When broadcasting sports events such as football, it is useful to be able to place virtual annotations on the pitch, to indicate things such as distances between players and the goal, or whether a player is offside. This requires the camera position, orientation, and focal length to be estimated in real time, so that the graphics can be rendered to match the camera view. Whilst this can be achieved by using sensors on the camera mount and lens, they can be impractical or expensive to install, and often the broadcaster only has access to the video feed itself. This paper presents a method for computing the position, orientation and focal length of a camera in real time, using image analysis. The method uses markings on the pitch, such as arcs and lines, to compute the camera pose. A novel feature of the method is the use of multiple images to improve the accuracy of the camera position estimate. A means of automatically initialising the tracking process is also presented, which makes use of a modified form of Hough transform. The paper shows how a carefully chosen set of algorithms can provide fast, robust and accurate tracking for this real-world application.

This paper was published in the Journal of Real Time Image Processing, Vol. 2, No. 2-3 November 2007, pp. 117-132. It is an extended version of a similar paper presented at CVMP2006, published as WHP 146.