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

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

>    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.
>
> Huh.  That's very interesting, and doesn't accord with my experience.
> I would expect stock 3.1.3 to be the least grainy, followed by the
> current repository and then 3.1.2.  3.1.2 might be denser, which would
> reduce the perceived grain.
>
Right, this is not what I meant. Saying that the current repository was lousy
was an overstatement. I meant it's too dark to tell. For 3.1.2 I meant a
version that I had modified. I would otherwise agree with you.


>    The greenish thing should not be difficult to fix. That's color
>    balance again and probably involves no specific flow in the
>    algorithms.
>
> I hope not, but it's been exceedingly difficult to get right thus far.

I think the density ratio between colors is simply not one. I'll send you a
patch over the week end, that mostly fixes grays. To get it absolutely right,
we need to address the grain issue first though. To fix red and blue, the only
thing I came up with were hue adjustements.

>
> Actually, gray IS printed as CMY (and LC and LM as appropriate) until
> it gets quite dark.  And currently, there's some CMY mixed in all the
> way to almost 100% black.
>
I know and I can't argue with that until I understand the new dithering a
little better. This was just a general thought.

>
> I think that the black threshold is a bit too low right now.

Yes indeed. The new k_lower in print-escp2 .138, looks better already. That
does not totally fix the problem, but helps very much.

> It's
> also possible that the density value that's being used to compute the
> size of the black dot is wrong.  Try changing this:
>
> 	  tk = print_color(d, &(d->k_dither), bk, bk, k, x, row, kptr,
> 			   NULL, bit, length, 0, 0, 0, 0);
>
> to
>
> 	  tk = print_color(d, &(d->k_dither), bk, bk - d->k_lower, k, x, row,
kptr,
> 			   NULL, bit, length, 0, 0, 0, 0);
>
> What this will do is decide what kind of black dot to print based on
> the difference between the black value and the lowest value at which
> black is printed, rather than the absolute black value.  So that would
> use smaller dots.  Let me know how it works.
>
Well this does not improve things in my opinion. It is lighter as you would
expect, but grain remains. The change in k_lower is much more useful.

Have you tried to print a black and white picture in color mode ? I've noticed
that, beyond the greenish tone, it shows diffuse magenta grains, particularly
around transitions and I could not figure out why. Interestingly, I've noticed
that (now that black is better), grain in color pictures was mostly magenta as
well and in fact localized in exactly the same regions. Is it just me ?



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