js实现web调用摄像头 js截取视频画面

<p> <button onclick="openMedia()">打开</button> <button onclick="closeMedia()">关闭</button> <button onclick="drawMedia()">截取</button></p><video id="video" class="bg"></video><canvas id="qr-canvas"></canvas>

<script type="text/javascript"> var video = document.querySelector('video'); var text = document.getElementById('text'); var canvas1 = document.getElementById('qr-canvas'); var context1 = canvas1.getContext('2d'); var mediaStreamTrack; // 一堆兼容代码 window.URL = (window.URL || window.webkitURL || window.mozURL || window.msURL); if (navigator.mediaDevices === undefined) { navigator.mediaDevices = {}; } if (navigator.mediaDevices.getUserMedia === undefined) { navigator.mediaDevices.getUserMedia = function(constraints) {  var getUserMedia = navigator.webkitGetUserMedia || navigator.mozGetUserMedia || navigator.msGetUserMedia;  if (!getUserMedia) {  return Promise.reject(new Error('getUserMedia is not implemented in this browser'));  }  return new Promise(function(resolve, reject) {  getUserMedia.call(navigator, constraints, resolve, reject);  }); } }  //摄像头调用配置 var mediaOpts = { audio: false, video: true, video: { facingMode: "environment"} // 或者 "user" // video: { width: 1280, height: 720 } // video: { facingMode: { exact: "environment" } }// 或者 "user" } // 回调 function successFunc(stream) { mediaStreamTrack = stream; video = document.querySelector('video'); if ("srcObject" in video) {  video.srcObject = stream } else {  video.src = window.URL && window.URL.createObjectURL(stream) || stream } video.play(); } function errorFunc(err) { alert(err.name); } // 正式启动摄像头 function openMedia(){ navigator.mediaDevices.getUserMedia(mediaOpts).then(successFunc).catch(errorFunc); } //关闭摄像头 function closeMedia(){ mediaStreamTrack.getVideoTracks().forEach(function (track) {  track.stop();  context1.clearRect(0, 0,context1.width, context1.height);//清除画布 }); } //截取视频 function drawMedia(){ canvas1.setAttribute("width", video.videoWidth); canvas1.setAttribute("height", video.videoHeight); context1.drawImage(video, 0, 0, video.videoWidth, video.videoHeight); }</script>