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

Peter Rolf indiego at gmx.net
Wed Nov 14 14:58:55 CET 2018


Hi Jan,

I donno if things have changed (ConTeXt is a fast moving target), so
take my answers with a grain of salt.

(1)
AFAIK there is no ACTIVE color management in ConTeXt in the sense that
there is an automatic color conversion from one profiled(!) colorspace
into another profiled color space. This means that all given colors
(objects) have to be in the correct (profiled) color space already!

Objects with embedded color profiles (e.g. a PDF bitmap graphic with icc
profile) will use those. Objects without a profile will use the
corresponding "default" color space profiles. These default color space
profiles can be defined via "profile" in \setupbackend.
This way every color in the PDF is defined properly.


(2)

I think so. Can't test, because my Windows 10 is beyond repair after
three "upgrades" and Acrobat is not working properly anymore.

(3)

Judging from the size of the final PDF, the icc profile is not embedded.
The profiles have to be installed in "the right place" (this includes
ConTeXt but also the PDF viewer software).

See "ICC profiles" section at

https://wiki.contextgarden.net/PDFX


(5) see (1) and (3)



Hope this helps,


Peter


Am 2018-11-14 um 11:39 schrieb Jan U. Hasecke:
> 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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