Re: [Gimp-print-devel] Tell me about CMYK vs. RGB

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

   Date: Fri, 22 Jun 2007 20:28:51 -0500
   From: Timur Tabi <ti...@ta...>

   I'm running Gutenprint 5.1.2 on OS X 10.3.9 with an Epson Stylus Color 3000.

   I printed this document:

	   http://www.druckerchannel.de/testdateien/dc_grafiktest_eps.zip

This document is mostly a synthetic test pattern with grayscale and
fully saturated colors, with one photographic image.

   via the Preview application (apparently, it can open an EPS and convert 
   it to PDF automatically).

   I printed this document twice.  Once with these settings:

   Media Type: Plain Paper
   Color Model: RGB Color
   Color Precision: Best
   Print Quality: Best
   Resolution: Automatic
   Shrink Page: Crop

   and again with these settings:

   Media Type: Plain Paper
   Color Model: CMYK
   Color Precision: Best
   Print Quality: Best
   Resolution: Automatic
   Shrink Page: Crop

   As you can see, the only difference is that I choose CMYK instead
   of RGB.

The choice of color model selects what's passed to Gutenprint, so for
any given document, it simply changes what color processing is done in
CUPS vs. what is done in Gutenprint.  So when you select CMYK, CUPS
does a generic RGB->CMYK conversion and Gutenprint accepts CMYK data;
when you select RGB, CUPS passes the RGB to Gutenprint.  If your
document itself is CMYK, CUPS will pass the CMYK to Gutenprint if you
select CMYK, but it will convert the CMYK to RGB if you select RGB.

Normally you should select the same color model as the document
itself, because CUPS's conversion to CMYK is generic, whereas
Gutenprint knows about the printer and paper characteristics, so it
can do a (presumably) better job of generating black (particularly in
gray areas).  Usually the choice of CMYK vs. RGB doesn't matter very
much in fully saturated colors (where there's no gray component) or in
100% black (where on most -- but not all -- printers, pure black ink
works fine).  However, in gray and unsaturated color regions, the
choice of how much black vs. composite (C+M+Y) gray is very important
to control gray balance and dot visibility.

   On the CMYK printout, the colors are richer and the dithering is
   less obvious.  The only exception is the part that says
   "CMY-K-Modell, 40 % Schwarz", which is German for "CMYK Model, 40%
   black".  On the CMYK printout, the dithering is very obvious and
   bottom 1/8th is lighter than the rest.  On the the RGB, it's
   consistently gray, although I can see some lightness on the bottom
   1/8th as well.

I'd have to look at the code to be certain, but what I think is going
on is that in RGB mode it's doing more color correction than in CMYK
mode.  The default color correction in RGB mode (which is really
"eyeball sRGB") is High Accuracy, which converts the input into pure
CMY+K, adjusts the CMY values in HSL space (correcting the hue,
luminosity, and saturation), and then does a final conversion to
printer CMYK space.  This produces more accurate colors, but they
aren't as rich -- in particular, green, cyan, blue, and purple/magenta
are lighter in fully corrected mode, but they're more accurate.

I believe that the default in CMYK is to not do this extra
adjustment.  The reasoning is that CMYK is a device-specific space,
and anyone generating CMYK has presumably already profiled their
printer.

A side effect of the lightening is that fewer dots are printed (less
ink is used), which makes them more visible, particularly on a printer
like the 3000 that uses large 4-color drops.

You could test this by selecting Uncorrected or Correct Hue Only in
the Color Correction box.  Uncorrected will probably give you very
similar output as CMYK, but better gray quality; Correct Hue Only will
adjust the hue, but not the luminosity or saturation, so you'll get
more accurate colors (particularly blues and purples) while retaining
the extra vividness.

   For example, on the CMYK printout, the cyan and magenta and yellow
   squares are pure, with no dithering.  On the RGB printout, I can
   see tiny red dots through the yellow square.  My guess is that
   because the printer has CMYK inks, and so if I chose RGB mode, then
   basically it's converting the Yellow to RGB, and then back to CMYK
   mode.

See above.

   When Gutenprint talks to my Epson 3000, does it send the color data
   as CMYK?  If so, then why isn't the default mode always CMYK?

The output will always be in the printer's space, whatever the input
may be.  It's usually better for Gutenprint to do the RGB->CMYK
conversion than CUPS, because Gutenprint has more information.

-- 
Robert Krawitz                                     <rl...@al...>

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 Gutenprint   --    http://gimp-print.sourceforge.net

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