About issues and terminology

Its in line with the definition of Dominant Wavelength (17-23-062 | CIE), but I confess it is a bit of a mouthful and isn’t as intuitive for colors along the line of purples.

It could also potentially be described as a “purity” transform (17-23-066 | CIE), e.g. “at higher luminances we reduce the (colorimetric excitation) purity”…

But this really is bikeshedding…

With regard to the vanilla default rendering transform, I’m of two minds about this. Yes it is good to keep the creative choices “choices”, but what aspect of fixing a blue light from turning magenta for example is “creative”? Hue linearity is an admirable goal, but the more critical aspect is generally “hue category constancy” to me. That is to say, objects should generally retain their hue category (red stays red-ish, blue stays blue-ish) but small deviations are generally acceptable, and even welcomed in some scenarios (pull in the red->orange fire debate here*). To completely cast aside a humans perception of hue or hue category constancy is kicking the can of perceptual corrections down the road to artists.

I would be curious to see a middle road. One in which we don’t create the umpteenth color appearance model, nor force the hand of artists to try and make their own.

If this is produced by a LMT + RRT combo, that’s fine too, I’m only talking about the “fall off the truck” version.

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I think that the group is referring to the underlying model on top of which you build the path-to-white. I’m hoping that it is well understood that desaturation will affect chromaticities as it is its job in the first place. It is certainly what I’m alluding to when I talk about chromaticity-preserving in this context. The lines can be straight in chromaticity space because some of the models are effectively chromaticity preserving at their core, separating chrominance from luminance.

White-light-mixing does not really tell much about the path taken by the chromaticities when colours are made achromatic, I much prefer dominant-wavelength-preserving here.

To be complete and pedantic, we should probably say something along the lines of a chromaticty-preserving based model with a dominant-wavelength-preserving chrominance-reduction-transform or something along those lines :).

We agree that this has nothing to do with perceptual issues, and everything to do with channel clipping?

Agree. Hence why chromaticity linear results in the most “patterned and predictable” behavior for the fundamental mechanic component?

See also “creative flourish”; something exterior / post of the fundamental?

I am not suggesting having a proper perceptual correction negotiation as a punt, but rather discussing the formalities of position of the flourish.

Specifically, if we think in terms of film emulsion-like terms, we calculate how much we need to correct the dechroma, based on the open domain light data. Once we have the corrections for the tonality, we can easily, as already demonstrated by Jed, evaluate the two linear light domains (open domain, pre image formation, and closed smaller domain, post image formation) and provide any hue constancy via whatever model desired.

That Abney-corrective aesthetic flourish could be selectively applied, as required; it is a perceptual negotiation between two radically different light transport domains, in accordance with simulated perceptual models.

This also would nicely firewall the need to say, despill / key pull / “hue grade” etc.

I believe firewalling as above allows for flexibility to swap out as newer models or developments happen.

Except it is in a stimulus chromaticity space. Imagine something further built atop of some idea of “wavelength” and then this moves into 2021 and everyone uses 2006 instead of the less than optimal 1931. Now the wavelengths have potentially changed.

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Can you clarify what you mean by this? Are you saying the Abney effect is a matter of aesthetics, rather than a matter of human perception of color?

Also isn’t the Abney effect currently addressed in Jed’s OT via the OKlab color model so that the path-to-white maintains “hue category constancy” as opposed to (for example) blue in CIELAB where blue appears to go into magenta in its path-to-white?

Nice interactive visualization of this here:

I’m saying that not every creative decision will want this potential flourish, and as such, it should not be considered a default flourish. Further still, if folks are using image referred tooling, all of the things like keying, despilling, “hue” grading selection / manipulation, albedo calculation evaluations etc. will all become vastly more challenging with this as a default, and as such, the flourish should be considered a post-image formed state flourish, where access to the underlying non-perceptually warped variant may be desirable.

Would help it if folks were to analyze why this “white” thing exists in the first place, and what the fundamental mechanic is behind it “working”. I still haven’t seen anyone appear to vocalize what it does, and as such, it would seem that there’s no mechanic driving the code to dechroma the light mixtures.

