1. A method comprising: determining at least one transfer function relating integral projection curves of a face region in a first frame, wherein the face region has a predefined position and a predetermined size, and of a first window in a second frame, wherein the first window is co-located in size and position with the face region; determining a scaling factor based upon the at least one transfer function; determining integral gradient projections for the face region in the first frame and for a second window in the second frame with the second window having a size that is based upon a size of the first window modified by the scaling factor; determining a distance between the integral gradient projections of the face region in the first frame and of the second window in the second frame; and determining whether the scaling factor is verified based on a relationship between the distance and a predefined threshold.