Skeleton-based Action Recognition Research Articles

SkelVIT: consensus of vision transformers for a lightweight skeleton-based action recognition system

SkelVIT: consensus of vision transformers for a lightweight skeleton-based action recognition system

A puzzle questions form training for self-supervised skeleton-based action recognition

This paper proposed a novel pretext task to address the skeleton-based video representation for self-supervised action recognition tasks. Instead of exploiting only the whole body, various levels of the skeleton structure (e.g., upper body, lower body, left arm, left leg, right arm, right leg, and torso) are employed to extract essential coarser-grained characteristics. This involves computing statistical representations like motion, orientation, trajectory, and magnitude shift from unlabeled skeleton configurations. Then a learning model is built and trained to yield these statistical representations given the sequence configuration as the input. Our approach is question-driven, where each question acts as a puzzle piece contributing to a deeper understanding of the skeleton joint configuration. It's inspired by the ability of the cognitive system observed in individuals to hypothesize unseen actions. This is accomplished by posing pertinent questions and envisioning plausible scenarios to recognize the actions taking place. The answers to these devised questions are derived from the statistical representation of skeleton configurations. To this end, we made 44 questions designed to encompass the broadest overview to the finest detail. Our experiments on the NTU RGB-D, NW-UCLA, and PKU-MMD datasets demonstrate outstanding results in action recognition, proving the superiority of our approach in learning discriminative characteristics.

Multi-scale sampling attention graph convolutional networks for skeleton-based action recognition

Skeleton-based action recognition has attracted increasing interest in recent years. With the flexibility of modeling long-range dependency of joints, the self-attention module has served as the basic component in skeleton-based action recognition. However, the global receptive field of self-attention is not conducive to the modeling of skeleton locality, and the self-attention model is imbued with less inductive bias, which leads to overfitting. In this paper, we propose an attention graph convolutional network (AGCN) with multi-scale sampling to effectively model the local and global features of the skeleton. Firstly, we propose two extreme sampling strategies for generating and ordering neighboring nodes of root nodes. A local-first sampling method is introduced to construct local graph windows, and a global-first sampling method is proposed to assemble long-range joints for constructing global graph windows. The local-first sampling and global-first sampling introduce more skeleton-specific inductive biases to regularize the model capacity. Secondly, the AGCN combines the self-attention mechanism with graph convolution operation, which alleviates the over-smoothing of graph convolution and preserves the translation invariant. Based on the multi-scale sampling strategy, the AGCN can effectively model the locality and non-locality of the skeleton. Finally, by coupling the aforementioned proposals, we develop a two-pathway model for multi-scale feature fusion. Extensive experiments demonstrate that our model could achieve comparable performance with state-of-the-art works on the NTU RGB+D 60, NTU RGB+D 120, the UAV-HUMAN and NW-UCLA datasets.

Exploring High-Order Skeleton Correlations with Physical and Non-Physical Connection for Action Recognition

Hypergraphs have received widespread attention in modeling complex data correlations due to their superior performance. In recent years, some researchers have used hypergraph structures to characterize complex non-pairwise joints in the human skeleton and model higher-order correlations of the human skeleton. However, traditional methods of constructing hypergraphs based on physical connections ignore the dependencies among non-physically connected joints or bones, and it is difficult to model the correlation among joints or bones that are highly correlated in human action but are physically connected at long distances. To address these issues, we propose a skeleton-based action recognition method for hypergraph learning based on skeleton correlation, which explores the effects of physically and non-physically connected skeleton information on accurate action recognition. Specifically, in this paper, spatio-temporal correlation modeling is performed on the natural connections inherent in humans (physical connections) and the joints or bones that are more dependent but not directly connected (non-physical connection) during human actions. In order to better learn the hypergraph structure, we construct a spatio-temporal hypergraph neural network to extract the higher-order correlations of the human skeleton. In addition, we use an attentional mechanism to compute the attentional weights among different hypergraph features, and adaptively fuse the rich feature information in different hypergraphs. Extensive experiments are conducted on two datasets, NTU-RGB+D 60 and Kinetics-Skeleton, and the results show that compared with the state-of-the-art skeleton-based methods, our proposed method can achieve an optimal level of performance with significant advantages, providing a more accurate environmental perception and action analysis for the development of embodied intelligence.

