HTML5可以通过调用navigator.getUserMedia来获取手机设备摄像头，兼容性写法如下
window.navigator.getUserMedia = navigator.getUserMedia || navigator.webKitGetUserMedia || navigator.mozGetUserMedia || navigator.msGetUserMedia;
但是navigator.getUserMediau已经从 Web 标准中删除，虽然部分浏览器可以使用，生产环境中还是要做好兼容。新的API更替为MediaDevices.getUserMedia。MediaDevices.getUserMedia可以通过video的facingMode属性指定调用手机的前置或后置摄像头
video:{ 'facingMode': "user" }//调用前置摄像头
video: { facingMode: { exact: "environment" } }//后置
具体代码：
<!DOCTYPE html>
<html lang="en">
<head>
<meta charset="UTF-8">
<meta name="viewport" content="width=device-width,initial-scale=1.0,maximum-scale=1.0,minimum-scale=1.0,user-scalable=no">
<title>Document1</title>
<style type='text/css'>
* { margin: 0; padding: 0; }
html, body { height: 100%; }
.flex { display: flex; }
.flex-row { flex-direction: row; justify-content: space-around; align-items: center; }
.flex-column { flex-direction: column; justify-content: flex-start; align-items: center; }
body { overflow: auto; background: #fff; }
.title { width: 920px; padding: 30px; align-items: flex-end; }
.title h1 { padding-bottom: 20px; font-size: 38px; color: #ffffff; text-shadow: 0 1px 3px #222222; }
.title p { font-size: 18px; color: #f5f5f5; text-shadow: 0 1px 3px #565656; }
.wrap { width: 1220px; }
.wrap .reference { position: relative; padding: 10px; background-color: rgba(255, 255, 255, 0); border-radius: 10px; box-shadow: 0 0 20px #a1a19f; }
.wrap .reference img.face { display: block; width: 320px; height: auto; border-radius: 10px; }
.wrap .reference img.toggle { position: absolute; right: 10px; top: 10px; width: 50px; height: 50px; }
.wrap .scan video { background-color: rgba(0, 0, 0, .8); border-radius: 10px; }
.wrap .control { justify-content: space-around; height: 456px; padding: 0 20px; }
.wrap .control p { width: 160px; height: 60px; background-color: #f9f9f9; text-align: center; line-height: 60px; color: #ffffff; font-size: 24px; border-radius: 8px; cursor: pointer; box-shadow: -8px -8px 150px -8px #b2b3b5 inset, 0 0 5px #222222; text-shadow: 0 0 1px #222222; transition: .5s; }
.wrap .control p:hover { box-shadow: -8px -8px 150px -8px #50c4f1 inset, 0 0 5px #ffffff; }
.wrap .scan { position: relative; overflow: hidden; }
.wrap .scan .strainer { position: absolute; top: 10px; width: 320px; z-index: 999; height: 3px; }
.wrap .scan .capture { width: 320px; height: 456px; }
.wrap .scan .strainer.on { background: linear-gradient(to left, transparent, #0bffb2, transparent); animation: scan 1s linear infinite; }
@keyframes scan {
0% { top: 10px; }
50% { top: 456px; }
100% { top: 10px; }
}
</style>
<script type="text/javascript" src="vconsole.min.js"></script>
</head>
<body>
<div class="title flex flex-column">
</div>
<div class="wrap flex flex-row">
<div class="control flex flex-column">
<p class="open">开启摄像头</p>
<p class="recognition">显示到Canvas</p>
<p class="close">关闭摄像头</p>
</div>
<div class="scan reference">
<div class="strainer"></div>
<video class="capture" width="320" height="456" src=""></video>
</div>
</div>
<script type="text/javascript">
var buffer;
var oCapture = document.querySelector(".capture"),
open = document.querySelector(".open"),
recognition = document.querySelector(".recognition"),
close = document.querySelector(".close");
var control = document.querySelector(".control");
window.navigator.getUserMedia = navigator.getUserMedia || navigator.webKitGetUserMedia || navigator.mozGetUserMedia || navigator.msGetUserMedia;
