Trying to place JS popup in center of page?

Question

I have centered the popup on this website here, however, it's only on the x-axis. I also need to center it on the y-axis, but I can't seem to make it happen.

Here's the JavaScript...

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 blanket_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)) {
        blanket_height = viewportheight;
    } else {
        if (document.body.parentNode.clientHeight > document.body.parentNode.scrollHeight) {
            blanket_height = document.body.parentNode.clientHeight;
        } else {
            blanket_height = document.body.parentNode.scrollHeight;
        }
    }
    var blanket = document.getElementById('blanket');
    blanket.style.height = blanket_height + 'px';
    var popUpDiv = document.getElementById(popUpDivVar);
    popUpDiv_height=blanket_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/2-200;//200 is half popup's width
    popUpDiv.style.left = window_width + 'px';
}
function popup(windowname) {
    blanket_size(windowname);
    window_pos(windowname);
    toggle('blanket');
    toggle(windowname);     

}

I thought it would work with this...

 popUpDiv_height=blanket_height/2-200;//200 is half popup's height

but it's not. Any ideas on how to fix this?

Edit: Here's a link to the site here.

Here's all the CSS attributed to this popup...

#blanket {
   background-color:#111;
   opacity: 0.65;
   filter: alpha(opacity=65)
   *background:none;
   position:absolute;
   z-index: 9001;
   top:0px;
   left:0px;
   width:100%;
}

#popUpDiv {
    position:absolute;
    background-image:url(../images/popup_bg.png);
    background-repeat:no-repeat;    
    top:50%;
    left: 50%;
    width:400px;
    height:400px;
    margin-top: -120px auto 0 auto; /* auto, centers horizontally and -120px is half your height to finish the centering vertically */
    border:5px solid #000;
    z-index: 9002;
}


#popUpDiv .close {
    background-image: url(../images/x.png); 
    background-repeat: no-repeat;
    position:absolute; 
    top:10px; 
    right:10px
}

Answer 1

You could accomplish this with CSS like so:

HTML

<div id="popUpDiv">
    Your content here
</div>

CSS

#popUpDiv {
    position:absolute;
    background-image:url(../images/popup_bg.png);
    background-repeat:no-repeat;    
    top:50%;
    width:400px;
    height:400px;
    margin: -200px auto 0 auto; /* auto, centers horizontally and -120px is half your height to finish the centering vertically */
    border:5px solid #000;
    z-index: 9002;
}

In conclusion, if you use this method it looks like you may just need to use javascript to dynamically set the height and margin-top value depending on your div and its content at the time.

More information here: http://blog.themeforest.net/tutorials/vertical-centering-with-css/