X-Invariant Contrastive Augmentation and Representation Learning for Semi-Supervised Skeleton-Based Action Recognition.

Semi-supervised skeleton-based action recognition is a challenging problem due to insufficient labeled data. For addressing this problem, some representative methods leverage contrastive learning to obtain more features from the pre-augmented skeleton actions. Such methods usually adopt a two-stage way: first randomly augment samples, and then learn their representations via contrastive learning. Since skeleton samples have already been randomly augmented, the representation ability of the subsequent contrastive learning is limited due to the inconsistency between the augmentations and representations. Thus, we propose a novel X-invariant Contrastive Augmentation and Representation learning (X-CAR) framework to thoroughly obtain rotate-shear-scale (X for short) invariant features by learning augmentations and representations of skeleton sequences in a one-stage way. In X-CAR, a new Adaptive-combination Augmentation (AA) mechanism is designed to rotate, shear, and scale the skeletons by learnable controlling factors in an adaptive way rather than a random way. Here, such controlling factors are also learned in the whole contrastive learning process, which can facilitate the consistency between the learned augmentations and representations of skeleton sequences. In addition, we relax the pre-definition of positive and negative samples to avoid the confusing allocation of ambiguous samples, and present a new Pull-Push Contrastive Loss (PPCL) to pull the augmenting skeleton close to the original skeleton, while push far away from the other skeletons. Experimental results on both NTU RGB+D and North-Western UCLA datasets show that the proposed X-CAR achieves better accuracy compared with other competitive methods in the semi-supervised scenario.