background-atttachment fixed polyfill test

function polyFillBgFixed($this, imgWidth, imgHeight, paddingTop) {
    var paddingTop = paddingTop || 0,
        thisHeight = $this.height();
    $this.css("background-attachment", "scroll");
    setInterval(function() {
        var $window = $(window),
            thisTop = $this.offset().top + (paddingTop || 0),
            windowBottom = $window.scrollTop() + $window.height(),
            w = $window.width() / imgWidth;
            y = (windowBottom - thisTop) / ($window.height());

        var imgPos = -imgHeight * w + (imgHeight * w * y);
        imgPos = Math.round(imgPos);
        $this.css("background-position-y",  imgPos + "px");
    }, 1);
}