public function scale($width = null, $height = null)
    {
        if ($width === null && $height === null) {
            throw new \LogicException('one of "width" or "height" must be set for "scale"');
        }

        if ($width !== null && $height !== null) {
            $this->resize($width, $height);
            return;
        }

        if ($width !== null) {
            $this->image->widen($width);
            return;
        }

        $this->image->heighten($height);
    }