1. A method of tracking objects on a plane within one or more video images of the objects on the plane captured by a video camera, the method comprising: processing the captured video images so as to extract one or more image features from each object, detecting each of the objects from a relative position of the objects on the plane as viewed from the captured video images by comparing the one or more extracted image features associated with each object with sample image features from a predetermined set of possible example objects which the captured video images may contain; generating object identification data for each object, from the comparing, which identifies the respective object on the plane; generating a three dimensional model of the plane, and logging, for each detected object, the object identification data together with object path data, the object path data providing a position of the object on the three dimensional model of the plane from the video images with respect to time; in which the logging includes: detecting an occlusion event in dependence upon whether a first image feature associated with a first of the objects obscures a whole or part of at least a second image feature associated with at least a second of the objects; and, if an occlusion event is detected, associating the object identification data for the first object and the object identification data for the second object with the object path data for both the first object and the second object respectively and logging the associations; identifying at least one of the objects involved in the occlusion event in dependence upon a comparison between the one or more image features associated with that object and the sample image features from the predetermined set of possible example objects; and updating the logged path data after the identification of at least one of the objects so that the respective path data is associated with the respective identified object.