Gil-Werman algorithm, that performs the calculations
   in a time that is independent of the size of the Sels.  Implementations
   of tophat and hdome are also given.

   We also provide grayscale rank order filters for brick filters.
   The rank order filter is a generalization of grayscale morphology,
   that selects the rank-valued pixel (rather than the min or max).
   A color rank order filter applies the grayscale rank operation
   independently to each of the (r,g,b) components.

4. Image scaling

  