第二个是设置在绘制插图的时候，设置透明透明（用到globalCompositeOperation属性）

代码如下：

<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('canvas')[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>

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