Deep Convolutional Neural Networks (CNN) have shown great success in supervised classification tasks such as character classification or dating. Deep learning methods typically need a lot of annotated training data, which is not available in many scenarios. In these cases, traditional methods are often better than or equivalent to deep learning methods. In this paper, we propose a simple, yet effective, way to learn CNN activation features in an unsupervised manner. Therefore, we train a deep residual network using surrogate classes. The surrogate classes are created by clustering the training dataset, where each cluster index represents one surrogate class. The activations from the penultimate CNN layer serve as features for subsequent classification tasks. We evaluate the feature representations on two publicly available datasets. The focus lies on the ICDAR17 competition dataset on historical document writer identification (Historical-WI). We show that the activation features we trained without supervision are superior to descriptors of state-of-the-art writer identification methods. Additionally, we achieve comparable results in the case of handwriting classification using the ICFHR16 competition dataset on historical Latin script types (CLaMM16).
The date of historical documents is an important metadata for scholars using them, as they need to know the historical context of the documents. This paper presents a novel attribute representation for medieval documents to automatically estimate the date information, which are the years they had been written. Non-semantic attributes are discovered in the low-level feature space using an unsupervised attribute learning method. A negative data set is involved in the attribute learning to make sure that our system rejects the documents which are not from the Middle Ages nor from the same archives. Experimental results on the basis of the Medieval Paleographic Scale (MPS) data set demonstrate that the proposed method achieves the state-of-the-art result.