1. A method of determining local relative z-ordering information, the method comprising: (a) obtaining a first image frame and second image frame containing digital image data; (b) dividing the first image frame and second image frame into segments; (c) matching segments of the first image frame to the second image frame, and matching segments of the second image frame to the first image frame; (d) determining relative offsets for the segments that represent a relative displacement of the segments between image frames; and (e) determining for either of the image frames which segments in that frame would overlap if the segments were moved by their relative offsets.