[Gimp-print-devel] Re: Epson 870

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

First off: the gray scale is much better than mine.

720 dpi looks pretty good.  1440 looks a bit distorted, and 1440
enhanced (pseudo-1440x1440) looks positively strange.  I think that
thaere's some kind of arithmetic overflow going on in darker regions;
light tones are OK.  It might be the ink budget stuff triggering it, though.

However, in general 1440 and 720 look noticeably different.

Try downloading http://www.tiac.net/users/rlk/colors.tif.  This is a
color sweep pattern.

   Date: Sun, 14 May 2000 22:39:49 +0000
   From: Jean-Marc Verbavatz <ver...@if...>

   I would like anyone with an Epson 870 to try what I came up
   with. This is totally experimental at this point and will break
   with any printer other than 6 colors/3 levels printers. In
   addition, my sources are based on ~ last week's version of
   gimp-print. All of the above is very easy to fix though as long as
   dithering doesn't change too much.

I'll be happy to leave this stuff alone for a while...

   2 - I made one major change to dithering. That's how we think about
   density.  In my understanding, current gimp-print scales everything
   by density/oversampling. That includes values to print and
   thresholds.  My approach is to take advantage of oversampling to
   use (more) lighter inks.  For that purpose, I'm scaling thresholds
   only by density (not oversampling).  This results in an increase in
   the usage of light inks as quality (oversampling) increases.

I adopted the approach of scaling everything by density, and ignoring
the oversampling issue per se (except for scaling density) between
3.1.2 and 3.1.3 because the behavior of the 3.1.2 code when the user
changed density (or resolution) was too unpredictable.  I wanted the
code to behave identically (as far as ink choice) for any choice of
density and resolution.

I'm not sure that using more light ink is the way to go.  If anything,
I think that at high resolution you're often better off using more
small dots, which means dark ink at some intermediate points, to avoid
laying down too much ink.

   That's all (or almost), but was not easy with the current
   dithering. I came across major (and unexpected) color problems. The
   reason I believe is that for any given density, dithering will use
   mostly one of 2 inks only (one darker one lighter). Without going
   into the details, that means that if the settings for only one ink
   level is not perfect, everything having a color density between the
   immediately lighter ink and the immediately darker ink will
   accumulate (and therefore amplify) this error, particularly in flat
   tones. I think there is a problem with that.

I think you're right.  I think that it should be possible for segments
to overlap, so that if a particular point lies within multiple
segments, they're each tried in turn.  I don't know if that would drop
too much ink, though.

Raph, are you investigating multi-toned inks for rinkj?

-- 
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."
--Eric Crampton