function getPosition() {
    var top    = document.documentElement.scrollTop;
    var left   = document.documentElement.scrollLeft;
    var height = document.documentElement.clientHeight;
    var width  = document.documentElement.clientWidth;
    return {top:top,left:left,height:height,width:width};
}
function showPop(id){
    var width  = 150;  //弹出框的宽度
    var height = 38;  //弹出框的高度
    var obj    = document.getElementById(id);
    obj.style.display  = "block";
    obj.style.position = "absolute";
    obj.style.width    = width + "px";
    obj.style.height   = height + "px";
    var Position = getPosition();
    leftadd = (Position.width-width)/2;
    topadd  = (Position.height-height)/2;
    obj.style.top  = (Position.top  + topadd)  + "px";
    obj.style.left = (Position.left + leftadd) + "px";
    window.onscroll = function (){
        var Position   = getPosition();
        obj.style.top  = (Position.top  + topadd)  +"px";
        obj.style.left = (Position.left + leftadd) +"px";
    };
}
function hidePop(id){
    document.getElementById(id).style.display = "none";
}