Ground Plane Rectification by Tracking Moving Objects

Most outdoor visual surveillance scenes involve objects moving on a ground plane. We present a new, fully automated technique for both affine and metric rectification of this ground plane (up to a scale factor) by simply tracking moving objects. In particular, we derive the necessary constraints on the image plane to ground plane projective transformation by observing objects which move at constant (world) velocity for some part of their trajectory. No knowledge of camera parameters is assumed. We describe a hierarchy of possible solutions, depending on the nature of the motion trajectories observed. We also show how to automatically detect degenerate cases where 2D rectification is not possible. Useful applications of the various types of rectification are presented. Our experiments demonstrate all the possible solutions on a variety of scenes, as well as some of the applications made possible by rectification.