Chapter 10. Color Management with GIMP
1. Color Management in GIMP
Many devices you use in your design or photography workflow, like digital
photo cameras, scanners, displays, printers etc., have their own color
reproduction characteristics. If those are not taken into account during
opening, editing and saving, harmful adjustments can be done to images.
With GIMP you can have reliable output for both Web and print.
Figure 10.1. Image Processing Workflow
1.1. Problems of a non Color Managed Workflow
The basic problem of image manipulation without color management is that
you do simply not see what you do. This affects two different areas:
There are differences in Colors caused by different color
characteristics of different devices like cameras, scanners,
displays or printers
There are differences in Colors caused by the limitations of the
colorspace a specific device is able to handle
The main purpose of color management is to avoid such problems. The
approach taken to do so involves the addition of a description of the
color characteristic to an image or devices.
These descriptions are called color profile. A
color profile is basically a look-up table to translate the specific
color characteristic of a device to a device-independent color space -
the so called working-space. All the image manipulation is then done to
images in the working-space. In addition to that the color profile of a
device can be used to simulate how colors would look on that device.
The creation of color profiles is most often done by the manufacturer
of the devices themselves. To make these profiles usable independent of
platform and operating system, the ICC (International Color Consortium)
created a standard called ICC-profile that describes how color profiles
are stored to files and embedded into images.
1.2. Introduction to a Color Managed Workflow
Most digital cameras embed a color profile to individual photo files
without user interaction. Digital scanners usually come with a color
profile, which they also attach to the scanned images.
Figure 10.2. Applying the ICC-profile
When opening an image with an embedded color profile, GIMP offers to
convert the file to the RGB working color space. This is sRGB by
default and it is recommended that all work is done in that color
space. Should you however decide to keep the embedded color profile,
the image will however still be displayed correctly.
In case for some reason a color profile is not embedded in the image
and you know (or have a good guess) which one it should be, you can
manually assign it to that image.
For the best results, you need a color profile for your monitor. If a
monitor profile is configured, either system-wide or in the Color
Management section of the GIMP Preferences dialog, the image colors
will be displayed most accurately.
One of the most important GIMP commands to work with color
management is described in
Section 5.8, “Display Filters”.
If you do not have a color profile for your monitor, you can
create it using hardware calibration and measurement tools. On
UNIX systems you will need
Argyll Color Management System™
to create color profiles.
220.127.116.11. Display Calibration and Profiling
For displays there are two steps involved. One is called calibration
and the other is called profiling. Also, calibration generally
involves two steps. The first involves adjusting external monitor
controls such as Contrast, Brightness, Color Temperature, etc, and
it is highly dependent on the specific monitor. In addition there
are further adjustments that are loaded into the video card memory
to bring the monitor as close to a standard state as possible. This
information is stored in the monitor profile in the so-called vgct
tag. Probably under Windows XP or Mac OS, the operating system loads
this information (LUT) in the video card in the process of starting
your computer. Under Linux, at present you have to use an external
program such as xcalib or dispwin. (If one just does a simple visual
calibration using a web site such as that of Norman Koren, one might
only use xgamma to load a gamma value.)
The second step, profiling, derives a set of rules which allow GIMP
to translate RGB values in the image file into appropriate colors on
the screen. This is also stored in the monitor profile. It doesn't
change the RGB values in the image, but it does change which values
are sent to the video card (which already contains the vgct LUT).
Using GIMP, you can easily get a preview of what your image will look
like on paper. Given a color profile for your printer, the display can
be switched into Soft Proof mode. In such a simulated printout, colors
that cannot be reproduced will optionally be marked with neutral gray
color, allowing you to correct such mistakes before sending your
images to the printer.