jQuery.fn.center = function () {
      this.css("position","absolute");
      this.css("top", ( $(window).height() - this.outerHeight() ) / 2+$(window).scrollTop() + "px");
      this.css("left", ( $(window).width() - this.outerWidth() ) / 2+$(window).scrollLeft() + "px");
      return this;
    }

    $(document).ready(function() {
      $(".mainDiv").center();
      $(window).bind('resize', function() {
        $('.mainDiv').center({transition:300});
      });

    });