public function getStills()
    {
        if (!isset($this->stills)) {
            $this->stills = [
                new Still($this->thumbnail_url, 'default'),
                new Still($this->thumbnail_url_lq, 'lq'),
                new Still($this->thumbnail_url_mq, 'mq'),
            ];
        }

        return $this->stills;
    }