1. A method of image processing for tracking a subject in a sequence of image frames using defined criteria for classifying image regions as potentially being part of the tracked subject, the method comprising the steps of: (a) obtaining, for a first image frame, a record of an image region previously designated as containing the tracked subject; (b) classifying, in graded form, image regions of a second image frame in accordance with the defined criteria to thereby generate classified image regions, (c) defining a spatially varying threshold level with reference to the previously designated image region; and (d) updating said designated image region associated with said second image frame with reference to both said classified image regions in said second image frame and the previously designated image region in said first image frame, excluding image regions whose classification grade is below the spatially varying threshold level from the updated designated image region.