function centerform() {
  o = $("#mydiv")
  o.css('top',  ((($(window).height()-o.outerHeight())/2) + (o.css('position') == 'absolute' ? $(window).scrollTop():0)   ) + 'px' );
  o.css('left', ((($(window).width()-o.outerWidth())/2)   + (o.css('position') == 'absolute' ? $(window).scrollLeft():0)  ) + 'px' );
}

function showform() {
  $("#mydiv").css('display', 'block');
}

$(document).ready(function() {

  centerform();
  $('.formtrigger').click( function() { centerform(); showform(); } );
  $('#closeform').click( function() { $('#mydiv').hide(); } );
  $(window).resize( function() {  centerform();  });


});