Online calibration of two zoom-pan-tilt units for planar dynamic events

In many sports and surveillance scenarios the action is dynamic and takes place on a planar surface, while being recorded by two or more zoom-pan-tilt cameras. Although their position is fixed, these cameras can typically rotate and zoom independently from each other. When rotation and zoom of each camera are known, one could reconstruct the dynamic event in 3D and generate different views of the action. Sensors exist which report zoom and orientation changes of pan-tilt units. In absence of such sensors, however, we prove that the varying zoom and rotation of two pan-tilt units can be extracted solely from the planar homography which exists between both cameras.