Active Rays : Polar { transformed Active Contours for Real { TimeContour

In this paper we describe a new approach to contour extraction and tracking, which is based on the principles of active contour models and overcomes its shortcomings. We formally introduce active rays, describe the contour extraction as an energy minimization problem and discuss what active contours and active rays have in common. The main diierence is that for active rays a unique ordering of the contour elements in the 2D image plane is given, which cannot be found for active contours. This is advantageous for predicting the contour ele-ments' position and prevents crossings in the contour. Furthermore, another advantage is that instead of an energy minimization in the 2D image plane the minimization is reduced to a 1D search problem. The approach also shows any{time behavior which is important with respect to real{time applications. Finally, the method allows for the management of multiple hypotheses of the object's boundary. This is an important aspect if concave contours shall be tracked. Results on real image sequences (tracking a toy train in a laboratory scene, tracking pedestrians in an outdoor scene) show the suitability of this approach for real{time object tracking in a closed loop between image acquisition and camera movement. The contour tracking can be done within the image frame rate (25 fps) on standard Unix workstations (HP 735) without any specialized hardware.