Object detection techniques are coming closer to the automatic detection and identification of objects in multimedia documents. Still, this is not sufficient for the understanding of multimedia content, mainly because a simple object may be related to multiple topics, few of which are indeed related to a given document. In this paper we determine the thematic categories that are related to a document based on the objects that have been automatically detected in it. Our approach relies on stored knowledge and a fuzzy hierarchical clustering algorithm; this algorithm uses a similarity measure that is based on the notion of context. The context is extracted using fuzzy ontological relations.