In Estonia, CMYK is mostly used for prepress, as almost everywhere. Spot colors are also used quite a lot. But I have never heard of anybody using CIE Lab or whatever else.
Øyvind> No, you cannot convert from CMYK to any RGB, CIE Lab, CIE XYZ or Øyvind> similar three dimensionsal color space, and back again to CMYK Øyvind> w/o losing information contained in the original CMYK buffer.
The fact that we only have three cones implies that any colour differences found in a CMYK file can be represented in XYZ and 3D spaces derrived therefrom, even if the raw math seems to loose data.
You are right about the fact, that CMYK _color_ can be represented using 3D color map. But you have to agree that some information gets unrecoverably lost. I will show this with a little example. Lets say we have solid background of cmyk(50,20,10,0) and we print black text on it. As Inkscape has no "Black overprint" option yet, we have to do it using right text color of cmyk(50,20,10,100). When you look at color separations, no letter cutouts should be visible. Now imagine the text color got converted to RGB, it ends up as rgb(0,0,0), which is correct representation on screen (when using color profiles, then not). When you convert it back to CMYK, you never know what it ends up like, whether cmyk(0,0,0,100); cmyk(96,78,89,100) or something else, definitely not original cmyk(50,20,10,100).
It is always healthy to look at things from several viewpoints and to debate, but inkscape really shouldn't convert to RGB internally except when the output is to be RGB.
In my humble opinion all effects and whatsoever manipulates colours, should use objects original colourspace, whenever possible. If not, then some fallback colorspace, but effect writers should be allowed to implement several algorithms for as many colour spaces they want, to preserve information. When fallback space ise used, the completion notice (on statusbar) should state it clear.
All the best Mattias