WebRTC–getUserMedia & Canvas

下面是一个使用getUserMedia接口和Canvas的drawImage方法实现的截图功能（截取视频中的一帧）。

基本思路是这样子的：

getUserMedia获取一个MediaStream, stream
stream作为video的输入源
提供一个button按钮。当用户点击时，使用canvas的drawImage方法绘制video的一帧数据

index.html代码

<!DOCTYPE html>
<html>
<head>
<meta name="keywords" content="WebRTC, HTML5, JavaScript" />
<meta name="description" content="WebRTC Demo." />
<meta http-equiv="Content-Type" content="text/html; charset=utf-8" />
<meta name="viewport" content="width=device-width, initial-scale=1.0, minimum-scale=1.0">
<meta http-equiv="X-UA-Compatible" content="chrome=1" />
<base target="_blank">
<title>getUserMedia to canvas - Snapshot</title>
<link rel="stylesheet" href="../css/main.css" />
</head>
<body>
<div id="container">

  <h1><a href="#" title="WebRTC Demo homepage">WebRTC Demo</a> <span>getUserMedia to canvas - 拍照</span></h1>

  <video autoplay></video>
  <button>拍照</button>
  <canvas></canvas>

  <p>使用canvas元素的<code>drawImage()</code>方法绘制video元素的一帧数据。</p>

  <p><code>canvas</code>, <code>video</code> and <code>stream</code>三个变量是全局的，用户可以从console中查看。</p>

  <a href="#" title="在gitlab上查看源代码" id="viewSource">查看源代码-gitlab.baidu.com</a>
  </div>

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

main.js代码

// 全局变量
button = document.querySelector("button");
video = document.querySelector("video");
canvas = document.querySelector("canvas");

canvas.width = 480;
canvas.height = 360;

// 点击事件处理
// 在canvas上画一帧画像
button.onclick = function(){
  canvas.getContext("2d").drawImage(video, 0, 0, canvas.width, canvas.height);
}

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);

drawImage方法定义如下：

context.drawImage(img,sx,sy,swidth,sheight,x,y,width,height);

img - 规定要使用的图像、画布或视频

sx - 开始剪切的x位置

sy - 开始剪切的y位置

swidth - 被剪切的宽度

sheight - 被剪切的高度