[NTG-context] Issues with RGB to CMYK conversion with color profiles

Henning Hraban Ramm texml at fiee.net
Mon Nov 19 10:56:53 CET 2018

Am 2018-11-18 um 19:26 schrieb Hans Hagen <j.hagen at xs4all.nl>:

> On 11/14/2018 4:51 PM, Wolfgang Schuster wrote:
>> Jan U. Hasecke schrieb am 14.11.18 um 11:39:
>>> 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.
>> Even without a color profile the results from the normal conversion of colors from cmyk to rgb
>> can result in differences between ConTeXt and Gimp because both use different formulas.
>> This is the formula from ConTeXt (attr-col.lua)
>> r = 1.0 - math.min(1.0,c+k)
>> g = 1.0 - math.min(1.0,m+k)
>> b = 1.0 - math.min(1.0,y+k)
>> and this is the one from Gimp (https://gitlab.gnome.org/GNOME/gimp/blob/master/libgimpcolor/gimpcolorspace.c) r = 1.0 - math.min(1.0,c*(1.0-k)+k)
>> g = 1.0 - math.min(1.0,m*(1.0-k)+k)
>> b = 1.0 - math.min(1.0,y*(1.0-k)+k)
> Are there more variants? Do we need to support variants?

Oh my, color separation is a science of its own, but not an exact science...

Any really usable color conversion algorithm uses color profiles.

It would make sense to outsource color calculations to a color management library like LittleCMS.

Greetlings, Hraban

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