[Digikam-users] Color Managed View - How?

[Hal V. Engel]

> Am Friday 24 August 2007 schrieb Paul Waldo:
> > Gerhard Kulzer wrote:
> > > Am Thursday 23 August 2007 schrieb Paul Waldo:
> > >> Hi all,
> > >>
> > >> I was reading through the Digikam docs relating to ICC and came across
> > >> this surprising sentence:
> > >>
> > >> "The Use color managed view is an alternative to using Xcalib or 
> >> > Argyll. 
> > >> Only your image will be color managed, not your entire screen!"
> > >>
> > >> How does Digikam do this?  I would think it would need to know how the
> > >> monitor behaves in order to render the colors properly...
> > >>
> > >> Paul
> > >
> > > You are correct, in order to color-manage, digiKam needs to have a
> > > monitor profile.
> > >
> > > Gerhard
> >
> > Yes, and monitor profiles are created by programs like Xcalib and
> > Argyll, aren't they?  I'm confused as to how profiling applications can
> > be taken out of the loop....
> >
> > Paul
>
> Xcalib and Argyll are programs that apply a color profile to X, so all 
> programs are automatically color-managed as far as the monitor is concerned 
> (that means that the monitor is somewhat more neutral in its color 
> rendering,  
> missing color lobes can't be displayed of course, and notebooks are terribly 
> bad when it come to photographs, no whatever sophisticated color management 
> can correct that).
> 
> Xcalib and Argyll don't manage other devices than screens. So they are 
> partial color managers.
>
> Gerhard

Paul and Gerhard,

After looking at the answers that Paul received to his question I concluded 
that there was a significant amount of misinformation in those replies.  As a 
result I joined this email list in the hope that I could clear up some of 
these misconceptions.  Let me start off with a very quick overview.

Color managing a display or monitor is divided into two parts but only one of 
these is really the color management part.  One of these is called 
calibration and the other is called characterization.   Characterization is 
the part that is really color management.  

This is a confusing point for most new users of color management in that they 
tend to think that color management is about calibration.   This, of course, 
is not the case.  Calibration is a process where a device it brought into 
some defined state by making adjustments to it's controls or some other 
physical means.  For example, the act of calibrating a monitor involves 
adjusting its white point, black level, luminosity and gamma to predetermined 
or standard values using the monitors controls and by altering the video card 
gamma ramp.  

In many cases software for profiling monitors, like LProf or the set of 
utilities in ArgyllCMS, also have the ability to calibrate displays as part 
of the profiling process and this is another reason that this is confusing to 
many users.  The main point is that these are different and that you as a 
user of color management need to have at least a basic understanding of the 
difference.  Note that calibration affects everything that is displayed (well 
almost since Xv video playback by passes the video card gamma tables in many 
current x11 drivers) not just applications that are color management aware 
like Digikam or Krita.

In contrast to calibration the process of creating a profile is a 
characterization of the device that does not involve making any changes or 
adjustments to the device.  Rather it is a measurement process that results 
in a file that contains a precise mathematical description of the devices 
color and tonality characteristics.  This file is an ICC profile.  These 
characteristics include the transfer function from the devices color space to 
a standardized absolute color space (this is called a Profile Color Space, 
PCS, in an ICC profile), the devices white point, black point, primaries and 
other information.  Displays are normally characterized (profiled) in their 
calibrated state.

To summarize calibration makes changes to the device to alter it's color 
reproduction characteristics to conform to some predetermined state.  
Profiling or characterization is a measurement process that results in a 
detailed description of the device's (normally calibrated) color reproduction 
characteristics. 

In ArgyllCMS dispcal is used to calibrate the display, dispread is used to 
create a set of measurements that are used to create a profile (IE. to 
characterize the display) and profile is used to create a profile using the 
files created by dispcal and dispread.   In LProf these functions are 
combined into a single GUI wizard rather than being separate programs.  

ArgyllCMS also includes a program named dispwin that can read either an ICC 
profile that contains gamma table calibration data (called a VCGT tag) or use 
the file created by dispcal directly to apply the calibration to the video 
card gamma table.  This is also what xcalib does only xcalib can only use an 
ICC profile with a VCGT tag.  LProf does not contain a gamma loader and it's 
users are encouraged to use either xcalib or dispwin.

The point here is that you can characterize (IE. profile) your display in 
either a calibrated or uncalibrated state and you can use the resulting 
profiles in any color management aware application.  The main differences are 
that if you calibrate your display before profiling all of your applications 
benefit, even those that are not color management aware, and the adjustments 
being made as a result of using the display profile in a CM aware application 
will be somewhat better because the color space transform will be making 
smaller adjustments to the image as it is displayed. 

Based on this I am not sure why Digikam would need to have some setting 
specifically for this.  In addition, even the sentence from the Digikam 
docs "The Use color managed view is an alternative to using Xcalib or Argyll. 
Only your image will be color managed, not your entire screen!" seems to 
contain some misinformation since with current implementations of x11, 
Windows and OS/X the most that you can achieve for the whole display is that 
the display is well calibrated but not color managed.  None of these systems 
do color management of the whole display and only color management aware 
software running on these systems can give you color managed results.

Also just for clarity xcalib is a video card gamma table loader and it does 
not have any functionality for creating profiles or for doing the actual 
calibration (IE. measurements, adjustments and gamma table creation).   In 
addition, ArgyllCMS is actually a complete CM solution.  It allows you to 
profile scanners, displays and printers if you have the proper equipment.  It 
is however a set of command line utilities and is not very user friendly.

I hope this helped.

Hal