Graph Sampling Based Deep Metric Learning for Generalizable Person Re-Identification

Recent studies show that, both explicit deep feature matching as well as large-scale and diverse training data can significantly improve the generalization of person re-identification. However, the efficiency of learning deep matchers on large-scale data has not yet been adequately studied. Though learning with classification parameters or class memory is a popular way, it incurs large memory and computational costs. In contrast, pairwise deep metric learning within mini batches would be a better choice. However, the most popular random sampling method, the well-known PK sampler, is not informative and efficient for deep metric learning. Though online hard example mining has improved the learning efficiency to some extent, the mining in mini batches after random sampling is still limited. This inspires us to explore the use of hard example mining earlier, in the data sampling stage. To do so, in this paper, we propose an efficient mini-batch sampling method, called graph sampling (GS), for large-scale deep metric learning. The basic idea is to build a nearest neighbor relationship graph for all classes at the beginning of each epoch. Then, each mini batch is composed of a randomly selected class and its nearest neighboring classes so as to provide informative and challenging examples for learning. Together with an adapted competitive baseline, we improve the previous state of the art in generalizable person re-identification significantly, by up to 24% in Rank-1 and 13.8% in mAP. Besides, the proposed method also outperforms the competitive baseline by up to 6.2% in Rank-1 and 5.3% in mAP. Meanwhile, the training time is significantly reduced by up to five times, e.g. from 12.2 hours to 2.3 hours when training on a large-scale dataset with 8,000 identities. Code is available at https://github.com/ShengcaiLiao/QAConv.