Posted by khk

The Gimp Color Manager

The Gimp Color Manager

Karl Heinz Kremer
khk@khk.net

2-May-2000
The source file color_manager.c contains the code for the Gimp Color Manager plugin. This plugin can be used to color correct images with ICC color profiles.

At this time the functionaliy is very limited, the plugin e.g. accepts only RGB images. There is more to come …

Prerequisites

You need the Linux version of LCMS ( http://www.littlecms.com/) to compile the plugin. Compile and install lcms and remember where you put it :-)

Now edit the Makefile and add the lcms directories.

make
make install

… and get yourself a couple of profiles.

Usage

The plugin will install a menu entry in the Filters menu.

< Load a RGB image and select the filter. Once you start the plugin you first have to select a directory that contains your ICM profiles. Once this is done you can select the transformation direction and after the list of profiles refreshes finally the profile to use can be selected. In addition to e.g. an input profile the system also needs a second profile to convert the image to the color work space:

   
   Input Color Space -&gt; Connection Color Space -&gt; Color Work Space
               Input Profile           Work Space Profile

So if you want to use sRGB as your color work space you would select the profile file sRGB.icm.

The last thing to do is to select the rendering intent.

If all this sounds too complicated, you should probably read up on color management in general. The best and most complete collection of everything color is at http://www.efg2.com/Lab/Library/Color/index.html

Hit “Apply” and enjoy the show… The plugin will then apply the profile to the image.

Color Correct a Scanned Image

To color correct a scanned image you need of course a profile for your scanner. EPSON does distribute profiles with their Windows drivers. These are usually in c:\windows\system\color. Other manufacturers may also distribute profiles, just check this directory. These profiles are of course generic and not made for your particular scanner. They are however better than no color correction at all.

Color Correct an Image for Printing

This one is a little more complicated. Even if your printer manufacturer distributed a profile this is only useful for the driver it was made for. Most likely this is a Windows driver.

Custom Made Profiles

If you want to apply profiles to print jobs or want a better profile for scaned images you have to use profiles made just for your environment.

Scanner Profiles

There is at least one package that runs under Linux and can create a profile for your scanner: http://www.scarse.org

If you have access to a Windows installation there are much more choices, even free (as in beer) ones: http://www.iphoto.coloraid.de

http://www.coloraid.de is a collection of links to a few colormanagement projects. You can also get IT8 targets via one of these links. Such targets are required to create scanner profiles.

If you have access to a Corel Draw/Ventura/Photo Paint installation on Windows then you can use the Color Profile Wizard that comes with the software to create a profile for your scanner and printer (more about this later).

And finally the WiziWYG software from http://www.wiziwyg.com for $80 can also generate profiles for scanners and printers.

Printer Profiles

It is much more complicated to generate printer (good) profiles than it is to create scanner profiles. This is problably the reason why there is no free software available. I know of two products that can create such profiles:

The Corel Color Profile Wizard can create printer profiles for RGB and CMYK processes. The plugin can at this time only use RGB profiles. Every printer that I know uses CMY(K) to get color on the paper, but it’s still possible to use RGB profiles.

Scanner profiles can be generated just with the scanner and a target. The Corel software does however require a spectrometer or a colorimeter to meassure the calibration prints that the software generates. The problem here is that the wizard only supports four different devices, if your colorimeter is not supported you have to resort to a manual process and create the measurement file line by line.

The version 7 of the wizard does use a different approach in addition to the spectrometer/colorimeter measurements: You can also use a calibrated scanner to measure the printer output. The version 7 is often also what you get as an OEM version 8. So if your scanner or printer came with Corel Draw 8 and without manual or the IT8 target it’s very likely that you have a genuine version seven.

I am using the WiziWYG software from http://www.wiziwyg.com to create both scanner and printer profiles. The software can be downloaded for free from their web site, but they are selling a special IT8 target for $80. Unfortunately the software only works with their version of the target (actually it’s the reference file that is different from reference files created by everybody else).