Introduction

Animenu

AWBMTools

CamCardSync

ImgTarget

IRMix

Separate

EPIA TVOut driver

EPIA TVTool

XvTestcard


A plugin providing rudimentary CMYK support for The GIMP


PLEASE NOTE:  While my version of the plugin is still available here, a much-improved version, maintained by Yoshinori Yamakawa, is available at this address: http://cue.yellowmagic.info/softwares/separate.html  -  you are strongly urged to use his version instead of mine!


DISCLAIMER:
As with most of my projects, this is experimental software, and you use it entirely at your own risk...

Version 0.3

Many thanks to:

• Sven Neumann for doing much of the work involved in cleaning up and porting the plugin to GIMP 2.0.
• Michael Schumacher for taking the time to do the Win32 build.
• Mat Caughron for doing the MacOS X build.

Background
One thing preventing The GIMP from being useful in a pre-press environment is the lack of support for the CMYK colour-space.  This plug-in goes some small way towards rectifying the situation, using a trick with layers to fake CMYK support.

The plugin is unfinished, but usable for its primary purpose, and since I'm unlikely to have time to develop it further in the near future, I'm releasing it as is.


What can it do?

• Convert an RGB image to individual CMYK layers, using specified source and destination Colour Profiles.
• Save such a collection of layers as a CMYK TIFF.
• Optionally restrict separation of pure black pixels to the black plate (useful for processing flattened PDFs with text).
• Perform a rudimentary background fill-in (Overprint) to accompany the Preserve Pure Black option.
  
• Proof the CMYK image on the monitor.  (This is currently done as an Absolute Colorimetric transform from the CMYK data back to the RGB profile; it should provide a pretty good side-by-side match with a print.)
• Perform a "duotone" separation on an RGB image, reducing it to just those colours achievable with red and black ink.  (This feature does *not* use colour-profiles!)

What can't it do?

• Load CMYK TIFFs into individual layers.  If you want to edit a CMYK image, save it in XCF format as well as CMYK TIFF, so the layers are preserved for future editing.
• Embed ICC profiles in the generated TIFF.
• Perform the "duotone" trick based on any colour other than red.

Requirements:

• The RGB -> CMYK conversion is performed with the truly excellent littlecms library, so you'll need that installed.  (The win32 version includes a pre-built dll, and for GPL-compliance, the source too...)
• LibTIFF is also used, but since The GIMP itself uses it, its unlikely to be missing!
• ICC profiles.   I recommend that you download Adobe's free colour profiles.  A Google search for "Adobe ICC Profiles" will find them...

Installation:
Installation instructions for each platform are included in the archives.


Separating an image:
To convert an RGB image to CMYK format, bring up the right-button menu, and go to "Image->"  If the plugin in installed correctly, there will be a new menu, "Separate".  From this new menu, select "Separate (normal)"; you will be prompted to select an RGB source profile, and a CMYK destination profile.
If you have installed the Adobe and sRGB profiles as per the instructions in the archive, you can just accept the defaults for testing, otherwise you'll have to locate the profiles manually.

A new image will be created with four greyscale layers, named "C", "M", "Y", and "K".

If you have loads of memory to spare, you can use the "Separate (colour)" option; this will perform the same operation, but the new image will contain five layers: The first, "Background" will be white, and the other four will be solid Cyan, Magenta, Yellow and Black, with the separated image data in layer masks.  In addition, the layer modes for the four colours will be set to "Darken Only".
This gives a rough reconstruction of the colours, and is the next best thing to a true CMYK painting mode, since you can paint on the layer masks, and see the results in realtime.
*NEW for 0.3* - the "primary" colours chosen for the "Separate (colour)" mode are now much more akin to the primaries used in printing, which are nowhere near as bright, saturated and downright lurid as pure screen Cyan and Magenta!  This gives a far more realistic preview of the colours.


Saving:
To save a CMYK TIFF for use in a DTP program, bring up the right-mouse-button menu, and select "Image->Separate->Save...".  Please note that saving an image this way won't clear the image's modified flag, so when closing, you'll be warned that the image hasn't been saved.  This won't be fixed until the plugin is capable of loading CMYK images. Until then, if you need to edit the CMYK image in the future, you need to save it off in XCF format, to preserve the layers.


Duotone:
This is a little bonus, included simply because I needed it at work.  It reduces the colours in the image to those available just using red and black ink, then creates a pseudo-CMYK image, with just Background, K and M layers.  The resulting image can be saved in the same way as a real CMYK image, but the "red" data will occupy the "magenta" channel of the resulting TIFF.


Download:

      Linux (source + binary) for GIMP 2.x

      MacOS X (source + binary) for GIMP 2.x

      Windows (source + binary) for GIMP 2.x