1. A method of tracking objects in a video sequence comprising a plurality of frames, each object being defined by a plurality of pixels, the method comprising: (a) identifying, in a first frame, first and second objects; (b) providing first and second appearance models respectively representing the first and second objects; (c) identifying in a second, subsequent, frame, a group object resulting from the merging of the first and second objects; (d) classifying each pixel of the group object as representing either the first or second object, the classification comprising (i) calculating first and second likelihood parameters respectively indicating the correspondence between said pixel's appearance and the first and second appearance models, (ii) applying a first and second weighting factor to the respective likelihood parameters respectively indicative of the depth order of the first and second objects, and (iii) identifying which of the first and second objects said pixel represents on the basis of the weighted first and second likelihood parameters.