[Digikam-users] Color managed view too bright?

[Milan Knížek]

Torsten Crass píše v Pá 11. 04. 2014 v 00:55 +0200:
> Hi there,
> 
> in spite of spending days and weeks on reading on various aspects of 
> color management, I still don't quite get the meaning of the "color 
> managed view" checkboxes in DigiKams CM settings. What puzzles me in 

It means that digiKam will apply on-the-fly conversion of the image data
from its embedded colour profile [1] to the colour space of your monitor
[2].

[1] If there is no profile embedded in the image, digiKam may use either
the default "working colour space" profile or the "default input colour
profile" as set in "Profiles" tab. I am not sure, since all my images
have some profile embedded.

[2] If the drop-down list of monitor profiles in "Profiles" tab is
greyed out and nothing can be chosen, then it means that some profile is
loaded into X atom by gnome color manager, dispcalgui, xicc or whatever
tool you have installed.

> particular is the following: I'm using a profiled (using a ColorHug 
> device) and calibrated (using xcalib with the derived profile) monitor. 

Hm, which system do you use? For KDE and GNOME there are built in tools,
which take care for both profiling with ColorHug and then also for
setting up the calibration and loading the profile to X atom.

Both major DEs should be able to use colord as system daemon and have
some "Colors" icon in their settings panel/centre.

In other words, using xcalib (and xicc and other CLI tools) should not
be neccessary unless you know what you are doing.

If you have more profiling tools installed (dispcalGUI + colord + gnome
color manager + kde color bla bla) you may not really know, which
profile actually gets loaded and setup after login. Check your
"autostart" apps in your profile and in /etc/xdg/autostart to see what's
run on login.

> When viewing a pure black image (generated with imagemagick) in DigiKam 
> and activating color managed view, the image is displayed -- brighter! 

It is difficult to help here since we cannot se what you see and cannot
check your system properly.

Do some further analysis yourself:

x does the image created by ImageMagick have any profile embedded (I
doubt so). You should assign it some profile.

x how does it look like in GIMP (choose manually both display and image
profiles to make sure)?

x is it displayed in digiKam the same in Image Preview and in the
editor?

x does the black point compensation (tab "Advanced" in digiKam) have any
effect?

x how about converting the image to the display colour space manually?
E.g. "convert my_image.jpg -intent Relative -profile /path/to/sRGB.icm
-profile /path/to/display.icc my_image_in_display_space.jpg"
and "display my_image_in_display_space.jpg"

> Well, admittedly as a *very* dark gray, but definitely brighter than 
> with color managed view deactivated. So how can that possibly be? I 
> would have thought that sRGB black should stay black, even when 
> converted into the monitor's gamut! I mean, the monitor obviously is 
> capable of displaying darker colors!

I am really not sure here: in general, ICC standard handles white point
only and it is up to colour engine (lcms, adobecms) to decide on what to
do with the black point, it may also vary with intent, though I guess
simple profiles offer relative colorimetric intent only.

My best guess in your case is that ColorHug does not think that your
display can manage such blacks and puts the "black" a bit higher on the
gray scale than neccessary.

Might be usefull to provide us with the display profile.

Regards,
Milan