Problem maintaining correct colour profiles
I am running GIMP 2.10.8 in Linux, with Darktable as the preferred RAW converter. I have been struggling with a problem of controlling the colour profile of images processed in GIMP and exported as sRGB JPEG files. There seem to be two distinct issues:
- Setting the Export Policy to Export the image's colour profile by default is not effective. Unless a profile is specifically assigned for each file in Image -> Color Management -> Assign (or Convert) Color Profile before exporting, the colour profile is not embedded in the image file.
- No matter which profile is assigned to the image, there is a problem with the exported JPEG file as described below.
If I export a JPEG file directly from Darktable in sRGB colour space, without using GIMP (i.e. not using the plug-in), the resulting file behaves perfectly and gives no problems. If I take that same file and load it into GIMP, I cannot find any setting which will produce a satisfactory result.
Files exported from GIMP as JPEGs invariably gave rise to errors when on a USB flash drive plugged into a top of the range Canon projector last weekend, much to my embarrassment. Another characteristic of these files is that when viewed in Gwenview (or DigiKam), the full-screen images look as they should do but the thumbnails are much darker. I have no means of correlating these two effects in further tests as I do not own the projector and have no access to it. However, I have done tests as listed below, which I hope illustrate the issues.
GIMP Settings: Monitor profile (Preferences): "None" Rendering intent: Perceptual File Open behaviour: "Keep embedded profile" Export Policies set to Export the image's colour profile by default. Test file is JPEG as exported from Darktable in sRGB
Test 1. Take exported file directly from Darktable without using GIMP
Gwenview thumbnail and full-size image are OK. Canon projector displays without error.
identify -verbose | grep "icc" reports: icc:copyright: Public Domain icc:description: sRGB icc:manufacturer: Darktable icc:model: sRGB Profile-icc: 8940 bytes
Test 2. Load file into GIMP: Image Properties reports profile is GIMP built-in Linear sRGB
Convert to GIMP built-in sRGB and export to JPEG: GIMP image is OK, Gwenview thumbnail is dark but full-size is OK. Canon projector gives error.
identify -verbose | grep "icc" reports: icc:copyright: Public Domain icc:description: GIMP built-in sRGB icc:manufacturer: GIMP icc:model: sRGB Profile-icc: 672 bytes
Test 3. Load file into GIMP: Image Properties reports profile is GIMP built-in Linear sRGB
Convert to sRGB (Darktable icc profile from disc) and export to JPEG: GIMP image is OK, Gwenview thumbnail is dark but full-size is OK. Canon projector gives error.
identify -verbose | grep "icc" reports: icc:copyright: Public Domain icc:description: sRGB icc:manufacturer: Darktable icc:model: sRGB Profile-icc: 8940 bytes
New GIMP settings As above, but File Open behaviour: "Convert to preferred RGB color profile". Preferred RGB profile: GIMP built-in sRGB
Test 4. Load file into GIMP: Image Properties still reports profile is GIMP built-in Linear sRGB - no conversion has apparently taken place.
Convert to GIMP built-in sRGB and export to JPEG: GIMP image is OK, Gwenview thumbnail is dark but full-size is OK.
identify -verbose | grep "icc" reports: icc:copyright: Public Domain icc:description: GIMP built-in sRGB icc:manufacturer: GIMP icc:model: sRGB Profile-icc: 672 bytes
Test 5. Load file into GIMP: Image Properties still reports profile is GIMP built-in Linear sRGB - no conversion has taken place.
Export to JPEG with no changes, in particular no specific assignment/conversion of colour profile: GIMP image is OK, Gwenview thumbnail is dark but full-size is OK. identify -verbose | grep "icc" reports no embedded profile.
I hope the above describes these two issues with sufficient clarity and I would be very grateful for information on a fix or a work-around.y