function popup(url, w, h) {
   var dx = Math.round((screen.availWidth - w) / 2);
   var dy = Math.round((screen.availHeight - h) / 2);
   window.open(url,'','width='+w+', height='+h+', left='+dx+', top='+dy+', toolbar=no, statusbar=no, scrollbars=yes, resizable=yes, menubar=yes');
}