Re: Issues with RGB to CMYK conversion with color profiles (was: Colormanagement)

[Peter Rolf <indiego@gmx.net>]

Am 2018-11-14 um 15:36 schrieb Jan U. Hasecke:
> 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?
>

Should work (the other part is invisible for ConTeXt).


>>
>> (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.
>


Mh, the file size of "ISOcoated_v2_300_eci.icc" is around 1.7MB. I
highly doubt that ConTeXt can compress it into a 10KB PDF.

But I'm not so sure if PDF/X-3 needs embedding (there was an exception).
Never used it (only PDF/X-4p) and I can't remember the specifications
(all those stuff was added over 8 years ago).

I just assume you have installed the icc profiles properly (needs a
manual installation!) :D


> 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
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>>
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> 
> ___________________________________________________________________________________
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