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摄像头访问在HTML和JS中是如何工作的?

camera javascript

Devansh Sharma · 技术社区 · 8 年前

我正在寻找一种在浏览器中实现网络摄像头的方法,当用户单击按钮时,可以对摄像头进行快照。有一个在线资源似乎使用了下面的代码,但我无法理解下面的代码。有人能解释一下吗?

navigator.getMedia = (navigator.getUserMedia ||
        navigator.webkitGetUserMedia ||
        navigator.mozGetUserMedia ||
        navigator.msGetUserMedia);

如果需要,完整代码如下。

编辑:另一个相关问题事实上 navigator.getMedia 代表以及它是如何工作的?

(function() {
  // Default Width set and height to be based on aspect ratio
  // Change to width/height of device (detect)
  var width = 320;
  var height = 0;

  //Status of stream
  var streaming = false;
  //Refers to <video> element
  var video = null;
  //Refers to <canvas> element
  var canvas = null;
  //Refers to <img> element
  var photo = null;
  //Refers to <button> element for trigger
  var startbutton = null;

  function startup() {
    video = document.getElementById('video');
    canvas = document.getElementById('canvas');
    photo = document.getElementById('photo');
    startbutton = document.getElementById('startbutton');

    navigator.getMedia = (navigator.getUserMedia ||
      navigator.webkitGetUserMedia ||
      navigator.mozGetUserMedia ||
      navigator.msGetUserMedia);

    navigator.getMedia({
        video: true,
        audio: false
      },
      function(stream) {
        if (navigator.mozGetUserMedia) {
          video.mozSrcObject = stream;
        } else {
          var vendorURL = window.URL || window.webkitURL;
          video.src = vendorURL.createObjectURL(stream);
        }
        video.play();
      },
      function(err) {
        console.log("An error occured! " + err);
      }
    );

    video.addEventListener('canplay', function(ev) {
      if (!streaming) {
        height = video.videoHeight / (video.videoWidth / width);

        // Firefox currently has a bug where the height can't be read from
        // the video, so we will make assumptions if this happens.

        if (isNaN(height)) {
          height = width / (4 / 3);
        }

        video.setAttribute('width', width);
        video.setAttribute('height', height);
        canvas.setAttribute('width', width);
        canvas.setAttribute('height', height);
        streaming = true;
      }
    }, false);

    startbutton.addEventListener('click', function(ev) {
      takepicture();
      ev.preventDefault();
    }, false);

    clearphoto();
  }

  // Fill the photo with an indication that none has been
  // captured.

  function clearphoto() {
    var context = canvas.getContext('2d');
    context.fillStyle = "#AAA";
    context.fillRect(0, 0, canvas.width, canvas.height);

    var data = canvas.toDataURL('image/png');
    photo.setAttribute('src', data);
  }

  // Capture a photo by fetching the current contents of the video
  // and drawing it into a canvas, then converting that to a PNG
  // format data URL. By drawing it on an offscreen canvas and then
  // drawing that to the screen, we can change its size and/or apply
  // other changes before drawing it.

  function takepicture() {
    var context = canvas.getContext('2d');
    if (width && height) {
      canvas.width = width;
      canvas.height = height;
      context.drawImage(video, 0, 0, width, height);

      var data = canvas.toDataURL('image/png');
      photo.setAttribute('src', data);
    } else {
      clearphoto();
    }
  }

  // Set up our event listener to run the startup process
  // once loading is complete.
  window.addEventListener('load', startup, false);
})();

<!doctype html>
<html>

<head>
  <script src="Camera.js"></script>
</head>

<body>
  <div>
    <div class="camera">
      <video id="video">Video stream not available.</video>
      <button id="startbutton">Take photo</button>
    </div>

    <canvas id="canvas"></canvas>

    <div class="output">
      <img id="photo" alt="The screen capture will appear in this box.">
    </div>

  </div>
</body>

</html>

2 回复 | 直到 8 年前

Barmar 8 年前

您显示的代码只是实现了对旧浏览器的向后兼容性。

在 navigator.getUserMedia() 功能是标准化的,每个主要浏览器都有自己的功能,传统的做法是使用浏览器前缀命名扩展功能: webkit 对于基于WebKit的浏览器(Chrome和Safari), moz 对于Mozilla Firefox,以及 ms 对于Microsoft浏览器。

逻辑OR运算符 || 从左到右计算其操作数,返回第一个具有truthy值的操作数。

所以这段代码所做的就是尝试使用 导航器。getUserMedia() 如果它存在(在现代浏览器中也会存在)。如果没有,则会尝试使用每个旧的浏览器特定功能作为后备功能。

Gustavo Topete 8 年前

代码本身正在验证每个案例:

    navigator.getUserMedia ||
    navigator.webkitGetUserMedia ||
    navigator.mozGetUserMedia ||
    navigator.msGetUserMedia

它将返回第一个非null或未定义的var,然后将其分配给navigator。getMedia。我认为,编写这些代码是为了确保它能够跨多个不同的浏览器和版本工作。如果我错了,请纠正我