An Active Tracking Method for Small Ships in Open Water Based on Fixed/PTZ Camera System

It is difficult to actively track and capture clear images of inland river ships with the current Closed Circuit Television (CCTV) system. To fill the gap, an active tracking method for small ships in open waters based on the fixed/pan-tilt-zoom (PTZ) camera system is proposed. A three-layer joint calibration model based on a virtual quadrilateral (VQ) is introduced to jointly calibrate the fixed camera and the PTZ camera, which matches the image coordinate with the pan and tilt angle of the PTZ camera one by one; The introduced VQ filters out the targets outside the quadrilateral, eliminating inference and improving the accuracy of detection. The mapping relationship between the image coordinates and the world coordinates can be obtained by using the Perspective-n-Point (PnP) algorithm and the vertices of the VQ; Fourthly, the world coordinates of the points in the VQ are transformed into the Pan-Tilt-Hight (PTH) coordinates via PTH model. Then, by calculating the coordinate of the ship (the centroid of the ship) in the VQ, the pan and tilt angle of the PTZ camera can be derived, achieving real-time active tracking and keeping the target at the center of the PTZ camera image. To validate the proposed method, two real scenes are introduced, namely Chunhui Lake in Xiaogan City and the Sino-French Bridge section of the Han River in Wuhan City, Hubei Province. The results indicate that, ① the F1 -Scores of the proposed method on the fixed camera are 96.82% and 97.62%, respectively; ② when the proposed method is applied to the PTZ camera for tracking the moving ships, the failure rate is 4.63%. In summary, the proposed active tracking method performs reasonably in practice with a high tracking rate of 18.55 fps.