Image Processing Reference
when shifting modes, you should avoid converting an image from RGB to
CMYK, and vice versa, unless it is necessary.
Since the CMYK color model has four color channels, it possesses a total
number of approximately 4.3 billion potential colors, which translates to a
color depth of 32 bits .
The colors of this model are subtractive primary colors. This means that
the CMYK model behaves inversely to the RGB model. For example, if you
apply the RGB model to CMYK, then 256 units of cyan, 256 units of magenta,
and 256 units of yellow should produce black. However, what you will actually
see is a dirty dark brown. To obtain real gray and black shades, you have to
add black. CMYK is actually an initialism for Cyan, Magenta, Yellow, Key, where
Key = Black.
Currently, GIMP does not have a feature for converting and editing images
directly in CMYK mode. However, it can produce the chromatic components
necessary for use in four-color printing processes. If you want to edit in CMYK,
you can use the Image > Mode > Decompose menu command to decompose
your image into the four color channels. Each of these channels can then be
saved, edited, and shared as separate images, which can be re-integrated
prior to printing.
Alastair M. Robinson's plug-in Separate+ offers a feature for color
separation as well as additional features for soft proofing and duotone
coloration. You can find information about the plug-in on the author's website:
http://www.blackfiveservices.co.uk/separate.shtml. The improved version can
be found at http://registry.gimp.org/node/471 an d on the DVD at the back of
After the installation, you will be able to separate an image into the four
color channels of the CMYK color model by using the menu command Image
> Separate . The separate channels will be generated as layers in a new picture
and can be retouched individually.
At least GIMP can separate the colors for the four-color printing by using
grayscale images from one image.
Note: Further information on plug-ins can be found in section 1.5.2. You
can find the download addresses in the link list on DVD. Most of the mentioned
plug-ins are gathered there as installable files.