function showDiv(div_id) {
    var div = document.getElementById(div_id);
    div.style.top = "50%";
    div.style.left = "50%";
    div.style.display = 'block';
    div.style.visibility = 'visible';
    div.style.left = "" + (div.offsetLeft - div.offsetWidth / 2) + "px";
    div.style.top = "" + (div.offsetTop - div.offsetHeight / 2) + "px";
}

function hideDiv(div_id) {
    var div = document.getElementById(div_id);
    div.style.display = 'none';
}