1. A method of selectively tracking an object appearing in a video sequence comprising a plurality of frames, each frame comprising a plurality of pixels, the method comprising: (i) comparing first and second frames of the video sequence to identify a region of pixels therein representing an object having inter-frame motion; (ii) determining whether said region appears in a predetermined number of subsequent frames, and, if so, assigning a motion parameter ζ m to said region indicative of the change in position thereof over said predetermined number of frames; (iii) comparing said motion parameter ζ m with a threshold value T ζ to determine whether or not said region is to be tracked; and (iv) if said motion parameter ζ m is below the threshold value T ζ , recording an intra-frame position of said region for subsequent frames in which said region is identified, wherein the motion parameter ζ m is defined as: ζ m = ( σ cx 2 σ vx 2 + τ + σ cy 2 σ vy 2 + τ ) / 2 where σ cx 2 and σ cy 2 are the positional variances, of a reference point on said region, in x and y directions, respectively, σ vx 2 and σ vy 2 are the velocity variances in x and y directions of movement of said region, respectively, and τ is a predetermined constant.