public function sharpen(array $params = null) {
        $options = array('preset', 'radius', 'sigma', 'threshold', 'gain');
        $transformation = array();

        foreach ($options as $param) {
            if (!isset($params[$param])) {
                continue;
            }

            $transformation[] = $param . '=' . $params[$param];
        }

        return $this->addTransformation(
            'sharpen' . ($transformation ? ':' . implode(',', $transformation) : '')
        );
    }