Mod: this thread has been extracted from the V6 daily builds discussion thread - https://forum.corona-renderer.com/ Corona Renderer 6 for 3ds Max - Daily Builds Discussion
just imagine how scenes will look now that the plastic feel of the materials will be gone very SOON forever
Why do you think so? There is fundamentally nothing wrong with current GGX behavior, it’s been fixed long time ago. The different energy conservation loss illustrated above only applies to metals.
The PBR will bring new conveniences like Metalness workflow, coating,etc.. and hopefully Sheen for fabrics, but otherwise all materials will look exactly the same.
That will 90perc. be related to GI (particularly clamping) and Tonemapping.
But also… F-Storm doesn’t have correct glossiness curve, in fact it’s wrong. It can make nice looking materials, but the are absolutely not PBR :- ). Ask Johannes or Daniel, they would love to change this.
So if you’re asking for even more tweaked GGX behavior, it will look just slightly bit more advanced than what Corona currently has (maybe 5perc. difference?) but absolutely not like F-Storm material.
Corona’s PBR will bring different UI and parameters. It will make a creation of materials easier and more like current industry standard.
They will look absolutely the same though (with exception of Sheen parameter for fabrics). Unless they change fundamentally something under the hood for BRDF, which they could…like Lambert Diffuse is getting pretty old.
Everyone would :- ) I asked for both almost 4 years ago.
But some people expect miracles…and there won’t be any miracles.
I’m curious to see the impact of multiple scattering in interiors tho! Not sure this would have a tremendous impact but we can’t conclude anything without testing it.
What makes you think that? It applies to every single material actually. Current BRDF only account for a single bounce of light. With High roughness, this causes a significant energy loss (up to 60%). This is particularly visible on metals because their reflectance is full specular but it applies to everything. It’s also pretty significant in rough transmissive materials.
Single vs multiple scattering :
The Heitz full evaluation method involves a massive overhead but recent works from sony image work and ILM approximate the effect quite well and are way cheaper to compute (ILM method is included in unity HDRP pipeline and in google filament, so really cheap but comes with some hiccups, good for real-time only). I submitted the ImageWorks’s method (based on the Kelemen-Szirmay-Kalos model) to Vlado on the chaos group forum and he was pretty enthusiastic about it.
So we basically need the Autodesk Standard Surface ( https://autodesk.github.io/standard-surface/ ) + multiple scattering approximation and we would end up with an insane shader.
For those who want to read more about the subject, Stephen hill made a great article series analyzing Imageworks’ multiple-scattering approximation :
part 1: A Multi-Faceted Exploration (Part 1)
part 2: A Multi-Faceted Exploration (Part 2)
part 3: A Multi-Faceted Exploration (Part 3)
part 4: A Multi-Faceted Exploration (Part 4)
I’m also interested in the multiple scattering stuff. I often have to deal with metals and need them to look really good. In high contrast situations metals are quite finicky to get right, and of course the artist is part of the problem/solution so not sure if it’s the current implementation or a wrong approach on my side.
Then again, I know from image reference that metals do react differently than what I get sometimes and I tend to think multiple scattering is a vital part. For example, a metal shader looks good in one lighting situation but needs a lot of tweaking in another which doesn’t make sense sometimes, especially with glossiness values at 0.5-0.7 I have to tweak a lot depending on the lighting angle already… hmm would certainly love to see how much multiple scattering could help here.
Very interesting reads indeed, i found the google way of doing things : Redirecting... Super interesting , but i dont understand at least 95% of it 
That’s all I meant exactly :- ). I only answered the critique that new PBR ( which isn’t promised to come with new BRDF !) material will solve materials looking “plastic”.
The energy loss issue is currently only trouble for Metals, and still fixable.
Don’t get me wrong, I would love new BRDFs implemented, esp. for Diffuse shading. But our current GGX doesn’t make materials look plastic any more than any other GGX implementation elsewhere and particularly F-Storm, has worse (their gloss doesn’t go below 0.4 on linear scale).
The new PBR shader will mainly reorganize UI to comply with industry standard by introducing Metalness workflow instead of Intensity&Gloss only, and adding Coat & Sheen parameters. Our leather material will still look like it does right now.
great links indeed. how does the new Advanced diffuse model change things for the corona material?
