@walter.arrighetti: I agree with many of your points, but do need to correct your comment that:
- some color-grading systems (e.g. Flame and Nucoda) use pre-baked LUTs for every color passage (including Input/Output Transforms),
In fact, Flame, Lustre, and Maya do not use 3d-LUTs to implement ACES. Instead they use what I would call more of an exact-math implementation of the original CTLs. This allows for greater accuracy across the full floating-point range and much better inversion.
I also wanted to comment on one of the other points in this thread. The word “Look” has become somewhat of a loaded term that connotes an intentional move away from the baseline for aesthetic purposes. As such, I feel it is a bit misleading to say that the ACES Output Transforms impart a “Look”.
There were many design constraints on the development of the ACES Output Transforms but I don’t think the desire for “a look” was one of them.
Rendering transforms for cinema (ACES, motion picture film, camera renderings, …) attempt (at least partially) to do something that is not strictly possible: to recreate the illusion of an outdoor scene within the constraints of a 48 nit white point, 2000:1 contrast ratio, P3 gamut, dark surround, etc.
So any solution is going to involve trade-offs and reasonable people may disagree over which they like. Many, many approaches were tried on the road to ACES 1.0 and, in my opinion, one of the top priorities was to try and be as faithful to the original scene as possible given the constraints. For example, a number of characteristics that are found in motion picture film that many find pleasing, but which could be considered biases to the scene, are not found in ACES.
The Output Transforms represent a difficult technical challenge and no doubt improvements will be made over time as more is learned. However, as Scott, Walter, and others have pointed out, the benefits to the industry of having a standard baseline rendering far outweigh any current limitations.
I also agree with the point that the full LMT system described in the ACES specifications is not widely deployed yet and that will need to happen to meet the needs of productions to communicate the custom looks needed for particular projects across their workflows in a standardized way.