var lastPopUpNode;
function popUp( node )
{
   lastPopUpNode = node;
   document.getElementById( node ).style.visibility = "visible";
   document.getElementById('screenoverlay').style.visibility='visible';
   centerHorizontal( node );
   centerVertical( node );
}
function popDown()
{
   document.getElementById( lastPopUpNode ).style.visibility = "hidden";
   document.getElementById('screenoverlay').style.visibility='hidden';
}
function makeVisible( obj )
{
   document.getElementById( obj ).style.visibility = "visible";
}
function makeHidden( obj )
{
   document.getElementById( obj ).style.visibility = "hidden";
}
function centerHorizontal( obj )
{
   documentWidth = document.documentElement.clientWidth;
   objectWidth = document.getElementById( obj ).clientWidth;
   leftPosition = ( documentWidth - objectWidth ) / 2;
   document.getElementById( obj ).style.left = leftPosition + 'px';
}
function centerVertical( obj )
{
   documentHeight = document.documentElement.clientHeight;
   scrollDistance = document.documentElement.scrollTop;
   objectHeight = document.getElementById( obj ).clientHeight;
   topPosition = scrollDistance + ( documentHeight - objectHeight ) / 2;
   document.getElementById( obj ).style.top = topPosition + 'px';
}