Static graph convolution with learned temporal and channel-wise graph topology generation for skeleton-based action recognition

Graph convolutional networks (GCNs) are widely used in skeleton-based action recognition. It is known that the graph topology is a vital part in GCNs, and different kinds of graph topologies have been proposed for skeleton-based action recognition, mostly based on a predefined topology and a dynamically learned one. The predefined topology is based on the human intuition for skeleton (the connectivity of joints) and has not been investigated whether it is optimal. In this paper, we focus on investigating this static graph topology and propose to generate a learned static graph topology for skeleton. To be specific, a temporal frame-wise and channel-wise topology-based GCNs (TC-GCNs) are developed, where, instead of using a predefined topology by human, a topology is learned for skeleton-based action recognition. The TC-GCNs consist of generating a temporal frame-wise topology and a channel-wise topology to formulate the relationship of skeleton joints in the temporal dimension and channel dimension, respectively. The proposed method can be integrated with the conventional dynamic topology by replacing the predefined graph topology with our generated one. Experimental results show that our method with learned static graph achieves better performance than the predefined graph and dynamic graph on three widely used benchmarks, namely the NTU-RGB+D, NTU-RGB+D 120 and UAV-Human.

Fusing Higher-Order Features in Graph Neural Networks for Skeleton-Based Action Recognition.

Skeleton sequences are lightweight and compact and thus are ideal candidates for action recognition on edge devices. Recent skeleton-based action recognition methods extract features from 3-D joint coordinates as spatial-temporal cues, using these representations in a graph neural network for feature fusion to boost recognition performance. The use of first- and second-order features, that is, joint and bone representations, has led to high accuracy. Nonetheless, many models are still confused by actions that have similar motion trajectories. To address these issues, we propose fusing higher-order features in the form of angular encoding (AGE) into modern architectures to robustly capture the relationships between joints and body parts. This simple fusion with popular spatial-temporal graph neural networks achieves new state-of-the-art accuracy in two large benchmarks, including NTU60 and NTU120, while employing fewer parameters and reduced run time. Our source code is publicly available at: https://github.com/ZhenyueQin/Angular-Skeleton-Encoding.

Spatial-temporal graph neural ODE networks for skeleton-based action recognition.

In the field of skeleton-based action recognition, accurately recognizing human actions is crucial for applications such as virtual reality and motion analysis. However, this task faces challenges such intraindividual action differences and long-term temporal dependencies. To address these challenges, we propose an innovative model called spatial-temporal graph neural ordinary differential equations (STG-NODE). First, in the data preprocessing stage, the dynamic time warping (DTW) algorithm is used to normalize and calculate 3D skeleton data to facilitate the derivation of customized adjacency matrices for improving the influence of intraindividual action differences. Secondly, a custom ordinary differential equation (ODE) integrator is applied based on the initial conditions of the temporal features, producing a solution function that simulates the dynamic evolution trend of the events of interest. Finally, the outstanding ODE solver is used to numerically solve the time features based on the solution function to increase the influence of long-term dependencies on the recognition accuracy of the model and provide it with a more powerful temporal modeling ability. Through extensive experiments conducted on the NTU RGB+D 60 and Kinetics Skeleton 400 benchmark datasets, we demonstrate the superior performance of STG-NODE in the action recognition domain. The success of the STG-NODE model also provides new ideas and methods for the future development of the action recognition field.

Enhancing action recognition from low-quality skeleton data via part-level knowledge distillation

Skeleton-based action recognition is vital for comprehending human-centric videos and has applications in diverse domains. One of the challenges of skeleton-based action recognition is dealing with low-quality data, such as skeletons that have missing or inaccurate joints. This paper addresses the issue of enhancing action recognition using low-quality skeletons through a general knowledge distillation framework. The proposed framework employs a teacher-student model setup, where a teacher model trained on high-quality skeletons guides the learning of a student model that handles low-quality skeletons. To bridge the gap between heterogeneous high-quality and low-quality skeletons, we present a novel part-based skeleton matching strategy, which exploits shared body parts to facilitate local action pattern learning. An action-specific part matrix is developed to emphasize critical parts for different actions, enabling the student model to distill discriminative part-level knowledge. A novel part-level multi-sample contrastive loss achieves knowledge transfer from multiple high-quality skeletons to low-quality ones, which enables the proposed knowledge distillation framework to include training low-quality skeletons that lack corresponding high-quality matches. Comprehensive experiments conducted on the NTU-RGB+D, Penn Action, and SYSU 3D HOI datasets demonstrate the effectiveness of the proposed knowledge distillation framework.

