A video data processing apparatus, that processes video data including a sequence of images, the apparatus including: 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 the 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 the first identity and a tracked object associated with the second identity have both been detected; and a similarity detector unit configured to determine whether two tracked objects are similar in appearance.