[Digikam-users] color management problem: bug? user error?
jrle1 at twcny.rr.com
Thu May 22 19:37:31 BST 2008
I hesitate to report this as a bug, because it might well be simple user
error. But when I view images in digikam, even with color management
enabled, the images don't reflect the embedded color profile's "gamma".
That is, all images are displayed as if the embedded color profile used a
gamma of 2.2 (my best guess as to the assumed gamma).
To test and confirm the problem, I created 3 versions of the same image,
using (1)"adobe rgb" with it's usual gamma 2.2; (2)"adobe rgb" but with
gamma set to 1 in the embedded color profile; and (3)"adobe rgb" but with
gamma set to 3 in the embedded color profile. I opened all three images in
Krita, Gimp, and digikam.
Krita and Gimp both display all three images exactly correctly (Gimp
thumbnails are off, to be expected if the thumbs are not generated using the
embedded profile; but when the image is opened for editing, it is displayed
Digikam displays image 1 (gamma=2.2) correctly. But Digikam displays image
2 (embedded profile has gamma 1) as VERY dark and displays image 3 (embedded
profile as gamma 3) as too light, exactly as one would expect if digikam
were assuming that all three images had embedded color profiles with gamma
set to 2.2.
I am not sure how conversant the digikam developers are regarding color
profiles. And please correct me if I am conveying misinformation. But my
understanding is that many color profiles (LUT profiles are more
complicated) are characterized by (1)"rgb primaries" (dictates the range of
colors that can be contained in the color profiles); (2)"white point"
(usually D50 or D65 - dictates the tonality dynamic range of the profile,
from 0,0,0 to the brightest possible white); and (3)"gamma." "Gamma"
dictates what "power transform" needs to take place to properly convert from
the embedded color profile to another color profile, such as the color
profile used to display the image on the screen.
It looks to me very much as if digikam is simply assuming that all embedded
color profiles are using a gamma of 2.2 (not true - Kodak prophoto, for
example, uses 1.8; afaik, all hdr software uses gamma 1 for image creation;
and so on).
Now I am a Linux newbie still trying to learn the ropes and I am constantly
making new mistakes. So very possibly I am doing something wrong in digikam
(the same problem holds in digikam 94 and 93). I would absolutely love it
if someone would please respond to my question. Perhaps I am not asking my
question about wrongly displayed images in digikam very clearly. But this
is the third time I've asked (each time investigating a little further
before asking), so I likely won't ask again. Is this a bug in digikam? Or
am I doing something wrong in digikam/linux, perhaps some configuration
setting problem or some permissions problem in my linux files.
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