1. A video data processing apparatus, that processes video data including a sequence of images, said apparatus comprising: an object tracker unit configured to detect a presence of one or more objects within an image and to track a detected object across successive images; an identity association unit configured to associate an identity with an object tracked by said object tracker unit; a counter unit configured to, for a first and second identity, count a number of images within which a tracked object associated with said first identity and a tracked object associated with said second identity have both been detected; and a similarity detector unit configured to determine whether two tracked objects are similar in appearance, wherein said identity association unit is further configured to change said identity associated with a first tracked object to said identity associated with a second tracked object if: said similarity detector unit determines that said first and second tracked objects are similar in appearance, and said count corresponding to said identities associated with said first and second tracked objects, as counted by said counter unit, is less than a predetermined threshold, and wherein said counter unit is further configured, when said identity association unit has changed a first identity associated with a first tracked object to a second identity associated with a second tracked object, not to count a number of images within which a tracked object associated with said first identity and a tracked object associated with any other identity have both been detected.