protected function initTransformUrl()
    {
        if (!$this->transform_url) {
            // security in a different format for transformations
            $security_str = $this->security ?
                sprintf('/security=policy:%s,signature:%s',
                    $this->security->policy,
                    $this->security->signature
                ) : '';

            $this->transform_url = sprintf(FilestackConfig::CDN_URL . '%s/%s',
                $security_str,
                $this->handle);
        }
    }