Ping @elle @Carmelo_DrRaw @ggbutcher
Have fun!
Claes in Lund, Sweden
I’ve always used darktable’s softproofing. Or loading the desired profile in RawTherapee, which for me is AdobeRBG.
Together with @Elle and few others, we have put quite some efforts in the soft-proofing code in PhotoFlow.
Here is an example of the interface. If you are interested, I can provide you lots of additional details 
Hi @Claes
Depending on the source and destination ICC profiles, you might or might not get the same and equally useful results from darktable, RawTherapee, GIMP, PhotoFlow, etc. The problem is knowing which source and destination profiles actually do work with default LCMS soft proofing.
PhotoFlow soft proofing code is designed to work around the problems with default LCMS soft proofing. So even if you decide you prefer to use darktable or GIMP or RawTherapee or whatever for soft proofing, it’s a good idea to compare results wtih PhotoFlow results, and if you see any discrepancies, perhaps report them here to be sorted out.
In rawproc, I have a very basic implementation committed in the github repository past the last release. I still don’t fully understand the concept, so I just declared a few properties and stuck the logic in the display panel, so the input profile is the one assigned to the working image specified for display, the proofing profile is specified in a property, and the output profile is the display profile. If a proofing profile is specified in the relevant property, the LittleCMS createProofingTransform is used for display rather than the default createTransform.
You can toggle the LCMS gamut check with a property. Yet to be incorporated is a property to set the gamut check colors. Just a straightforward insertion of the LCMS tool…
Just for information, the latest RT (definitely in the current dev, maybe already in 5.4? – I actually don’t remember) uses the code “stolen” from PhotoFlow for softproofing (and actually, I think there was one bug in it that got fixed in the process: New RT Out Of Gamut function in RT5.4 is too sensitive · Issue #4438 · Beep6581/RawTherapee · GitHub )
Feedback on the accuracy/usefulness/usability of this is always welcome! 
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Awesome that RawTherappe incorporated that code!
Even when I posted my initial comment I had a feeling I wasn’t being clear. But I didn’t mean to imply that any discrepancies would mean “PhotoFlow is right, the other software is wrong” - rather I wanted to emphasize that user feedback regarding different soft proofing results in different image editors is critically important for making sure that everyone’s preferred image editing software is working properly when doing soft proofing. As an example, recently I found an issue in the PhotoFlow code (that @Carmelo_DrRaw already fixed), while comparing results with GIMP soft proofing - comparisons are good! feedback is even better!
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