[Gimp-user] GIMP 2.10 embedded color profiles

[docrob <robin lenkeninternational co uk>]

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.

Rob



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