jquery.parallax-1.1.3.js

/*
Plugin: jQuery Parallax
Version 1.1.3
Author: Ian Lunn
Twitter: @IanLunn
Author URL: http://www.ianlunn.co.uk/
Plugin URL: http://www.ianlunn.co.uk/plugins/jquery-parallax/

Dual licensed under the MIT and GPL licenses:
http://www.opensource.org/licenses/mit-license.php
http://www.gnu.org/licenses/gpl.html
*/

(function( $ ){
  var $window = $(window);
  var windowHeight = $window.height();

  $window.resize(function () {
    windowHeight = $window.height();
  });

  $.fn.parallax = function(xpos, speedFactor, outerHeight) {
    var $this = $(this);
    var getHeight;
    var firstTop;
    var paddingTop = 0;
    
    //get the starting position of each element to have parallax applied to it    
    $this.each(function(){
        firstTop = $this.offset().top;
    });

    if (outerHeight) {
      getHeight = function(jqo) {
        return jqo.outerHeight(true);
      };
    } else {
      getHeight = function(jqo) {
        return jqo.height();
      };
    }
      
    // setup defaults if arguments aren't specified
    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 to be called whenever the window is scrolled or resized
    function update(){
      var pos = $window.scrollTop();        

      $this.each(function(){
        var $element = $(this);
        var top = $element.offset().top;
        var height = getHeight($element);

        // Check if totally above or totally below viewport
        if (top + height < pos || top > pos + windowHeight) {
          return;
        }

        $this.css('backgroundPosition', xpos + " " + Math.round((firstTop - pos) * speedFactor) + "px");
      });
    }   

    $window.bind('scroll', update).resize(update);
    update();
  };
})(jQuery);