This is why its an option. But it doesnt make sense to have it off by default. It never did.
Afaik all common pathtracers have it on by default except cycles.
Yeah youâre right. Are there even any Situations where you wouldnât want random Seed except from Denoising? Maybe it could have a third option âautoâ that is the default and depends on wether Denoising is checked or not.
@cgslav
None of those features require the entire renderer to be spectral. They can be done on per-BSDF basis.
@numen22
I am fairly sure Octane is not completely spectral. It, at least by default, doesnât to that characteristic RGB colored noise. As for all the other commercial spectral renderers youâve named; pretty much all of them are irrelevant and half of them are dead. Same goes for the FOSS ones.
I know this is only Archviz related, but itâs still a good pool of data that will likely be at least somewhat similar in other CG rendering fields too: http://www.cgarchitect.com/2018/02/2018-architectural-visualization-rendering-engine-survey
I only can remember one shot i worked on where i used a static seed and this was with a static camera. But for 99% of the shots i use an animated seed.
Oh, my mistake, for some reason I thought you were talking about just having the button to make render seed random every frame (because it didnât used to exist some time ago).
Also, mineâs animated by default, didnât realize default has it disabled.
Doing thin film in RGB is possible but again, isnât as accurate as doing it spectrally. Most of the time it is good enough, but for some applications, it is better off to be done properly.
From what Iâve read it does do its rendering everything spectrally. It can manage to remove most colour noise by using hero sampling, which resolves chromatic noise much faster, and generally resolves much quicker than a naive single wavelength approach.
What information from there is relevant here?
That almost no spectral renderers are relevant in mainstream market, at least in the visualization field 
Well, thatâs just an opinion, so letâs move on with evolving and striving to give a helping hand or try and be quiet AKA non-intrusive and non-persuasive 
How is that an opinion? Thatâs actual hard data right there, in that link.
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I do not understand much about technical issues. But if problem is basically performance, why could not it be optional to use it only when it is useful? Choosing spectral mode from Render tab settings, different kernel, I do not know.
By sole definition relevance has biased, a purely subjective value. It comes down to the task needed to be performed.
Remember, Fish cannot be judged by how fast it can climb a tree.
The whole humanity & along the industry, strive towards complexity to perform so called seeming simplicity. The valley is vast and pioneers most welcome.
yes, but that bias is and of itself quantifiable by the ebbs and flows of the industry. If no one is using it, no one needs it.
If nothing else, same was happening with PTs decades ago⌠from Marcos Fajardo (Arnold)
â⌠but we made it and now they use it.â
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Multispectral rendering also opens up a can of worms.
How many spectral textures do you have in your asset library? To really make use of this enhanced realism, you need to step up the complexity of the assets that you are using.
If RGB is good enough for all of our textures, and the output files themselves, and every monitor or printed page our renders end up on, what difference is spectral rendering going to make?
Outside of some pretty strained example cases, the benefits seem pretty slender.
Of course when we all have quantum computers embedded in our eyeballs, we can throw in all of the advanced techniques our hearts desire, but for now, Iâm not convinced of the real utility.
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Would a spectral part in Cycles allow better caustics ? (I think I red that somewhere but I am not sure 
)
Not much, itâs still a path tracer. There are some optimizations, but really, if you want usable caustics, you need bidirectional tracing.
Can you show me some examples? Please just donât show me OSL method or the ones that are present on BA forum (I was even active in those topics), also not faking those effects.
I just made a list and had no intention to value the usefulness or relevance. That is something everyone has to decide for himself with the specific project at hand. That the main archviz renderer are not spectral and have no intention to becoming it is evident. However, in some cases people want to have the option of spectral transport e.g. when you want to simulate thin Gold coatings on a perfume flacon depending on layer thickness or when simulating exact light extinction and dispersion in liquids from measured data.
Should Cycles become a spectral renderer, my personal opinion is no, but I commend the work of @smilebags to give it a try. Cycles does well following the path of Arnold with an emphasis on animation and corresponding feature set.
Should Cycles get bidirectional pathtracing or secondary GI caching to make it more suitable to the archviz community? This is for the community to decide and my personal opinion is not really relevant to this.
Whatâs wrong with OSL? If the result is visually indistinguishable from the one rendered by spectral renderer, does it really matter then? As for other effects, glass dispersion in V-Ray and Corona renderer is a good example, itâs the spectral effect yet the renderers themselves are not.
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I donât disagree. If you go up a few pages in this very topic, you will see my post where I explicitly said I hope @smilebags will be successful with his efforts and prove me wrong. If the performance hit would be negligible, then thereâd be absolutely nothing wrong about Cycles being spectral.
Also, regarding your last paragraph, I would not put secondary GI caching and bidir path tracing in the same bag. While secondary GI caching would speed up interior cases tremendously, bidirectional in most cases slower than pure unidirectional path tracing. Itâs just better at rendering caustics, at the cost of rendering pretty much everything else slower. One would make rendering much faster than it is, while other one would most likely make it slower than it is now, if weâre talking Cycles.