On Sat, 2008-04-05 at 17:46 +0100, norman wrote:
> < snip >
> > It sounds like some color management problem, but I'm not sure exactly
> > what.  To start, have you profiled your monitor?  Are you choosing a
> > profile in gimp?   The image viewer almost certainly is not using any
> > profile information except thaqt which may be loaded into the video card
> > memory.  Gimp may be doing something different.
> I have not profiled my monitor and I am not choosing any special profile
> in GIMP. Is it complicated to profile a monitor?

Profiling a monitor can be fairly complicated.   You can find a general
discussion of it at www.normankoren.com/color_management_2A.html.

It involves two steps: calibrating the monitor thru hardware controls,
and adjusting what is sent to the monitor from the RGB values in the
image file, ie.e. the color balance of the image as it appears on the

You can do a basic calibration by setting contrast and/or brightness
(depending on whether the monitor is a CRT or LCD) and then setting
gamma using images on Koren's web site.  (See also

You can set the gamma under Linux using either xgamma or xcalib.
The second part of the proces involves color profiling, for which you
really need a device such as the Eye One Display or better the Eye One
Pro.  the latter is much more expensive but also allows you to profile
your printer. Under Linux, you can both calibrate and profile your
monitor with such a device using the program lprof or using the argyll
suite of programs.  The latter can also do your printer if you have the
right device. Lprof may be already available as a package under ubuntu;
it is under Fedora.  The argyll suite must be donwloaded.  The
documentation is pretty clear, but it must be read very carefully, and
you have to know what the words mean.  It also has a very active users
group which can be very helpful.  Try googling "argyll color management"
for more information,  But I should warn you that the learning curve for
these tools, and for color management in general is very steep.  If you
are determined to understand it, you should start with Norman Koren's
web page, but you would be well advised to study a book like Real World
Color Management.  It covers the basics very well, but most of the book
assumes you are using commercial software available under Windows or
MacOS.  Some of  this can be adapted to Linux, but you it is hard to dig
out just that part.  I've been working on understanding color management
for many months, and I don't think I've completely mastered it yet.
Unfortunately, the gimp manual documentation on the subject is pretty
sketchy and some of it, I think, is misleading.  I suggested some
improvements to the gimp-doc group, but the last time I looked, they had
not been adopted.

You should probably do a minimal adjustment for gamma, as described on
Norman Koren's web page, but you will probably find it doesn't make much
difference.  The default calibration and profile---sRGB---is probably
not that far from what careful profiling would yield.  

I suspect that this is not the problem in your case.  As I suggested
earlier, it sounds as if the image viewer and gimp are using different
profiles or assuming different color space models, which amounts to the
same thing. It is remotely possible  that ubuntu has added some color
profiling to its interface, but I really doubt that.   Gimp will by
default use sRGB, which is supposed to be an average profile for
monitors, and the image viewer is probably also using it.  If a profile
for a color space is embedded in your image file,  gimp may be choosing
that instead, and then what it shows will be different from what the
image viewer shows.  You can check that by using Image properties>Color
Management in gimp.   Also, just what gimp will do with an embedded
profile it finds in an image is determined under Preferences>Color

> Norman

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