1. Video data processing apparatus, said video data comprising a sequence of images, said apparatus comprising: an object tracker operable to detect the presence of one or more objects within an image and to track a detected object across successive images; an identity associator operable to associate an identity with an object tracked by said object tracker; a counter operable, for a first and second identity, to count the 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 operable to determine whether two tracked objects are similar in appearance; said identity associator being operable to change said identity associated with a first tracked object to said identity associated with a second tracked object if: (a) said similarity detector determines that said first and second tracked objects are similar in appearance and (b) said count corresponding to said identities associated with said first and second tracked objects, as counted by said counter, is less than a predetermined threshold.