Abstract--A new technique for the implementation of a single hardware structure capable of computing any rank order filter is presented in this paper. The proposed technique, which is based on the majority gate, achieves faster extraction of setting flag signals and, therefore, shorter processing times are attained. A pipelined systolic array, suitable for performing rank order filtering, is also presented. Applications of rank order filters include digital image processing, speech processing and coding and digital TV applications. ~ 1997 Pattern Recognition Society. Published by Elsevier Science Ltd.
Nonlinear filtersComputer visionRank order filters are a class of nonlinear filters. The input of such filters is a window of data with an odd number of elements. These elements are sorted in ascending order and the output of the rank order filter with rank r is the rth element (rth order statistic)J 1) Special cases of rank order filters are median, minimum and maximum filters, where the outputs are the median, the minimum and the maximum values of the input data window, respectively. Rank order filters exhibit excellent robustness properties and provide solutions in many cases where linear filters are inappropriate. They can suppress high-frequency and impulse noise in an image, avoiding at the same time extensive blurting of the image, since they have good edge preservation properties. They have found numerous applications, such as in digital image analysis, in speech processing and coding, in digital TV applications, etc. (~'2) Median and rank order filters are strongly related with morphological filters, another class of nonlinear illters. (3'4) It has been shown that erosions and dilations are special cases of rank order filters and that any rank order filter can be expressed either as a maximum of erosions or as a minimum of dilations. °) Therefore, algorithms which originally have been devised for rank order and median filters can be used for realization of morphological operators. (5'6) Several algorithms have been proposed for the realization of rank order filters, such as tree sorts, shell sorts and quick sorts. (1) Although these algorithms are suitable for software implementation, they result in inefficient hardware structures, since they handle the numbers in wordlevel. Rank order filters can be implemented in VLSI