Self-Supervised Skeleton Action Representation Learning: A Benchmark and Beyond

Self-supervised learning (SSL), which aims to learn meaningful prior representations from unlabeled data, has been proven effective for label-efficient skeleton-based action understanding. Different from the image domain, skeleton data possesses sparser spatial structures and diverse representation forms, with the absence of background clues and the additional temporal dimension. This presents the new challenges for the pretext task design of spatial-temporal motion representation learning. Recently, many endeavors have been made for skeleton-based SSL and remarkable progress has been achieved. However, a systematic and thorough review is still lacking. In this paper, we conduct, for the first time, a comprehensive survey on self-supervised skeleton-based action representation learning, where various literature is organized according to their pre-training pretext task methodologies. Following the taxonomy of context-based, generative learning, and contrastive learning approaches, we make a thorough review and benchmark of existing works and shed light on the future possible directions. Our investigation demonstrates that most SSL works rely on the single paradigm, learning representations of a single level, and are evaluated on the action recognition task solely, which leaves the generalization power of skeleton SSL models under-explored. To this end, a novel and effective SSL method for skeleton is further proposed, which integrates multiple pretext tasks to jointly learn versatile representations of different granularity, substantially boosting the generalization capacity for different downstream tasks. Extensive experiments under three large-scale datasets demonstrate that the proposed method achieves the superior generalization performance on various downstream tasks, including recognition, retrieval, detection, and few-shot learning.