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From: lydia@cs.kun.nl (Lydia Meesters)
Subject: CMYK color mixing and conversion to RGB
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Date: Tue, 15 Nov 1994 08:51:48 GMT
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Xref: glinda.oz.cs.cmu.edu comp.publish.prepress:3254 sci.image.processing:10890


Hi there!

I have two questions:

 1. How can I transform CMYK color values into RGB values?
 2. How can I mix CMYK colors values?

The CMYK colors in question are not academic values in the sense that the
K value simply is used for undercolor removal (i.e. K = min(C,M,Y) and then
C=C-K, M=M-K and Y=Y-K), but are real life pre-press values. This means that
the black channel K might contain values to support the other color values or
to support overprinting of colors. We're talking real ink here.
Callibration of the CMYK channels might also be necessary, but is not of
primary concern to me at this moment.

Mixing of CMYK values is a problem also because of the above mentioned
reasons (the black channel). Transforming to RGB first, mixing and then
backtransforming is not a solution because it is too time consuming and
because of the callibration problems that are likely to occur.
The mixing problems I have result from overlaying two CMYK images with
partial transparency. E.g. 20% of one CMYK value should be added to 80%
of the other CMYK value. Simply adding percentages of the separated channels
result in undesired effects in luminance, mainly because of the black channel.

If you could be of help to me I would greatly appreciate it. Please send
mail to me directly at lydia@cs.kun.nl, as I don't always read this newsgroup.

Oscar.

