<!DOCTYPE html>
<html>
<head>
<meta charset="utf-8">
<title></title>
<style type="text/css">
* {
margin: 0;
padding: 0;
}
</style>
</head>
<body>
<div id="box">
<img src="img/1.jpg" style="width: 700px;height: 500px;" />
<canvas id="myCanvas" width="700" height="500" style="position: absolute; left: 0px;top: 0px;"></canvas>
</div>
</body>
<script type="text/javascript">
var myCanvas = document.getElementById(‘myCanvas‘);
var context = myCanvas.getContext(‘2d‘);
//设置一个灰色矩形的画布 将图片盖住
context.fillStyle = ‘gray‘;
context.fillRect(0, 0, 700, 500);
myCanvas.onmousedown = function(e) {
//获取鼠标按下时鼠标的坐标
var X = e.clientX - box.offsetLeft;
var Y = e.clientY - box.offsetTop;
//绘制起点
context.beginPath();
context.moveTo(X, Y);
myCanvas.onmousemove = function(e) {
//移动的时候开始绘制 获取此时的鼠标坐标
var x = e.clientX - box.offsetLeft;
var y = e.clientY - box.offsetTop;
//让绘制区域重合部分显示透明
context.globalCompositeOperation = "destination-out";
context.lineTo(x, y);
//修饰一下线条
context.lineCap = ‘round‘;
context.lineWidth = 30;
context.stroke();
//当刮到一半的时候,显示全图
//获取画布上的所有数据
var imgData = context.getImageData(0, 0, 700, 500);
//从图片数据中把像素获取出来
var pixels = imgData.data;
var num = 0;
//被刮的区域显示透明 遍历一下所有色素块 用num存一下有多少个像素显示透明(即被刮区域)
for(var i = 0; i < pixels.length; i += 4) {
if(pixels[i + 3] == 0) {
num++;
}
}
//当(被刮区域)超过 百分之多少时 让整个刮刮乐 全部显示
if(num / (pixels.length / 4) > 0.3) {
myCanvas.style.opacity = 0;
}
}
};
//当鼠标抬起时 让移动事件失效
myCanvas.onmouseup = function() {
myCanvas.onmousemove = null;
}
</script>
</html>
开始用用矩形或球形来当笔触,如果移动的过快浏览器有一个反应时间,会让刮的区域中间形成间隙.用线条来当笔触,适当的修饰笔触,可避免这个现象.
原文：http://www.cnblogs.com/wangziye/p/6014196.html