public function convertBackgroundPosition($css, &$res)
    {
        // init the res
        $res = false;

        // explode the value
        $css = explode(' ', $css);

        // we must have 2 values. if 0 or >2 : error. if 1 => put center for 2
        if (count($css)<2) {
            if (!$css[0]) return null;
            $css[1] = 'center';
        }
        if (count($css)>2) return null;

        // prepare the values
        $x = 0;
        $y = 0;
        $res = true;

        // convert the first value
        if ($css[0]=='left')        $x = '0%';
        else if ($css[0]=='center') $x = '50%';
        else if ($css[0]=='right')  $x = '100%';
        else if ($css[0]=='top')    $y = '0%';
        else if ($css[0]=='bottom') $y = '100%';
        else if (preg_match('/^[-]?[0-9\.]+%$/isU', $css[0])) $x = $css[0];
        else if ($this->convertToMM($css[0])) $x = $this->convertToMM($css[0]);
        else $res = false;

        // convert the second value
        if ($css[1]=='left')        $x = '0%';
        else if ($css[1]=='right')  $x = '100%';
        else if ($css[1]=='top')    $y = '0%';
        else if ($css[1]=='center') $y = '50%';
        else if ($css[1]=='bottom') $y = '100%';
        else if (preg_match('/^[-]?[0-9\.]+%$/isU', $css[1])) $y = $css[1];
        else if ($this->convertToMM($css[1])) $y = $this->convertToMM($css[1]);
        else $res = false;

        // return the values
        return array($x, $y);
    }