An overview of Graph Convolutional Networks in skeleton-based action recognition

The field of research is currently focused on human activity recognition. Hence, numerous pertinent literature reviews have expounded upon the multifaceted nature of data, the process of selecting feature vectors, and the advantages and disadvantages of classification networks. Graph Convolutional Networks (GCNs) have demonstrated significant efficacy in the domain of human action recognition. In recent years, with the rapid development of 3D skeleton data collection, a plethora of studies in action recognition based on skeleton data have emerged. Skeleton data consists of three-dimensional coordinates of multiple spatiotemporal skeletal joints, making it an effective representation of kinematics. It can be easily acquired through low-cost depth sensors and also directly extracted from two-dimensional images using video-based pose estimation algorithms, attracting widespread attention. As relational networks continue to evolve, GCNs have been applied to various fields, including human action recognition. GCNs have demonstrated significant advantages in feature extraction from skeleton data. However, using GCNs alone may have various limitations. Therefore, in recent years, many enhancement measures for GCNs have emerged. This review aims to summarize the research achievements of Graph Convolutional Network improvements in the field of human action recognition in recent years. It intends to assist future researchers in quickly organizing their research ideas, facilitating the generation of new results.

SCD-Net: Spatiotemporal Clues Disentanglement Network for Self-Supervised Skeleton-Based Action Recognition

Contrastive learning has achieved great success in skeleton-based action recognition. However, most existing approaches encode the skeleton sequences as entangled spatiotemporal representations and confine the contrasts to the same level of representation. Instead, this paper introduces a novel contrastive learning framework, namely Spatiotemporal Clues Disentanglement Network (SCD-Net). Specifically, we integrate the decoupling module with a feature extractor to derive explicit clues from spatial and temporal domains respectively. As for the training of SCD-Net, with a constructed global anchor, we encourage the interaction between the anchor and extracted clues. Further, we propose a new masking strategy with structural constraints to strengthen the contextual associations, leveraging the latest development from masked image modelling into the proposed SCD-Net. We conduct extensive evaluations on the NTU-RGB+D (60&120) and PKU-MMD (I&II) datasets, covering various downstream tasks such as action recognition, action retrieval, transfer learning, and semi-supervised learning. The experimental results demonstrate the effectiveness of our method, which outperforms the existing state-of-the-art (SOTA) approaches significantly. Our code and supplementary material can be found at https://github.com/cong-wu/SCD-Net.

Dynamic Semantic-Based Spatial Graph Convolution Network for Skeleton-Based Human Action Recognition

Graph convolutional networks (GCNs) have attracted great attention and achieved remarkable performance in skeleton-based action recognition. However, most of the previous works are designed to refine skeleton topology without considering the types of different joints and edges, making them infeasible to represent the semantic information. In this paper, we proposed a dynamic semantic-based graph convolution network (DS-GCN) for skeleton-based human action recognition, where the joints and edge types were encoded in the skeleton topology in an implicit way. Specifically, two semantic modules, the joints type-aware adaptive topology and the edge type-aware adaptive topology, were proposed. Combining proposed semantics modules with temporal convolution, a powerful framework named DS-GCN was developed for skeleton-based action recognition. Extensive experiments in two datasets, NTU-RGB+D and Kinetics-400 show that the proposed semantic modules were generalized enough to be utilized in various backbones for boosting recognition accuracy. Meanwhile, the proposed DS-GCN notably outperformed state-of-the-art methods. The code is released here https://github.com/davelailai/DS-GCN

