function popupwindow(url, title, w, h) {
  var left = (screen.width/2)-(w/2);
  var top = (screen.height/2)-(h/2);
  return window.open(url, title, 'toolbar=no, location=no, directories=no, status=no, menubar=no, scrollbars=no, resizable=no, copyhistory=no, width='+w+', height='+h+', top='+top+', left='+left);
}