Learning Representations by Contrastive Spatio-Temporal Clustering for Skeleton-Based Action Recognition

Self-supervised representation learning has proven constructive for skeleton-based action recognition. For better performance, existing methods mainly focus on (1) multi-modal data augmentations and (2) triplet contrastive samples construction. However, designing these strategies is always heuristics and hard. Instead of exploring more similar strategies, this paper addresses this issue with a different view and proposes a novel Contrastive Spatio-Temporal Clustering (CSTC) module. CSTC constructs a supervised signal (pseudo-label) of action sequences in an online clustering manner, and it is complementary to the recent data augmentations or triplet contrastive samples construction strategies. Specifically, CSTC can be formulated as an optimal transport problem. we introduce the spatio-temporal regularizations into the original optimal transport term to guide the pseudo-label generation, i.e., a semantic regularization learned by frame index is proposed to constrain the frame order, and a prior normal distribution regularization based on sampling characteristics of samples is proposed to maintain the dependability of spatial cluster assignments. Furthermore, to enhance the learning of latent features, we propose a Bidirectional Cross-modal Clustering Consistency Objective (B3CO) to enforce cluster assignments consistency for different modalities of the same sample. Last, since fusing spatial and temporal clustering losses directly during back-propagation will confuse the learned dimension-specific semantics, we propose a simple yet effective training strategy to fix it by training the model using these two losses alternately. By integrating the above designs into the MoCo framework, we propose a Contrastive Spatio-Temporal Clustering Network (CSTCN), which can excavate cross-modal discriminative spatio-temporal features in the clustering space. Experimental results on NTU RGB+D 60, NTU RGB+D 120, and PKU-MMD II datasets show that CSTCN achieves state of-the-art performance in both single- and multi-modal models, especially in the KNN and semi-supervised evaluation protocols. Besides, the key module CSTC shows good generalization capability, and achieves consistent performance improvement on the basis of several state-of-the-art methods which focus on data augmentations and triplet contrastive samples construction.