Re: Dithering (off-topic)

[Bill McClain <wmcclain@salamander.com>]

On Tue, 04 Nov 2003 14:34:54 +0100
Michal Kvasnicka <quasar@econ.muni.cz> wrote:

> BTW, is there some easy way to set the size of the dithering points
> (eg. 2x2 pixels, 3x3 pixels), or should I do it ex post in some image
> editor (Gimp, imagemagick)?

I haven't studied the Floyd-Steinberg algorithm, but it seems to be an
error-difusion technique that handles that automatically. The
resulting pixel dimensions of the image are the same as the
original. There are other halftoning methods you can experiment with in
pgmtopbm.

Although I did some experimentation, I am by no means an expert at this.
My method is simply to expand the orginal image to so that a pixel in
the image will be a dot on the output. Using the example I posted
before, the image was 3.5 inches wide, the printer was 600 dots per
inch, so I sized the graphic to 3.5 x 600 = 1950 pixels before
halftoning. I've dealt with only one printer so far, but the results
have been good.

Depending on the original grayscale image, it may need to be normed
(pgmnorm) or smoothed (pgmsmooth) before halftoning.

-Bill
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