSuccess(pc) {
  console.log(`${getName(pc)} addIceCandidate success`);
}

function onAddIceCandidateError(pc, error) {
  console.log(`${getName(pc)} failed to add ICE Candidate: ${error.toString()}`);
}

localVideo.addEventListener('loadedmetadata', function () {
  console.log(`Local video videoWidth: ${this.videoWidth}px,  videoHeight: ${this.videoHeight}px`);
});

remoteVideo.addEventListener('loadedmetadata', function () {
  console.log(`Remote video videoWidth: ${this.videoWidth}px,  videoHeight: ${this.videoHeight}px`);
});

// 挂断
function hangup() {
  console.log('挂断');
  pc1.close();
  pc2.close();
  pc1 = null;
  pc2 = null;
  hangupBtn.disabled = true;
  callBtn.disabled = false;
  codecPreferences.disabled = false;
}

获取可用codec

先判断浏览器是否有RTCRtpTransceiver，并且要能支持setCodecPreferences方法

const supportsSetCodecPreferences = window.RTCRtpTransceiver &&
  'setCodecPreferences' in window.RTCRtpTransceiver.prototype;

通过RTCRtpSender.getCapabilities('video')获取可支持的codec。
然后把它们放进列表codecPreferences里

  if (supportsSetCodecPreferences) {
    const { codecs } = RTCRtpSender.getCapabilities('video');

    codecs.forEach(codec => {
      if (['video/red', 'video/ulpfec', 'video/rtx'].includes(codec.mimeType)) {
        return;
      }
      const option = document.createElement('option');
      option.value = (codec.mimeType + ' ' + (codec.sdpFmtpLine || '')).trim();
      option.innerText = option.value;
      codecPreferences.appendChild(option);
    });
    codecPreferences.disabled = false;
  }

呼叫的时候，把选中的codec交给transceiver

  if (supportsSetCodecPreferences) {
    // 获取选择的codec
    const preferredCodec = codecPreferences.options[codecPreferences.selectedIndex];
    if (preferredCodec.value !== '') {
      const [mimeType, sdpFmtpLine] = preferredCodec.value.split(' ');
      const { codecs } = RTCRtpSender.getCapabilities('video');
      const selectedCodecIndex = codecs.findIndex(c => c.mimeType === mimeType && c.sdpFmtpLine === sdpFmtpLine);
      const selectedCodec = codecs[selectedCodecIndex];
      codecs.splice(selectedCodecIndex, 1);
      codecs.unshift(selectedCodec);
      console.log(codecs);
      const transceiver = pc1.getTransceivers().find(t => t.sender && t.sender.track === localStream.getVideoTracks()[0]);
      transceiver.setCodecPreferences(codecs);
      console.log('选择的codec', selectedCodec);
    }
  }

codec

观察控制台的log，打印出了可用codec信息。

0: {clockRate: 90000, mimeType: 'video/VP8'}
1: {clockRate: 90000, mimeType: 'video/rtx'}
2: {clockRate: 90000, mimeType: 'video/VP9', sdpFmtpLine: 'profile-id=0'}
3: {clockRate: 90000, mimeType: 'video/VP9', sdpFmtpLine: 'profile-id=2'}
4: {clockRate: 90000, mimeType: 'video/H264', sdpFmtpLine: 'level-asymmetry-allowed=1;packetization-mode=1;profile-level-id=42001f'}
5: {clockRate: 90000, mimeType: 'video/H264', sdpFmtpLine: 'level-asymmetry-allowed=1;packetization-mode=0;profile-level-id=42001f'}
6: {clockRate: 90000, mimeType: 'video/H264', sdpFmtpLine: 'level-asymmetry-allowed=1;packetization-mode=1;profile-level-id=42e01f'}
7: {clockRate: 90000, mimeType: 'video/H264', sdpFmtpLine: 'level-asymmetry-allowed=1;packetization-mode=0;profile-level-id=42e01f'}
8: {clockRate: 90000, mimeType: 'video/AV1'}
9: {clockRate: 90000, mimeType: 'video/H264', sdpFmtpLine: 'level-asymmetry-allowed=1;packetization-mode=1;profile-level-id=4d0032'}
10: {clockRate: 90000, mimeType: 'video/H264', sdpFmtpLine: 'level-asymmetry-allowed=1;packetization-mode=1;profile-level-id=640032'}
11: {clockRate: 90000, mimeType: 'video/red'}
12: {clockRate: 90000, mimeType: 'video/ulpfec'}

clockRate 是codec的时钟频率，单位hz

sdpFmtpLine 是codec的SDP里a=fmtp的参数信息

mimeType 里说的是视频编码类型，常见的有VP8和H264等等。下面是视频格式的一些介绍。

VP8 VP9

2010年5月Google收购了On2 Technologies，获得了VP8。
Opera，FireFox，Chrome和Chromium支持HTML5中的video播放VP8视频。

WebM作为一个容器格式，图像部分使用VP8，音频使用Vorbis和Opus。

VP9由Google开发，一个开放的无版权费的视频编码标准。开发初期曾用名“Next Gen Open Video”。VP9也被视为是VP8的下一代视频编码标准。

H264

H.264，又称为MPEG-4第10部分，高级视频编码是一种面向块，基于运动补偿的视频编码标准。
到2014年，它已经成为高精度视频录制、压缩和发布的最常用格式之一。

优势：

• 1）网络亲和性，即可适用于各种传输网络
• 2）高的视频压缩比

目前我们用的比较多的还是H264。

效果预览

网页做好，效果请参考

扩展阅读

• WebRTC模拟传输视频流，video通过本地节点peer传输视频流
• H.264简介