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From: geof@smiley.stanford.edu (Geof Cooper)
Newsgroups: comp.lang.postscript
Subject: Re: Color separations
Message-ID: <834@shelby.Stanford.EDU>
Date: 12 Nov 89 04:17:06 GMT
References: <443@cs.columbia.edu>
Sender: news@shelby.Stanford.EDU
Reply-To: geof@smiley.UUCP (Geof Cooper)
Organization: Stanford University, Stanford Ca.
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In article <443@cs.columbia.edu> djk@cs.columbia.edu (David Kurlander) writes:
>I have a set of color postscript drawings that need to be converted
>into color separations for a book.

This is comparatively easy to do in a limited way.  If you don't like the
results you get, it is somewhat more complicated to tune the results to
the printing process (that is what color pre-press is all about, and is
essentially an unsolved problem).

Obstacle #1 is that older printers do not support the PostScript color
operators, like "setcmykcolor" and "colorimage".  If you use either of
these, you will likely have to simulate them in PostScript.

Obstacle #2 is that color postscript has a built in mechanism for doing
"black generation", i.e., converting a CMY signal into a CMYK signal.  You
may need to simulate this, too.  This may not be a problem if your artwork
has little saturated black in it.

The technique is to replicate the postscript master three times (4 times
for CMYK), with three special headers prefixed that redefine the operators
"setgray", "setrgbcolor", ... etc. to set a gray level that is equal to the
component you want.  I.e., to generate the cyan pass from "setcmykcolor",
you would do something like:
    /setcmykcolor { pop pop pop 0 0 0 setcmykcolor } bind def
with analogous definitions of the other operators.

You also want to call setscreen in the prefixed code to set different
rotations for the different separations.  Something like a 60 degree
separation works well.

I would suggest that you contact Adobe on this one.  I have noticed
that they have ProcSets that do essentially what you want.  Perhaps
they will release them.  They have also come up with good setscreen
values for typesetters.

At the very least, the "Color PostScript Extensions" document is a must
to understand how to simulate the color operators and do black generation.

- Geof Cooper