From jonathan@jvc.UUCP Fri Mar 31 15:05:24 1989
Path: leah!csd4.milw.wisc.edu!bionet!ames!hc!lll-winken!uunet!jvc!jonathan
From: jonathan@jvc.UUCP (Jonathan Hue)
Newsgroups: comp.graphics
Subject: Re: Computer Graphics Academy Awards.
Message-ID: <478@jvc.UUCP>
Date: 31 Mar 89 20:05:24 GMT
References: <28994@sri-unix.SRI.COM> <96848@sun.Eng.Sun.COM>
Organization: JVC Laboratory of America
Lines: 43

In article <96848@sun.Eng.Sun.COM>, falk@sun.Eng.Sun.COM (Ed Falk) writes:
> Announcing, the first annual comp.graphics awards for most frequently asked
> questions.
> 
> Nomination #1:

I would go even farther than this.  I claim that there is no such thing
as an RGB->CMYK conversion.  The reason is that you can't completely
describe a color as an RGB triplet or CMYK quad.  Describing a color
as RGB or CMYK numbers is device/media dependent.  The numbers don't
mean anything outside the context of the device.  Now, if you were to fully
qualify the RGB numbers with the chromaticity coordinates of the primaries,
and gamma (and viewing conditions for a monitor - as far as I know no standards
exists for "soft" proofing"), and the CMYK numbers as some sort of
ink/paper/press/??? (my understanding is that there are variances due
to which point in a press run something is printed, so that is yet another
variable, there are others) combination, you could come up with an RGB->CMYK
conversion for going between those two devices.

Actually, the relationship between intensity and voltage on a monitor only
approximates I=kV^gamma, so you really need to find out the luminance/voltage
transfer function for each component.

I think the best way to figure out the conversion is to map the color spaces
of the devices into/out of a universal color space.  I would probably pick
Luv, so I could measure color differences as Euclidean distance in L*u*v*
space.  I know this is nothing new, Eikonix has been pushing universal
color spaces for years, but I am dense so I'm just catching on now.

Two useful devices for figuring out the conversion are a spectrophotometer
(Minolta makes a cheap one, the CM-1000 for $20K, Hunter also except the head
isn't separate) and a color TV analyzer (again, Minolta makes a cheap one).
If your requirements are not too high, you can use a colorimeter instead of a
spectrophotometer.  You can get a reasonable colorimeter for about $6K
(Minolta CR221).  All of these devices can measure color in xyY, and you can
convert that to uvL easily.  I've been recommending that users of our color
printer purchase these instruments.

If anyone knows of instruments which offer better price/performance, I'd like
to know about them so I can pass that information on to our users.


-Jonathan Hue		uunet!jvc!jonathan