JS居中弹出div，背景变暗

//背景css：
#fullbg{
position:absolute;
cursor:pointer;
background-color: #000000;
display: none;
z-index: 100;
left: 0px;
top: 0px;
filter: Alpha(Opacity=50);
-moz-opacity: 0.5;
opacity: 0.5;
}

//弹出背景html：
<div id="fullbg"></div>

//弹出框div的id：

tishi_info

function show_tishi()
{
var bH = $(document).height();
var bW = $(document).width();
$("#fullbg").css({ width: bW, height: bH, display: "block" });

var x=$(window).width()/2;
var y=$(window).height()/2;

var div_w=$("#tishi_info").width()/2;
var div_h=$("#tishi_info").height()/2;

var stop=document.documentElement.scrollTop;

var sleft=$("tishi_info").scrollLeft();
var left=x-div_w+sleft+"px";
var top=y-div_h+stop+"px";
$("#tishi_info").css({"position": "absolute",left:left,top:top});

$("#tishi_info").show('block');

}