刮刮卡的效果和这个一样，只是我这个是画的雾，擦雾擦着擦着雾就消失的效果，

html代码:

.cloud{
width: 100%;
height: 100%;
position: absolute;
left: 0;
top: 0;
z-index: 999999;
}

<div class="cloud hide">
<canvas></canvas>
</div>

js代码：

function doCa(){
var img = new Image();
var canvas = document.querySelector('canvas');
canvas.style.backgroundColor='transparent';
canvas.style.position = 'absolute';
canvas.style.width = '100%';
canvas.style.height = '100%';
var imgs = ['/images/cloud-animate.png'];//我这里用的是图片，半透明的云
img.src = imgs[0];

img.addEventListener('load', function(e) {
var ctx;
var w = img.width,
h = img.height;
var offsetX = canvas.offsetLeft,
offsetY = canvas.offsetTop;
var mousedown = false;
var key = 0;//滑动的点的个数，我是根据这个个数来判断什么时候隐藏掉雾的这个div层的
function layer(ctx) {
var imgs=document.getElementById("cloud-animate");
var pat=ctx.createPattern(imgs,'no-repeat');
ctx.fillStyle=pat;
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) {
if(e.changedTouches){
e=e.changedTouches[e.changedTouches.length-1];
}
var x = (e.clientX + document.body.scrollLeft || e.pageX) - offsetX || 0,
y = (e.clientY + document.body.scrollTop || e.pageY) - offsetY || 0;
with(ctx) {
beginPath()
arc(x, y, 50, 0, Math.PI * 2);
fill();

//滑动的时候，没动一下记录一次
if(key >= 150){
$(".cloud").addClass('hide');
key = 0;
}else{
key++;
}
}
}
}

canvas.width=w;
canvas.height=h;
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);
});
}