function toggle(div_id) {
    var el = document.getElementById(div_id);
    if ( el.style.display == 'none' ) {
        el.style.display = 'block';
    }
    else {
        el.style.display = 'none';
    }
}

function cover_size(popupdivVar) {
    if (typeof window.innerWidth != 'undefined') {
        viewportheight = window.innerHeight;
    } else {
        viewportheight = document.documentElement.clientHeight;
    }
    if ((viewportheight > document.body.parentNode.scrollHeight) && (viewportheight > document.body.parentNode.clientHeight)) {
        cover_height = viewportheight;
    } else {
        if (document.body.parentNode.clientHeight > document.body.parentNode.scrollHeight) {
            cover_height = document.body.parentNode.clientHeight;
        } else {
            cover_height = document.body.parentNode.scrollHeight;
        }
    }
    var cover = document.getElementById('cover');
    cover.style.height = cover_height + 'px';

    var popupdiv = document.getElementById(popupdivVar);
    popupdiv_height=screen.height/2-200;//200 is half popup's height
    popupdiv.style.top = popupdiv_height + 'px';
}

function window_pos(popupdivVar) {
    if (typeof window.innerWidth != 'undefined') {
        viewportwidth = window.innerHeight;
    } else {
        viewportwidth = document.documentElement.clientHeight;
    }
    if ((viewportwidth > document.body.parentNode.scrollWidth) && (viewportwidth > document.body.parentNode.clientWidth)) {
        window_width = viewportwidth;
    } else {
        if (document.body.parentNode.clientWidth > document.body.parentNode.scrollWidth) {
            window_width = document.body.parentNode.clientWidth;
        } else {
            window_width = document.body.parentNode.scrollWidth;
        }
    }
    var popupdiv = document.getElementById(popupdivVar);
    window_width=window_width/2-220;//220 is half popup's width
    popupdiv.style.left = window_width + 'px';
}

function popup(windowname) {
    cover_size(windowname);
    window_pos(windowname);
    toggle('cover');
    toggle(windowname);
}