“…To this end, there have been increasing efforts made to learn a distance metric in recent years [6,10,37,39,41,50]. Metric learning methods can be categorized into unsupervised [50], semisupervised [5] and supervised ones [1,11,25,27,34,53], according to the availability of the labels of training samples. Metric learning has been proved to successfully improve the clustering and recognition performance in information retrieval [29,30], bioinformatics [47] and computer vision tasks [6,10,12,16,39,37].…”