This code creates artistic halftones using a very simple
(quick-and-dirty) algorithm.  

The algorithm:

Input: a motif image M (a gray-scale image)
Input: an ordered dither matrix D (Bayer, void-and-cluster, etc)
Output: an ordered dither matrix Q

1. Let S be the set of all pixels in M.
2. Count the number n of gray levels in M (example: if M is a black-and
   white image like a line drawing or some text, then there are two
   gray levels, black and white).
3. Define the n subsets Si (i from 1 to n): Si is the set of all
   points in S having the i-th gray level.
4. Define n lists Li (i from 1 to n): Li is all the thresholds in D,
   at the pixels in subset Si.
5. For each i, sort the pixels in Li in order of increasing threshold
   value. 
6. Define L by concatenating all the sorted lists Li.
7. Define Q from L: each pixel gets threshold i/|Q|, where i is the
   index in the list L of the pixel.