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