canvas 星星闪烁的效果

代码实例：

<!DOCTYPE html>
<html lang="en">
<head>
	<meta charset="UTF-8">
		<style>
		canvas{
		background:#eee;
		}
		</style>
		<title>星星</title>
		<meta charset="utf-8">

		<script>
			window.onload=function  () {
			 var canvas=document.getElementById("canvas");
			 var cobj=canvas.getContext("2d");

		     var starArr=[];
			 for (var i=0; i<30; i++) {
              var starObj={
			   radius1:20+10*Math.random(),radius2:8+7*Math.random(),x:30+(canvas.width-60)*Math.random(),y:30+(canvas.height-60)*Math.random(),num:Math.ceil(4+4*Math.random()),color:"rgb("+parseInt(255*Math.random())+","+parseInt(255*Math.random())+","+parseInt(255*Math.random())+")",angle:360*Math.random(),changeAngle:-5+10*Math.random()
			  }
			  starArr.push(starObj);
			 }
              setInterval(function  () {
			    cobj.clearRect(0,0,500,500);
				for (var i=0; i<starArr.length; i++) {
				starArr[i].angle+=starArr[i].changeAngle;
				cobj.save();
                cobj.beginPath();
				cobj.translate(starArr[i].x,starArr[i].y);
				cobj.rotate(starArr[i].angle*Math.PI/180);
				cobj.scale(Math.sin(starArr[i].angle*Math.PI/180),Math.sin(starArr[i].angle*Math.PI/180))
				cobj.globalAlpha=Math.abs(Math.sin(starArr[i].angle*Math.PI/180));
				drawStar (0,0,starArr[i].radius1,starArr[i].radius2,starArr[i].num,"fill",starArr[i].color);
				cobj.restore();
				}
			  },60)

			 function drawStar (x,y,radius1,radius2,num,drawType,color) {
				    var angle=360/(num*2);
					var arr=[];
					for (var i=0; i<num*2; i++) {
					 var starObj={};
					  if(i%2==0){
					  starObj.x=x+radius1*Math.cos(i*angle*Math.PI/180);
					  starObj.y=y+radius1*Math.sin(i*angle*Math.PI/180);
					  }else{
					   starObj.x=x+radius2*Math.cos(i*angle*Math.PI/180);
					   starObj.y=y+radius2*Math.sin(i*angle*Math.PI/180);
					  }
					  arr.push(starObj);
					}
					cobj.beginPath();
					cobj.fillStyle=color;
					cobj.strokeStyle=color;
					cobj.moveTo(arr[0].x,arr[0].y);
					for (var i=1; i<arr.length; i++) {
					cobj.lineTo(arr[i].x,arr[i].y);
					}
					cobj.closePath();
					if(drawType=="fill"){
					cobj.fill();
					}else{
					cobj.stroke();
					}
                 }
			}
		</script>
	</head>
	<body>
		<canvas id="canvas" width=500 height=500>
		</canvas>
	</body>
</html>

　　效果：