Aesthetic intent - human perception vs filmic reproduction

[ChrisBrejon (Christophe Brejon)]

First of all, thank you for having this conversation. ACEScentral is exactly meant for this kind of exchange, as a sharing community.

@garrett.strudler

By HVS as the goal I meant maintaining the captured scene (to the extent possible) as opposed to injecting some kind of filmic response to the data.

This is exactly what we are trying to do here, going from scene to display in the most faithful way. If a colour cannot be displayed (because of limitations), let’s try to pick the closest one.

@garrett.strudler

The whole concept of “scene-referred” data is to capture photometrically exactly what the scene was, correct? If you had a theoretical display that could reproduce all of the captured values then you would have a virtual copy of the actual scene that would look identical to actually being there (3D/spatial issues aside).

We obviously don’t have any such displays, but what I’m getting at is this: are we are trying to faithfully reproduce the scene itself, or are we trying to reproduce the scene as film would have captured (and/or displayed) it?

We are trying to reproduce faithfully a scene by taking in account the display limitations. The effectiveness of this method will mostly rely on the quality of the gamut compression. Lars Borg explained that during meeting#5 if I recall correcly.

@garrett.strudler

I fully understand that you have to apply some kind of tonescale to map to the display, but how are you determining the effectiveness or success of that operation? You have to have a target to say “that looks correct” or “expected”, but what is that target?

What is tonescale ? Let’s have a look at two mac beth charts. Which one looks closer to the actual data scene ?

@garrett.strudler

Currently it is “this is how a film camera would reproduce the image” (or I suppose “this is how photographic reproduction has looked in the past”), but I’m questioning if that is the correct target. We’re working from a preconceived notion of how a visual representation should look based on a (flawed) chemical process; it is not wholly representative of the physical scene that it captured.

Is this really what we are doing here ? There was a beautiful reaction by Alex Fry about Jed’s experiment :

@alexfry

I feel the frame of the CG woman’s face looks far less ‘cinematic’ through the ‘naive DRT’, yet provides a clearer view of what’s actually going on with the asset. More true to ‘the eye’ than ‘the camera’.

I am not sure that Jed’s experiment can be qualified at reproducing a flawed chemical process but rather acknowledging that we are dealing with gamut volumes. The core question in my opinion.

@garrett.strudler

Respectfully I disagree to an extent. Why expend the effort on a solution to something that doesn’t need to be solved?

I am pretty sure that everyone would agree that “something” needs to be solved. It is the core reason of this group.

@garrett.strudler

With a single chromaticity and no relationships to maintain, presumably it would just map to display max (255,0,0).

Now of course what happens when you add different chromaticities at different luminance ranges both within and beyond the display range? If you are attempting to keep scene “intent”, then in my mind it should be chromaticity-preserving with compression of the luminance only. Because scene luminance is potentially unlimited (especially if computer generated), you would have to specify an upper threshold/clamp to keep a realistic ratio of the values under the clamp, which also gives you a ceiling to map all values equally. Looking at the “red christmas” example from Chris’s post it appears this is more or less what the current ACES transform is doing: it is trying to respect the chromaticity of the scene-referred data. However, it is clamping too quickly and just clipping a lot of potentially useful data.

The “naive drt” and other transforms like IPP2 instead of being chromaticity-preserving follow a “path to white” where high luminance colors instead “burn out” to white to mimic what happens with film. It’s generally a pleasing look and what we are all used to seeing in film, but it’s also not realistic.

If I may, there are so many expressions in your answer that it is hard to follow :

• what is chromaticity-preserving ?
• what is filmic ?
• what is realistic ?
• what is tonescale ?

1. How could an output transform deal only with compression of the luminance ? We need to go to from one volume to another volume, right ?

2. How could the current ACES Output Transform be chromaticity-preserving if an overexposed blue goes purple ?

Animated gif from Nick :

Per-channel lookup is not chromaticity-preserving. Or hue-preserving ? At this point I am not sure which is which. We have done some plots of the ACES Output Transform :

I don’t know what realistic is but I do know that displays have limitations and that we cannot display 120% of red emission… I don’t think we are trying to mimic anything yet but rather take in account gamut volumes. And this issue is so complex (for me at least) that we need to break down into simple questions, just like Troy did :

@Troy_James_Sobotka

Ignore film. Ignore semantics. Focus on the simple example.

I am more than happy to follow this conversation and provide examples if needed. Thanks Garrett !

PS : About the example you provided, it is great to show images. It makes the conversation much easier. But I am not sure what your point is by decreasing the exposure of 2 stops ? Do not hesitate to put as much explanation as you can when you provide an image so it is obvious to others when comparing.

Update : You see in your example the bulbs have reached the peak display white (they are not red hard clipped) with their faces being red. Hence the question :

@Troy_James_Sobotka

Imagine we have a BT.709 red primary. It goes to some very high emission at the camera / render that exceeds the display range. A simple, single chromaticity, at some intensity.

Explain what to do? Any ideas?

Regards,
Chris