[Jquery] layer 를 화면 중간에 띄우기

<script type="text/javascript">
jQuery.fn.center = function () {
    this.css("position","absolute");
    this.css("top", Math.max(0, (($(window).height() - $(this).outerHeight()) / 2) + $(window).scrollTop()) + "px");
    this.css("left", Math.max(0, (($(window).width() - $(this).outerWidth()) / 2) + $(window).scrollLeft()) + "px");
    return this;
}

function open(layerId){
	$("#"+layerId).show();
	$("#"+layerId).center();//fn
}