Thanks Sean, that helps tremendously.

I don’t really understand the “except” part here nor the relevance of the Standard Observer change. If we swap the Observer, everything will change as it defines the foundations, i.e. the basis, on top of which pretty much everything we do is built.

Because the line traced is between two chromaticities, not a wavelength?

If we were tracing a line on a longitudinal / latitude map, we wouldn’t turn around and say “city dominant linear” or such?

Seems to be an odd way to describe the plotting, and one that will likely have too broad of an overstep in the near future.

Well, you can certainly describe something by what it is or its effect/quality. We are not saying that the path-to-white is implemented by tracing lines between dominant wavelengths and the whitepoint, instead we are saying that one of its quality is that it preserves the dominant wavelength of any chromaticity coordinates it affects. I honestly cannot think about a better concise way to describe it using colorimetry terminology.

See how you had to use “chromaticity” to define what is happening? That strikes at the root of the issue with layering “wavelength”, which is completely unrelated, into a term. If a descriptive component becomes superfluous, because it is unrelated, it is likely a poor choice.

Also, there is a mechanic there. “Path” doesn’t do the mechanic justice I reckon.

Speaking purely from an artist’s perspective, I find “wavelength” to be confusing/unhelpful as a descriptor of what the intended goal is. That is, it does not communicate meaningfully to the non-scientist visual artist end-user.

I also find Chromaticity-preserving to be confusing as I understand chroma to mean colorfulness or saturation, being the opposite of achromatic. To speak of a “chromaticity- preserving path to achromatic white” thus sounds inherently contradictory.

If we are wanting to describe "“hue category constancy” in the path-to-white so that blue stays blue and orange stays orange to our eyes, I’d vote for hue-preserving, possibly with the modifier of “perceived hue-preserving” or “hue appearance preserving.”

My 2 cents. Worth every penny! :slight_smile:


I only used chromaticity because the context was using it, I could have used colour and a domain expert or colour scientist would have understood equally well.

My take on this is that if the artist is willing to understand a domain he is new to, he must learn its terminology and definitions. We are not at a point where we are talking about surfacing an interface or documentation to an artist so we should not refrain from using terminology that describes precisely the behaviour of the algorithms.

As mentioned earlier, the chromaticity-preserving part is not related to the chrominance reduction, it only implies that the luminance mapping will not affect the chromaticities of the input colours.

Hue-preserving is certainly appropriate, been seen and used a few times (I certainly do). It is mostly less precise because hue, in its definition, implies appearance similarity, not a colorimetric match which is an important difference: the former is tied to colour appearance, i.e. advanced colorimetry, and the latter is about basic colorimetry.

For aesthetics reasons, we actually might want to modify them, so we will have to amend the vocabulary anyway.

@Thomas_Mansencal that all sounds quite reasonable. I’m inclined to agree on all points. Thanks for the explanations!

Oh, I had thought the goal was to preserve appearance similarity… color appearance model stuff. Is that not the case?

Could you define aesthetic? By “aesthetic” do you mean “for sake of human perception of color” i.e. the way humans perceive color? Or do you mean “aesthetic” in terms of “personal preference and taste in visual art”?

Excitation Purity perhaps.

This is an excellent question! Let’s start here:

For proper colour appearance, the colorimetry of the displayed image must always be altered from that of the scene or the original reproduction.

The implication is that, discounting a path-to-white transform, imagery chromaticities must be changed.

Maintaining colour appearance implies that the SDR imagery appears the same on a HDR display or conversely. While this might be desirable, it is not a requirement and TV makers would argue that this is actually highly non-desirable: Why buying a HDR TV if the image appears the same than on a SDR TV? Why not, for example, let the higher luminance levels naturally increase image colourfulness, i.e. Hunt Effect, so that content appears more colourful and intense? You will find people in both camps and some in-between, at which point it is about aesthetics and an exercise in subjectivity.



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Thanks Thomas, that’s super helpful!

Yes thnx, I got my wires crossed above, this is the more pertinent form of “purity”.

Speaking of issues and terminology, could someone define the term “flare” and “diffuse-flare”?

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