function showpopup() {
	if(document.getElementById('popup_background').style.display != 'block') {
		document.getElementById('popup_background').style.display = 'block';
		var container_height = document.getElementById('container').offsetHeight;
		var container_width = document.getElementById('container').offsetWidth;
		var viewport_height = returnViewPortHeight();
		var viewport_width = returnViewPortWidth();	
		if(container_height > viewport_height) {
			document.getElementById('popup_background').style.height = container_height+'px';
		} else {
			document.getElementById('popup_background').style.height = viewport_height+'px';
		}
		if(container_width > viewport_width) {
			document.getElementById('popup_background').style.width = container_width+'px';
		} else {
			document.getElementById('popup_background').style.width = viewport_width+'px';
		}
		
		//center image
		document.getElementById('popup').style.top = (parseInt(viewport_height/2)-276)+'px';
		document.getElementById('popup').style.left = (parseInt(viewport_width/2)-195)+'px';
		//alert('viewport_height: '+viewport_height+', viewport_width: '+viewport_width);
	} else {
		document.getElementById('popup_background').style.display = 'none';
	}
	
}