没有固定高度宽度的层垂直居中

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