color management problem: bug? user error?

[Elle Stone-3]

Hi, all,

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 correctly).  

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.

Yours respectfully,
Elle

[Elle Stone-3]

elle stone wrote

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 . . . 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.

If anyone would like sample images with different color profiles and gammas, I have made four jpegs (each sized at approximately 110 kb) of the same image, each with a different color profile - sRGB with gamma 2.2 (more or less, sRGB not having an exact gamma), Kodak ProPhoto with gamma 1.8, AdobeRGB modified to have gamma 1, and AdobeRGB modified to have gamma 3.  All four images look identical on my system when viewed using the gimp and krita.  I'd be happy to email these images to anyone who would be willing to see if they all display the same on digikam/gimp/krita on their system.  If they all look the same on someone else's system, then I can start trouble-shooting my own system for user error.  Otherwise, I will file an official bug report.

Elle

[Lawrence Plug]

Hi Elle,

If you send me the test jpegs, I'll test krita/gimp/digikam
(0.9.3 and 0.9.4svn) on my ubuntu systems. I don't normally use
colour managment in digikam (yet) but would be interested to
help.

Lawrence

please email the jpegs  to ljplug at fastmail dot fm




On Fri, May 23, 2008 at 05:15:18AM -0700, elle stone wrote:

>
>
> elle stone wrote:
> >
> >
> > 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 . . .
> > 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.
> >
> >
>
> If anyone would like sample images with different color profiles and gammas,
> I have made four jpegs (each sized at approximately 110 kb) of the same
> image, each with a different color profile - sRGB with gamma 2.2 (more or
> less, sRGB not having an exact gamma), Kodak ProPhoto with gamma 1.8,
> AdobeRGB modified to have gamma 1, and AdobeRGB modified to have gamma 3.
> All four images look identical on my system when viewed using the gimp and
> krita.  I'd be happy to email these images to anyone who would be willing to
> see if they all display the same on digikam/gimp/krita on their system.  If
> they all look the same on someone else's system, then I can start
> trouble-shooting my own system for user error.  Otherwise, I will file an
> official bug report.
>
> Elle
>
> --
> View this message in context: http://www.nabble.com/color-management-problem%3A-bug--user-error--tp17410833p17424328.html
> Sent from the digikam-users mailing list archive at Nabble.com.
>
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[Elle Stone-3]

Lawrence Plug wrote

Hi Elle,

If you send me the test jpegs, I'll test krita/gimp/digikam
(0.9.3 and 0.9.4svn) on my ubuntu systems. I don't normally use
colour managment in digikam (yet) but would be interested to
help.

Lawrence

Hi Lawrence,

Thanks VERY much for offering to check out the test images.  I've sent an email to your fastmail account.

Elle