protected function getImageRequestId($requestString, $source = null)
    {
        if (!isset($this->cachedNames[$requestString])) {

            $this->cachedNames[$requestString] = $this->processCache->createKey(
                $source,
                $requestString,
                $this->config->cache_prefix,
                pathinfo($source, PATHINFO_EXTENSION)
            );
        }

        return $this->cachedNames[$requestString];
    }