[scribus] Color management conclusions (was: Re: No real CMYK black possible??)

Jean Ghali jghali at libertysurf.fr
Mon Jun 23 22:58:06 CEST 2008


Tino Schwarze-2 wrote:
> Hi there,
> I observed the following:
> - black image with ISOCoated profile looks different on screen than
>   black RGB image, even though they come out the same in CMYK (ISOCoated
>   image looks a lot blacker on screen). IMO that shouldn't be the case.

This is perfectly expected and not an issue. The RGB and CMYK ISOCoated
black points are simply completely different. A RGB monitor profile will
usually have its black point set at L*0, the CMYK ISOCoated black point is
around L* 14, which means that RGB black is *way* darker than ISOCoated
black and explain why they look different on screen. When RGB black is
converted to ISOCoated, well it just does not fit in the ISOCoated gamut and
is usually mapped to ISOCoated black point ("black point compensation").
Which explain why RGB black looks identical to ISOCoated black after

> - RGB 0/0/0 black became CMYK 0/0/0/100 black.
>   -> this does not match the black image, so I cannot combine a black
>   RGB background with a black RGB image for printing!

This is in fact the result of a very old design decision (1.2.x or even
older, not sure tho). It reflects how most preflight softwares used in
printing houses will handle RGB black and more generally RGB greys (eg: RGB
55/55/55) conversions to CMYK. Why so? Because use of RGB greys are in more
than 99% of the cases a designer error and result in time loss on press and
sometime hardly printable jobs! Fyi, according to the logs of our workflows,
those kind of errors affect tens of files *by day* where i work.

However, i think those mapping from RGB greys to black ink should be handled
more selectively. In 1.3.6 i plan to give more control to users about how
conversion from RGB greys to CMYK is done. All i can say now is that direct
mapping from RGB greys to black ink will be the default at least for text

> Should I file a bug report on these issues or are they known? 

There's is already one : #6844

Hope i was clear enough in my explanations,


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