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scratchCard.html
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<!DOCTYPE html>
<html>
<head>
<meta http-equiv="Content-Type" content="text/html; charset=utf-8">
<meta name="viewport" content="width=device-width, initial-scale=1.0, minimum-scale=1.0, maximum-scale=1.0, user-scalable=no">
<style>
html, body {
margin: 0;
padding: 0;
background: #000;
}
.wrapper {
position: relative;
width: 1024px;
margin: auto;
}
.blowImg {
width: 600px;
height: 400px;
}
img {
width: 600px;
height: 400px;
}
.blowImg, #myCanvas {
position: absolute;
border: 0px solid #000;
}
</style>
</head>
<body>
<div class="wrapper">
<div class="blowImg">
<img src="http://b.hiphotos.baidu.com/image/h%3D768%3Bcrop%3D0%2C0%2C1024%2C768/sign=f8101107b33533faeab6912890e89e6d/cf1b9d16fdfaaf51121619b68e5494eef11f7ac6.jpg" alt=""恭喜中奖！>
</div>
<canvas id="myCanvas" width='1024' height='768' style="border:0px solid red;">your browser donot support the canvas</canvas>
</div>
<script>
var init = function () {
var canvas = document.getElementById('myCanvas');
var ctx = canvas.getContext('2d');
// 遮盖要显示的图层
ctx.globalCompositeOperation = 'source-over';
ctx.fillStyle = '#aaa';
ctx.beginPath();
ctx.rect(0, 0, canvas.width, canvas.height);
ctx.fill();
ctx.closePath();
ctx.stokeStyle = 'rgba(0, 0, 0, 255)';
ctx.lineWidth = 10;
ctx.lineCap = 'round';
ctx.lineJoin = 'round';
ctx.globalCompositeOperation = 'destination-out';
var firstClick = false;
canvasOffsetLeft = getElementPosition(canvas).x;
canvasOffsetTop = getElementPosition(canvas).y;
// 触摸事件
var touchStart = function (e) {
e.preventDefault();
var touch = e.touches[0];
var startX = touch.pageX - canvasOffsetLeft;
var startY = touch.pageY - canvasOffsetTop;
console.log(touch.pageX, touch.pageY);
ctx.beginPath();
ctx.moveTo(startX, startY);
};
// 计算touchmove的触发事件
var count = 0;
var touchMove = function (e) {
e.preventDefault();
var touch;
// ctx.moveTo(e.touches[0].pageX - canvasOffsetLeft, e.touches[0].pageY.canvasOffsetTop);
// for (var i = 0; i < e.touches.length; i++) {
touch = e.touches[0];
count++;
console.log(touch.pageX, touch.pageY, count, e.touches.length);
posX = touch.pageX - canvasOffsetLeft;
posY = touch.pageY - canvasOffsetTop;
ctx.lineTo(posX, posY);
// }
ctx.stroke();
/*if (!canvas.style.opacity) {
canvas.style.opacity = 0.09;
}else {
canvas.style.opacity = '';
}*/
// 计算刮开面积
// var percentage = areaOK();
// console.log(percentage);
};
var touchEnd = function (e) {
e.preventDefault();
ctx.closePath();
return false;
};
// 绑定事件
document.addEventListener('touchstart', touchStart, false);
document.addEventListener('touchmove', touchMove, false);
document.addEventListener('touchend', touchEnd, false);
// 判断刮开区域达到一定的程度
function areaOK () {
var data = ctx.getImageData(0, 0, canvas.width, canvas.height).data;
var pixels = canvas.width * canvas.height, clearPixels = 0;
for (var i = 0; i < data.length; i += 4) {
if (data[i+3] == 0) {
clearPixels++;
}
}
return clearPixels / pixels;
}
areaOK();
};
// 获取当前元素在页面上的偏移位置
function getElementPosition (e) {
var x = 0, y = 0;
while (e != null) {
x += e.offsetLeft;
y += e.offsetTop;
e = e.offsetParent;
}
// console.log(x, y);
return {
x : x,
y : y
};
}
init();
</script>
</body>
</html>