2011/3/21 Jacek Poplawski <jacekpoplaw...@gmail.com>:
> On Mon, Mar 21, 2011 at 11:30 PM, gespert...@gmail.com
> <gespert...@gmail.com> wrote:
>> Most of the people ask for CMYK because:
> I need CMYK support for photo retouch, to create better colors.
> CMYK is no different than LAB, HSV or RGB.

Well, CMYK is quite different than LAB actually.
Could you please elaborate about how you can create better color in a
colorspace which is device dependent and has tipically a smaller gamut
than most of the RGB working spaces?
When you work in CMYK mode in a program like Photoshop, the visual
feedback you get is an on-screen soft-proof of the CMYK color
converted to your working RGB profile. So what you see isn't really
what you get, and it's probably better idea to do your photo
retouching in RGB soft-proofing to your target CMYK.
The good thing about this is that you'll keep the larger gamut and
your colors won't be unnecessarily and irreversibly clipped to a
smaller gamut.
CMYK (and it's not just me saying this) is an output colorspace to
send images to four-inks process printers. With color management the
CMYK mode should be legacy.

> It is colorspace like
> others, but uses 4 channels instead 3.

That's not completely true. If inks were perfect, the fourth channel
wouldn't be necessary.
Black was added to compensate CMY inks imperfections and also to save
ink and make prints cheaper.
Tell me if that's not a declaration of device-dependent colorspace!
When it comes to monitors, it's obvious you need to work in a device
dependent colorspace. You can't use another device to see your images
when you manipulate them, so there's a reasonable excuse to use device
dependent colorspaces as working profiles in that case.
But how your RGB image is separated to CMYK is defined in the target
profile. Messing with those separations individually is modifying the
way they were separated by the profile, which is the one in charge of
converting the colors to the best possible values for the target
Despite it's a pretty common practice, it doesn't sound correct.

> Instead focusing on CMYK I would give Gimp access to use any defined
> colorspace in realtime, just as RGB.
> So adding support to display group of layers as RGB/LAB/HSV/CMYK could
> be good first step.

As far as I know (please correct me if I'm wrong), the idea with GEGL
is to work in device-independent, 32bpc float linear space and then go
to Lab, RGB (or even CMYK) depending on the need.
So the first step you mention is on the works. What I discussed in my
previous message was an interim solution mostly for black generation
and pure CMYK primaries, the things that management won't solve
exactly as the user wants.

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