Re: [Gimp-print-devel] New dither codes

[Robert L K. <rl...@al...>]

   Date: Sat, 29 Apr 2000 16:05:32 +0200
   From: Thomas Tonino <tt...@bi...>

   Robert L Krawitz wrote:

   > For some reason, the 360 dpi prints seem to be much lighter on every
   > printer.  Try bumping up the density to 1.5 at 360 dpi and see if it
   > helps.  How fast was this mode?

   360 DPI on the Stylus Color 600 seems to produce smaller dots than 720
   or for that matter 1440 HQ.

720 and up produce the same size dots on my EX, no matter what I set
for the dot size.  360 does seem to be capable of producing even
smaller dots.  I haven't figured this one out at all.

   I think this is really strange: the code that selects the smallest dots
   on 800 and IIs (and more) does not do so on the 600. I would expect more
   discrepancies with other printers.

In this case, even the Photo and the Photo EX have different settings
in this regard.  The doc for the EX says that 0x4 is "super micro
dots", but that doesn't appear to be the case; 0x4 produces larger
dots than 0x3.  I think we're just going to have to try empirically
with each printer to figure out what works best.

   Other results: I went from rotating and inverting the matrix to just
   shidting it in X and Y directions. For ordered dither, this made no big
   difference, but the splotches are not colored any more. I think I know
   where the splotches originate from. If I print a white-to-black wedgeit
   really prints as a white-to-grey, then grey-to-darkgreen, then
   green-to-darkgrey (that dark grey is a bit lighter than the green) and
   then to black. This is on Canon coated paper, but plain paper is
   similar. It seems black has to be added earlier than it is now - at
   least it has to be added in larger amounts.

The current default is to start adding black at 30%.  With plain
paper, it goes to all black at 50%; with glossy film (which works well
for glossy paper, too), it goes to all black at 80%.  At anything much
less than 30%, the black dots start to become very noticeable.  It's
probably worth a try, though (play with the dither_set_black_lower and
dither_set_black_upper calls in print-escp2.c).

I also get slightly greenish medium to dark grays.  I can partially
correct gray with the color controls, but it's imperfect.

   If CMY print all at 70 % and you add 30% black, the black has very
   little effect on overall density, because it overlaps most of the CMY. I
   guess this is what is happening.

Not really; no CMY is ever printed if black is.  This works fine for
single dot size, but it may not work right for variable dot size.

   1440x720 is very ugly in ordered dither mode, as only half the dots are
   not printed. 1440 mode thus is only useable with some kind of error
   diffusion or other mechanism that can use half dot spacings. Ordered
   dither inherently cannot do this because a dot position that is printed
   at level X will also be printed at a level higher than X.

Hmm.  I found 1440x720 quite usable with the iterated-2 ordered dither
-- much better than 720x720.  However, it does band a lot.

-- 
Robert Krawitz <rl...@al...>      http://www.tiac.net/users/rlk/

Tall Clubs International  --  http://www.tall.org/ or 1-888-IM-TALL-2
Member of the League for Programming Freedom -- mail lp...@uu...
Project lead for The Gimp Print --  http://gimp-print.sourceforge.net

"Linux doesn't dictate how I work, I dictate how Linux works."
--Eric Crampton