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Jun 24, 2016 am 11:44 AM

The last time I wrote about the scratch-off effect was a year ago. At that time, I found the source code from Baidu and modified it. I was actually confused. This time, I asked a girl to write it, and then I thought Reorganize it yourself.

<!DOCTYPE html><html><head>    <meta charset="UTF-8">    <meta name="viewport" content="width=device-width, user-scalable=no, initial-scale=1.0, maximum-scale=1.0, minimum-scale=1.0">    <title>刮奖效果</title>    <style type="text/css">    #canvas{        display: block;        margin:0 auto;    }    </style></head><body>    <canvas id="canvas"></canvas>    <script type="text/javascript">    // 初始化，设置canva的宽(width)、高(height),涂抹画笔的直径(r)，背景图片(img)（即底层的图片）,上层遮罩层    var width=440;    var height=440;    var r=20;    var img = new Image();    img.src="1.jpg";    var canvas=document.getElementById("canvas");    canvas.style.backgroundImage='url('+img.src+')';    canvas.width=width;    canvas.height=height;    var ctx=canvas.getContext("2d");    ctx.fillStyle="#ccc";    ctx.fillRect(0,0,width,height);                    //绘制上层的涂层    ctx.globalCompositeOperation="destination-out";    //在源图像外显示目标图像。只有源图像外的目标图像部分会被显示，源图像是透明的。    var isMove=false;    var el=new Array();    el.x=canvas.offsetLeft;    el.y=canvas.offsetTop;    /************************************    方式一：利用arc来模拟两端圆角的直线    ***********************************/    //按下时    function touchStart(e){        e.preventDefault();        isMove=true;        ctx.closePath();    }    // 移动时    function touchMove(e){        e.preventDefault();        var x,y;        if(isMove){            if(e.targetTouches){                    //移动端                var touch=e.targetTouches[0];                x=touch.clientX-el.x;                //坐标矫正                y=touch.clientY-el.y;            }else{                                    //pc端                x=e.pageX-el.x;                y=e.pageY-el.y;            }            ctx.beginPath();            ctx.arc(x,y,r,0,Math.PI*2);                //绘制笔触            ctx.fill();            ctx.closePath();                        //每次闭合路径        }    }    // 离开时    function touchEnd(e){        e.preventDefault();        isMove=false ;        ctx.closePath();        //涂抹区域百分比        var imgData=ctx.getImageData(0,0,width,height);//返回ImageData对象，该对象拷贝画布指定的像素数据        var pixles=imgData.data;        var transNum=0;        //rgba以数组形式储存在imageData中，        // r=imgData.data[0];        // g=imgData.data[1];        // b=imgData.data[2];        // a=imgData.data[3];        for(var i=3,j=pixles.length;i<j;i+=4){            var a=pixles[i];            if(a==0){                transNum++            }        }        var transPercent=transNum/(pixles.length/4);//透明度比        console.log(transPercent);        //超过某个值显示所有        if(transPercent>0.5){            ctx.fillRect(0,0,width,height);        }    }    /**************************************************************************    方式二：ctx.lineTo(x,y)来画直线，用ctx.lineCap="round";来设置直线两端的圆角    当滑的很快的时候，这种效果比第一种好    ***************************************************************************/    //按下时    // function touchStart(e){    //     e.preventDefault();    //     isMove=true;    //     var x,y;    //     if(e.targetTouches){                    //移动端    //         var touch=e.targetTouches[0];    //         x=touch.clientX-el.x;    //         y=touch.clientY-el.y;    //     }else{                                    //pc端    //         x=touch.clientX-el.x;    //         y=touch.clientY-el.y;    //     }    //     ctx.beginPath();    //     ctx.lineCap="round";    //     ctx.lineJoin="round";    //     ctx.lineWidth=r;    //     ctx.moveTo(x,y);    // }    // // 移动时    // function touchMove(e){    //     e.preventDefault();    //     var x,y;    //     if(isMove){    //         if(e.targetTouches){                    //移动端    //             var touch=e.targetTouches[0];    //             x=touch.clientX;    //             y=touch.clientY;    //         }else{                                    //pc端    //             x=e.pageX;    //             y=e.pageY;    //         }    //         ctx.lineTo(x,y);                        //绘制笔触    //         ctx.stroke();    //     }    // }    // // 离开时    // function touchEnd(e){    //     e.preventDefault();    //     isMove=false ;    //     ctx.closePath();    // }    // 事件监听    canvas.addEventListener("touchstart",touchStart);    canvas.addEventListener("mousedown",touchStart);    canvas.addEventListener("touchmove",touchMove);    canvas.addEventListener("mousemove",touchMove);    canvas.addEventListener("touchend",touchEnd);    canvas.addEventListener("mouseup",touchEnd);    </script></body></html>

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