Effet parallax avec jQuery

/*
* Adds a parallax effect to a background image
* (position:fixed;no-repeat;background-size:42%;)
*
* element : the element with a background image
* percent : percent of the background-size (0.42)
* height : height of the background image
* width : width of the background image
* factor : factor of speed (the lower the faster)
* reference : the element bottom-placed to get the total
*             height of the page
*
* author : Guillaume Litaudon  <guillaume.litaudon@gmail.com>
*/

function parallax(element,percent,height,width,factor,reference){
	var winWidth = $(window).width();
	var winHeight = $(window).height();

	var sizePercent = percent == 1 ? height : ((winWidth*percent)/width)*height;

	var maxScroll = -(sizePercent - winHeight);
	var speed = factor*($(reference).offset().top / winHeight);
	var yPos = -($(window).scrollTop() / speed);
	yPos = yPos >= 0 ? 0 : yPos;
	yPos = yPos >= maxScroll ? yPos : maxScroll;
	var coords = '0 '+ yPos + 'px';

	$(element).css({ backgroundPosition: coords });
}


function goGoParallaxEffect(){
	/* La taille dépend de si l'on est en-dessous
	* de pictureWidth ou non
	*/
	var pictureHeight = 1080;
	var pictureWidth = 388;

	if($(window).width() > pictureWidth)
		parallax('#page',0.42,pictureWidth,pictureHeight,5,footer);
	else
		parallax('#page',1,pictureWidth,pictureHeight,5,footer);
}

$(window).scroll(function() {
	goGoParallaxEffect();
});

$(window).resize(function() {
	goGoParallaxEffect();
});