Octane Standalone Documentation

All Octane materials allow for the choice of specific BRDF (Bidirectional Reflectance Distribution Function) models to best match the type of material being recreated within Octane. BSDF (Bidirectional Scattering Distribution Function) is a broader umbrella that describes how light interacts with a surface both reflective and transmissive. BRDF is a subset of BSDF that only determines how light bounces off or reflects from a surface. 

Diffuse (Opaque) BRDF Types

Octane - This is the BRDF model available in Octane prior to version 3.07, before support for other BRDF models was introduced. It is not physically accurate.

Lambertian - This model renders simple matte and diffuse surfaces, but does not consider mode complex surface characteristics, such as subsurface scattering or varying roughness values. For more complex surfaces, Oren-Nayar is superior.

Oren-Nayar - This model is a directional microfacet diffuse model, and is an extension of the Lambertian model meant for rough surfaces using diffuse reflection. It can handle surfaces with irregularities auch as concrete, fabric and so on. Oren-Nayar does support directional scattering as well as light bounces between microfacets. It is good for clay, sand, unfinished wood and concrete, but the other models discussed in the following section offer far more ability. That said, those models also are a bit slower to render.

Reflective & Transmissive BRDF Types

Octane mimics the natural roughness of a surface while redefining the surface at the micro geometry level according to the microfacet functions in five of the BRDF choices, Beckmann, GGX,  Energy Preserving GGX, STD and Ward BRDFs. Unlike the original Octane BRDF, these five models allow you to create features such as Fresnel effect and anisotropic roughness.

Octane - This is the BRDF model available in Octane prior to version 3.07, before support for other BRDF models was introduced. It is not physically accurate.

Beckmann - This model is popular for PBR rendering and produces realistic highlights and reflections. Like Ward and GGX, Beckmann uses a microfacet model and may differ in terms of realism and render speed. Beckmann calculates the specular component based on the Fresnel approach, reflecting light differently with different angles of incidence. The microfacets may occlude light and reflections from surrounding microfacets. Beckmann is a good choice for recreating polished metal surfaces, like steel or aluminum; glossy surfaces like automotive paint or varnished wood; dielectric materials, which are often smooth, such as plastics and ceramics and glass.

GGX and GGX Energy Preserving  - This model also uses microfacets, and is often a better choice for very rough surfaces than Beckmann, as this model handles broader highlights and stronger roughness values more realistically than Beckmann. Unlike other models, GGX is energy perserving (when GGX Energy Preserving is chosen as the BRDF type in a given material), which ensures that the total reflected light from a surface does not exceed the incoming light (which other BRDF models allow). Like Beckmann and others, GGX and GGX Energy Preserving calculates specular reflections with the Fresnel equation, considering changes in the angle of incidence. GGX Energy Preserving uses multiple scattering between microfacets, which prevents surfaces from incorrectly getting darker (no energy is lost), which tends to occur with surfaces that have high roughness values in other models. Overall, GGX Energy Preserving is a good, all-around BRDF model for metals and rough plastics as well as glossy surfaces. Figure 1 compares the specular materials of the old GGX model (top row) versus the energy preserving GGX model (bottom row) with increasing roughness from left to right:

Figure 1: Comparing GGX models

Figure 2 compares the metallic materials of the old GGX model (top row) versus the energy preserving GGX model (bottom row) with increasing roughness from left to right:

Figure2: Comparing GGX models for metallic surfaces

STD - This model also uses microfacet distribution, via a Normal Distribution Function. Essentially, this model uses statistical modeling for the microfacet distribution on a given surface, and is suitable for rough, porous surfaces with more pronounced light scattering. As a result, the STD model is great for materials that are very rough, such as rough metallic surfaces, rough glass and stone, and is considered superior to Beckmann or GGX when roughness values are extreme. Like the other models, STD also uses the Fresnel effect for dielectric and metallic surfaces, and is also energy perserving. The STD model provides an additional parameter, Spread, that provides some control over the shape and slope of the falloff of the distribution/spread of reflected rays. The BRDF is available in glossy, specular, metallic and universal materials. In figure 3, the material ball with the STD BRDF with roughness 0.2 and 4 different spread values and one version with a GGX BRDF for comparison:

Figure 3: Comparing STD BRDF Spread values

Ward  - This model uses anisotropic reflections, making it a good choice for material like fabric (satin, velvet), hair, and brushed metals, and uses energy conservation Ward is available only for the Glossy Material and the Metallic Material. 

The primary difference between these five 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 Figure 4, the specular lobes of all five 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.

Figure 4: Specular lobe comparisions

The microfacet BRDFs were implemented as part of the OSL materials to support the default microfacet BRDF closures in OSL. They are also useful for importing materials from other applications because microfacet models (especially GGX) are widely adopted across different applications. With the BRDF models integrated in the OctaneRender® core, OctaneRender achieves a similar look from importing materials.

The original OctaneRender BRDF is done by doing the BRDF sampling based on the light direction. In the additional BRDF models, the BRDF sampling is done based on the microfacet normal. These microfacet models try to mimic the surface's roughness, reconstructing the surface bumpiness at the microgeometry level and enabling the render core to achieve material properties like glossy Fresnel, which reduces the Fresnel effect at grazing angles for high roughness surfaces. These additional models also allow anisotropic roughness, which help simulates anisotropic surface reflectance.