1. A method for compositing motion picture and video images by automatically generating inner and outer boundary lines on either side of a subject-to-background transition area comprising the steps of: a) storing in a memory the signal levels of each signal channel comprising the signals at each pixel location in a clear image frame with a subject absent, b) storing in a memory the signal levels of each signal channel comprising the signals at each pixel location in a current image frame with said subject in place, c) subtracting the signal levels of pixels in said clear image frame from signal levels of corresponding pixels in said current image frame to obtain their difference, d) storing in a memory a 0.0 at all pixel addresses where said difference approximates zero, and storing a 1.0 for all remaining pixels in said image frame to form a field of zeros in the background area and a field of ones in the subject and transition areas, e) defining an outer boundary line as the addresses of those pixels forming a continuous line of pixels having been assigned a 0.0 and located adjacent to pixels having been assigned a 1.0, thereby forming an outer boundary line on the background adjacent to the edge of the subject-to-background transition area, f) shrinking said outer boundary line inward to form an inner boundary line on the subject at a sufficient distance from said subject's edge to avoid including elements of the background, thereby automatically generating said inner and outer boundary lines.