From jlg@hpfcdq.HP.COM Mon Apr 10 21:00:44 1989
Path: leah!csd4.milw.wisc.edu!mailrus!tut.cis.ohio-state.edu!ucbvax!hplabs!hpfcdc!hpfcdq!jlg
From: jlg@hpfcdq.HP.COM (Jeff Gerckens)
Newsgroups: comp.graphics
Subject: Re: RGB -> CYMK conversion
Message-ID: <390028@hpfcdq.HP.COM>
Date: 11 Apr 89 01:00:44 GMT
References: <28994@sri-unix.SRI.COM>
Organization: Hewlett-Packard - Fort Collins, CO
Lines: 54

> / hpfcdq:comp.graphics / hutch@net1.ucsd.edu (Jim Hutchison) / 11:38 pm  Apr  5, 1989 /
> 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.

No, [ R G B ] = [ 1 1 1 ] - [ C M Y ].

> Unfortunately C,M,Y=1,1,1 is a lousy black.  It has too much gunk

Yes.

> 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.

K=min(CMY) , preferably with some threshold (&& K > .6),
C=C-K, M=M-K, Y=Y-K as before.

> 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
                                   ^^^^^^^^

Actually this is best accomplished by primary conversion using
the CIE 1931 XYZ space as a standard, measurable intermediate.
A brief description of the analogous procedure for phosphors
appears in Rogers (Procedural Elements for Computer Graphics).


> 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.

Or call the manufacturer to get the CIE (x,y) chromaticities of the
gunks and your monitor.  Or use a colorimeter.
> 
> /*    Jim Hutchison   		{dcdwest,ucbvax}!ucsd!net1!hutch  */

-- Jeff Gerckens jlg%hpfcrg@hplabs.hp.com