[scribus] RGB to CMYK not using ICC profile for R=G=B colors

[Patrick Noffke]

Hello,

I am trying to create a document with some gray patches, and convert them
to CMYK using two different ICC profiles (with different levels of GCR).
 In order to use the BToA transforms in the profiles, I created some RGB
rectangles.  I want Scribus to treat these as sRGB, then do the conversion
to L*a*b* (the PCS), then apply the ICC profile to convert to CMYK.

In my document setup, I have assigned sRGB IEC61966-2.1 to RGB Solid Colors
(and RGB Images, and the Monitor), and one of my ICC profiles to CMYK
Images, CMYK Solid Colors, and the Printer.  I have Absolute Colorimetric
selected for Images and Solid Colors.

When I export to PDF, I have the same one of my ICC profiles selected for
the Output Profile.

I have 5 rectangles, with the following RGB values:

1:  (191, 191, 191)
2:  (127, 127, 127)
3:  (64, 64, 64)
4:  (0, 0, 0)
5:  (0, 1, 0)

I created custom RGB colors for rects 1, 2, 3, and 5, and for rect 4, I
used the predefined "Black" color.

For rects 1, 2, 3, 5, it is converting the colors to CMYK using only black.
 The black values in the PDF are 0.25, 0.50, 0.75, and 1.0, for rects 1, 2,
3, 5, respectively.  For rect 4, the CMYK value is:  (0.99216, 0.97647,
0.96078, 0.98039).  (I created the profile with a max TAC of 400%, so
colors near perfect black will get approximately this CMYK value.)

I expect all colors to use the profile to convert to CMYK, but for RGB
colors when R=G=B, it is only using black.  I don't think this is the
correct behavior.

If I use Matlab to convert the RGB colors to L*a*b*, then CMYK, I get the
following:

 rgb = [191 191 191;128 128 128;64 64 64;0 0 0;0 1 0]

rgb =

   191   191   191
   128   128   128
    64    64    64
     0     0     0
     0     1     0

>> lab = applycform(rgb/255, makecform('srgb2lab'))

lab =

   77.3397    0.0153    0.0002
   53.5840    0.0114    0.0001
   27.0923    0.0071    0.0001
         0         0         0
    0.1982   -0.3783    0.2856

>> cmyk = applycform(lab, makecform('clut', iccProfile, 'BToA1'))

cmyk =

    0.2664    0.1938    0.1971    0.0240
    0.5274    0.4277    0.4274    0.1267
    0.7748    0.6952    0.6869    0.4728
    1.0000    0.9950    0.9604    0.9880
    0.9913    0.9793    0.9611    0.9828

This profile has a relatively low amount of GCR (compared to my other one),
so I expect the 25-75% blacks to have more CMY than K, as they do with
Matlab's conversion.  These are the CMYK values I expected from Scribus.

I also wonder how it will treat images when there are equal RGB values near
non-equal values.  Will it switch from black-only to profile-converted CMYK
within the image, for very similar RGB colors?

I am somewhat new to Scribus, and this is my first post.  Please let me
know if there is a more suitable place to ask the question.

FWIW, I created these profiles from ProfileMaker, based on the ISO 15339
RPC#6 data set.

Please let me know if you have any questions, or if I'm doing something
wrong.

Best regards,
Patrick
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