2. The method of claim 1 , the parent rectangle comprising two or more sub-rectangles.
2. The method of claim 1 , the parent rectangle comprising two or more sub-rectangles. 3. The method of claim 2 , the two or more sub-rectangles comprising a first sub-rectangle and a second sub-rectangle, the numeric value calculated by summing values of pixels in each of the first sub-rectangle and the second sub-rectangle, and subtracting a sum for the first sub-rectangle from a sum for the second sub-rectangle.