//absolute pagina lengte
function getPageHeight() {     
	 var yScroll;
	if (window.innerHeight && window.scrollMaxY) {	
		yScroll = window.innerHeight + window.scrollMaxY;
	} else if (document.body.scrollHeight > document.body.offsetHeight){ // all but Explorer Mac
		yScroll = document.body.scrollHeight;
	} else { // Explorer Mac...would also work in Explorer 6 Strict, Mozilla and Safari
		yScroll = document.body.offsetHeight;
	}
	var windowHeight;
	if (self.innerHeight) {	// all except Explorer
		windowHeight = self.innerHeight;
	} else if (document.documentElement && document.documentElement.clientHeight) { // Explorer 6 Strict Mode
		windowHeight = document.documentElement.clientHeight;
	} else if (document.body) { // other Explorers
		windowHeight = document.body.clientHeight;
	}	
	// for small pages with total height less then height of the viewport
	if(yScroll < windowHeight){
		pageHeight = windowHeight;
	} else { 
		pageHeight = yScroll;
	}
	return pageHeight;
}

//huidige locatie
function getScrollY() {
	  var scrOfY = 0;
	  if( typeof( window.pageYOffset ) == 'number' ) {
		//Netscape compliant
		scrOfY = window.pageYOffset;
	  } else if( document.body && ( document.body.scrollLeft || document.body.scrollTop ) ) {
		//DOM compliant
		scrOfY = document.body.scrollTop;
	  } else if( document.documentElement && ( document.documentElement.scrollLeft || document.documentElement.scrollTop ) ) {
		//IE6 standards compliant mode
		scrOfY = document.documentElement.scrollTop;
	  }
	  return  scrOfY ;
}

//onderdeel van openWindow
function openBack() {
	document.getElementById('backgroundOverlay').style.height= getPageHeight() +'px';
	document.getElementById('backgroundOverlay').style.display='';
}

//onderdeel van closeWindow
function closeBack() {
	document.getElementById('backgroundOverlay').style.display='none';
}

//scherm openen
function openWindow(){
	openBack();
	document.getElementById('overlay').style.display = '';
}

//scherm weer sluiten
function closeWindow(){
	document.getElementById('overlay').style.display = 'none';
	closeBack();
}