Self-Supervised Skeleton-Based Human Action Recognition under Occlusions: A Two-Stage Approach

Concetti Chiave

This research paper introduces a novel approach to improve the performance of self-supervised skeleton-based human action recognition models in the presence of occlusions, a common challenge in real-world applications.

Sintesi

• Bibliographic Information: Chen, Y., Peng, K., Roitberg, A., Schneider, D., Zhang, J., Zheng, J., ... & Stiefelhagen, R. (2024). Exploring Self-Supervised Skeleton-Based Human Action Recognition under Occlusions. arXiv preprint arXiv:2309.12029v2.
• Research Objective: This paper aims to address the challenge of occlusions in self-supervised skeleton-based human action recognition by proposing a new method that combines data imputation and an adaptive spatial masking strategy.
• Methodology: The authors propose a two-stage approach:
  1. KNN-Imputation: This stage uses K-means clustering to group similar skeleton sequences and then employs K-Nearest Neighbors (KNN) to impute missing joint coordinates within each cluster, effectively completing occluded skeletons.
  2. Occluded Partial Spatio-Temporal Learning (OPSTL): This stage builds upon the existing Partial Spatio-Temporal Learning (PSTL) framework by introducing Adaptive Spatial Masking (ASM). ASM leverages the distribution of missing joints in the dataset to mask joints during training, forcing the model to learn more from intact skeleton data.
• Key Findings:
  • The proposed KNN-Imputation method significantly improves the performance of various self-supervised action recognition methods on occluded datasets.
  • OPSTL, with its ASM strategy, further enhances performance compared to the baseline PSTL method, demonstrating its effectiveness in handling occlusions.
  • Experiments on occluded versions of the NTU-RGB+D 60 and NTU-RGB+D 120 datasets show consistent improvements across different evaluation settings (linear evaluation, semi-supervised, and fine-tuning).
• Main Conclusions: The two-stage approach presented in this paper effectively addresses the challenge of occlusions in self-supervised skeleton-based action recognition. The KNN-Imputation method provides a computationally efficient way to complete missing skeleton data, while OPSTL with ASM leverages the occlusion patterns to improve feature learning.
• Significance: This research contributes to the field of computer vision and robotics by enabling more robust action recognition in real-world scenarios where occlusions are common. This has implications for applications like human-robot interaction, healthcare, and surveillance.
• Limitations and Future Research: The authors acknowledge that the proposed method relies on the availability of a pre-trained model and a dataset with occlusion annotations. Future work could explore unsupervised or weakly-supervised approaches for occlusion handling. Additionally, investigating the generalization ability of the method to different types of occlusions and environments would be beneficial.

Statistiche

OPSTL improves performance by 1.59% and 1.95% on cross-subject and cross-view evaluations of NTU-60 with realistic occlusion, respectively.
OPSTL achieves a performance gain of 1.47% and 2.28% on cross-subject and cross-set evaluations of NTU-120 with realistic occlusion.
Using ASM in the first stage of pre-training yields accuracy improvements of 1.47% for cross-subject and 2.28% for cross-set over CSM on the NTU-120 dataset.
In the second stage, continuing to use ASM yields gains of 0.59% for cross-subject and 0.73% for cross-set compared to using CSM.

Citazioni

"To empower models with the capacity to address occlusion, we propose a simple and effective method."
"Imputing incomplete skeleton sequences to create relatively complete sequences as input provides significant benefits to existing skeleton-based self-supervised methods."
"The new proposed method is verified on the challenging occluded versions of the NTURGB+D 60 and NTURGB+D 120."

Approfondimenti chiave tratti da

Exploring Self-Supervised Skeleton-Based Human Action Recognition under Occlusions

by Yifei Chen, ... alle arxiv.org 10-24-2024

https://arxiv.org/pdf/2309.12029.pdf

Domande più approfondite

How could this approach be adapted to handle dynamic occlusions, where the occluding objects are moving?

Handling dynamic occlusions, where the occluding objects are moving, presents a significant challenge for the proposed approach. Here's a breakdown of potential adaptations and considerations:
Challenges:

Temporal Inconsistency: KNN-Imputation, as described, relies on finding similar static poses within a cluster to fill in missing data. Dynamic occlusions introduce temporal inconsistencies, making it difficult to find suitable matches as the occluded regions change over time.
Increased Uncertainty: Moving occlusions make it harder to determine the true underlying pose. The imputation process needs to account for a wider range of possible joint positions, increasing uncertainty.
Adaptations:

Temporal Context Integration:

Sliding Window Approach: Instead of imputing missing joints frame-by-frame, incorporate temporal context by using a sliding window across multiple frames. This can help identify similar movement patterns even with intermittent occlusions.
Recurrent/Temporal Convolutional Networks: Integrate recurrent neural networks (RNNs) or temporal convolutional networks (TCNs) into the imputation process. These networks can learn temporal dependencies and predict missing joint trajectories more effectively.

