Tracking with the Kinematics of Extremal Contours

This paper addresses the problem of articulated motion tracking from image sequences. We describe a method that relies on an explicit parameterization of the extremal contours in terms of the joint parameters of an associated kinematic model. The latter allows us to predict the extremal contours from the body-part primitives of an articulated model and to compare them with observed image contours. The error function that measures the discrepancy between observed contours and predicted contours is minimized using an analytical expression of the Jacobian that maps joint velocities onto contour velocities. In practice we model people both by their geometry (truncated elliptical cones) and with their articulated structure – a kinematic model with 40 rotational degrees of freedom. We observe image data gathered with several synchronized cameras. The tracker has been successfully applied to image sequences gathered at 30 frames/second.