Rank filter

Rank filters in image processing sort (rank) the greyvalues in some neighbourhood of every pixel in ascending order, and replace the centre pixel by some value in the sorted list of greyvalues. If the middle value is chosen, this is the median filter. If the smallest or largest value is chosen, the filter is called a minimum or maximum filter, respectively; the latter are also used in greylevel morphological transforms ( Morphological Operations).