[NTG-context] Issues with RGB to CMYK conversion with color profiles (was: Colormanagement)

Jan U. Hasecke juh+ntg-context at mailbox.org
Wed Nov 14 11:39:14 CET 2018


Hi all,

attached you find my colormanagement tests and I need your help to
understand and debug the situation.

I am testing together with a friend of mine, a professional
colormanager, who does not know ConTeXt.


There are several files in the archive:

The differences in the files are the color definition and the color
profile used.

- rgb-xxx.tex – colors are defined with RGB values only.

- rgb-cmyk-xxx.tex – colors are defined with RGB and CMYK values.

  The cmyk colors are derived from the rgb values using gimp and the
  color profiles sRGB/ISOCoated

- cmyk-xxx.tex – colors are defined with CMYK values only.

The xxx stand for two color profiles: iso-coated and swop.

What I see is this:

Colors in pdfs are the same no matter what color profile I choose.

With rgb values and rgb/cmyk values I get the same colors in pdf (the
wrong ones).

With cmyk values only I get other colors in pdf. Next week my friend
will test if these are the right ones.

I have many questions:

1. Is colormanagement working in ConTeXt at all?
2. Is my code correct?
3. Why do pdfs with different color profiles show the same colors?
4. Why does ConTeXt not take the cmyk colors from \definecolor when
there are rgb values as well?
5. Where can I define a coresponding rgb profile? In Scribus you have to
define two profiles (eg. sRGB and ISOCoated) to make colormanagement work.

TIA
juh


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