Navigating Open Set Scenarios for Skeleton-Based Action Recognition

In real-world scenarios, human actions often fall outside the distribution of training data, making it crucial for models to recognize known actions and reject unknown ones. However, using pure skeleton data in such open-set conditions poses challenges due to the lack of visual background cues and the distinct sparse structure of body pose sequences. In this paper, we tackle the unexplored Open-Set Skeleton-based Action Recognition (OS-SAR) task and formalize the benchmark on three skeleton-based datasets. We assess the performance of seven established open-set approaches on our task and identify their limits and critical generalization issues when dealing with skeleton information.To address these challenges, we propose a distance-based cross-modality ensemble method that leverages the cross-modal alignment of skeleton joints, bones, and velocities to achieve superior open-set recognition performance. We refer to the key idea as CrossMax - an approach that utilizes a novel cross-modality mean max discrepancy suppression mechanism to align latent spaces during training and a cross-modality distance-based logits refinement method during testing. CrossMax outperforms existing approaches and consistently yields state-of-the-art results across all datasets and backbones. We will release the benchmark, code, and models to the community.

Representation modeling learning with multi-domain decoupling for unsupervised skeleton-based action recognition

Skeleton-based action recognition is one of the basic researches in computer vision. In recent years, the unsupervised contrastive learning paradigm has achieved great success in skeleton-based action recognition. However, previous work often treated input skeleton sequences as a whole when performing comparisons, lacking fine-grained representation contrast learning. Therefore, we propose a contrastive learning method for Representation Modeling with Multi-domain Decoupling (RMMD), which extracts the most significant representations from input skeleton sequences in the temporal domain, spatial domain and frequency domain, respectively. Specifically, in the temporal and spatial domains, we propose a multi-level spatiotemporal mining reconstruction module (STMR) that iteratively reconstructs the original input skeleton sequences to highlight spatiotemporal representations under different actions. At the same time, we introduce position encoding and a global adaptive attention matrix, balancing both global and local information, and effectively modeling the spatiotemporal dependencies between joints. In the frequency domain, we use the discrete cosine transform (DCT) to achieve temporal-frequency conversion, discard part of the interference information, and use the frequency self-attention (FSA) and multi-level aggregation perceptron (MLAP) to deeply explore the frequency domain representation. The fusion of the temporal domain, spatial domain and frequency domain representations makes our model more discriminative in representing different actions. Besides, we verify the effectiveness of the model on the NTU RGB+D and PKU-MMD datasets. Extensive experiments show that our method outperforms existing unsupervised methods and achieves significant performance improvements in downstream tasks such as action recognition and action retrieval.

Independent Dual Graph Attention Convolutional Network for Skeleton-Based Action Recognition

Graph convolutional networks (GCNs) have been widely adopted in skeleton-based action recognition, achieving impressive outcomes. However, the convolution operations in GCNs fail to make full use of the original input data, which restricts its ability to accurately capture the correlation within the skeleton. To solve this issue, this study introduces an independent dual graph attention convolutional network (IDGAN). Specifically, IDGAN additionally incorporates an instinctive attention module that leverages self-attention to capture the correlation among the joints in the original input skeleton. In addition, two independent convolutional operations are used to process two self-attention modules, respectively, to further refine the relationship between skeleton joints. Extensive experiments on several publicly available datasets show that IDGAN outperforms most state-of-the-art algorithms.

Spatio-temporal neural network with handcrafted features for skeleton-based action recognition

The task of human action recognition (HAR) can be found in many computer vision practical applications. Various data modalities have been considered for solving this task, including joint-based skeletal representations which are suitable for real-time applications on platforms with limited computational resources. We propose a spatio-temporal neural network that uses handcrafted geometric features to classify human actions from video data. The proposed deep neural network architecture combines graph convolutional and temporal convolutional layers. The experiments performed on public HAR datasets show that our model obtains results similar to other state-of-the-art methods but has a lower inference time while offering the possibility to obtain an explanation for the classified action.

