private function set_dynamic_sizes()
    {
        foreach (Config::get('images.image_sizes') as $size => $data) {
            if (! isset($data[3]) || ! $data[3]) {
                self::$dynamic_sizes[ $size ] = true;
            }
        }

        if (! empty(Config::get('images.dynamic_image_sizes'))) {
            foreach (Config::get('images.dynamic_image_sizes') as $size => $data) {
                self::$dynamic_sizes[ $size ] = true;
            }
        }
    }