Statistical shape theory for activity modeling

Monitoring activities in a certain region from video data is an important surveillance problem today. The goal is to learn the pattern of normal activities and detect unusual ones by identifying activities that deviate appreciably from the typical ones. In this paper we propose an approach using statistical shape theory (based on Kendall´s shape model) [D.G. Kendall et al., 1999]. In a low resolution video each moving object is best represented as a moving point mass or particle. In this case, an activity can be defined by the interactions of all or some of these moving particles over time. We model this configuration of the particles by a polygonal shape formed from the locations of the points in a frame and the activity by the deformation of the polygons in time. These parameters are learnt for each typical activity. Given a test video sequence, an activity is classified as abnormal if the probability for the sequence (represented by the mean shape and the dynamics of the deviations), given the model is below a certain threshold. The approach gives very encouraging results in surveillance applications using a single camera and is able to identify various kinds of abnormal behaviors.