Craft\ImageResizerService::getImageQuality PHP Method

    public function getImageQuality($filename, $quality = null)
    {
        $desiredQuality = !$quality ? craft()->imageResizer->getSettings()->imageQuality : $quality;
        if (IOHelper::getExtension($filename) == 'png') {
            // Valid PNG quality settings are 0-9, so normalize and flip, because we're talking about compression
            // levels, not quality, like jpg and gif.
            $quality = round($desiredQuality * 9 / 100);
            $quality = 9 - $quality;
            if ($quality < 0) {
                $quality = 0;
            }
            if ($quality > 9) {
                $quality = 9;
            }
        } else {
            $quality = $desiredQuality;
        }
        return $quality;
    }