Jacek - you don't need CMYK for photos ["I need CMYK support for photo
retouch, to create better colors"].

CMYK eventually will kill some nuances - being dependent on the paper
(or other support) color.
RGB colors on screen make use of luminance of the screen pixels  - you
can have many nuances of a base color
because you can control the pixel luminosity.
CMYK is made for help transferring as much color as possible from
screen (other color spaces) to paper - the only white (read light)
from the printing support (paper, plastic, etc). True, there are some
special colors with fluorescent additives - but they can't go
everywhere. That's why CMYK is not so equal with other screen based
color spaces.
On short, CMYK can not reproduce the same number of colors.

The assumption that 4 channels is better than 3 is wrong on this case.

You can only make sense working in pure CMYK when you want to have a
very precise reproduction
- but for this goal you need to know exactly the paper they use
(printers) and color profiles they use for the
offset.   The best is to  send them your work in RGB [16 bits /
channel - for smoothest gradients ] and your monitor
profile, then they will know how to get it right. Think that for RGB
to CMYK you loose something anyway.

2011/3/22 Jacek Poplawski <jacekpoplaw...@gmail.com>:
> On Tue, Mar 22, 2011 at 4:52 AM, Alexandre Prokoudine
> <alexandre.prokoud...@gmail.com> wrote:
>> On 3/22/11, Jacek Poplawski wrote:
>>>>> I need CMYK support for photo retouch, to create better colors.
>>>>> CMYK is no different than LAB, HSV or RGB. It is colorspace like
>>>>> others, but uses 4 channels instead 3.
>>>> Right, all colorspaces are equal, but some are more equal than others
>>>> :-) The willingness to go from a wider gamut to a narrower gamut for
>>>> editing what will then go to a different color space once again is,
>>>> er, equally amazing :)
>>> I just mean that they should be treated similarly :)
>> For photography? I very much doubt that. When it comes to all things
>> related to photography, the point is to preserve as many colors as
>> possible. Which is how all those ProPhotoRGB and the like were
>> introduced all those years ago. Jumping between wide and narrow gamuts
>> effectively kills useful information. Hardly better colors, sorry.
> I was influenced by Dan Margulis. I try to follow his ideas in Gimp,
> instead Photoshop.
> He generally assumes that photography is made from 10 channels: R, G,
> B, L*, A*, B*, C, M, Y, K and you can use any subset of them to
> generate good quality image with good colours.
> (http://en.wikipedia.org/wiki/Dan_Margulis)
> And everything works as expected, with the exception of realtime
> preview. I just decompose image to LAB or CMYK then use these layers
> for increasing contrast, masking, etc... but using curves in LAB or
> CMYK is very hard without preview, because you have to "imagine"
> colors. The good thing is that GMIC has support for these colorspaces
> now, and RawTherapee is developing fast.
> PS. sorry for offtopic
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Nemes Ioan Sorin
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