private function move(ball:animation_ch15_Ball3D):void
    {
     var radius:Number =  ball.radius;
     
     ball.xpos += ball.vx;
     ball.ypos += ball.vy;
     ball.zpos += ball.vz;
     
     if(ball.xpos + radius > right)
     {
      ball.xpos = right - radius;
      ball.vx *= -1;
     }
     else if(ball.xpos - radius < left)
     {
      ball.xpos = left + radius;
      ball.vx *= -1;
     }
     if(ball.ypos + radius > bottom)
     {
      ball.ypos = bottom - radius;
      ball.vy *= -1;
     }
     else if(ball.ypos - radius < top)
     {
      ball.ypos = top + radius;
      ball.vy *= -1;
     }
     if(ball.zpos + radius > front)
     {
      ball.zpos = front - radius;
      ball.vz *= -1;
     }
     else if(ball.zpos - radius < back)
     {
      ball.zpos = back + radius;
      ball.vz *= -1;
     }