Person Re-identification (ReID) aims at matching a person of interest across images. In convolutional neural networks (CNNs) based approaches, loss design plays a role of metric learning which guides the feature learning process to pull closer features of the same identity and to push far apart features of different identities. In recent years, the combination of classification loss and triplet loss achieves superior performance and is predominant in ReID. In this paper, we rethink these loss functions within a generalized formulation and argue that triplet-based optimization can be viewed as a two-class subsampling classification, which performs classification over two sampled categories based on instance similarities. Furthermore, we present a case study which demonstrates that increasing the number of simultaneously considered instance classes significantly improves the ReID performance, since it is aligned better with the ReID test/inference process. With the multi-class subsampling classification incorporated, we provide a strong baseline which achieves the state-of-the-art performance on the benchmark person ReID datasets. Finally, we propose a new meta prototypical N-tuple loss for more efficient multi-class subsampling classification. We aim to inspire more new loss designs in the person ReID field.
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