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Colour management issues in GIMP/Darktable

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Colour management issues in GIMP/Darktable Robin Simmons 23 Jan 10:37
Robin Simmons
2019-01-23 10:37:02 UTC (over 5 years ago)

Colour management issues in GIMP/Darktable

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.