function invokingCarera(){
if(navigator.mediaDevices&&navigator.mediaDevices.getUserMedia){
navigator.mediaDevices.getUserMedia({
'audio':true,
'video':{ 'facingMode': "user" }//调用前置摄像头，后置摄像头使用video: { facingMode: { exact: "environment" } }
})
.then(function(mediaStream) {console.log(555);getVideoStream(mediaStream)})
.catch(function(error) { console.log(666);console.log(error) })
}else if(navigator.getUserMedia){
navigator.getUserMedia({
'video':true,
'audio':true
},getVideoStream,getFail)
}else{
alert('不支持摄像头调用！')
}
}
//调用成功
function getVideoStream(stream){
buffer = stream;
if(oCapture.mozSrcObject !== undefined){
oCapture.mozSrcObject = buffer;
}else{
oCapture.src = window.URL && window.URL.createObjectURL(buffer);
}
oCapture.play();
}
function getFail(){
}
recognition.onclick = function(){
}
control.addEventListener('click',function(e){
e = e || window.event;
var className = e.target.className;
switch(className){
case 'open':
invokingCarera();
break;
case 'close':
closeCamera();
break;
case 'recognition':
screenShot();
break;
default:
break;
}
})
function closeCamera(){
buffer&&buffer.getTracks()[1].stop();//关闭摄像头
}
window.requestAnimFrame = (function(){
return window.requestAnimationFrame ||
window.webkitRequestAnimationFrame ||
window.mozRequestAnimationFrame ||
function( callback ){
window.setTimeout(callback, 1000 / 60);
};
})();
function screenShot(){
var canvas = document.createElement('canvas');
canvas.width=320,canvas.height = 456;
document.querySelector(".wrap").appendChild(canvas);
var ctx = canvas.getContext('2d');
function drawVideo(){
ctx.drawImage(oCapture,0,0,320,456);
ctx.font = "30px sans-serif";
ctx.fillStyle = "blue";
ctx.fillText("请眨眼", 50, 50);
requestAnimationFrame(drawVideo);
}
window.requestAnimationFrame(drawVideo);
}
</script>
</body>
</html>
以上代码并没有进行太多的错误处理，比如用户拒绝授权访问摄像头或浏览器不支持等情况，生产环境使用需添加错误处理代码
以上代码在PC端和Android手机端和微信测试通过，IOS下浏览器均不支持，据说IOS11会开放权限。
注意，以上HTML需要在HTTPS下访问方可正常工作
查看在线DEMO，如果使用微信访问可能被屏蔽，微信打开链接后点击右下角“访问原网页”
如需本地测试请使用Chrome插件：web-server;
webserver使用方法：chrome://apps > web-server > choose folder 勾选 Show Advanced Settings下Set CORS Headers即可
使用input type=”file”调用摄像头拍照可以参考camera API
补充下，获取vedio的实际尺寸，可以通过：this.videoWidth，this.videoHeight获取。
HTML5之drawImage函数
可用函数
drawImage(image, x, y) //按原图片大小绘制。
drawImage(image, x, y, width, height) //按指定大小绘制。
drawImage(image, sourceX, sourceY, sourceWidth, sourceHeight,
destX, destY, destWidth, destHeight) //常用于图片裁剪
参数
image:所要绘制的图像。这必须是表示 标记或者屏幕外图像的 Image 对象，或者是 Canvas 元素。
x和y：图片在文档中的坐标位置。
width和height：图片的宽高。
对于drawImage(image, sourceX, sourceY, sourceWidth, sourceHeight,
destX, destY, destWidth, destHeight) 常用有图片的裁剪。其参数含义是原来image上从某一个位置开始(sourceX,sourceY)，指定长宽进行剪切(sourceX,sourceY),然后将剪切的内容放到位置为(destX,destY),宽度为(destWidth),高度为(destHeight)的位置上，当然裁剪后的会覆盖原来的图片。<!DOCTYPE html>
<html lang="en">
<head>
<meta charset="UTF-8">
<title>drawImage</title>
</head>
<body>
<canvas id="myCanvas" width="500" height="400"></canvas>
<script type="text/javascript">
var canv=document.getElementById("myCanvas");
var ctx = canv.getContext("2d");
img = new Image();
img.src = "2.jpg";
//当图片加载完毕的时候在drawImage,否则可能图片还没有加载完毕
//当然画不上去喽，这就和浏览器的性能有关了。
img.onload=function(){
ctx.drawImage(img,0,0,500,500);
//ctx.drawImage(img,0,0,100,100,300,100,100,100);
}
</script>
</body>
</html>
除此之外，drawImage()还可以画video，参考链接