add readme

skimage/rank/README.rst

@@ -7,7 +7,29 @@ To use this to build your Cython file use the commandline options:
 
 **To do**
 
-* add simple examples
+* add simple examples, adapt documentation on existing examples
 
 * add/check existing doc
 
+* adapting tests for each type of filter
+
+**General remarks**
+
+Basically these filters compute local histogram for each pixel. Histogram is build using a moving window in
+order to limit redundant computation. The path followed by the moving window is given hereunder
+
+ ...-----------------------\
+/--------------------------/
+\-------------------------- ...
+
+A comparison is proposed with cmorph.dilate algorithm to show how computation costs evolve with respect to image size or
+structuring element size. This implementation gives better results for large structuring elements.
+
+A local histogram is update at each pixel by introducing pixel entering the structuring element border and
+by removing those leaving it. The histogram size is 8bit (256 bins) for 8 bit images and 2 to 12 bit (up to 4096 bins)
+for 16bit image depending on the image maximum value. Image with pixels higher than 4095 raise a ValueError.
+
+The filter is applied up to the image border, the neighboorhood used is adjusted accordingly. The user may provide
+a mask image (same size as input image) where non zero value are the part of the image participating the the
+histogram computation. By default all the image is filtered.
+