Resource efficient action recognition in videos

This thesis traces an innovative journey in the domain of real-world action recognition, in particular focusing on memory and data efficient systems. It begins by introducing a novel approach for smart frame selection, which significantly reduces computational costs in video classification. It further optimizes the action recognition process by addressing the challenges of training time and memory consumption in video transformers, laying a strong foundation for memory efficient action recognition. The thesis then delves into zero-shot learning, focusing on the flaws of the currently existing protocol and establishing a new split for true zero-shot action recognition, ensuring zero overlap between unseen test classes and training or pre-training classes. Building on this, a unique cluster-based representation, optimized using reinforcement learning, is proposed for zero-shot action recognition. Crucially, we show that a joint visual-semantic representation learning is essential for improved performance. We also experiment with feature generation approaches for zero-shot action recognition by introducing a synthetic sample selection methodology extending the utility of zero-shot learning to both images and videos and selecting high-quality samples for synthetic data augmentation. This form of data valuation is then incorporated for our novel video data augmentation approach where we generate video composites using foreground and background mixing of videos. The data valuation helps us choose good composites at a reduced overall cost. Finally, we propose the creation of a meaningful semantic space for action labels. We create a textual description dataset for each action class and propose a novel feature generating approach to maximise the benefits of this semantic space. The research contributes significantly to the field, potentially paving the way for more efficient, resource-friendly, and robust video processing and understanding techniques

A robust and efficient video representation for action recognition

This paper introduces a state-of-the-art video representation and applies it to efficient action recognition and detection. We first propose to improve the popular dense trajectory features by explicit camera motion estimation. More specifically, we extract feature point matches between frames using SURF descriptors and dense optical flow. The matches are used to estimate a homography with RANSAC. To improve the robustness of homography estimation, a human detector is employed to remove outlier matches from the human body as human motion is not constrained by the camera. Trajectories consistent with the homography are considered as due to camera motion, and thus removed. We also use the homography to cancel out camera motion from the optical flow. This results in significant improvement on motion-based HOF and MBH descriptors. We further explore the recent Fisher vector as an alternative feature encoding approach to the standard bag-of-words histogram, and consider different ways to include spatial layout information in these encodings. We present a large and varied set of evaluations, considering (i) classification of short basic actions on six datasets, (ii) localization of such actions in feature-length movies, and (iii) large-scale recognition of complex events. We find that our improved trajectory features significantly outperform previous dense trajectories, and that Fisher vectors are superior to bag-of-words encodings for video recognition tasks. In all three tasks, we show substantial improvements over the state-of-the-art results

Mining Mid-level Features for Action Recognition Based on Effective Skeleton Representation

Recently, mid-level features have shown promising performance in computer vision. Mid-level features learned by incorporating class-level information are potentially more discriminative than traditional low-level local features. In this paper, an effective method is proposed to extract mid-level features from Kinect skeletons for 3D human action recognition. Firstly, the orientations of limbs connected by two skeleton joints are computed and each orientation is encoded into one of the 27 states indicating the spatial relationship of the joints. Secondly, limbs are combined into parts and the limb's states are mapped into part states. Finally, frequent pattern mining is employed to mine the most frequent and relevant (discriminative, representative and non-redundant) states of parts in continuous several frames. These parts are referred to as Frequent Local Parts or FLPs. The FLPs allow us to build powerful bag-of-FLP-based action representation. This new representation yields state-of-the-art results on MSR DailyActivity3D and MSR ActionPairs3D