jQuery视差滚动插件，（附原理分析，调用方法）

演示地址：http://www.jq22.com/jquery-info1799

jquery插件，使用验证过可用。

分析源代码后总结原理：设置background样式为fixed，判断浏览器滚动距离，当在视窗范围内时，调用$(window).scroll。并根据所设置的速度比例，进行同步滚动。

调用时，设置三个必要参数：data-speed，data-type，background样式为fixed

<div data-speed="4" data-type="background" style="background:url(...) 50% 0 no-repeat fixed">    

js会根据data-speed来计算滚动速度，越小越快。

可以根据不同的data-type作不同处理，可自定义写代码，很方便。

background必须是fixed。

parallax.js源代码（需先引入jQuery）

$(document).ready(function(){

    // Cache the Window object
    $window = $(window);

    // Cache the Y offset and the speed of each sprite
    $('[data-type]').each(function() {
        $(this).data('offsetY', parseInt($(this).attr('data-offsetY')));
        $(this).data('Xposition', $(this).attr('data-Xposition'));
        $(this).data('speed', $(this).attr('data-speed'));
    });

    // For each element that has a data-type attribute
    $('section[data-type="background"]').each(function(){

        // Store some variables based on where we are
        var $self = $(this),
            offsetCoords = $self.offset(),
            topOffset = offsetCoords.top;

        // When the window is scrolled...
        $(window).scroll(function() {

            // If this section is in view
            if ( ($window.scrollTop() + $window.height()) > (topOffset) &&
                 ( (topOffset + $self.height()) > $window.scrollTop() ) ) {

                // Scroll the background at var speed
                // the yPos is a negative value because we're scrolling it UP!
                var yPos = -($window.scrollTop() / $self.data('speed')); 

                // If this element has a Y offset then add it on
                if ($self.data('offsetY')) {
                    yPos += $self.data('offsetY');
                }

                // Put together our final background position
                var coords = '50% '+ yPos + 'px';

                // Move the background
                $self.css({ backgroundPosition: coords });

                // Check for other sprites in this section
                $('[data-type="sprite"]', $self).each(function() {

                    // Cache the sprite
                    var $sprite = $(this);

                    // Use the same calculation to work out how far to scroll the sprite
                    var yPos = -($window.scrollTop() / $sprite.data('speed'));
                    var coords = $sprite.data('Xposition') + ' ' + (yPos + $sprite.data('offsetY')) + 'px';

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

                }); // sprites

                // Check for any Videos that need scrolling
                $('[data-type="video"]', $self).each(function() {

                    // Cache the video
                    var $video = $(this);

                    // There's some repetition going on here, so
                    // feel free to tidy this section up.
                    var yPos = -($window.scrollTop() / $video.data('speed'));
                    var coords = (yPos + $video.data('offsetY')) + 'px';

                    $video.css({ top: coords });                                                    

                }); // video    

            }; // in view

        }); // window scroll

    });    // each data-type

}); // document ready