Relay-Version: version B 2.10 5/3/83; site utzoo.UUCP
Path: utzoo!watmath!clyde!caip!rutgers!lll-crg!nike!ll-xn!mit-eddie!genrad!decvax!decwrl!amdcad!cae780!leadsv!eps2!msunix!jon
From: jon@msunix.UUCP
Newsgroups: net.graphics
Subject: CMYK <-> RGB
Message-ID: <282@msunix.UUCP>
Date: Wed, 22-Oct-86 02:12:27 EDT
Article-I.D.: msunix.282
Posted: Wed Oct 22 02:12:27 1986
Date-Received: Thu, 23-Oct-86 06:17:55 EDT
Organization: Via Visuals Inc.
Lines: 40
Keywords: conversion needed


Does anyone have [info|references] on how to convert CMYK to and from
RGB?  CMYK are the subtractive colors used by printers, CMY are Cyan,
Magenta, and Yellow, K is the black, since CMY inks make a dark grey
and not a black. Going to RGB, it seems like subtracting the black from
the RGB using an NTSC-like luminance function works ok, though not quite
weighted as heavily toward green and red as NTSC.

It seems that everyone else doing the conversions uses a bunch
of lookup tables because the formulas derived from the theory
don't quite get the colors right.  I may end up doing the same
thing, putting a bunch of palettes on an armload of magnetic
tapes and making a bunch of Cromalins, then measuring them with a
reflection densitometer (chicken way out).

When I approached this from the theoretical end, I measured pure
cyan, magenta, and yellow inks with a densitometer and as I had
expected, found that the inks are far from ideal, i.e. cyan doesn't
just absorb red, it absorbs a good deal of green also and a little
bit of the blue.  I plugged this into my formulas and got slightly
better results than assuming the inks were ideal, but still the
results were not quite satisfactory.

I have a book called "Color Separation Techiniques" by Miles Southworth
from the RIT School of Printing.  I am told this is the "Bible" of
color seps, and the people who made the first electronic scanners
tried to emulate the functions he mentions in the book.  The book does
have a lot of useful information, but unfortunately there isn't a
chapter called "How to convert to and from RGB densities to CMYK dot
sizes for every type of ink used in the printing industry". :-)

Oh well, if anyone has any useful information they'd like to tell
me, please send me mail.


"Remember, it's not important how 	  Jonathan Hue
much you can drink, only that you  	  Via Visuals Inc.
drink as much as you can"                 sun!sunncal\
						      >!leadsv!msunix!jon
	            			amdcad!cae780/