vertical center div with overflow to clicked child div

function center(el, parent) {
    var el = $(el),
        el_height = el.outerHeight(),
        el_width = el.outerWidth(),
        parent = parent || el.parent(),
        parent_height = parent.height(),
        parent_width = parent.width(),
        top = (parent_height/2) - (el_height/2),
        left = (parent_width/2) - (el_width/2);
    el.css({
        top: top,
        left: left,
        position: 'fixed'
    });    
}