Thank you so much for clarifying those issues. Let me see if I understand regarding the practical side of this. You wrote,
Until that happy day when GIMP finally allows to embed the built-in sRGB profile, here’s how I handle the situation: If, for example, I export an sRGB file from darktable, it will be at 32f precision, so I could do an actual ICC profile conversion from the darktable sRGB profile to GIMP’s sRGB profile. But because the two profiles are functionally “the same profile”, there’s no reason to do an actual ICC profile conversion, all that’s really needed is to assign GIMP’s built-in sRGB profile - again the two profiles are functional equivalents. Then I edit the image, and when ready to export, I assign whatever “on disk” version of sRGB that seems appropriate, usually my own V2 sRGB profile for uploading to the web, or else the ArgyllCMS sRGB.icm profile - both are “free/libre” profiles. If I want to continue editing the XCF file, then after exporting I reassign the built-in sRGB profile.
So the correct recipe is:
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Assign the GIMP profile to an existing file before opening it in GIMP, to avoid any conversion weirdness.
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Open in GIMP, edit it, and save it.
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At this point the GIMP profile is still assigned, so now assign a new one, say the ArgyllCMS one.
Since the GIMP profile is a functional match to the ArgyllCMS profile, which is apparently (hopefully?) a functional match to the sRGB IEC61966-2.1 profile used by Lightroom/my OS, this should produce “technically correct” results.
Do I have this right? And if I do, how do you recommend performing the assignment of the profiles in steps 1 and 3?