[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 15:36:14 CET 2018


Thanks a lot, Peter.

Am 14.11.18 um 14:58 schrieb Peter Rolf:
> 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.

This would clarify some of the issues.

When I define rgb colors AND use not-print-modus I get the right rgb
colors in the pdf.

When I define cmyk colors AND use print modus I get colors that might be
correct (tested next week)

But obviously it is not possible to define both color spaces in
\definecolor because I don't get the same colors in the pdf as with the
cmyk-only defined file.

Maybe I misunderstood this, I've got a hint here, that I could define
rgb and cmyk in the same \definecolor and choose the right color space
in \setupcolors.

The correct method of defining rgb AND cmyk in the same file would be to
put the definecolor command inside a startstopmode-environment.

Can someone confirm this?

> 
> (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
> 

With --mode=print (which uses the color profiles) the pdf get bigger.
>10.000 compared to 8.000 without the mode. So I would guess that they
are included.

I hope I get this sorted out, because I want to print a series of
folders and other print products with ConTeXt in the near future.

juh

> (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
>> ___________________________________________________________________________________
>> If your question is of interest to others as well, please add an entry to the Wiki!
>>
>> maillist : ntg-context at ntg.nl / http://www.ntg.nl/mailman/listinfo/ntg-context
>> webpage  : http://www.pragma-ade.nl / http://context.aanhet.net
>> archive  : https://bitbucket.org/phg/context-mirror/commits/
>> wiki     : http://contextgarden.net
>> ___________________________________________________________________________________
>>
> 
> ___________________________________________________________________________________
> If your question is of interest to others as well, please add an entry to the Wiki!
> 
> maillist : ntg-context at ntg.nl / http://www.ntg.nl/mailman/listinfo/ntg-context
> webpage  : http://www.pragma-ade.nl / http://context.aanhet.net
> archive  : https://bitbucket.org/phg/context-mirror/commits/
> wiki     : http://contextgarden.net
> ___________________________________________________________________________________
> 



More information about the ntg-context mailing list