parallax.js

(function (jQuery) {
  var jQuerywindow = jQuery(window);
  var windowHeight = jQuerywindow.height();
  jQuerywindow.resize(function () {
    windowHeight = jQuerywindow.height();
  });
  jQuery.fn.parallax = function (xpos, speedFactor, outerHeight) {
    var jQuerythis = jQuery(this);
    var getHeight;
    var firstTop;
    var paddingTop = 0;
    jQuerythis.each(function () {
      firstTop = jQuerythis.offset().top;
    });
    if (outerHeight) {
      getHeight = function (jqo) {
        return jqo.outerHeight(true);
      };
    } else {
      getHeight = function (jqo) {
        return jqo.height();
      };
    }
    if (arguments.length < 1 || xpos === null) xpos = "50%";
    if (arguments.length < 2 || speedFactor === null) speedFactor = 0.1;
    if (arguments.length < 3 || outerHeight === null) outerHeight = true;
    function update() {
      var pos = jQuerywindow.scrollTop();
      jQuerythis.each(function () {
        var jQueryelement = jQuery(this);
        var top = jQueryelement.offset().top;
        var height = getHeight(jQueryelement);
        if (top + height < pos || top > pos + windowHeight) {
          return;
        }
        jQuerythis.css(
          "backgroundPosition",
          xpos + " " + Math.round((top - pos) * speedFactor) + "px"
        );
      });
    }
    jQuerywindow.bind("scroll", update).resize(update);
    update();
  };
})(jQuery);