Rhino library material inconsistencies

[FlynnAD]

Could someone from Chaos Group explain the reasoning behind the creation - and differences - between the aluminum and steel in the default Vray Rhino 6 materials?

For example, why does "Steel_Polished" use Blinn BRDF, while "Steel_Brushed_5cm" uses Ward, and "Steel_Blurry" uses Microfacet GTR? And then, comparatively, why does "Aluminum_Polished" NOT use Blinn like the polished steel version, but Microfacet GTX, while "Aluminum_Brushed_5cm" uses Blinn and not Ward like the steel version?

Lastly, why would "Aluminum_Anodized_Clear" have a different Reflection IOR ( than Aluminum_Polished at 20? Wouldn't a particular metal have a consistent reflection IOR no matter what its finishing?

Thanks; I'm trying to understand so I can create better materials of my own.

Matt

[Shane C]

I'm pretty new to vray, but I think it's mostly legacy stuff. blinn and ward etc are older methods of approximating materials. (or maybe these methods give faster approximations and reduce render time for blurry reflections?) I'm pretty sure GGX is the newest method of calculating metals, and is based on the work of Grant Warwick. (he's got some excellent training stuff for Vray.) it gives subtle falloff to the highlight area. I use this for metals now pretty much exclusively, with an IOR of 25.
If you use the metal preset in the dropdown slider these are the settings that you get.

[Peter.Chaushev]

Hello, Matt,

The library preset materials are mostly an artistic interpretation of real materials. While certain parameter values are scientifically based, others are subjective design decisions based on photographic references.
Regarding the BRDF types, we realize the current info provided in our documentation is insufficient and will amend this soon.

GGX is the most modern and flexible BRDF type and is able to better represent a broad range of materials thanks to its ability to control the shape of the specular lobe. There currently isn't any particular performance difference between models and there is little reason to choose any of the other types.

Historically, the Phong, Blinn, Ward and GGX are successive reflectance models developed over the years in computer graphics where each model aimed to improve on the limitations of the previous ones. For example, the specular highlights with the Phong model have a very narrow and bright center with no falloff, but it doesn't work well with anisotropic reflections. The Blinn model has broader highlight center with a tight falloff. The Ward model has an even broader center and falloff. The GGX model has a bright center and an even longer falloff (at default settings). In the past, each model's characteristics resembled more closely a certain type of material, for example Phong could be used for plastics, Ward for cloth and metals, and Blinn for other common surfaces. However with the introduction of the GGX model, all of these surfaces can be approximated well, thus reducing the need for using the other models.

It should be noted that no principled model is able to represent all possible materials entirely accurately, and where those models fail - for example when the material isn’t viewed frontally - only approaches such as that of VRscans are able to capture the correct material representation.

Kind regards,
Peter

[FlynnAD]

Thanks for the explanation of the discrepancies, Peter.