Dielectric specular value? Tweet

[lance_uppercut]

I'm a little confused about Octane's universal material, specifically about what is the appropriate value to use in the 'Specular' color input.

https://marmoset.co/posts/physically-based-rendering-and-you-can-too/
https://help.otoy.com/hc/en-us/articles/360051320052-Universal-Material-Channels-Deep-Dive

Based on the first article, when modeling a dielectric, the specular value should be in the ballpark of 0.04 (linear space). Is the default value of 1.0 appropriate, or should we be locking it to 0.04 for 99% of use cases? I have read somewhere that Unreal engine, for example, defines the reflectance of dielectric (non-metal) materials from 0 % (0.00) to 8 % (0.08) via a remapped value of 0 to 1. Is Octane remapped this way as well, hence the default of 1.0?

Second, as the Marmoset article notes "IOR is another way to define reflectivity, and is equivalent to the specular and metalness inputs". How does octane relate the specular term to IOR? Does it derive specular from IOR and then multiply it by the specular input?

Thanks!

[elsksa]

Hi,

It's one of those simple questions that call for long answers.

Briefly:
The universal material could be defined nowadays (and since some years) as a "über shader material" (all around versatile material), featuring various lobes ("albedo" "diffuse", specular reflection, transmission, coating, etc). It's the same principle in nearly any modern renderers.
Yet, Octane does have "sort of" "individual lobes" (diffuse material has no reflection, or specular material as no diffuse, glossy being a mini über material). This allows for simpler node network and more freedom when "layering/mixing" BxDFs materials.

On top of that, Octane does support the online rendering "Metalness Workflow" and offline rendering "Specular Workflow". The latter is recommended.

Honestly, I wouldn't worry about all of that. Some look-developers follow or break the "specular rule".
I've been there myself and that's even the case at some VFX studios. Unless at the end of the chain, there is a specific request, the importance is matching the reference or goal from the inception.

There is also x2 "IOR", one for specular reflection, one for transmission. It's simply stand for "Index of Refraction" and for specular reflection, can be accordingly set with Specular to 1 (for more or less reflection, basically).

That said, I would recommend consulting these links:
Official Documentation (here and here)
Octane Material Guide (WIP)
Warning: some of the online content is potentially misleading and misinforming (even some official documentation pages).

Welcome to the rabbit hole!

[lance_uppercut]

Thank you! Your website is extremely helpful, I'm bookmarking it! These kinds of practical examples should make their way into Octanes documentation. (Don't get me started on IES lights)