Path: utzoo!attcan!uunet!tut.cis.ohio-state.edu!pacific.mps.ohio-state.edu!zaphod.mps.ohio-state.edu!usc!elroy.jpl.nasa.gov!ames!dftsrv!drax!buck
From: buck@drax.gsfc.nasa.gov (Loren (Buck) Buchanan)
Newsgroups: comp.graphics
Subject: Re: RGB ---> YMCK
Message-ID: <864@dftsrv.gsfc.nasa.gov>
Date: 12 Feb 90 15:38:20 GMT
References: <1990Feb7.220805.27260@allgfx.agi.oz> <260002@hplred.HP.COM> <1990Feb10.151544.19088@druid.uucp>
Sender: news@dftsrv.gsfc.nasa.gov
Reply-To: buck@drax.gsfc.nasa.gov (Loren (Buck) Buchanan)
Organization: Computer Sciences Corporation @ NASA Goddard Space Flight Center
Lines: 41

In article <1990Feb10.151544.19088@druid.uucp> darcy@druid.UUCP (D'Arcy J.M. Cain) writes:
>In article <260002@hplred.HP.COM> piety@hplred.HP.COM (Bob Piety) writes:
>>Given components in the 0..1 range,
>>
>>C = 1 - R
>>M = 1 - G
>>Y = 1 - B
>>K = .59R + .30G + .11B = Luminance
>
>Is the above generally accepted.  None of
>the texts I have seen really details this algorithm.

This is actually a very complicated problem that is not totally
solved.  I imagine that several PhD's can be earned researching
this topic.  It really boils down to what are the CMYK values going
to be used for.  The conversion to CMY is correct.  The problem
with K is that if you use these four values for percentages of ink
to be put to paper your image will be much to dark.

The computation of K is typically on the order of:
  1. Set K to the smallest value of CMY
  2. Subtract K from C, M, and Y

The problem with the above computation is that it does not take
into account the various interactions of the four different inks,
the paper it is applied to, and the printing process used.  These
interactions are usually non-linear.

The thing to do is find a printer who can give you the values you
need (for a particular paper/ink set combination).  

Welcome to the confusing world of printing.  (I am not involved
in the printing industry, but I have had discussions with an expert 
in the field)

B Cing U

Buck
Loren "Buck" Buchanan | internet: buck@drax.gsfc.nasa.gov | standard disclaimer
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