private function init():void
    {
     stage.align = StageAlign.TOP_LEFT;
     stage.scaleMode = StageScaleMode.NO_SCALE;
     
     balls = new Array();
     for(var i:uint = 0; i < numBalls; i++)
     {
      var ball:animation_ch15_Ball3D = new animation_ch15_Ball3D(15);
      balls.push(ball);
      ball.vx = Math.random() * 10 - 5;
      ball.vy = Math.random() * 10 - 5;
      ball.vz = Math.random() * 10 - 5;
      addChild(ball);
     }
     addEventListener(Event.ENTER_FRAME, onEnterFrame);
    }