GIMP/CMYK support

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This article will show how to enable rudimentary CMYK support in The GIMP using the Separate plug-in.
It will also cover more general topics on CMYK colors and DTP.

Before you need

Before you install CMYK support for The GIMP, you need to know if you really need it.

There has been much debate about the merits of using The GIMP. Most of the heated discussions revolves around the fact that The GIMP does not support CMYK mode. However, you have to understand that the topic is more important to DTP professionals than other users (photographers, web artists, home users). CMYK color model (or CMYK mode) is used mostly by DTP professionals that need to output images intended for printing on a commercial press. For an average home user or even professional photographers, support for separating images using CMYK color is not necessary. Most ink-jet and color laser printers print color images using sRGB color, so you don't really need CMYK color for that either.

Required software

You need The GIMP (of course), and lcms (Little CMS). Both can be obtained from the Arch package repositories. You also need to download the source tarball for Separate plugin.

Root access

You will need root access in order to install the plugin.

Note on Adobe ICC profiles

Adobe Systems offer a nice set of standard color profiles for professional use. However, before you use the products, please read the end user license agreement supplied with the .zip file. The zip files (Windows version is a zip file, and it is the preferred way to get ICC profiles) are located at Adobe's support page.

About CMYK color model

If you are not interested in the theory, you may skip straight to the heading on CMYK color support in The GIMP.

First off, the proper name for CMYK mode, as it is commonly known, is CMYK color model. It is called a color model, because it represents a standard way of describing colors.

The color model is also called a subtractive color model, as opposed to additive (that is RGB) color model. Words additive and subtractive suggest that light, which is essential for perception of color, is either added or subtracted before it reaches the eye. The choice of color is based on belief that the combination of colors Red, Green, and Blue (for RGB) or Cyan, Magenta, Yellow (for CMYK) produce the most visible colors.

Ideally, subtraction of all light, that is when Cyan, Magenta, and Yellow are of full density, we should get black. However, this is usually not true in the real world. The use of additional Black ink in printing (K in CMYK stands for Key, or blacK) is due to this fact. It adds the necessary density to the image and makes black a black.

When printing an image on a commercial press, it needs to be printed one primary (plus Black) at a time. Therefore the original (usually a digital RGB image, or a printed photograph) needs to be separated into Cyan, Magenta, Yellow, and Black components.

The lack of support for this kind of separation made The GIMP unattractive to DTP professionals.

ICC color profiles

Since reproduction of both RGB and CMYK colors are specific to the device (or inks) used to produce images, a concept of color-spaces was invented. Color-spaces describe that describes the relationship of physical color and the model that we use to describe them. Those relationships (functions) can be packaged as a file in the form of ICC profiles.

The ICC profiles are used to describe the way colors are reproduced in a system, be it a monitor, a scanner, or a printing press. When separating images for press, we use the source profile (the color-space of the image to be separated) and the target profile (the color-space of the printing press the image is intended for).

CMYK color and The GIMP

The GIMP still lacks full CMYK color model support. The ability to separate and then edit an image in CMYK mode is still a long way down the list of features to be added (if on the list at all). However, there is a plugin called Separate that offers a partial solution to the problem.

Separate plugin has following abilities:

  • separate a RGB image
  • color management (using ICC profiles and lcms)
  • soft-proofing colors

Installing and using Separate plug-in

Installing the plug-in

Example RGB-to-CMYK conversion

Soft-proofing using Separate plug-in

Other ways to do RGB-to-CMYK conversion