| Re: Postscript Color RGB to CMYK [message #35669] |
Thu, 03 July 2003 04:16  |
George White
Messages: 10
Registered: November 1998
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Junior Member |
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On Tue, 1 Jul 2003, Esa Riihonen wrote:
> Hi all!
>
> I wonder whether someone might have bumped on this problem before and
> even perhaps have somekind of solution.
>
> As I understand, Postscript files produced by IDL have color coded as
> RGB. However several (propably most) journals require CMYK-code. I have
> a clumsy solution, load the file in CorelDraw and make the changes there
> - this is far from perfect as it among other things usually require many
> trials e.g., going back to IDL for changing the line thicknesses and such.
>
> So has anyone a better solution?
I find Adobe tools (Illustrator and Photoshop) do a better job of the
RGB->CMYK conversion. I've tried doing conversions using the textbook
formulas, but often the colors come out looking "muddy" and don't come out
as well in print. I suspect the problem is that many of our images use
colors that are outside the printer gamut. The same RGB LUT will produce
different images on different CRT's (e.g., PC vs Mac or SGI due to the
different gamma values used on these systems). See, e.g.,
http://www.fcaglp.unlp.edu.ar/~esuarez/gmt/1998/0651.html for more on this
issue.
In principle, using a PS interpreter such as ghostscript you can rewrite a
PS file to replace RGB colors with CMYK. Jason Olszewski's aimaker script
does a number of color manipulations (using awk scripts) after converting
the PS file to AI format (so the operators have a consistent form), but
doesn't deal with images.
There are other issues in creating pre-press files (color separation,
trapping, gamut mapping). The IDL display devices are RGB, and doing CMYK
properly seems to require more than just the textbook formula. We often
create reports with images processed using multiple tools (IDL, GMT,
matlab). All of these support RGB LUT's so you can preserve consistent
color scales across the packages. These LUT's are tuned for CRT's, and
have become the "norm" for viewing certain data sets.
RGB to CMYK conversion seems to be a "hard" problem. Does it make sense
to try to solve this in IDL or is it better to look for a more universal
tool that will give consistent results for PS from multiple sources?
--
George White <aa056@chebucto.ns.ca>
Head of St. Margarets Bay, Nova Scotia
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