HTML5 Canvas实现多层同心圆绘制示例

<!DOCTYPE html>
<html>
 <head>
  <title>demo</title>
  <style type="text/css">
   #canvas {
    background:#F2F2F2; height:500px; height:500px; margin-top:100px; margin-left:200px;
   }
  </style>
 </head>
 <body>
  <canvas id="canvas" width="500px" height="500px" ></canvas>
 </body>
 <script type="text/javascript">
  (function() {
    var dyl = {};
    dyl.getDom = function(id) {
        return document.getElementById(id);
    }
    dyl.getContext = function(canvasID) {
        var canvas = this.getDom(canvasID);
        if(!canvas) {
            return null;
        }
        return canvas.getContext("2d");
    }
    if(!window.dyl) {
        window.dyl = dyl;
    }
  })();
  cache = {};
  cache.context = dyl.getContext('canvas');
  // 每个圈的圆个数控制
  cache.scaleNmb = 6;
  cache.createColor = function() {
   var color = "rgb(";
   color += Math.round(Math.random()*255);
   color += ",";
   color += Math.round(Math.random()*255);
   color += ",";
   color += Math.round(Math.random()*255);
   color += ")";
   return color;
  };
  cache.draw = function() {
   cache.context.fillRect(-10, -10, 20, 20);
   for(var i=1; i<10; i++) { 
    cache.context.save();
    for(var j=0; j<cache.scaleNmb*i; j++) {
     cache.context.rotate(Math.PI*2/(cache.scaleNmb*i));
     cache.context.fillStyle = cache.createColor();
     cache.context.beginPath();
     cache.context.arc(0, 20*i, 5, 0,Math.PI*2, true);
     cache.context.closePath();
     cache.context.fill();
    }
    cache.context.restore();
   }
  };
  cache.init = function() {
   cache.context.translate(250, 250);
   cache.draw();
  };
  cache.init();
 </script>
</html>