About issues and terminology

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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