Good evening,
You may want to test another dithering algorithm: its is entitled "A Simple and
Efficient Error-Diffusion Algorithm" and it is available on this web page:
http://www.iro.umontreal.ca/~ostrom/publications/abstracts.html

A source code is also provided. (There are also more recent publications about
error diffusion on the same page).

Another recent work that I am aware of, with some pictures to have an idea of
the result:
http://people.scs.carleton.ca/~hli1/publication/Contrast-Aware-Halftoning/CAHalftoning.htm

As mentioned by other people, I would suggest to avoid the "it took 3 days to
render and I have used the wrong dither method" effect. My proposals are:
1. If the user wants a dithered output, then POV-Ray outputs the original image
AND ALL the dithered images (using ALL the available algorithms). It would be up
to the user to select the one the user likes. A "more than 8 bits per component"
image would always be saved "just in case".
2. Add the dither as a postprocessing step, for instance calling POV-Ray with a
"--dither" argument. It would process an existing picture, and it would not
render anything new.

I hope this helps,
Lionel


=?UTF-8?B?IkrDqXLDtG1lIE0uIEJlcmdlciI=?= <jeberger@free.fr> wrote:
> clipka wrote:
> > Am 04.12.2010 19:43, schrieb "Jérôme M. Berger":
> >>     You probably already know it, but for others who may not here are
> a
> >> couple of points to keep in mind:
> >>   - Dithering may cause very visible moire artifacts when the images
> >> are printed;
> >
> > Hm, I didn't think of that. Actually I'd expect it to improve print
> > quality.
> > I also guess it depends on the dithering algorithm; maybe that's where
> a
> > benefit of random dithering can be found? Printers should be quite
> > immune to random noise in images to be printed I guess.
> >
>  The problem is that the printer will also use a dither algorithm
> when printing and you can have unwanted interactions between the
> two. However, since you implemented both Bayer dithering and error
> diffusion it should be possible to avoid the issues by choosing the
> other method than the printer. However, this is something to keep in
> mind when rendering because you don't want to realize you made the
> wrong choice after a one week high resolution render ;)
>
>   Jerome
> --
> mailto:jeb### [at] freefr
> http://jeberger.free.fr
> Jabber: jeb### [at] jabberfr

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