效果如下：
![4C2Y`_$MP(PDU4_X[FQE@K.png
话不多说直接上代码：
<!DOCTYPE html>
<html lang="en">
<head>
<meta charset="UTF-8">
<title>Document</title>
</head>
<body>
<div class="msg">
<button onclick="window.location.reload()">再试一次</button>
</div>
<canvas class="canvas"></canvas>
</body>
<script type="text/javascript">
var bodystyle = document.body.style;
bodystyle.mozUserSelect = "none";
bodystyle.webkitUserSelect = "none";
var img = new Image();
var canvas = document.querySelector('canvas');
canvas.style.backgrountColor = "transparent";
canvas.style.position = 'absolute';
var imgs = ['http://labfile.oss.aliyuncs.com/courses/133/p_0.jpg','http://labfile.oss.aliyuncs.com/courses/133/p_1.jpg'];
var num = Math.floor(Math.random()*2);
img.src = imgs[num];
img.addEventListener('load',function(e){
var ctx;
var w = img.width,
h = img.height;
var offsetX = canvas.offsetLeft,
offsetY = canvas.offsetTop;
var mouseDown = false;
function layer(ctx){
ctx.fillStyle = "gray";
ctx.fillRect(0,0,w,h)
}
function eventDown(e){
e.preventDefault();
mouseDown = true;
}
function eventUp(e){
e.preventDefault();
mouseDown = false;
}
function eventMove(e){
e.preventDefault();
if(mouseDown){
var x = e.pageX -offsetX;
var y = e.pageY - offsetY;
with(ctx) {
beginPath();
arc(x,y,10,0,Math.PI *2);
fill();
}
}
}
canvas.width = w;
canvas.height = h;
canvas.style.backgroundImage = 'url(' + img.src + ')';
ctx = canvas.getContext('2d');
ctx.fillStyle = 'transparent';
ctx.fillRect(0,0,w,h);
layer(ctx);
ctx.globalCompositeOperation = 'destination-out';
canvas.addEventListener('touchstart',eventDown);
canvas.addEventListener('touchend',eventUp);
canvas.addEventListener('touchmove',eventMove);
canvas.addEventListener('mousedown',eventDown);
canvas.addEventListener('mouseup',eventUp);
canvas.addEventListener('mousemove',eventMove);
},false)
</script>
</html>
