Rough Specular reduces specularity amount?

[Ihno]

Very interesting thread!
Might be one of those where something very cool happens in the end.

I 'm wondering if VRScans could possibly result in some kind of ChaosGroup BRDF at any time.
Wouldn't be supprised if they're already researching in that direction.

[Midiaeffects]

Or if we still want to use fresnel, perhaps we just raise the facing colour a tiny bit higher than normal and lower the edge colour quite a bit so there's just less difference between the front on and edge strength of the reflections?

What type of controls are used in Iray to set how rough the microfaceting is? Do they use units of measurement to set how big the high and low value of the facets are?

For flat colors, adjusting falloff manually seems fine. But not so much if you map roughness. But having a falloff which doesn't respond to the microfacets is not necessarily a bad thing, can be useful for artistic reasons. Also, when changing roughness in all the quoted renderers (Iray, ART, FStorm, etc), the fresnel curve doesn't change when used as a map.

What you mean by controlling microfacets in Iray? There's no control, just glossiness, as any other renderer. AFAIK, a BRDF is an analytical function which simulates the light reflectance distribution "as if" the surface had a micro-structure built from mirror like microfacets. There's no size. This is why no matter how close or big you render a burry reflection/refraction, it always appears smooth, despite the fact that in reality, when seen from close enough, we should see the real surface bump imperfections that create the effect. So when I say that Vray's fresnel must "see" the microfacets (instead of the face normal), is that the fresnel term is usually part of the BRDF function itself, and not something external to it.

I don't see why there couldn't be an option added, the current method which gives you the option of adding a falloff texture. Or a checkbox that does the "rough fresnel" version, which doesn't allow the option. Since the reason people would use the "rough fresnel" is if they wanted the software to take over and give more automatic realistic results, I don't think they'd miss the ability to add a custom falloff map in that mode.

- Neil

Yeah I agree. In fact, once I thought that a VrayFresnel map having all the bells and whistles like simple Fresnel (just IOR), Complex Fresnel, a Custom Curve like Output map and whatever makes sense PLUS a dropdown with Vray BRDF models and roughness would be ideal. But after trying Redshift, I like it's solution better. It's mostly the same, but instead of the usual Fresnel IOR field, there's a dropdown for how to input Fresnel.

Nice tests Adan! I'm providing the gradients, could you please generate the graphs also for Iray, ART and Redshift too? Results seem to vary considerably between renderers. My guess is that it's likely caused by different BRDF models/variations.

https://dl.dropboxusercontent.com/u/...GlossyTEST.rar

-Eugenio

[joconnell]

Car companies that do crash repairs or floor polishing places would be great for this - they've got a cheap device called a glossometer that measures the sharpness of a surface's polish - if they're trying to match in a section of a lacquer to an existing one, they can use this small handheld device to measure the focus / polish / sharpness of reflections and buff their new section of lacquer to match it precisely!

[soulburn3d]

Yes, but I don't see why they couldn't take a material of a specific ior and make it more and more matte, taking photos/measurements as they go. It would be a lot of work, but a table could be built and hopefully some sort of algorithm would be found that generally fit the results.

Our specular models of course are just an approximation, and to get the real results you need something that's specific per material, I'm looking to see if we can get something that's half way between these two extremes.

- Neil

[joconnell]

Exactly - hence the result of anyone trying to measure it in the real world for dimming amount would only work for the specific sample measured at the specific scale of roughness and not an overall rule.

[soulburn3d]

I would assume the object's IOR is identical in all cases, its just the fresnel equation that needs to be dimmed by a much more complex equation. But I can't image the material properties have really changed.

- Neil

[joconnell]

Is the amount of dimming down to the level of roughening on the surface and not anything to do with the material itself or it's fresnel properties?

For example, if I take the same object you had and sand two different balls with different grades of sandpaper, they're both made of the same material so have the same fresnel amount but it's just the difference in scale of the physical distortions on the surface that would determine the amount dimming? Thus it's kind of similar to a lot of other measuring based approaches - you'd have an accurate amount of dimming for only that one example and it's surface roughness, but that doesn't mean that the same material settings would be the right amount of dimming on a surface that had a larger or finer scale of roughening detail?

[soulburn3d]

Hi. Last week i made a quick test to compare the diferences on masking/shadowing of 5 diferent BRDF: Vray GGX, Vray GGX + OSL from Rens, Octane, Corona and FStorm. The curves on the botton represents the reflectivity. Maybe this helps to have some visual reference. .

Great test. The thing I'm most concerned about is that it looks like everyone has their own opinion of how much dimming should happen. And I still haven't found someone replicating this test in the real world. It seems if we had real world results as our benchmark, that would inform the cg shading model.

- Neil

[soulburn3d]

I guess for Vray to simulate this effect is just a matter of it's Fresnel to "see" the microfacets of the BRDF. The downside though, as Vlado said we can no longer use a Falloff texture in Fresnel mode to mask reflections.

I don't see why there couldn't be an option added, the current method which gives you the option of adding a falloff texture. Or a checkbox that does the "rough fresnel" version, which doesn't allow the option. Since the reason people would use the "rough fresnel" is if they wanted the software to take over and give more automatic realistic results, I don't think they'd miss the ability to add a custom falloff map in that mode.

- Neil

[adanmq]

I finish the chart for the Vray AlMtl GGX and upload to my FLickr album to compare:
https://www.flickr.com/photos/adanmq...h/28559201220/



Best.

[Rens]

Thanks! : )

[adanmq]

Hi, and thanks for your OSL i will make further test using it.

To get the gradient i use a very simple scene, 90 planes 1º rotation each and 1.0 on a full Vray dome and orthographic camera. I share the scene in case you want to do some test. It´s not 100% accurate because render noise and because you can´t render 99.99º planes but i like to "visualize" this kind of data.

To get the curves i use Nuke, crop the black border and sampler to sample the gradient.

MAX 2014 Scene Link:
https://drive.google.com/open?id=0By...WZZSmpiMUJGSjQ

Best.

[Rens]

Ha! Look at that! Thanks Adan. : )
Looks like I need to check the code on the shader, it seems odd that it would lower the base reflection with lowered glossiness.
How did you do the gradient renders and the graphs?

The raising comes from the rgh_darken_brighten parameter. Depending on how you feel what should happen when a surface gets rougher you can raise or lower the base value. If you even out / average the 0-90 degree reflection curve you have a value that is slightly higher than the 0 degree value. Think of it as water in a fish tank with the water on the right higher like in Adan's graph. After it will slosh down and settle the level will be higher than the original level on the left. That's why it could be brighter overall. Now the shadowing of the microfacets comes into play with increased roughness, which will darken the reflection overall. You can brighten it with values above 0.5 and darken it with values below 0.5.

The roughness_falloff parameter controls how fast the curve will flatten out with increased roughness / decreased glossiness.

[adanmq]

I made the 90º planes test, will make the complete chart including visual references son.



Base reflectivity maintain at 4% between 0-0.5 roughness and diminishes between 0.5 - 1 roughness to a minimum of 1.11%
Max Reflectivity diminishes constantly.

I never hear about this VrayALMtl before :O

Best.

[vlado]

Can you also make one with the VRayALMtl material in GGX mode?
https://github.com/ChaosGroup/vray_al_surface/releases

I'm curious how it compares.

Best regards,
Vlado