[Gimp-print-devel] Epson 870

[Jean-Marc V. <ver...@if...>]

You haven't heard from me for over a week; I was trying to fix grain/smoothness
for the Epson 870. Here is my first shot.

I have basically implemented my previous suggestions for gimp-print 3.1.3/4.
This has been more difficult than I had expected.

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 have tried a lot of different things and this is what seemed to work:

1 - First of all I had to implement my "ink budget" thing for the following to
work. That was the easy part.

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.

3 - the above change involved chnages in the computation of levels.

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. Nevertheless I think I came up with reasonable settings
(which could be improved further). The problem particularly occurs as we use
lighter inks more often because the error amplifies faster then.

Anyway, I would like one or more people to try it (again, only with an epson
750, 870 or another variable size 6 color epson printer). The sources are not
yet very clean, they do not include recent changes (such as tom margin
improvements), but I'm looking for comments on grain/smoothness, particularly
at highest qualities on photo paper by comparison with lower qualities and
current gimp-print.

Instead of posting patches, that would break against current versions, I've put
my sources (print-dither.c, print-escp2.c and print.h are all you need I hope)
at:
"ftp://perse.saclay.cea.fr/pub/incoming/verbavatz/"

Thanks,



-- 
--
Jean-Marc Verbavatz		<ver...@if...>
5, rue La Fontaine		"http://perso.cybercable.fr/verbavat"
F-75016 Paris