simple background-image parallax

parallax

$(function() {

	var offset = [], height = [], windowHeight = 0;
	var $window = $(window);
	var $parallax = $('.parallax');
	
	$window.resize(measure).bind('scroll', update);
	measure();
	
	function measure() {	
		$parallax.each(function(i) {
			$parent = $(this).parent('div');
			offset[i] = $parent.offset().top;
			height[i] = $parent.height();
		});
		
		windowHeight = $window.height();
		
		update();
	}
	
	function update() {
		var pos = $window.scrollTop(); 
		
		$parallax.each(function(i) {
			if(pos + windowHeight > offset[i] && pos - height[i] < offset[i]) {
				var translate = 'translate3d(0, '+ Math.round((pos - offset[i]) * 0.7) + 'px, 0)';
				$(this).css({'-webkit-transform':translate,'transform':translate});
			}
		});
	}
}