html5 WebRTC无法实现调用摄像头

<!DOCTYPE html>

<html>



<head>



  <title>Realtime communication with WebRTC</title>



  <link rel="stylesheet" href="css/main.css" />



</head>



<body>



  <h1>Realtime communication with WebRTC</h1>



  <video autoplay></video>



  <script src="js/main.js"></script>



</body>



</html>

'use strict';



navigator.getUserMedia = navigator.getUserMedia ||

    navigator.webkitGetUserMedia || navigator.mozGetUserMedia;



var constraints = {

  audio: false,

  video: true

};



var video = document.querySelector('video');



function successCallback(stream) {

  window.stream = stream; // stream available to console

  if (window.URL) {

    video.src = window.URL.createObjectURL(stream);

  } else {

    video.src = stream;

  }

}



function errorCallback(error) {

  console.log('navigator.getUserMedia error: ', error);

}



navigator.getUserMedia(constraints, successCallback, errorCallback);