JS居中彈出div，背景變暗

//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;
}

//彈出背景html：
<div id="fullbg"></div>

//彈出框div的id：

tishi_info

//彈出js函數：

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');

}