[Scribus] CMYK - one last try

[David Purton]

One last effort to try and understand what Scribus is doing, since I'm
not convinced so far that people on this list have followed me :(

So here follows a *really* long explanation, complete with lots of great
examples :) And I do eventually get to the point too.


Grab this file:  http://marshwiggle.net/CMYK.eps

This is a simple eps with four rectangles each in a different process
colour (C, M, Y, K). View it in a text editor if you like.

Now, convert it to pdf using this ghostscript command:

        ps2pdf -dEPSCrop -dProcessColorModel=/DeviceCMYK CMYK.eps

Don't leave out the ProcessColorModel option, otherwise ghostscript will
convert this to RGB colour without using colour management.

Open it up in acroread 5, or 6 if you are cursed with using one of the
lesser operating systems. The colours you will see are correct and match
reasonably closely what you would get if you sent this eps to an offset
press or a nice colour postcript laser printer. (Trust me on this one -
I work for a publising company and we frequently produce two colour
books with Process Cyan as the second colour. I know what is is supposed
to look like.)

The problem is not with Little CMS, because it too can produce the
colours I want. Try this:

First convert the eps to a CMYK tiff using ImageMagick:

        convert -colorspace CMYK CMYK.eps CMYK.tif

Use tiffinfo to confirm you have a tiff in CMYK colour space.

Now produce a proof in RGB colour space using Little CMS (Replace
"CMYKProfile" with the path to a CMYK profile on your machine.):

        tifficc -i CMYKProfile CMYK.tif CMYK-proof.tif

This looks pretty similar to what acroread produces.

I want to see these colours on screen in Scribus.


For comparison you can also produce a proof of an RGB tif using Little
CMS directly:

First convert our original eps to and RGB tiff (without colour
correction):

        convert -colorspace RGB CMYK.eps RGB.tif

Now use Little CMS to make a proof (again replace CMYKProfile with the
path to a CMYK profile on your machine):

        tifficc -p CMYKProfile RGB.tif RGB-proof.tif

This looks like what Scribus is giving me with colour management turned
on.



Now, open Scribus (with colour management enabled). Import CMYK.eps
(this is one nice thing about Scribus - eps import is great).

If anyone has Scribus producing colours similar to acroread, then let me
know what your settings are.

To me the colours look like RGB colours that have been converted to the
nearest available colour in the CMYK colour space - like the
RBG-proof.tif image produced above.

But Scribus does seem to be at least partially treating them as CMYK
colours, because if you print separations from Scribus, you will get one
black square on each of your four pages. (Whether you have "Apply ICC
Profiles" selected in the advanced printing options or not does not seem
to make any difference.)

To me this is completely inconsistent. Either a colour is RGB or it is
CMYK, not both at the same time. If Scribus is treating these four
colours as RGB, then the separations that it is producing are incorrect
(Open up RGB.tif which you created above in the GIMP and use the
separate plugin to produce the correct separations).  If Scribus is
treating these four colours as CMYK, then the on screen display is
incorrect.


So has this made any sense to anyone? Can anyone understand why I'm
confused about colour management in Scribus?


cheers

dc


-- 
David Purton
dcpurton at chariot.net.au
 
For the eyes of the LORD range throughout the earth to
strengthen those whose hearts are fully committed to him.
                                 2 Chronicles 16:9a
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