Gimp Color Profile (from Rawtherapee)

Reading around internet I don’t understand which is the best output color profile to use in Rawtherapee in order to continue working on the image with Gimp.
With Gimp (2.10), on the other hand, it’s not clear to me whether it’s better to use the built-in color profile or convert to the GIMP one.
Whitch is the correct procedure?

(Sorry for my “Google Traslate-English”).

Ciao @FAb7D!
The ‘correct’ reply would be:
It depends on a lot of things…
Start by studying Elle Stone's well-behaved ICC profiles and code

It is dinner time up here, but when I am back at my main computer again I will send a better reply…

Now after dinner:
Basically — if you want to export 8-bits, use RTv4_sRGB as output profile. If you want to export 16-bits, use RTv4_Large as output profile.

Also web search for Andrew Rodney’s Color Management for Photographers.

Cordiali saluti,
Claes a Lund, Svezia

In the meantime, thanks for the very useful advice, links and the book, I will need some time to finish reading them and understand them well, but I still have a doubt Gimp side, which originated (the first time) when I read this article on Guide to High Bit Depth GIMP 2.9.2,particularly in passages like this one about space color:

“For best results when using GIMP 2.9.2, only edit sRGB images.
GIMP 2.8 has hard-coded sRGB parameters that make many editing operations produce wrong results for images that are in RGB working spaces other than sRGB. GIMP 2.9.2 still has these hard-coded sRGB parameters. Almost certainly GIMP 2.10 also will have these same hard-coded sRGB parameters.”
Is this still true (with 2.10)?

So my Pentax actualy produce file in sRGB (but I can choice Adobe RGB too). Rawtherapee work in Prophoto, it can export RTv4_Large, but Gimp seem to prefer again sRGB 32 or 16 bit floating point.

So Is better convert or to use embeded RTv4_Large while working with GIMP?

And to avoid all these profile and color space changes from the camera to Gimp, via Rawtherapee, couldn’t it be better to always stay only in sRGB 16 or 32 bit?

This is only for a jpg output from the camera…has no bearing on a raw file…

I can’t speak to Gimp’s internal routines and how they may be hard wired for sRGB… basically if you have high bit depth and small color space much of the extra data will likely just be mapped to the same color… To me it depends on what you want to do with gimp…obviously for the most fidelity you would want the most bit depth in the widest gamut but maybe you don’t need that…

I have come across this before and it explains it pretty well…

Essentially, you want to keep the image in the largest gamut practical until you make the rendition JPEG, only then do you go down to sRGB.

So, export from RT in the RTv4_Large, retain that when you open in GIMP, do the GIMP thing, then save the JPEG converted to sRGB. I just looked in my installed GIMP and the input thing is straightforward, but the export thing is not, to me. Someone more familiar with GIMP needs to tell how you how to get it to export a JPEG with a particular color profile.

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Morning, @ggbutcher!

According to my memory:
The only (?) dilemma is whether to Assign or Convert or Discard a profile inside The Gimp
— or keep it as it was when you opened it.
When saving, keep Save color profile enabled.

Didn’t Elle Stone write a nice description of this?
I’ll scout for the link … will be back…

… … … …

I’m back! Here: Convert, Assign
Hmmmm. I feel I’d better re-read all of her work.

Have fun!
Claes in Lund, Sweden

Ciao, @FAb7D!

These tutorials deal with high bit depth in The Gimp 2.10: Articles and tutorials on Color management

Actually, all of Elle Stone’s site is full of valuable information!

Cordiali Saluti,
Claes a Lund, Svezia

Thank you very much all, very kind! You have clarified several doubts providing many useful insights too with a very interesting sitography that I did not know.

Goodbye - Hejdå!

I did an experiment.
As I read here somewhere I installed the Prophoto icc taken here, I put it in the preferences of Rawtherapee and those of Gimp.
Then I exported a photo from rawtherapee in prophoto format, so opening it in Gimp I chose to keep the Prophoto profile, the photo opens and appears identical. Well, but if I then tell Gimp to convert it (or even just assign it) (Image → Color Management → Convert… or assign…) in the Prophoto profile (which it actually already has) the photo becomes dark.
The same thing also happens in other similar cases where I export to RTV4_Large and convert from Gimp to Prophoto.
Where am I wrong?

Prophoto profiles typically have linear TRCs, so that darkness would be consistent with that. Your rendition conversion needs to have a so-called “gamma TRC” for most displays.

Files like TIFF, JPG, PNG encode pixels as collections of numbers. One popular encoding records the values of the red, green and blue (RGB) components. However, without knowing how to interpret the numbers, you will be unable to view (or process) the image. That is what colour spaces are for: RGB colour spaces have different notions of what ‘pure red’, ‘pure green’ and ‘pure blue’ (the primary colours) are; they may also differ on how the numbers encode energy (‘brightness’, ‘amount’).

Think of this as temperature. If I say, the temperature is 32, you won’t know if I’m in a cold or hot environment. If I tell you it’s in °F or °C, then you can interpret the number.

In Gimp, when you open an image, the software will check if it has an embedded profile (information to tell you how to interpret the value). If not, the standard is to assume it’s sRGB.
Then, Gimp will ask you whether to keep the profile (don’t change the numbers, and keep in mind they have to be interpreted using that profile), or convert to the built-in profile (convert the numbers, and remember to interpret them according to the built-in rules).

Later, when you assign a profile, it means keeping the numbers, and changing the interpretation. You would normally not do that. It can be useful if you work with low-level tools that don’t specify a colour space, or make experiments. ‘My thermometer reported a reading of 32; please interpret it as °C, even if you so far assumed it was K or °F.’

When you convert to a profile, it means ‘change the numbers so that they represent the same colour as they do now (as far as possible), and remember this interpretation’. So, ‘take my 32 °C, convert it into 89.6 °F, and remember that, from now on, the readings are in °F’.
(Because of the different red, green and blue primaries, some colour spaces are more limited in the colours that can be mixed from their primaries than others. sRGB is fairly limited, AdobeRGB, ProPhoto, Rec 2020 are less so. That’s why I added ‘as far as possible’ above.)

The conversions are not always as straight-forward, since some spaces encode values in a linear fashion (200, 200, 200 represents twice as much energy as 100, 100, 100), while others do not (with the standard 8-bit sRGB encoding, 124, 124, 124 would represent twice as much energy as 89, 89, 89).

So, if you just assign a profile, the interpretations can change drastically: brightness, as well as colours, can change.


Thank you very much! As usual helpful and clear explanations.
So, in my case the conversion was from Prophoto to Prophoto and in theory nothing should have changed. Instead it gets darker, so it means that “change the number”.
With exiftool I haven’t seen any other differences.
So I searched better for my mistake and finally found it! (…not that there is anything to celebrate … :roll_eyes:).
Screenshot from 2023-02-19 09:50:26
I’ve to chose like rendering intent “Absolute colorimetric” not “Relative colorimetric” (as “usaly” I did), which makes sense since the two color spaces are the same and I guess even in the case they were similar (like RTv2_Large and Prophoto), if I had switched to sRGB, instead (I suppose) I would have had to use “Relative colorimetric”.

@kofa That Fahrenheit/Centigrade analogy was a good one!

Have fun!
Claes in Lund, Sweden