var velocity = 0.2;
function fn_bgScroll(){ 
    var pos = $(window).scrollTop(); 
    var height = $('#main').height();
    var img = $('#main').css("backgroundPosition");
    var x = img.split(' ');
	var x2 = x[1];
    var h_pos = x2.indexOf("p");
    var imgHeight = x2.substr(0, h_pos);
    $("#main").css('backgroundPosition', '50% ' + Math.round((imgHeight*1 - pos) * velocity) +  'px');
};

$(window).bind('scroll', fn_bgScroll);