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Weakly-Supervised Temporal Action Detection for Fine-Grained Videos with Hierarchical Atomic Actions (2207.11805v1)

Published 24 Jul 2022 in cs.CV and cs.AI

Abstract: Action understanding has evolved into the era of fine granularity, as most human behaviors in real life have only minor differences. To detect these fine-grained actions accurately in a label-efficient way, we tackle the problem of weakly-supervised fine-grained temporal action detection in videos for the first time. Without the careful design to capture subtle differences between fine-grained actions, previous weakly-supervised models for general action detection cannot perform well in the fine-grained setting. We propose to model actions as the combinations of reusable atomic actions which are automatically discovered from data through self-supervised clustering, in order to capture the commonality and individuality of fine-grained actions. The learnt atomic actions, represented by visual concepts, are further mapped to fine and coarse action labels leveraging the semantic label hierarchy. Our approach constructs a visual representation hierarchy of four levels: clip level, atomic action level, fine action class level and coarse action class level, with supervision at each level. Extensive experiments on two large-scale fine-grained video datasets, FineAction and FineGym, show the benefit of our proposed weakly-supervised model for fine-grained action detection, and it achieves state-of-the-art results.

Citations (12)

Summary

  • The paper introduces a novel weakly-supervised method that models actions as hierarchical atomic actions for fine-grained video analysis.
  • It employs self-supervised clustering to discover atomic actions, constructing a four-level visual representation for improved classification with only video-level labels.
  • Experiments on large-scale datasets demonstrate state-of-the-art performance in detecting subtle temporal actions compared to existing methods.

The paper "Weakly-Supervised Temporal Action Detection for Fine-Grained Videos with Hierarchical Atomic Actions" presents a novel approach to detecting fine-grained actions in videos using weak supervision. The authors propose a method that models actions as combinations of reusable atomic actions, aiming to capture both the commonality and individuality of fine-grained actions through a four-level hierarchy of visual representations.

Key Contributions and Methodology

  1. Problem Definition and Objective: The focus of the paper is on weakly-supervised temporal action detection in fine-grained videos. This approach requires only video-level labels and does not necessitate detailed temporal annotations of action start and end times. The aim is to efficiently detect subtle differences between fine-grained actions that are label-efficient and reflect real-world complexities, such as similar human behavior.
  2. Hierarchical Atomic Actions: The core innovation is the modeling of actions through hierarchical atomic actions. These atomic actions are self-discovered from the data using self-supervised clustering, which positions them as fundamental building blocks in recognizing fine-grained differences between actions.
  3. Visual Representation Hierarchy: The paper introduces a visual representation hierarchy comprising four levels:
    • Clip level
    • Atomic action level
    • Fine action class level
    • Coarse action class level The supervision is applied at each of these levels, allowing for a nuanced understanding and classification of actions.
  4. Self-Supervised Clustering for Atomic Action Discovery: The authors employ a self-supervised clustering technique to discover atomic actions from the input data. Atomic actions are proposed to represent distinct, semantically meaningful components within an action sequence, enhancing the model's ability to differentiate between similar fine-grained actions.
  5. Semantic Label Hierarchy: By leveraging the semantic relationships in label hierarchies (from coarse to fine), the model can boost learning through shared characteristics among actions, effectively utilizing the semantic structure inherent in fine-grained data.
  6. Experimental Validation: The effectiveness of the model is validated through extensive experiments on two large-scale fine-grained video datasets—FineAction and FineGym. The proposed method achieves state-of-the-art results, showcasing its superiority over existing weakly-supervised models for general action detection.

Detailed Components and Results

  • Multiple Instance Learning (MIL): The model uses MIL to facilitate action classification where unknown parts of a video contain specific actions. The classifier operates through pooling methods to generate video-level predictions.
  • Visual Concept Learning: This component focuses on capturing fine-grained temporal details by learning visual concepts through a clustering process.
  • Coarse-to-Fine Semantic Hierarchy: The structure is used to enhance the learning process by modeling correlations and compositional relationships between fine-grained and coarse-grained actions.
  • Inference and Detection: The model employs a two-step process to first classify and then detect action segments, using clip scores and a threshold to establish action boundaries.

Conclusion

The authors conclude by highlighting the novelty of their approach in tackling weakly-supervised fine-grained temporal action detection. They demonstrate through their experiments that the Hierarchical Atomic Action Network (HAAN) effectively utilizes hierarchical modeling, visual concept learning, and semantic label hierarchical structure to achieve high performance in detecting complex, fine-grained actions, significantly outperforming prior approaches in this domain.

The research opens up new avenues in improving action detection methodologies within complex video datasets, contributing richly to both academic research and practical applications in areas such as skill acquisition and automated surveillance.