Mick <michaelkintz...@gmail.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
