Re: [Gimp-print-devel] Dithers in 3.1.3

[Thomas T. <tt...@bi...>]

Robert L Krawitz wrote:

> Epson printers can only print dots at a spacing of 1/720 (or in some
> cases 1/360).  However, the carriage can be positioned to 1/1440 in
> many of these printers.  So what it does is generate the line at
> 1/1440 resolution, and then split it into two or four pieces by
> de-interleaving and printing each row separately.  The high and
> highest quality modes go further, up to a total of four passes per
> row.  In order to further reduce banding, each subpass is printed with
> a different jet.  It turns out that for the photo-quality glossy film
> it's essential to do something like this -- print too much ink too
> quickly and it all beads together, and the result is awful.  Printing
> the same image at 720 dpi and at 1440 dpi highest quality yields very
> different results -- at 720 dpi, there are heavy blotches; at 1440 dpi
> things are smooth.
> 
> It's possible to put ink at every position, but it's not a great
> idea to do so.  Most of the printers seem to be designed for about 60%
> coverage ("density") at 720 dpi, so at 1440 dpi the density is divided
> by two.

Yup. My point is that it matters which pixels you leave out to get at
the 60% coverage. It may be possible to have the Floyd-Steinberg
algorithm handle this, but it is also possible to do postprocessing of
some kind. I once had a dot matrix printer that did allow 360 DPI
horizontally, more or less. The problem was it would only print every
other dot. So when you printed:

XXXXX
XXXXXX

Out came:

X X X
X X X

Dot overlap with these printers was huge of course. But I got much
better text by sending the following to the printer, which would print
exactly as sent:

X X X
X X  X

It may be worthwhile to do something like this after the error diffusion
steps, which then could build to a 100 % covered area that later gets
transformed to something that doesn't soak the paper. This may not be a
huge win compared to Floyd-Steinberg doing this work for us. It is more
of a knee-jerk reaction of me to the comment that for 1440 DPI we only
print every other dot - but I realize that it probably works out just
fine if you print lines in alternating directions. But then it might
not, as the resulting pattern may be interfering in horrible manners
with any chequerboard patterns that might be in the generated dither.

I thionk that I used a very simple algorithm for the dot matrix: if 3
dots are 'on' sequentially, and only then, take out the middle dot.

> BTW, can we have this discussion on the list, so that others can
> participate?

Here we are. I just got CVS set up. When the 'plain matrix' is in the
source, I'll experiment a lot with them on my lowly Epson 600. Well, at
least the printers shows any artefacts really well ;)


Thomas