windowHeight = window.getSize().y
topVisible = false;

function prepare(){	
	$$('.section_blank').setStyle('height',windowHeight);
	$('line_3').setStyle('height',windowHeight-350);
	$('line_6').setStyle('height',windowHeight-420);
	
	elements = [$('intro'),$('roeien'),$('schoenen'),$('vereniging'),$('ejz'),$('programma'),$('leden'),$('foto_leden'),$('sleepinn'),$('veertje')];
	
	if(windowHeight > 800){
		roeiAdjustment = windowHeight+200;
		spetterAdjustment = windowHeight + 4000;		
	} else {
		roeiAdjustment = -500;
		spetterAdjustment = windowHeight + 4000;		
	}

	
	adjustments = [1500,roeiAdjustment,spetterAdjustment,4000,5000,6500,7600,8400,10000,11300];
	inertias = [0.3,0.1,0.3,0.3,0.5,0.3,0.2,0.5,0.3,0.5];
	
	topButtonFx = new Fx.Tween($('bottom_buttons'),{
		duration: 'short',
		property: 'bottom'
	});

	window.addEvent('scroll',Move);
	Move();

}
	

function newPos(windowHeight,pos,adjuster,inertia){
	return (-((windowHeight+pos)-adjuster)*inertia);
}

function Move(evt){
	pos = window.getScroll().y;

	if(pos > 650 && topVisible == false){
		topButtonFx.start(30);
		topVisible = true;
	}
	if(pos < 650 && topVisible == true){
		topButtonFx.start(-25);
		topVisible = false;
	}	
	
	
	count = 0;
	elements.each(function(el){
		
		if(inView(el)){
			el.setStyle('background-position',"50% "+newPos(el.getStyle('height').toInt(),pos,adjustments[count],inertias[count])+"px");
		}
		count++;
	});
}


function inView(el) {
	scrollTop = window.getScroll().y;
	elheight = el.getStyle('height').toInt();
	eltop = el.getPosition().y;
	if (scrollTop > (eltop + elheight) || scrollTop + windowHeight < eltop) {
		el.removeClass('inview');
		return false;
    } else if (scrollTop < (eltop + elheight)) {
		el.addClass('inview');
		return true;
    }
}