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List:       gimp-user
Subject:    [Gimp-user] Colour management issues in GIMP/Darktable
From:       Robin Simmons <robin () docrobin ! co ! uk>
Date:       2019-01-23 10:37:02
Message-ID: f43f6c74-22ee-b86f-a9f6-84fb895f92a5 () docrobin ! co ! uk
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I am new to this group, but was referred to it after posting a description of the \
problem below to the Darktable forum.  I now believe this to be a GIMP issue, but I \
cannot be sure whether the behaviour described is effectively a bug in GIMP or \
whether I am missing something.

This issue refers to GIMP 2.10.8 and Darktable 2.4.4, running in Linux (KDE).

When using GIMP's Darktable plugin, I think I am right in saying that that closing \
Darktable always imports the image into GIMP in the form of a floating point EXR file \
with profile linear Rec709 RGB. This is logical since it is desirable to work with \
floating point precision in GIMP 2.10.

In most cases the desired final output from GIMP is a JPEG with sRGB profile but I \
can find no settings which give me confidence that this is working properly. In my \
case my starting point is Nikon NEF files. The Colorspace information for the image \
that Darktable exports shows up in Image -> Metadata -> View Metadata as: \
Exif.Nikon3.ColorSpace    sRGB Exif.Photo.ColorSpace      Uncalibrated

I have been advised that some images that I sent out as JPEG files did not contain an \
embedded profile, although GIMP's Image Properties insist that they have the GIMP \
built-in sRGB. To add to the doubt, the Gwenview image viewer, while it shows the \
full-sized image correctly, displays the thumbnails much too dark.

I have found the only way to show the thumbnail correctly in Gwenview is to reload \
the JPEG into GIMP, ensuring that the option to promote imported images to floating \
point precision is unchecked, specifically assign the GIMP built-in sRGB profile, \
then re-export as a JPEG. Curiously, this does not change either of the two \
ColorSpace tags above.

This is obviously unsatisfactory in itself, but owing to the Exif.Photo.ColorSpace \
tag remaining as Uncalibrated, I cannot be sure the image has the sRGB profile \
embedded. By contrast, if I load a JPEG file from the camera into GIMP, both tags are \
set to sRGB and there is no problem with thumbnails.

I would be grateful if someone could confirm whether the above behaviour is expected \
and if so, what would be a suitable work-around.

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