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

[Patrick Noffke]

On Tue, Dec 20, 2011 at 12:19 PM, john Culleton <John at wexfordpress.com>wrote:

> On Tue, 20 Dec 2011 12:06:11 -0600
> Patrick Noffke <patrick.noffke at gmail.com> wrote:
>
> > >
> > Because I want to experiment with different ICC profiles for the same
> > sRGB colors (which will include images in addition to these
> > rectangles).  I want the profile to define the conversion to CMYK for
> > all sRGB colors.
> >
>
> I obtained a profile that seems to do the conversion OK. Why do
> you need to create your own? I use the following profile
> which also limits the ink density to 240%:
> SWOPcoated5_240.icc
> Available here:
> http://wexfordpress.com/tex/SWOPcoated5_240.icc
>
>
That doesn't change the problem.  Scribus is still converting colors with
R=G=B to black only, and nearby colors (e.g. R=0, G=1, B=0) to 4-color CMYK
values.  With real inks, even with a 240 max TAC, there will be a big color
difference in these CMYK values.

Here is the portion of the (uncompressed) PDF that results using the
SWOPcoated5_240 profile:

q
   1 0 0 1 0 0 cm
   q
      0 0 0 0.25098 k  0 0 0 1 K  1 w  [] 0 d  0 J  0 j  1 0 0 1 40.83465
751.49291 cm  0 0 m  17.0079 0 l  17.0079 -17.0079 l  0 -17.0079 l  0 0 l
 h  f*
      0 0 m  17.0079 0 l  17.0079 -17.0079 l  0 -17.0079 l  0 0 l  h  S
   Q
   q
      0 0 0 0.50196 k  0 0 0 1 K  1 w  [] 0 d  0 J  0 j  1 0 0 1 60
751.49291 cm  0 0 m  17.0079 0 l  17.0079 -17.0079 l  0 -17.0079 l  0 0 l
 h  f*
      0 0 m  17.0079 0 l  17.0079 -17.0079 l  0 -17.0079 l  0 0 l  h  S
   Q
   q
      0 0 0 0.74902 k  0 0 0 1 K  1 w  [] 0 d  0 J  0 j  1 0 0 1 78.84252
751.49291 cm  0 0 m  17.0079 0 l  17.0079 -17.0079 l  0 -17.0079 l  0 0 l
 h  f*
      0 0 m  17.0079 0 l  17.0079 -17.0079 l  0 -17.0079 l  0 0 l  h  S
   Q
   q
      0 0 0 1 k  0 0 0 1 K  1 w  [] 0 d  0 J  0 j  1 0 0 1 97.68504
751.49291 cm  0 0 m  17.0079 0 l  17.0079 -17.0079 l  0 -17.0079 l  0 0 l
 h  f*
      0 0 m  17.0079 0 l  17.0079 -17.0079 l  0 -17.0079 l  0 0 l  h  S
   Q
   q
      0.43529 0.4 0.51373 0.99608 k  0 0 0 1 K  1 w  [] 0 d  0 J  0 j  1 0
0 1 116.52756 751.49291 cm  0 0 m  17.0079 0 l  17.0079 -17.0079 l  0
-17.0079 l  0 0 l  h  f*
      0 0 m  17.0079 0 l  17.0079 -17.0079 l  0 -17.0079 l  0 0 l  h  S
   Q
Q


> Any conversion from RGB to CMYK will involve some shift in
> colors. Not all RGB colors can be expressed in CMYK.
> But you knew that already, right?
>
>
Yes, I realize that.  I want to convert my document to CMYK using profiles
with the same gamut, but other differences that I am enforcing (e.g.
different GCR).

You ask why.  I design color measurement devices for offset and newspaper
printing presses.  Lately, I'm researching how to measure TVI from an
image.  I need to enforce known TVI differences in the color separations
from a document I create.  I can do this with Matlab using (CMYK) TIFFs,
but it is tedious to design my document that way, so I wanted to do it with
Scribus.  (I can also enforce TVI differences by modifying the ICC
profile's output curves.)

Thanks,
Pat
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