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

Wolfgang Schuster wolfgang.schuster.lists at gmail.com
Wed Nov 14 16:51:13 CET 2018


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)

Wolfgang



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