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Optical Flow Guided Feature: A Fast and Robust Motion Representation for Video Action Recognition (1711.11152v2)

Published 29 Nov 2017 in cs.CV

Abstract: Motion representation plays a vital role in human action recognition in videos. In this study, we introduce a novel compact motion representation for video action recognition, named Optical Flow guided Feature (OFF), which enables the network to distill temporal information through a fast and robust approach. The OFF is derived from the definition of optical flow and is orthogonal to the optical flow. The derivation also provides theoretical support for using the difference between two frames. By directly calculating pixel-wise spatiotemporal gradients of the deep feature maps, the OFF could be embedded in any existing CNN based video action recognition framework with only a slight additional cost. It enables the CNN to extract spatiotemporal information, especially the temporal information between frames simultaneously. This simple but powerful idea is validated by experimental results. The network with OFF fed only by RGB inputs achieves a competitive accuracy of 93.3% on UCF-101, which is comparable with the result obtained by two streams (RGB and optical flow), but is 15 times faster in speed. Experimental results also show that OFF is complementary to other motion modalities such as optical flow. When the proposed method is plugged into the state-of-the-art video action recognition framework, it has 96:0% and 74:2% accuracy on UCF-101 and HMDB-51 respectively. The code for this project is available at https://github.com/kevin-ssy/Optical-Flow-Guided-Feature.

Citations (283)

Summary

  • The paper introduces OFF as a novel motion representation that captures temporal dynamics with high efficiency through feature-level gradient computation.
  • The CNN with OFF achieves 93.3% accuracy on UCF-101 and reaches 96.0% when integrated with state-of-the-art frameworks, rivaling two-stream methods.
  • The method reduces computational overhead by 15 times compared to conventional optical flow while providing robust performance for video action recognition.

Optical Flow Guided Feature: An Advanced Motion Representation for Video Action Recognition

This paper introduces the Optical Flow guided Feature (OFF), a novel motion representation that aims to enhance the performance of video action recognition models by effectively capturing temporal dynamics with improved efficiency. OFF offers a significant contribution to action recognition, a classical problem in computer vision, by allowing convolutional neural networks (CNNs) to extract temporal features directly from input frames in a computationally efficient manner.

The proposed OFF is derived through a theoretical framework based on the classical definition of optical flow, yet it distinguishes itself by operating orthogonally to optical flow on the feature level. This orthogonality allows for capturing motion dynamics without directly computing optical flow vectors, traditionally known to be computationally intensive. Concurrently, OFF calculates pixel-wise spatio-temporal gradients on feature maps, providing theoretical backing for the practical use of frame differences to capture motion information.

Key numerical results demonstrate the competitive advantage of the proposed method. Specifically, the CNN using OFF with only RGB inputs achieves an impressive accuracy of 93.3% on the UCF-101 dataset. This performance is comparable to more computationally demanding two-stream methods that use both RGB and optical flow data, yet OFF provides a speedup that makes the model 15 times faster. When the proposed ONN is integrated with state-of-the-art frameworks for action recognition, accuracy climbs to 96.0% on UCF-101 and 74.2% on HMDB-51, reflecting the complementarity of OFF with other motion representations such as optical flow.

Furthermore, the paper outlines the implementation details of incorporating OFF within CNNs. The architecture comprises myriads of interconnected sub-networks, including feature generation sub-networks, followed by OFF sub-networks and classification sub-networks. The OFF sub-network contains several OFF units that effectively compute spatial and temporal features, leveraging concepts from 2D and 3D convolutions to refine output features progressively.

In practical terms, this work has two significant implications. First, it provides a robust alternative to optical flow that maintains competitive accuracy while offering substantial computational benefits. Second, it proposes a new design pattern for CNN architecture that harnesses gradients at the feature level to better encapsulate motion dynamics.

Looking forward, this research opens avenues for further improvements in the domain of action recognition. The efficient capture and use of spatio-temporal features could potentially be extended to more complex video understanding tasks such as video captioning or activity forecasting. Moreover, given its complementary nature, OFF might be integrated into other complex CNN models to enhance their efficiency without notably sacrificing accuracy.

Overall, the Optical Flow guided Feature presents both theoretical advancements and practical improvements for real-time video analysis, providing valuable insights for researchers and practitioners focused on optimizing computational efficiency without compromising precision.