private function onEnterFrame(event:Event):void
    {
     var angleX:Number = (mouseY - vpY) * .001;
     var angleY:Number = (mouseX - vpX) * .001;
     for(var i:uint = 0; i < numBalls; i++)
     {
      var ball:animation_ch15_Ball3D = balls[i];
      rotateX(ball, angleX);
      rotateY(ball, angleY);
      doPerspective(ball);
     }
     sortZ();
    }