Re: [gentoo-user] CMYK comparison to sRGB between platforms

Mick <michaelkintzios@×××××.com> wrote:

> On the same hardware I noticed that a CMYK photograph converted to
> sRGB looked mostly the same (indistinguishable) on Linux, but the
> sRGB colours were brighter on MSWindows.
>
> I tried this by dual booting between MSWindows and Linux.
>
> Then I tried it by running MSWindows within a VM on a Linux host and
> the MSWindows showed a clear difference in brightness between the two
> formats.
>
> Finally, I checked on an AppleMac and the difference between the CMYK
> and sRGB photographs was even more prominent than MSWindows.
>
> So, the Linux renedering seems to be misleading the user.  Have you
> noticed the same?
>
> BTW, both Linux machines that I tried this on are running radeon
> drivers - are these to blame?  The AppleMac is running Intel graphics
> with its 'retina' monitor.  Is it a matter of somehow tuning the Xorg
> settings on my Linux PCs?

First I must say that even though I'm working as a photographer I'm not 
an expert on Color Models. The professional exposure and print service 
that I use only accepts RGB Color Models. They use laser projectors to
expose photographic papers. No conversion to CMYK is necessary. 
If I order fine art prints, they are doing the conversion by them self. 
All I have to do is softproofing my pictures in Lightroom using their 
different ICC profiles, to make sure that I don't deliver pictures that 
are out of the destination gamut.
So I don't have any practical experiences with CMYK pictures. I only 
have some incomplete theoretical knowledge about it.

CMYK is a subtractive color model and RGB is an additive color model,
they are working completely different. It is not possible to convert 
one in to the other by just simply adjust some gamma curves or using a 
LUT as it is done by color management systems like lcms. 

When you are watching a CMYK picture, your picture viewer has to convert
it to a RGB color space (sRGB or AdobeRGB or similar), because that is
what your monitor needs. And I think there are not much picture viewers
that are able to display a CMYK picture.

This conversion can not be done by the graphics driver, regardless what 
kind of OS you use. Indeed Linux drivers can only use 8 bits per color
channel (that's really poor IMHO) and Windows can use 10 bits per channel
(depends on the graphics card), but this can't make big differences in 
brightness or saturation. It only leads to smother color transitions in 
some pictures.
So I don't think that the drivers have anything to do with your problem.

Apart from the different color models (CMYK vs RGB) there exist different
color spaces (eg. AdobeRGB and sRGB). When you convert one color space in 
to an other, there are parameters like black point compensation and 
different rendering intents (perceptual and relative or absolute 
colorimetric), that can make a difference in the resulting picture.

You didn't told exactly what you have done. This makes it difficult to 
find a reason for the problem. But I can think of different reasons for 
the phenomenon you observed:

Different picture viewers and/or different color management systems and/or
different color spaces (including different rendering intents respectively 
black point compensations). :-)

--
Regards
wabe