Object Knowledge Integration for Skeleton-Based Action Recognition

3D-Skeleton-based action recognition has been widely adopted due to its efficiency and robustness to complex backgrounds. While it is capable of conveying a significant amount of information regarding the dynamics of human poses, we argue that its performance is curtailed when confronted with actions involving interactions between humans and objects due to the absence of the study of the surrounding objects. It is of great importance to delve deeper into the study of human-object interactions for skeleton-based action recognition. This paper proposes a novel approach to represent the spatial-temporal skeleton features, along with the present nearby objects and their dynamics. To accomplish this, a new formulation named object knowledge is presented, which entails the categorization of object characteristics, based on whether or not the object necessitates a motion analysis. With a piece of prior knowledge, in cases where it is required, the motion is calculated, while for those where it is not necessary, only the category of object is considered. This object knowledge is then early-fusion along with the skeleton representation, in such a way that it fits into the self-attention model. The experimental results on different popular action recognition datasets (NTU RGB+D 60, NTU RGB+ D 120 illustrate that the proposed approach outperforms the current state-of-the-art methods.

Multi-stream P&U adaptive graph convolutional networks for skeleton-based action recognition

Multi-stream P&U adaptive graph convolutional networks for skeleton-based action recognition

Graph Neural Networks for Skeleton-based action recognition

One of the important directions of the application of artificial intelligence based on human bone behavior recognition is also a research hotspot in the field of computer vision in recent years. Human image video not only contains complex backgrounds, but also uncertain factors such as changes in illumination and changes in the appearance of the human body, which makes behavior recognition based on image videos have certain limitations. Compared with image video, human skeleton video can well overcome the influence of these uncertain factors, so be- havior recognition based on human skeleton has received more and more attention. The human skeleton sequence not only contains the tempo- ral features, but also the spatial structure features of the human body. How to effectively extract the discriminative spatial and temporal fea- tures from the human skeleton sequence is a problem to be solved. In recent years, many methods have been applied to bone-based behavior recognition, such as Recurrent Neural Network (RNN), Convolutional Neural Network (CNN) and Graph Neural Network (GCN). This article will introduce the content and characteristics of these three methods one by one. , And conduct a comparative analysis on it.

Fourier analysis on robustness of graph convolutional neural networks for skeleton-based action recognition

Using Fourier analysis, we explore the robustness and vulnerability of graph convolutional neural networks (GCNs) for skeleton-based action recognition. We adopt a joint Fourier transform (JFT), a combination of the graph Fourier transform (GFT) and the discrete Fourier transform (DFT), to examine the robustness of adversarially-trained GCNs against adversarial attacks and common corruptions. Experimental results with the NTU RGB+D dataset reveal that adversarial training does not introduce a robustness trade-off between adversarial attacks and low-frequency perturbations, which typically occurs during image classification based on convolutional neural networks. This finding indicates that adversarial training is a practical approach to enhancing robustness against adversarial attacks and common corruptions in skeleton-based action recognition. Furthermore, we find that the Fourier approach cannot explain vulnerability against skeletal part occlusion corruption, which highlights its limitations. These findings extend our understanding of the robustness of GCNs, potentially guiding the development of more robust learning methods for skeleton-based action recognition.

Motion Complement and Temporal Multifocusing for Skeleton-Based Action Recognition

Modeling sequences with spatial-temporal graph convolutional networks has become a mainstream paradigm in skeleton-based action recognition. However, many existing methods adopt redundant or cluttered structures to mine the key action features, thus making it difficult to achieve a balanced or leading performance in accuracy and efficiency. In this paper, we propose a novel framework, referred to as Motion Complement and Temporal Multifocusing Network (MCTM-Net), to capture the relationships within skeleton sequences by means of an efficient decomposition of the spatiotemporal graph model. Specifically, for spatial modeling, we introduce a motion-related relational descriptor that extends the channel dimension so as to enhance the modeling of motion salient regions as a complement to the conventional physical adjacency relationships. An improved parameterized physical relationship model is also proposed to better fit the data characteristics. As for temporal modeling, we propose an efficient multi-focus temporal information acquisition strategy that aggregates the information from multiple temporal spans and adjacent regions. We conduct extensive experiments on multiple representative datasets, including NTU-RGB+D (60&120), Northwestern-UCLA, and UWA3D Multiview Activity II, to validate our innovations. The experimental results show the effectiveness of our method. The code will be available at https://github.com/cong-wu/MCMT-Net.