var id_popup = '';

function popup(id)
{
	id = '#'+id;
	//Get the screen height and width  
	var maskHeight = $(document).height();  
	var maskWidth = $(window).width();  
	//Set height and width to mask to fill up the whole screen  
	$('#mask').css({'width':maskWidth,'height':maskHeight});
	//transition effect       
	$('#mask').fadeTo("fast",0.8);    
	//Get the window height and width  
	var winH = $(window).height();
	var winW = $(window).width();  
	//Set the popup window to center  
	$(id).css('top',  winH/2-$(id).height()/2 + $(window).scrollTop());  
	$(id).css('left', winW/2-$(id).width()/2);  
	//transition effect
	if (id_popup == '')
		$(id).fadeIn(700, function() { id_popup = id; });   
}

function close_popup()
{
	$('.window').fadeOut(300);  
	$('#mask').fadeOut(300);
	id_popup = '';
}

function center_popup(id)
{
	$(id).animate({'top':$(window).height()/2-$(id).height()/2 + $(window).scrollTop()}, 800, 'easeOutBack');
}

$(window).scroll(
	function()
	{
	 if (id_popup != '')
		$(id_popup).stop().animate({'top':$(window).height()/2-$(id_popup).height()/2 + $(window).scrollTop()}, 800, 'easeOutBack');  
	}
 );