Re: [Gimp-print-devel] Stylus photo 870

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

> OK, probably the inks are different.  I should put an Epson cartridge
> in my printer and see what happens.

Aha, that would explain. I'm using Epson inks.

>
>    Primarily, red looks too yellow and blue too magenta to me. Gray is
>    not quite gray either. This can probably be fixed by adjusting the
>    respective densities but I haven't tried yet.
>
> I've noticed that too.  Does your gray turn out slightly greenish?

Yup.

>
>    Wow, much better ! I can see color again :) The thresholds look excellent.
>    Looks a bit grainy (Dither=2), but that's photo quality again.
>
> Try adaptive random (dither=4).  I need to take a closer look at this
> grain issue, since the stuff as of this morning seems to have
> regressed a bit.
>

Same thing with dither=4. The rendenring is slightly different but about as
grainy.

I just printed the exact same photograph (a rather difficult one from previous
experience) with my version of gimp-print-3.1.2 (a), with stock 3.1.3 (b),
yesterday's 3.1.3 with dither=2 (c) and with dither=4 (d).
b, compared to others is lousy. That's history.
c and d don't differ significantly. They're good. A bit grainy, slightly
oversaturated, greenish grays (see above).
At this point (a) is best in my opinion. A little less saturated than c,d (but
whether this is worse or better is rather subjective and can be adjusted).
Better grays. Less grainy.

Why (a) is better, I don't know yet, but I might find out by examining the
ouput. I'll report later. I have some thoughts though:

The greenish thing should not be difficult to fix. That's color balance again
and probably involves no specific flow in the algorithms.

One issue in the grain, I've seen it while working on 3.1.2, is what I would
call "undersampling of smallest drops". Because I'm new here I don't know
whether this has been discussed before, sorry. Anyway, I think that smallest
drops in variable size (or smallest ink levels) are so tiny that they don't
fill
in. Take 50% gray. If printed with only smallest dot size black will shoot
everytime at 720 dpi and yet will be grainy because the diameter of the drop is
less than 35 microns. This particularly shows for dark inks (K,C,M). The fix is
to mix in larger volumes of lighter inks (Y, LC, LM) instead and to use some
CMY instead of K (larger volumes again) for midtone grays. The risk then is to
damp the paper. So we need to keep track of the volume of ink laid down. I can
tell for sure that this problem occurs and I can tell where exactly from the
printout, particularly for grays, because it shows despite randomization.

Of course there might be other issues as well.

Let me know if I am too verbose. I realize that the above is rather descriptive
and does not actually fix the problem. I'm still a bit overwhelmed by changes
in dithering in 3.1.3. But I realize they're for the better; dithering plays a
huge role in a good rendering.


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