Automatic building categorization and analysis are particularly relevant for smart city applications and cultural heritage programs. Taking a picture of the facade of a building and instantly obtaining information about it can enable the automation of processes in urban planning, virtual city tours, and digital archiving of cultural artifacts. In this paper, we go beyond traditional convolutional neural networks (CNNs) for image classification and propose the HierarchyNet: a new hierarchical network for the classification of urban buildings from all across the globe into different main and subcategories from images of their facades. We introduce a coarse-to-fine hierarchy on the dataset and the model learns to simultaneously extract features and classify across both levels of hierarchy. We propose a new multiplicative layer, which is able to improve the accuracy of the finer prediction by considering the feedback signal of the coarse layers. We have quantitatively evaluated the proposed approach both on our proposed building datasets, as well as on various benchmark databases to demonstrate that the model is able to efficiently learn hierarchical information. The HierarchyNet model is able to outperform the state-of-the-art convolutional neural networks in urban building classification as well as in other multi-label classification tasks while using significantly fewer parameters.
