Towards accurate group activity analysis in videos: Robust saliency detection and effective feature modeling

Human activity analysis is an important area of computer vision research today. The goal of human activity analysis is to automatically analyze ongoing activities from an unknown video. The ability to analyze complex human activities from videos has many important applications, such as smart camera system, video surveillance, etc. However, it is still far from an off-the-shelf system. There are many challenging problems and it is still an active research area. This dissertation focuses on addressing two problems: various camera motions and effective modeling of group behaviors.;We propose a unified and robust framework to detect salient motions from diverse types of videos. Given a video sequence that is recorded from either a stationary or moving camera, our algorithm is able to detect the salient motion regions. The model is inspired by two observations: 1) background motion caused by orthographic cameras lies in a low rank subspace, and 2) pixels belonging to one trajectory tend to group together. Based on these two observations, we introduce a new model using both low rank and group sparsity constraints. It is able to robustly decompose a motion trajectory matrix into foreground and background ones. Extensive experiments demonstrate very competitive performance on both synthetic data and real videos.;After salient motion detection, a new method is proposed to model group behaviors in video sequences. This approach effectively models group activities based on social behavior analysis. Different from previous work that uses independent local features, our method explores the relationships between the current behavior state of a subject and its actions. An interaction energy potential function is proposed to represent the current behavior state of a subject, and velocity is used as its actions. Our method does not depend on human detection, so it is robust to detection errors. Instead, tracked salient points are able to provide a good estimation of modeling group interaction. We evaluate our algorithm in two datasets: UMN and BEHAVE. Experimental results show its promising performance against the state-of-art methods.