I will try to get to the bottom of this, but if anyone has any ideas in the meantime, I would very much appreciate it! There is clearly a difference between using Darktable stand-alone or invoking it via GIMP. So the spotlight shifts back to Darktable rather than the image viewers. but it makes no difference if I change it to "sRGB (web-safe)" before closing Darktable. In this case the Darktable export profile defaults to "linear Rec709 RGB". NEF file via GIMP and in turn Darktable, then export by closing Darktable that the Colour Space tags change and all the information in my original post applies. The thumbnails in both viewers are happy and reflect the appearance of the full-sized image. I can then load the exported JPEG file into GIMP and the tags remain the same (although these will be subject to the GIMP settings). Both Colour Space tags are now "sRGB" and all is well. Changing this profile to "sRGB (web-safe)" makes no difference whatever to the exported JPEG. NEF file directly in Darktable and export it from Darktable, the default profile is "image settings", which seems to set an sRGB profile. NEF) is opened via GIMP, which in turn invokes Darktable. The problem only occurs when the RAW file (which I should have stated was a Nikon. I have now done some more tests now and the results are interesting. I would be very grateful for any explanation and hopefully a fix for this problem. However, apart from the irritation of having very dark thumbnails displayed in the viewer I am concerned about the two conflicting EXIF tags and wonder whether this might cause a problem where the images are displayed on another computer, e.g. I am baffled by this behaviour, which is admittedly more of a nuisance than a serious problem, as each image displayed singly is fine and subsequent processing to print works perfectly. So it looks as though Darktable is doing something to the output file which affects one of the two EXIF Color Space tags and the appearance of the thumbnails in a viewer the latter can be rectified via GIMP. However, if I take the exported JPEG derived from the RAW file via Darktable and re-load it into GIMP, then add "Assign Color Profile", the thumbnails in Gwenview (or DigiKam) display correctly, but the two Color Space EXIF tags remain as "Uncalibrated" and "sRGB". If before exporting from GIMP, I deliberately click Image -> Color Management -> Assign Color Profile it makes no difference to the situation described above. Exactly the same behaviour occurs with DigiKam as the viewer. Subsequent processing in GIMP does not affect the EXIF tags or the thumbnail display in Gwenview. If I take a JPEG straight from the camera, both these EXIF tags show "sRGB" and the thumbnails display correctly. If I look at the EXIF data in Gwenview I find there are not one, but two EXIF tags titled "Color Space" - one shows "Uncalibrated", the other "sRGB". Here again the full-sized image is displayed as expected, but the thumbnails are much darker. The colours appear exactly as expected during the processing and the completed image is generally exported as a JPEG. In GIMP, Colour Management is enabled and the preferred RGB profile set to GIMP built-in sRGB. Most of my work is done in sRGB colour space so Darktable's output profile is set to sRGB (web-safe). I work with RAW (NEF) files and use Darktable for RAW conversion. I am using Darktable 2.4.4 and GIMP 2.10.8 on Linux and I have a problem with colour management.
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