Motion Prediction:

Motion Models: Incorporate simple motion models (e.g., linear or Kalman filters) to predict the likely trajectory of occluded joints based on their recent movement history.
Generative Adversarial Networks (GANs): Train GANs to generate realistic skeleton sequences. The generator can be used to synthesize plausible joint movements in occluded regions, conditioned on the visible skeleton data.

Data Augmentation:

Synthetic Dynamic Occlusions: Generate training data with synthetic dynamic occlusions to expose the model to a wider range of occlusion patterns and improve its robustness.

Considerations:

Computational Complexity: Incorporating temporal context and more sophisticated imputation techniques will increase computational complexity. Trade-offs between accuracy and efficiency need to be carefully considered.
Evaluation Metrics: Standard action recognition metrics might not fully capture the performance degradation caused by dynamic occlusions. New evaluation metrics that consider temporal consistency and uncertainty are needed.

While the proposed method shows promising results, could relying on data imputation introduce biases or artifacts that might negatively impact the model's performance in certain scenarios?

Yes, relying on data imputation, while effective, can introduce biases or artifacts that might negatively impact the model's performance in certain scenarios:
Potential Biases and Artifacts:

Over-Smoothing of Motions: KNN-Imputation tends to average out joint positions from similar poses, potentially leading to over-smoothing of rapid or subtle movements. This could hinder the model's ability to recognize actions that rely on these fine-grained motion details.
Bias Towards Training Data: Imputed data reflects patterns present in the training set. If the training data lacks diversity in certain action variations or occlusion patterns, the model might exhibit biases and perform poorly on unseen examples.
Unrealistic Poses: In cases of severe or prolonged occlusions, the imputed joint positions might not accurately represent realistic human movements. This can lead to the model learning from artificial poses, degrading its generalization ability.
Scenarios of Negative Impact:

Actions with Subtle Motions: Actions that rely on small, fast, or highly-coordinated movements (e.g., sign language, playing musical instruments) are more susceptible to performance degradation due to over-smoothing.
Limited Training Data: If the model is trained on a dataset with limited occlusion diversity, it might not generalize well to real-world scenarios with more complex or unexpected occlusions.
Safety-Critical Applications: In applications where action recognition informs critical decisions (e.g., autonomous driving, healthcare), biases or artifacts introduced by imputation could have significant consequences.
Mitigation Strategies:

Diverse and Representative Training Data: Use a training dataset that encompasses a wide range of action variations, occlusion patterns, and viewpoints to minimize bias.
Hybrid Approaches: Explore combining imputation with other techniques like motion prediction or temporal context integration to reduce reliance on static pose matching.
Uncertainty Estimation: Implement methods to estimate the uncertainty associated with imputed joint positions. This information can be used to weight the importance of different body parts during action recognition.
Evaluation on Realistic Data: Rigorously evaluate the model's performance on datasets specifically designed to capture real-world occlusions and challenging scenarios.

What are the ethical implications of using skeleton-based action recognition in real-world applications, particularly in terms of privacy and potential misuse?

The use of skeleton-based action recognition in real-world applications raises significant ethical concerns, particularly regarding privacy and potential misuse:
Privacy Concerns:

Sensitive Information Inference: Even without visual data, skeletal information can reveal sensitive attributes like age, gender, health conditions (e.g., gait disorders), and even emotional states. This raises concerns about unauthorized inference and potential discrimination.
Surveillance and Tracking: Skeleton-based action recognition can enable continuous monitoring and tracking of individuals without their consent, potentially chilling free expression and autonomy.
Data Security and Access: Storage and transmission of skeletal data require robust security measures to prevent unauthorized access, identity theft, or misuse for malicious purposes.
Potential Misuse:

Discriminatory Practices: Action recognition systems trained on biased data could perpetuate or exacerbate existing societal biases, leading to unfair or discriminatory outcomes in areas like law enforcement, employment, or access to services.
Erosion of Trust: Widespread deployment of action recognition technology without clear guidelines and transparency can erode public trust and create a chilling effect on individual freedoms.
Unforeseen Consequences: As with any powerful technology, there is a risk of unforeseen consequences and unintended uses that could have negative societal impacts.
Ethical Considerations and Mitigation:

Transparency and Explainability: Develop transparent and explainable action recognition models to understand how decisions are made and address potential biases.
Data Minimization and Anonymization: Collect and store only the minimal amount of skeletal data necessary for the specific application and implement robust anonymization techniques.
Informed Consent and Control: Obtain informed consent from individuals before collecting or using their skeletal data and provide mechanisms for them to access, control, or delete their data.
Purpose Limitation: Clearly define and limit the use of action recognition technology to specific, legitimate purposes and prohibit use for mass surveillance or discriminatory practices.
Regulation and Oversight: Establish clear legal frameworks and regulatory oversight to govern the development, deployment, and use of action recognition technology, ensuring ethical considerations are paramount.
Public Dialogue and Engagement: Foster open public dialogue and engage with diverse stakeholders to address ethical concerns, build trust, and ensure responsible innovation in this rapidly evolving field.