html5实现刮刮卡效果

<!DOCTYPE html>
<html>
<body>
<img src="images/2.jpg" id="lamp" width="100%" />
<canvas/>
<script>
    (function(bodyStyle) {
        bodyStyle.mozUserSelect = ‘none‘;
        bodyStyle.webkitUserSelect = ‘none‘;
 
        var img = new Image();
        var canvas = document.querySelector(‘canvas‘);
        canvas.style.backgroundColor=‘transparent‘;
        canvas.style.position = ‘absolute‘;
 
        img.addEventListener(‘load‘, function(e) {
            var ctx;
            var w = img.width,
                h = img.height;
            var offsetX = canvas.offsetLeft,
                offsetY = canvas.offsetTop;
            var mousedown = false;
 
            function layer(ctx) {
                //ctx.fillStyle = ‘red‘;
                //ctx.fillRect(0, 0, w, h);//颜色图层
                var img=document.getElementById("lamp")
                var pat=ctx.createPattern(img,‘repeat‘);
                ctx.rect(0,0,w,h);
                ctx.fillStyle=pat;
                ctx.fill();
            }
 
            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, 10, 0, Math.PI * 2);
                        fill();
                    }
                }
            }
 
            canvas.width=w;
            canvas.height=h;
            canvas.style.backgroundImage=‘url(‘+img.src+‘)‘;
            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);
        });
        img.src = ‘images/1.jpg‘;//被遮住的背景图
    })(document.body.style);
</script>
</body>
</html>