Energy Conservation-节能

Energy Conservation

There are four BRDF models to choose from in OctaneRender®. Three are physically correct: Beckmann, Ward and GGX. The fourth is the original Octane BRDF, which does not consider microfacets or anisotropy, and only partially considers the fresnel effect. Still, the original Octane BRDF is useful for backwards compatibility for older scenes and for the ability to cheat a little, if you need it.

Let’s compare the BRDF models in the Live Viewer. Create a sphere and put it anywhere in your scene. Then create a Texture Environment from the Live Viewer. Select the RGBSpectrum 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 Live Viewer. You will not be able to see anything but white, even though the sphere is there. Why? We see a white image because the original Octane BRDF equalizes the reflected light energy and the incoming light energy — it’s always %100.

Now change the BRDF to “Beckmann”. You will see that Sphere is a little more defined against the environment, as the reflected light energy from the sphere is less than the incoming light’s energy. The illustration below shows all of the available BRDFs in the same context.

MICROFACETS IN OCTANE (ROUGHNESS)

Octane mimics the natural roughness of a surface while redefining the surface at the micro geometry level according to the “microfacet” functions in three of the BRDF choices, “Beckmann”, “GGX” and “Ward” BRDFs. Unlike the original Octane BRDF, these 3 models allow you to create features such as “Fresnel Effect” and “Anisotropic Roughness.”

Which BRDF should you choose? 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.

You can get more detailed information from the below link:
https://www.cs.cornell.edu/~srm/publications/EGSR07-btdf.pdf.

ANISOTROPY IN OCTANE

Some surfaces reflect light in a non-circular or spherical manner, but instead distorts the reflected light in a manner driven by the physical components of the surface as well as its surface smoothness and grain direction. This characteristic is known as “Anisotropy”. You can get complex metallic surfaces thanks to this feature as well as polished metal, human hair, fur and wood. The look of the reflection will vary depending upon which BRDF model that you choose.