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A N^2-level grey scale dithering for a 2-colour screen (B&W) may be performed with an NxN integer matrix. If N is a power of 2, the most expensive operation would be masking the X, Y ordinates to the range 0 .. N-1, and reading the dither threshold value.
Unless you are dealing with giant pictures, how would the dithering take that long?
(You could even speed the rendering by dithering a partial block of the picture in memory, and then bitblt-ing the block to the screen. The size of the block would depend on the memory available.)
As for colour dithering, assuming that you have an 8-colour screen (on/off for each of R,G,B), perhaps you could use a B&W dithering algorithm on each level - R, G, B, and then combine them. I do not know about the quality of the colours, though...
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Woman goes into a pharmacy and asks the pharmacist for poison.
I want to kill my husband.
Why I can't give you poison that is not only illegal but I would be an accomplice.
She reaches in her purse and takes out a picture of her husband and his wife in an intimate embrace and hands it to the pharmacist.