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List:       koffice-devel
Subject:    Re: Problems with the conversion formulas of RGB to CMYK and back.
From:       Thomas Zander <zander () kde ! org>
Date:       2005-12-22 12:06:55
Message-ID: 200512221306.57114.zander () kde ! org
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On Thursday 22 December 2005 12:05, Tim Beaulen wrote:
> Hello,
>
>
> I tried to use the Krita color dockers in Karbon and this went well,
> but there was no Krita CMYK color docker yet. So I tried to create
> one.
>
> I used the formulas in the following PDF to convert between RGB and
> CMYK: http://www.poynton.com/PDFs/coloureq.pdf
>
> But something is going wrong when calculating the black level (k).
>
> When all three C, M and Y sliders are not 0 anymore, the K level will
> change (according to the formulas). But this is, in my opinion, wrong.
> The result is that if I change C and M and Y are not 0, then all the
> sliders will move.

I agree with Boudewijn that you should not use this, but I can tell you 
the formula is correct.

CMYK is for printing and has some features that make it more usefull, but 
are not immediately obvious, the K being the biggest of them.

The K traditionally stands for key and is not really needed for a full 
gamut.  You can get all colors with CMY as well.
The problem with mixing CMY is that you get 3 colors ink very fat on top 
of each other if you want black. This is ugly and should be replaced with 
black ink instead.
Which is exactly what the K does; it levels the cyan, magenta and yellow 
and moves that to the K.  So 10,15,14,0 => 0,5,4,10 (roughly).

This is perfectly OK and gives better visual results (on paper)
-- 
Thomas Zander

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