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)


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