In recent years, hierarchical compositional models have been shown to possess many appealing properties for the object class detection. These range from efficient inference, possibility of incremental training, to coping with potentially large number of object categories. The reason is that they encode categories by hierarchical vocabularies of parts which are shared among the categories. On the downside, the sharing and purely reconstructive nature causes problems when categorizing visually-similar categories and separating them from the background. In this paper we propose a novel approach that preserves the appealing properties of the generative hierarchical models, while at the same time improves their discrimination properties. We achieve this by introducing a network of discriminative nodes on top of the existing generative hierarchy. The discriminative nodes are sparse linear combinations of activated generative parts. We show in the experiments that the discriminative nodes consistently improve a state-of-the-art hierarchical compositional model. Results show that our approach considers only a fraction of all nodes in the vocabulary (less than 10%) which also makes the system computationally efficient.



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