On Fri, Jun 20, 2008 at 4:25 AM, Andy Fitzsimon <andyfitz@...400...> wrote:
Its a really tricky question when blending from multiple colour spaces.. I guess convert all to display as rgb while screen ( using normal colour profile ) and then convert all to cmyk on export? I'm no expert.
The problem is that by converting the colors from CMYK to RGB you reduce the dimensionality, different CMYK color will map to the same RGB triplets, thus you do not preserve color fidelity in this transformation. CMYK -> RGB -> CMYK, most probably do not lead to the original CMYK values (unless they've already been "normalized" by a similar conversion first.)
Mixing of RGB is well defined for CMYK on the other hand things are a bit murkier since the color resulting from mixing two CMYK values depends on the inks used, the paper used and other physical attributes. GEGL currently makes the assumption that it is doing compositing of light, and it considers CMYK to be something to be done at export. Essentially it doesn't support native CMYK processing but allows color managed workflows optimized for RGB inputs, accepting CMYK inputs with the color precision loss, with the ability to then tweak and tune the separation of CMYK on the final composite (or leave it to a DTP or similar program that decides what to do with well defined RGB data.)
For GEGL I think this assumption is a valid one for now, but for cairo to go down this route seems more wrong, it would probably be more natural to convert RGB to CMYK to avoid losing the dimensionality of the colors.
/Øyvind K.