Intro

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 left to right, down, right to left, down etc

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.