Look at the first changelog entry of the new arnold 6.0 release : Help
yep yep yep. awesome stuff! is the new corona diffuse model gonna implement Oren-Nayar reflectance model, like the Oren-Nayar roughness parameter in Arnold, which should remove the darkening effect Lambert creates at grazing angles which in fact result in the plastic look? if yes then that would be a great step forward for corona material.
examples
Sorry, i don’t get it… In my opinion / habits, Lambertian looks better here…
Can you explain why i’m wrong ?
Oren Nayar is perfect for rough, dirty, dusty, non-shiny surfaces - mud, soil, stucco walls, concrete, rubber etc. It’s something you can see when you look at the moon - it doesn’t look round, it looks almost flat, because of the rough surface. Oren Nayar is quite old though, I believe there are newer, better shading models available now. Still, it would be great to have anything that helps with these surfaces.
The most recent PBR shaders also adjusted roughness of diffuse BRDF automatically with specular BRDF.
So not like old Vray shader where Diffuse Roughness simply interpolated between Lambert (default 0) and Oren (1), and enabled monstrosities like fully rough diffuse + fully specular GGX response on top :- ).
Instead, setting the general roughness parameter to 0.4 example would adjust both BRDF of the diffuse and the specular part.
This sort of approach, not giving user the option to created artificial crap is the right way to go.
But I also believe simply adopting Oren-Nayar would be half solution at this time. There must be something new, better, more universal. The difference between Lamber and Oren-Nayar is too radical even when interpolated in-between.
But it’s still better than having only Lambert.
I’m a little dubitative while looking at the results shown above. I wouldn’t expect that much of an impact from oren-nayar alone. Those are marmoset toolbag renders right?
The old Vray model was not even oren-nayar, it’s a custom gamma-based model that looks like… em… Oren-nayar is way better ! I wouldn’t even consider the Disney one. Even if they manage to get the same roughness as GGX, it is not energy conservative…
The only issue is that Oren-nayar and GGX roughness don’t match and as far as I know, there is no good conversion between those two. Some attempts have been made in the real-time space, but it does come with trades-off.
On my opinion Lambertian is much better
yes exactly, these at best are artistic pseudo-mathematical representations of Oren-Nayar, any google images search for Oren-Nayar would give many images comparing Oren-Nayar’s implementation with physical models and then Lambert, it’s very clear Oren-Nayar is closer to reality. if somebody doesn’t like how a shader looks if it’s closer to reality I think it’s very clear where the problem is!
I admire your insight Fluss, I think we are at a point where theory is experiencing a schism, one side applies to glossy materials while the other applies to rough materials. this is not pretty. a pure shader is out of reach for now, the best we can hope for is a blend that will break at many instances. unify the forces!! lol the theory of everything anyone?
the Trello page says Clear Coat, Sheen, and Advanced diffuse model.
Well, there is really nothing to admire here, just sharing stuff I read and liked. Some will share the same point of view, others won’t and it’s totally fine that way. But you are right if the shader is being rewritten, it should integrate the last advances in the field. And once the hype around the Disney shader as fallen a bit (I was there too), I have to admit that it’s not the best one to look at for the aforementioned reasons ( it’s principled but not technically phisically accurate).
In that regard, the Arnold Autodesk standard surface is way better. The diffuse model is good, the sheen is way better (physically based → Making sure you're not a bot! , not like Disney one) and it has everything we need (additional specular lobe, thin-walled surfaces for both diffuse and specular, thin film etc…).
What’s more it is shared as a standard for material exchange, OSL and materialX available here: standard-surface/reference at master · Autodesk/standard-surface · GitHub.
add to that :
_ multiple scattering for every micro-faceted lobes (links some posts above)
_ real Shadow terminator Fix (micro-facets based shadowing function) → https://link.springer.com/content/pdf/10.1007%2F978-1-4842-4427-2_12.pdf
_ micro-facets based normal maps → https://cg.ivd.kit.edu/publications/2017/normalmaps/normalmap.pdf
_ same for bump maps (“bump to roughness”)
and you get a pretty solid shader.
I’m not part of the development team, should I therefore sh**up? 
cheers bro
