<!doctype html>
<html>
<head>
<meta charset="utf-8">
<title>无标题文档</title>
<meta name="viewport" content="width=device-width,initial-scale=1,maximum-scale=1,user-scalable=no" />
<style>
.content,.cover{width:400px; height:400px; position:absolute; left:50%; top:50%; margin:-200px 0 0 -200px;}
.content{ font-size:48px; line-height:400px; text-align:center;}
h3{ text-align:center; line-height:200px;}
</style>
</head>
<body>
<h3>快来刮开！！！</h3>
<div class="content" >中奖啦~！</div>
<canvas id="cover" class="cover" width="400" height="400"></canvas>
</body>
<script>
var isdown = false,
cover = document.getElementById("cover"),
covercanvas = cover.getContext("2d");
//
covercanvas.fillStyle="transparent";
covercanvas.fillRect(0,0,400,400);
function fillter( canvas ){
canvas.fillStyle="#ccc";
canvas.fillRect(0,0,400,400);
}
function isDown(e){
e.preventDefault();
isdown=true;
}
function isUp(e){
isdown=false;
}
function draw( e ){
e.preventDefault();
if(isdown){
if(e.changedTouches){
e=e.changedTouches[e.changedTouches.length-1];
}
var _height= parseInt((window.innerHeight-400)/2),
_width= parseInt((window.innerWidth-400)/2),
touchTop=e.clientY - _height,
touchLeft=e.clientX - _width;
with(covercanvas){
beginPath();
arc(touchLeft, touchTop, 10, 0, Math.PI * 2);
fill();
}
}
//alert(touchTop);
}
fillter(covercanvas);
covercanvas.globalCompositeOperation = 'destination-out';
cover.addEventListener('touchstart',isDown);
cover.addEventListener('touchmove',draw);
cover.addEventListener('touchend',isUp);
cover.addEventListener('mousemove',draw);
cover.addEventListener('mousedown',isDown);
cover.addEventListener('mouseup',isUp);
</script>
</html>