Developing the Path Signature Methodology and Its Application to Landmark- Based Human Action Recognition

Landmark-based human action recognition in videos is a challenging task in computer vision. One key step is to design a generic approach that generates discriminative features for the spatial structure and temporal dynamics. To this end, we regard the evolving landmark data as a high-dimensional path and apply path signature techniques to provide an expressive, robust, non-linear, and interpretable representation for the sequential events. We do not extract signature features from the raw path, rather we propose path disintegrations and path transformations as preprocessing steps. Path disintegrations turn a high-dimensional path linearly into a collection of lower-dimensional paths; some of these paths are in pose space while others are defined over a multi-scale collection of temporal intervals. Path transformations decorate the paths with additional coordinates in standard ways to allow the truncated signatures of transformed paths to expose additional features. For spatial representation, we apply the non-linear signature transform to vectorize the paths that arise out of pose disintegration, and for temporal representation, we apply it again to describe this evolving vectorization. Finally, all the features are joined together to constitute the input vector of a linear single-hidden-layer fully-connected network for classification. Experimental results on four diverse datasets demonstrated that the proposed feature set with only a linear shallow network is effective and achieves comparable state-of-the-art results to the advanced deep networks, and meanwhile, is capable of interpretation.