From: Guillermo Espertino <gespert...@gmail.com>
   Date: Mon, 23 Mar 2009 19:59:53 -0300

   CMYK exists because, though is possible theoretically, it isn't possible
   to generate black from mixing CMY inks. As the C, M and Y inks aren't
   perfect and have some contaminants, mixing them ends up in a dirty brown
   instead of pure black.

Or dirty green/cyan, or dirty magenta, depending upon the colorants...

   That's why CMYK exists, and that's why it isn't so simple to print an
   RGB image.
   The problem resides mostly in the generation of black and gray shades.
   Although there are systems that do a great job converting a photo to
   CMYK on the print side, it's still a problem to do a simple task as
   placing a pure black overprint on a solid color background without
   destroying some underlaying information on the separation.
   I'm a designer, not a photographer, and an image manipulation program is
   an essential part of my workflow. And placing some black text, or
   editing a large image with a black or gray background, adding black drop
   shadows, aren't rare at all. And it's a pain without the ability of
   editing the separated CMYK.
   It's not about if the printer will handle the file or not, is about
   creative control. Sometimes you NEED to control the black overprint.
   Sometimes you need to use spot colors and you need to control the
   channels and how they overprint.
   Even with Adobe software, before having spot channels in Photoshop, it
   was a common practice to separate to CMYK to make 2, 3 or for 2 inks
   prints (replacing the corresponding plate's ink for a custom ink).
   Simply because other programs didn't support the Adobe's custom
   multitone files but did support CMYK tiffs.

This really sounds like you're using black as a spot color rather than
going generic editing in CMYK space.

I question whether doing this in an image editing application is
really the right thing to do.  If you're doing black text, you
probably want the text to be vector rather than raster anyway --
printing an image scaled to 240 DPI is fine, but the text won't look
so good at that resolution.  In that case, you're better off using
something like Scribus to do that kind of overlay, at least until GIMP
has vector layers.

   Well, I can't do duotones with Gimp to insert in a 2 inks flyer.

Which again is a spot color kind of use case rather than a CMYK use
case.

   CMYK editing would help. I can't control the black generation of a
   separation, because the separate+ plugin doesn't support that. It
   just support existing profiles and there is no control. I can't
   control CMYK curves. And trust me, that's extremely common.

Does that indicate that separate+ is what needs to be enhanced, rather
than the core application?

-- 
Robert Krawitz                                     <r...@alum.mit.edu>

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

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