canvas绘制五星红旗

代码

<!DOCTYPE html>
<html lang="en">
<head>
  <meta charset="UTF-8">
  <title>canvas绘制五星红旗</title>
</head>
<body>
  <canvas id="canvas" width="600" height="400"></canvas>
  <script>
    var canvas = document.getElementById('canvas');
    var ctx = canvas.getContext('2d');
    // 绘制背景
    var width=canvas.width;
    var height=width*2/3;
    ctx.fillStyle="red";
    ctx.fillRect(0,0,width,height);

    var maxR=0.15,minR=0.05;                //0.15大五角星的半径，0.05小五角星的半径
    var maxX=0.25,maxY=0.25;                //大五角星的位置
    var minX=[0.50,0.60,0.60,0.50];         //小五角星的X坐标
    var minY=[0.10,0.20,0.35,0.45];         //小五角星的Y坐标
    var ox=height*maxX,oy=height*maxY;        //大五角星的中心坐标
    drawStar(ctx,ox,oy,height*maxR,"#ff0",0);        //绘制五角星
    for (var idx = 0; idx < 4; idx++) {
            var  sx = minX[idx] * height, sy = minY[idx] * height;
            var  theta = Math.atan((oy - sy)/(ox - sx));
            drawStar(ctx,sx, sy, height * minR, "#ff0",-Math.PI/2+theta);
     }

     /*五角星的坐标为（sx,sy）,半径为radius,rotate为0时一个顶点在对称轴上*/
     /*rotate:绕对称轴旋转rotate弧度*/
    function drawStar(ctx,sx,sy,radius,color,rotate){
      ctx.save();
      ctx.fillStyle=color;
      ctx.translate(sx,sy);                //移动坐标原点
      ctx.rotate(Math.PI+rotate);          //Math.PI等于圆周率3.14
      ctx.beginPath();
      //360度分成5份，2/5*PI,但底下是PI/5*4
      var dig=Math.PI/5*4;
      for(var i=0;i<5;i++){           //画五角星的五条长边
        var x=Math.sin(i*dig);        //点的x坐标
        var y=Math.cos(i*dig);        //点的y坐标
        ctx.lineTo(x*radius,y*radius);
      }
      ctx.closePath();
      ctx.strokeStyle = "red"
      ctx.stroke();
      ctx.fill();
      ctx.restore();
    }
  </script>
</body>
</html>

效果

http://www.360doc.com/content/16/0809/10/10978981_581855158.shtml