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

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

On May 5, 10:22pm, Robert L Krawitz wrote:
> Subject: Re: [Gimp-print-devel] Stylus photo 870
>    Date: Fri, 5 May 2000 23:51:30 +0000
>    From: Jean-Marc Verbavatz <ver...@if...>
>
>    > 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 current repository prints too dark?  I thought I reduced the
> density.

Well I should think twice before typing. I meant the lastest release (3.1.3) as
of last week printed too much black to tell about grain. The current repository
is grainy but getting better everyday and 3.1.2, as I had modified it, was
best.
Let's forget about this statement I have some news.

>    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.
>
> I think you're probably correct.  See my next message, though, about
> grain.

I read the message, got the new version and it doesn't seem to have changed a
lot in my particular case.

I've tried to work on grays. The patch below improves the gray balance for me.
As you can see the magenta density will increase by ~17% (0.85 vs 1.0) and the
cyan density by ~11% (0.9 vs 1.0). Now grays look gray. Of course, the color
balance changes slightly as well. As the values are absolutely empirical, they
may need to be tweaked, but they look good to me.

--- ../print-escp2.c	Sun May  7 02:26:31 2000
+++ print-escp2.c	Sun May  7 02:31:12 2000
@@ -206,6 +206,26 @@
   { 1.0,   0x3, 1 }
 };

+static simple_dither_range_t variable_c_dither_ranges[] =
+{
+  { 0.137, 0x1, 0 },
+  { 0.228, 0x2, 0 },
+  { 0.342,  0x3, 0 },
+  { 0.45,   0x1, 1 },
+  { 0.60,  0x2, 1 },
+  { 0.9,   0x3, 1 }
+};
+
+static simple_dither_range_t variable_m_dither_ranges[] =
+{
+  { 0.129, 0x1, 0 },
+  { 0.215, 0x2, 0 },
+  { 0.323,  0x3, 0 },
+  { 0.425,   0x1, 1 },
+  { 0.567,  0x2, 1 },
+  { 0.85,   0x3, 1 }
+};
+
 static simple_dither_range_t standard_dither_ranges[] =
 {
   { 0.5,   0x1, 1 },
@@ -1219,8 +1239,8 @@
       dither_set_k_ranges_simple(dither, 3, dot_sizes, v->density);
       if (escp2_has_cap(model, MODEL_6COLOR_MASK, MODEL_6COLOR_YES))
 	{
-	  dither_set_c_ranges(dither, 6, variable_dither_ranges, v->density);
-	  dither_set_m_ranges(dither, 6, variable_dither_ranges, v->density);
+	  dither_set_c_ranges(dither, 6, variable_c_dither_ranges, v->density);
+	  dither_set_m_ranges(dither, 6, variable_m_dither_ranges, v->density);
 	}
       else
 	{


>    Yes indeed. The new k_lower in print-escp2 .138, looks better already.
That
>    does not totally fix the problem, but helps very much.
>
> We can always tweak it a bit more.

I think k_lower is fine. I tried to play with k_upper. In vain, but this gave
me some ideas that I'll try to play with in the next couple of days.

>    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 ?
>
> You know, I've seen some oddities with (not so diffuse) magenta in
> some things at extremely low densities.  At this point, I don't know
> if it's due to the image or if magenta alone is somehow being handled
> incorrectly.
>
I still see it with the latest version even on black and white pictures
(printed in color). Therefore, it is not due to the images. Moreover It did not
happen with gimp-print 3.1.2 (even on the same photographs). It occurs at low
densities indeed (although not extremely low), but only in proximity of higher
densities.
Therefore not on flat tones, but either at transitions or in regions of very
variable density. I haven't yet looked if it occurred with all the dithering
algorithms (only 3 and 4).


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