jquery计算出left和top,让一个div水平垂直居中

if($("#cont1").css("position")!="fixed"){
$("#cont1").css("position","absolute");
var dw = $(window).width();
var ow = $("#cont1").outerWidth();
var dh = $(window).height();
var oh = $("#cont1").outerHeight();
var l = (dw - ow) / 2;
var t = (dh - oh) / 2 > 0 ? (dh - oh) / 2 : 10;
var lDiff = $("#cont1").offset().left - $("#cont1").position().left;
var tDiff = $("#cont1").offset().top - $("#cont1").position().top;
l = l + $(window).scrollLeft() - lDiff;
t = t + $(window).scrollTop() - tDiff;
$("#cont1").css("left",l + "px");
$("#cont1").css("top",t + "px");
}