标签：
<!DOCTYPE html>
<html>
<head>
<meta charset="utf-8">
<meta name="viewport" content="initial-scale=1.0, width=device-width,minimum-scale=1.0,maximum-scale=1.0, user-scalable=no" />
<title> 刮刮卡 </title>
</head>
<body>
<canvas id="canvas" width="300" height="150"></canvas>
<script>
var oCanvas = document.getElementById(‘canvas‘),
aPrizeList = [‘images/1.jpg‘, ‘images/2.jpg‘],//控制中奖几率
prizeImg = aPrizeList[Math.floor(Math.random() * aPrizeList.length)];
scratch(oCanvas, prizeImg, 0.3, function(){
alert(‘成功！‘);
});
/**
* 刮刮卡
* @param oCanvas canvas绘图对象
* @param prizeImg 中奖图片
* @param scale 刮开区域占比
* @param callback 刮开后执行的函数
*/
function scratch(oCanvas, prizeImg, scale, callback){
if(!oCanvas.getContext) return;
var w = oCanvas.getAttribute(‘width‘),
h = oCanvas.getAttribute(‘height‘),
ctx = oCanvas.getContext(‘2d‘),
fn = callback || function(){},
bEnd = false; // 防止touchend多次执行回调
// 设置奖品图片
oCanvas.style.background = ‘url(‘+ prizeImg +‘) 0 0/cover no-repeat‘;
// 绘制灰色刮奖区域
ctx.fillStyle = "#999";
ctx.fillRect(0,0, w, h);
// 设置重叠显示方式
ctx.globalCompositeOperation="destination-out";
// 绑定事件
oCanvas.addEventListener(‘touchstart‘, function(){
// 按下并滑动
oCanvas.addEventListener(‘touchmove‘, function(e){
e.preventDefault();
var x = e.targetTouches[0].clientX + document.body.scrollLeft - oCanvas.offsetLeft,
y = e.targetTouches[0].clientY + document.body.scrollTop - oCanvas.offsetTop;
ctx.beginPath();
ctx.arc(x, y, 15, 0, Math.PI*2);
ctx.fill();
ctx.closePath();
});
oCanvas.addEventListener(‘touchend‘, function(e){
e.preventDefault();
var count = 0,
datas = ctx.getImageData(0,0,w,h);
for (var i = 0, len = datas.data.length; i < len; i++) {
if (datas.data[i] == 0) {
count++;
}
}
if (count >= datas.data.length * scale && !bEnd) {
fn();
ctx.fillRect(0,0,w,h);
bEnd = true;
}
});
});
}
</script>
</body>
</html>
效果如图：
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原文地址：http://www.cnblogs.com/beiz/p/5069682.html
