public function vignette($scale = null, $outerColor = null, $innerColor = null) {
        $params = array();

        if ($scale) {
            $params[] = 'scale=' . $scale;
        }

        if ($outerColor) {
            $params[] = 'outer=' . $outerColor;
        }

        if ($innerColor) {
            $params[] = 'inner=' . $innerColor;
        }

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