Recognizing action events from multiple viewpoints

A first step towards an understanding of the semantic content in a video is the reliable detection and recognition of actions performed by objects. This is a difficult problem due to the enormous variability in an action's appearance when seen from different viewpoints and/or at different times. In this paper we address the recognition of actions by taking a novel approach that models actions as special types of 3D objects. Specifically, we observe that any action can be represented as a generalized cylinder, called the action cylinder. Reliable recognition is achieved by recovering the viewpoint transformation between the reference (model) and given action cylinders. A set of 8 corresponding points from time-wise corresponding cross-sections is shown to be sufficient to align the two cylinders under perspective projection. A surprising conclusion from visualizing actions as objects is that rigid, articulated, and nonrigid actions can all be modeled in a uniform framework.