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

[Jean Ghali]

----- "Patrick Noffke" <patrick.noffke at gmail.com> a écrit :
> 
> 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.

The behavior here is explained by quality considerations. When using vector objects with r=g=b, icc conversions does not work well when document is targetted to be printed on a press (digital or not). In those case, icc base cmyk conversion will usually produce a separation using all 4 cmyk channels and this is the problem :
- such color has very bad colorimetric stability on press : unless the color is very dark, any shift on a single color will trigger the gray to be shifted. As the shift depends on the ink percentage, the result will also be a poor gray balances.
- using all 4 cmyk colors for grays on text and lines will deteriorate output quality by making text and lines look more 'fat' than they should be. Any ink registration problem will make the problem even worse.
- such a document will need better ink registration and cause higher make-ready times if printed on traditional press, those higher make-ready times will however not really improve printed quality in this case
- higher make-ready times will cause higher production costs

Conclusion : in most case, traditional icc conversion is to be avoided for gray color with r=g=b. Scribus behavior has been implemented in view of those practical considerations. Preflight tools such as Pitstop implements similar handling of rgb grays btw.

Hopes that answer your questions,

Jean