参考：
https://segmentfault.com/q/1010000006717959
问题：
a.获取的好像是第一帧的图？第一帧为透明图时，获取的个透明图片
b.得先加载视频到video,做视频上传的时候体验不太友好
c.qq空间能截取视频图片，不知道怎么实现的
<!DOCTYPE html>
<html>
<head>
<meta charset="utf-8" />
<title></title>
</head>
<body>
<video >
<source src="myvideo.mp4">
</video>
<br><br>
视频第一帧图片：
<div ></div>
<script type="text/javascript">
(function(){
var video, output;
var scale = 0.8;
var initialize = function() {
output = document.getElementById("output");
video = document.getElementById("video");
video.addEventListener('loadeddata',captureImage);
};
var captureImage = function() {
var canvas = document.createElement("canvas");
canvas.width = video.videoWidth * scale;
canvas.height = video.videoHeight * scale;
canvas.getContext('2d').drawImage(video, 0, 0, canvas.width, canvas.height);
var img = document.createElement("img");
img.src = canvas.toDataURL("image/png");
img.width = 400;
img.height = 300;
output.appendChild(img);
};
initialize();
})();
</script>
</body>
</html>