h5canvas实现刮刮乐效果代码

<script>    var arr=[        {name:"iphone7 磨砂黑",color:"rgba(255,255,0,1)"},        {name:"iphone7 磨砂黑",color:"rgba(0,255,0,.9)"},        {name:"iphone7 磨砂黑",color:"rgba(10,255,255,1)"},        {name:"iphone7 磨砂黑",color:"rgba(10,255,100,1)"}    ]    var r=Math.random();    var rIndex= Math.floor(r*arr.length);    var rPrice=arr[rIndex];    var cv=document.getElementsByTagName(&#39;canvas&#39;)[0];    var isDown=false;    cv.height=400;    cv.width=600;    var ctx=cv.getContext("2d");    function toAngle(radian){        return radian/Math.PI*180;    }    function toRadian(angle){        return angle/180*Math.PI;    }    ctx.textAlign="center";    ctx.textBaseline="middle";    ctx.font="30px consolas";    ctx.fillStyle=rPrice.color;    ctx.fillText(rPrice.name,cv.width/2,cv.height/2);    var dataURL=cv.toDataURL("image/jpg",1);    cv.style.background="url("+dataURL+")";    //覆盖层    ctx.beginPath();    ctx.fillStyle="#eee";    ctx.fillRect(0,0,cv.width,cv.height);    cv.addEventListener("mousedown",function(){        isDown=true;    })    cv.addEventListener("mouseup",function(){        isDown=false;        ctx.globalCompositeOperation="source-out"    })    cv.addEventListener("mousemove",function(e){        if (isDown){            ctx.globalCompositeOperation="destination-out";            ctx.beginPath();            var posObj=cv.getBoundingClientRect();            var left=posObj.left;            var top=posObj.top;            var x= e.clientX-left;            var y= e.clientY-top;            ctx.arc(x,y,50,0,Math.PI*2);            ctx.fill();        }    })</script>