From hutch@rawfish.UUCP Wed Apr  5 14:59:08 1989
Path: leah!csd4.milw.wisc.edu!lll-winken!uunet!seismo!esosun!cogen!celerity!rawfish!hutch
From: hutch@rawfish.celerity (Jim Hutchison)
Newsgroups: comp.graphics
Subject: Re: RGB -> CYMK conversion
Message-ID: <275@celerity.UUCP>
Date: 5 Apr 89 18:59:08 GMT
References: <28994@sri-unix.SRI.COM> <390026@hpfcdq.HP.COM>
Sender: news@celerity.UUCP
Reply-To: hutch@rawfish.UUCP (Jim Hutchison)
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Organization: FPS Computing
Lines: 32

RGB -> CMYK is not as simple as it might initially appear.
Note for convenience all color values below are normalized.
Gunk is the non-technical term I will use for ink/wax/jam.

If you are taking RGB to perfect CMYK, the operation is trivial,
you set K=0, C=1-(B+G)/2, M=1-(B+R)/2, Y=1-(R+G)/2.

Unfortunately C,M,Y=1,1,1 is a lousy black.  It has too much gunk
on the page, and is a color which I've seen to vary from bluish to
brownish black.  So, in comes black.  You want black to be a good
solid black without having 3 times the amount of gunk on the page
to do it.  An initial guess would be to make the transformation
with as much black as possible; K=1-min(RGB), C=C-K, M=M-K, Y=Y-K.
This puts the least ink on the page, and works.  Unfortunately
this really brings out the detail of the print screen used and
makes the print look a little garish (atleast to me).  So a next
obvious step is to do the black by percentage.  60-80% seems to
be what Hell suggests in their GCR (under color correction) poster.

Now that that is over with, on to gunk correction.  The unfortunate
thing about gunk (be it ink, wax, or some other pigment) is that
it is not pure in its absorption of color.  So you need to do a map
from RGB to CMY.  Luckily this is R3->R3 so there should be only 1
right answer, ignoring sampling frequency.  Unfortunately I can't
tell you the best way I know to do this, as I don't own it.  The
gist of the problem is to figure out how much of each R, G, and B
are absorbed by each C, M, and Y.  Black is excellent, you should
not have to ever worry about black (with any of today's standard
black pigments).  Ramps of uncorrected R, G, and B checked with a
densitometer should get you in the ballpark.
/*    Jim Hutchison   		{dcdwest,ucbvax}!ucsd!celerity!hutch  */
/*    Disclaimor:  I am not a official spokesman for FPS computing    */