[texhax] cmyk colors are auto changed!

Steve Schwartz s.schwartz at imperial.ac.uk
Sun Apr 17 20:10:16 CEST 2011

On Sun, 2011-04-17 at 15:23 +0100, Steven Woody wrote:
> I found if I define a color in cmyk mode, the resulted color with
> pdlatex will be anther one when I measure it with a screen color
> picker. For example, the cmyk (1,0,0,0) will become (1,0.27,0,0.07).
> On the other hand, if I define colors in rgb mode, the result colors
> will be exactly same as what I defined.

I am not a colour scheme expert, but in testing prior to a reply I gave
a day or three ago, I googled around and read that the translation
between rgb and cmyk is not unique, so you shouldn't expect an exact
numerical match, although you should end up with a very similar colour.
>From memory, the implication was that rgb->cmyk was well-defined and
always yields the same numbers, but cmyk->rgb does not. 

>From a purely mathematical perspective I can see why this might be so.
cmyk requires 4 numbers. When you reduce this to 3 numbers you must lose
some information, so any algorithm has to find a good approximation in
rgb-space. Unless there is a single universal algorithm that every
application and operating/screen system adheres to, the process is
destined to give slightly different answers.


Professor Steven J Schwartz        Phone: +44-(0)20-7594-7660
Head, Space & Atmospheric Physics  Fax:   +44-(0)20-7594-7772
The Blackett Laboratory            E-mail: s.schwartz at imperial.ac.uk
Imperial College London            Office: Huxley 6M67A 
London SW7 2AZ, U.K.               Web: www.sp.ph.ic.ac.uk/~sjs

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