Towards Understanding Action Recognition

TLDR

Action recognition methods often fail on real‑world datasets, and it is unclear which factors most influence performance. This paper systematically evaluates action recognition methods using a thoroughly annotated dataset to identify key factors for improvement. The authors annotate human joints in HMDB to generate ground‑truth optical flow and segmentation, then evaluate existing methods by substituting algorithm outputs with ground truth to isolate component contributions. High‑level pose features, especially temporal pose, dominate performance, yet pose estimation is unreliable; improving low‑level optical flow and human detection can also boost accuracy, and the J‑HMDB dataset supports further analysis.

Abstract

Although action recognition in videos is widely studied, current methods often fail on real-world datasets. Many recent approaches improve accuracy and robustness to cope with challenging video sequences, but it is often unclear what affects the results most. This paper attempts to provide insights based on a systematic performance evaluation using thoroughly-annotated data of human actions. We annotate human Joints for the HMDB dataset (J-HMDB). This annotation can be used to derive ground truth optical flow and segmentation. We evaluate current methods using this dataset and systematically replace the output of various algorithms with ground truth. This enables us to discover what is important - for example, should we work on improving flow algorithms, estimating human bounding boxes, or enabling pose estimation? In summary, we find that high-level pose features greatly outperform low/mid level features, in particular, pose over time is critical, but current pose estimation algorithms are not yet reliable enough to provide this information. We also find that the accuracy of a top-performing action recognition framework can be greatly increased by refining the underlying low/mid level features, this suggests it is important to improve optical flow and human detection algorithms. Our analysis and J-HMDB dataset should facilitate a deeper understanding of action recognition algorithms.