[Sent earlier to individual instead of list by accident - remarkably
similar to a subsequent mail on the list!]

At 01:54 AM 5/25/00 +0200, you wrote:
>Yes, I know this, but I only need an easy tool to convert this CMYK colors
>to RGB.  Not for an exact representation -- only for a start!
>So is there anyone with such a tool, or the basic transformation rules so
>that I can write my own?

If you only want an approximation, it is relatively straightforward.

The C, M and Y channels of the CMY colour space are effectively just the R,
G and B channels flipped.

R = 1 - C
G = 1 - M
B = 1 - Y

(assuming they are specified in the range 0 - 1).

The main reason for CMYK is that the CMY inks used in printing are not pure
filters corresponding to the spectra of the R, G and B channels. In order
to print good blacks and greys, some proportion of C, M and Y is removed
and replaced with black (K).

The simplest technique for this is Grey Component Replacement (GCR), which
simply removes common proportions of C, M and Y and replaces them with the
same proportion of black. For example (moving to percentages now), 30C 20M
60Y would become 10C 0M 40Y 20K.

There are other techniques, and many more issues than this, including the
exact inks used, the printing processes, local colour calibration, printing
stock absorbency and colour, whether the printing is to be back-lit and so
on, but  this is just a very brief overview. Real conversion seems to be
based on curves which effectively map each combination of RGB to a
corresponding CMYK, taking into account as many of these issues as
possible. There is no real meaning in a set of CMYK values in isolation
(although the same can be said about RGB...).

Using this poor approximation, you could convert from CMYK to RGB using the
following rules:

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

Hopefully I haven't slipped up anywhwere.

Hope this helps,


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