The answer to the question is anything but simple.
I’m linking again to the announcement I made in September 2024… Not much reaction.
Gamut compression
I’m also linking to the discussions we had with @Lawrence37 during the PR.
Pull Request Gamut compression
The subject is complex, but being afraid doesn’t avoid danger.
To summarize to the extreme:
The starting point is the camera’s maximum gamut, which (unless you have “inputs” at Nikon, Canon, Sony, etc.) is unknown. As a reminder, a perceived color is the matrix product of three concepts:
- illuminant (daylight, LED, etc.)
- the nature of the subject’s colors (theoretically measured with a Spectro) - independent of any illuminant
- the observer - what we saw with our phisiology (eye / brain), or here “observing from the camera”… This is what is unknown. In the version we have put online the reference cameras are those (cinema) chosen by ACES…which indirectly refer to “Maximum distance limits”, but it’s not Nikon Z6II or Canon MKIII, etc.
The second point is what do we want to do? Is the compression to be used to produce a TIF to be edited in Photoshop or Gimp, or to be viewed on screen? It’s not the same thing at all. Also, if your screen is sRGB or Rec2020 (you’re lucky) this is the profile you need to choose.
The coefficients that appear in “Threshold” depend of course on the profile you want to go to. By default they are those of ACES relative to Aces AP1, which of course will not be “perfectly” suitable if you start from Prophoto (default working profile) to go for example to DCI-P3.
@Lawrence37 and I had long debates about whether or not to adapt these coefficients according to the users’ choices. The calculations are not simple, because what to take as a reference - ACES uses Colorchecker24 which for me has a big flaw, its colors are close to sRGB, so we “extrapolate”… and what’s more, we ignore the ‘observer’ of the cameras.
The answer could be found in the documentation with “recommended” coefficients, but on the one hand, Rawpedia is “at a standstill,” to say the least, and on the other, I’m a sick old man… So creating documentation that won’t be published or in X months is problematic.
Still, it’s a very good product—not because @Lawrence37 and I ported it to RT, but the basic concept of ACEs is more than relevant.
Paul, excuse me if the answer is not compact and simple
Jacques