Re: [Gimp-print-devel] Epson 740

[Robert L K. <rl...@al...>]

   Date: Tue, 25 Apr 2000 20:19:07 +0200
   From: Frank van Maarseveen <F.v...@in...>

   I created a test picture containing RGBCMY colored rectangles at
   various angles (but not the 0, 90, 180, 270 degree special cases).
   Within a rectangle the selected color intensity varies from 0 to 100%.
   The low intensity end (i.e. paper is almost white) seems to be a stress
   test case for dithering. Most algorithms produce all kinds of small
   artifacts due to regularities in the dispersion of dots. This is exactly
   what I see when printing camera pictures with light areas (paper on a
   desk, tropical beach, light blue sky with a few clouds).

Yup, pale areas are really nasty.  I have a number of test images that
have pale areas.  What Epson seems to do in some cases is to use
almost a step function between nothing and slightly darker regions,
which results in better smoothness at the expense of color fidelity.

   I've given it a few thought and my guess is that these artifacts
   (kind of curves, waving patterns or something) might disappear
   when the error dispersion is done with a probability inverse
   to [the square] of the distance of the center of the pixel using
   a random angle. kind of. I'm sure there are statistical functions
   (from calculus) which are applicable to my idea. But maybe this
   is all silly or is already being done in a way.

That helps (I use a triangular function rather than an inverse square
because it's easier to compute), but it doesn't entirely solve the
problem, either.  The other thing that helps somewhat is to reverse
direction each pass.  But I've not been able to do anything yet that
eliminates the patterns altogether.

-- 
Robert Krawitz <rl...@al...>      http://www.tiac.net/users/rlk/

Tall Clubs International  --  http://www.tall.org/ or 1-888-IM-TALL-2
Member of the League for Programming Freedom -- mail lp...@uu...
Project lead for The Gimp Print --  http://gimp-print.sourceforge.net

"Linux doesn't dictate how I work, I dictate how Linux works."
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