So is there anyone with such a tool, or the basic transformation rules so
   that I can write my own?

Hi Uwe,

The map from RGB to CMYK is not unique because it depends on a
printing policy that specifies the percentage black that should be in
a pixel.  This is often referred to as GCR (or gray componenet
replacement).  It is important for a variety of reasons including how
much ink is needed to print an image (and thus the speed at with the
print will dry) the size of printable dots on a page, etc.

However, the conversion from CMYK to RGB is unique and is described by
the following formulas:

Presuming that C, M, Y, and K are expressed with values in [0,1] then:

 R = 1-(C+K)
 G = 1-(M+K)
 B = 1-(Y+K)

If C, M, Y, and K are not in [0,1], normalize the equations so they

Best regards,


PS You may want to read the discussion on CMYK given here:

I also recommend this section to those attending GimpCon as it
describes those features needed in the GIMP for it to become CMYK
capable.  This would finally make the GIMP a viable tool for the
pre-press world.

Dr. Carey Bunks                     
Senior Scientist                    
BBN Technologies
70 Fawcett St, 15/2A                
Cambridge,  MA 02138                
tel: 617-873-3028  fax: 617-873-2918
email:  [EMAIL PROTECTED]              

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