Implementation and Evaluation of Real-Time Pan-Tilt-Zoom Camera Calibration

Pan-tilt-zoom (PTZ) cameras are frequently used in surveillance applications as they can observe a much larger region of the environment than a fixed-lens camera while still providing high-resolution imagery. The pan, tilt, and zoom parameters of a single camera may be simultaneously controlled by online users as well as automated surveillance applications. To accurately register autonomously tracked objects to a world model, the surveillance system requires accurate knowledge of camera parameters. Due to imprecision in the PTZ mechanism, these parameters cannot be obtained from PTZ control commands but must be calculated directly from camera imagery. This paper describes the efforts undertaken to implement a real-time calibration system for a stationary PTZ camera. The approach continuously tracks distinctive image feature points from frame to frame, and from these correspondences, robustly calculates the homography transformation between frames. Camera internal parameters are then calculated from these homographies. The calculations are performed by a self contained program that continually monitors images collected by the camera as it performs pan, tilt, and zoom operations. The accuracy of the calculated calibration parameters are compared to ground truth data. Problems encountered include inaccuracies in large orientation changes and long algorithm execution time.