(function ($) {
    $.extend($.fx.step, {
        backgroundPosition: function (fx) {
            if (fx.state === 0 && typeof fx.end == 'string') {
                var start = $.curCSS(fx.elem, 'backgroundPosition');
                start = toArray(start);
                fx.start = [start[0], start[2]];
                var end = toArray(fx.end);
                fx.end = [end[0], end[2]];
                fx.unit = [end[1], end[3]];
            }
            var nowPosX = [];
            nowPosX[0] = ((fx.end[0] - fx.start[0]) * fx.pos) + fx.start[0] + fx.unit[0];
            nowPosX[1] = ((fx.end[1] - fx.start[1]) * fx.pos) + fx.start[1] + fx.unit[1];
            fx.elem.style.backgroundPosition = nowPosX[0] + ' ' + nowPosX[1];

            function toArray(strg) {
                strg = strg.replace(/left|top/g, '0');
                strg = strg.replace(/right|bottom/g, '100%');
                strg = strg.replace(/([0-9\.]+)(\s|\)|$)/g, '$1px$2');
                var res = strg.match(/(-?[0-9\.]+)(px|\%|em|pt)\s(-?[0-9\.]+)(px|\%|em|pt)/);
                return [parseFloat(res[1], 10), res[2], parseFloat(res[3], 10), res[4]];
            }
        }
    });
})(jQuery);