The Quantile User Image Macro Language Extension UMQuantile.p 
defines two filter macros, described below.  To be sure that the
extension is installed, include RequiresUser('Quantile',1) in your
macro.  This will give a reasonable error message if someone
tries to use the macro with a copy of Image that does not contain
the package.

- XYQuantile, which is an extension of the median filtering,
- ZQuantile, which does the same filtering "vertically" on a stack,

XYQuantile   
---------- 
  The XYQuantile routine allows you to effect a median filtering on a      
rectangle of chosen dimensions.  The user is asked to specify the values   
of  filter width (FW), filter height (FH), and quantile (Q).  For each     
pixel, the routine selects a rectangle of FWxFH pixels, sorts all the      
points in this rectangle,  and replaces the original pixel by the          
quantile. The median value is given by Q=((FWxFH) div 2) +1. For           
instance, in a 5x3 rectangle, the median is the 8th quantile of the        
sorted values.   
  To show the potenial of this filter, I added a "Noisy Image". Compare    
the filtering of this image with  the Reduce Noise function if NIH, and    
with a median filter with the following parameters:  Filter Width=5,       
Filter Height=1, Quantile=3.   
   You can also specify that the routine retains another value than the    
median, for instance the local  minimum (Quantile=1) or maximum            
(Quantile=FHxFW).   

Macro call: RequiresUser('Quantile',1); XYQuantile(FilterW, FilterH, Quantile);

ZQuantile   
---------   
  The ZQuantile filter works exclusively on stacks. It perfomrs            
basically the same operation than the XYQuantile  procedure, but           
vertically. It gives similar results as the Average function of the        
Stack menu, but there  is a weaker influence of the extreme values on      
the median than on the average.   

  Note that both XYQuantile and ZQuantile filterings are slow to           
process, and that the speed is inversely propotional to the size of the    
filter.   

Macro call:  RequiresUser('Quantile',1); ZQuantile(Quantile);

If you find some use of or have some comment about these filters,   
please contact jvanheld@dbm.ulb.ac.be