Metric-Learning-Based Deep Hashing Network for Content-Based Retrieval of Remote Sensing Images

Hashing methods have recently been shown to be very effective in the retrieval of remote sensing (RS) images due to their computational efficiency and fast search speed. Common hashing methods in RS are based on hand-crafted features on top of which they learn a hash function, which provides the final binary codes. However, these features are not optimized for the final task (i.e., retrieval using binary codes). On the other hand, modern deep neural networks (DNNs) have shown an impressive success in learning optimized features for a specific task in an end-to-end fashion. Unfortunately, typical RS data sets are composed of only a small number of labeled samples, which make the training (or fine-tuning) of big DNNs problematic and prone to overfitting. To address this problem, in this letter, we introduce a metric-learning-based hashing network, which: 1) implicitly uses a big, pretrained DNN as an intermediate representation step without the need of retraining or fine-tuning; 2) learns a semantic-based metric space where the features are optimized for the target retrieval task; and 3) computes compact binary hash codes for fast search. Experiments carried out on two RS benchmarks highlight that the proposed network significantly improves the retrieval performance under the same retrieval time when compared to the state-of-the-art hashing methods in RS.