[Gimp-print-devel] Stylus photo 870

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

Hi everyone,

My name is Jean-Marc Verbavatz. I have joined the developers of gimp-print a
few days ago and would like to introduce myself.

I first came across gimp-print, as a ghostscript plugin, to drive my stylus
photo 700 a few months ago and was quite impressed. Just over a month ago, I
got a new stylus photo 870 instead and naturally tried gimp-print. That was
with versions 3.1.1 and later 3.1.2. It worked but exhibited a number of
dithering weaknesses and bugs. I fixed some of them and tried to improve the
rendering (mainly for photographs) until I obtained real photo quality, hardly
distinguishable from true photographs. My fixes were against print-3.1.2 and I
reported them to Robert Krawitz who kindly advised me that 3.1.3 was out with
totally different dithering algorithms. He suggested that I should join and I'm
back to the drawing board so to speak.

I am not an expert in printers or in dithering but I am dedicated to quality
and I am ready to spare some ink and time on this.

I have now started testing print-3.1.3 with my stylus photo 870.
It works flawlessly and some of the major bugs I had found in 3.1.2 appear to
be gone. Good. Quality needs to be improved though.
Last night, I made qualitative density comparisons of all the inks and sizes
and this is the best (pretty good actually) setting I could find for
variable_dither_ranges:

{ 0.152, 0x1, 0 },
{ 0.255, 0x2, 0 },
{ 0.38,  0x3, 0 },
{ 0.5,   0x1, 1 },
{ 0.67,  0x2, 1 },
{ 1.0,   0x3, 1 }

The last 3 values also go for K and Y (simple case).
On my STP870, my values yield absolutely identical densities for each of the
inks/levels. The color balance still needs to be improved.

I see that these have already been modified in print-escp2.c v 1.134 in the
meantime and in the same direction except for dark level 2. In particular we
went from 5 to 6 levels (light 3 was not being used).
If I understand well, this should therefore apply as well to print-escp2.c:

1217,1218c1217,1218
<         dither_set_c_ranges(dither, 5, variable_dither_ranges, v->density);
<         dither_set_m_ranges(dither, 5, variable_dither_ranges, v->density);
---
>         dither_set_c_ranges(dither, 6, variable_dither_ranges, v->density);
>         dither_set_m_ranges(dither, 6, variable_dither_ranges, v->density);

Finally as of the version before today's black was too dark.

Thanks for your time,


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