EMPEROR'S NEW CLOTHES

Let's look at the new BRDF models in Octane. Until the V3.08 version, Octane was using its original BRDF. There was conservation of energy in this model, but it was not fully functioning. In other words, the amount of reflected light was the same as the incoming light (in other words, always 100%). Along with the new BRDF models, Octane's material arsenal is much more advanced. The materials you will create from now on will be more plausible and accurate materials that conform to physical laws.


You can see the difference between the original Octane BRDF and the new BRDF models as shown in the figure below.




ENERGY CONSERVATION IN OCTANE

The biggest difference between the new BRDF models is the compliance with the law of conservation of energy. In the original Octane BRDF this was always 100%. So energy conservation was not fully achieved. We can test it with new BRDF models. For example, create a sphere and put it anywhere in your scene. Then create a Texture Environment from the Live Viewer. Go to RGB Spectrum in Texture Environment settings and make the color white (230, 230, 230). Then create an Octane Glossy Material. Change the following in the material settings.

Diffuse: Full Black (0, 0, 0)

Specular: White Color (230, 230, 230)

Roughness: 0.5

Index: 1 (To disable Fresnel)


Finally, go to the Settings/Kernel and select Pathtracing.

Now run LV. You will not see anything other than a white screen. But the sphere is there. Why?. Even if the material and the environment are the same color, the total energy of the light reflected from sphere must not be more than the total energy of the incoming light, according to the law of conservation of energy. We see a white image because the original Octane BRDF is equalizes the reflected light energy and the incoming light energy (it's always %100).  


Now change the BRDF models this time, for example choose "Beckmann". You will see that Sphere is a little more visible. What happens here is that the reflected light's energy is less than the incoming light's energy. So the new BRDF models fit exactly into the Law of Conservation of Energy. You can see the differences in the picture below.


MICROFACETS IN OCTANE (Roughness)

In the previous chapter, we briefly mentioned microfacets. With the new BRDF models, Octane tries to mimic the roughness while redefining the surface at the micro geometry level according to the "Microfacet" functions. These functions are the commonly used "Beckmann", "GGX" and "Ward" BRDF functions. Unlike the original Octane BRDF, these 3 models allow you to create features such as "Fresnel Effect" and "Anisotropic Roughness" not previously available in Octane. These features make very nice and realistic results in Octane material creation.


The biggest difference between these 3 microfacet models is the Specular Lobe. These specular lobes are defined by the microfacet NDF (Normal Distribution Function). NDF describes the distribution of microfacets for the surface and unique to each BRDF model. Also this function is most responsible for the size and shape of the specular highlight. In the pictures below, you see the Specular Lobes of all 3 models with a roughness value of 0.2. GGX produces more specular tail than other models. This is because the angle of the Microfacet normal differs from the Surface Normal, so the GGX does not fall below a certain value.


Microfacet is a very complex theory. You can get more detailed information from the below link:

https://www.cs.cornell.edu/~srm/publications/EGSR07-btdf.pdf.


ANISOTROPY IN OCTANE

One of the new material properties of octane is "Anisotropy". You can get complex metallic surfaces thanks to this feature, which is a plus for new BRDF models. As we have already mentioned, the anisotropy is the reflection value changes by turning the surface around its normal. Examples of anisotropic surfaces include polished metal, human hair, fur and wood. In Octane, you can use the anisotropy feature for 3 new BRDF models. You can increase or decrease the roughness according to the BRDF model you choose. The results will be different for every 3 BRDF because of their Microfacet functions.