Octane for Houdini

The Specular material is used for transparent dielectric materials such as glass and water (figure 1). When light hits a surface, it is either reflected or absorbed or refracted, but light changes its behavior when it transitions from one medium (for example air) into another medium (for example glass). These changes depend on the optical and topological properties of the surface. In specular transmission when light enters another medium, it reduces speed, and changes direction.

Figure 1: The Specular material and it's associated parameters 

Specular Material Parameters

Reflection - Determines the glossy finish of the mesh. Reducing this value increases its ability to transmit light through the object volume. Reflectivity of specular materials is also affected by the Index Of Refraction. For example, with Transmission set to black or a value of 0 and an Index Of Refraction set to 0, the material produces a mirror result.

Transmission - Controls the light passing through the surface of the material via refraction.

BRDF - The BRDF (Bidirectional Reflectance Distribution Function) determines the amount of light that a material reflects when light falls on it. For Glossy materials, you can choose from six BRDF models. Specific geometric properties (the micro-facet distribution) of the surface affects each BRDF, which describes the surface's microscopic shape (i.e. micro-facet normals) and scales the brightness of the BRDF's reflections. 

Figure 2: Examples of the BRDF types 

Roughness - Determines the sharpness of the reflection present on the surface. The reflection becomes more blurry as the Roughness value increases. The lowest value, 0.0, produces the sharpest and mirror-like reflection. It accepts a value, color, or image-based texture.

Anisotropy - Controls the material's reflectance uniformity. Reflectance changes based on surface orientation, or if the rotation is Anisotropic. If the reflectance is uniform in all directions and doesn't change based on the surface's orientation or rotation, then it is Isotropic. This parameter's default value is 0, which sets the Metallic material as Isotropic. Non-zero values mean the material exhibits Anisotropic reflectance, where -1 is horizontal and 1 is vertical.

Figure 3: Anisotropic roughness exemplified in materials like brushed metal

Rotation - Controls the orientation of the Anisotropy effect.

Spread - Determines the tail spread for the specualr BSDF (Bidirectional Scattering Distribution Function) model of the specular layer.

Index of Refraction - Controls the Fresnel effect of the reflection and refraction of light when it enters or exits the material. You can find standard values of Index Of Refraction (IOR) by searching the internet. Glass has a value of 1.53, and water has a value of 1.33.

Allow Caustics - If enabled, the photon tracing kernel will create caustics for light reflecting or transmitting through the object.

Film Width - This controls the thickness of the optical, thin film on the material. This is useful in creating rainbow or oil slick effects.

Film IOR - This controls the Index Of Refraction of the thin film.

Dispersion Coefficient  - The dispersion in Octane is based on Cauchy’s equation which has two terms: A, which is the index of refraction; and B, which is the dispersion coefficient. Increasing the value increases the amount of coloration and dispersion in the object and in caustics.

Dispersion Mode - Determines how the IOR and dispersion inputs are interpreted.

Medium Pin - There are three primary types of mediums used with materials: Absorption, Random Walk, and Scattering. Details can be found in the Mediums and Volumes section. 

Opacity - Opacity sets the material's transparency. Although this parameter has the option to accept values and colors as inputs, a texture map is the most appropriate input parameter. 

Fake Shadows - Sets the architectural glass option for all meshes sharing that material. This setting is off by default. When enabled, the Specular material exhibits the characteristics of architectural glass with its transparent feature allowing light to illuminate enclosed spaces or frame an exterior view.

Refraction Alpha - This option makes refractions affect the alpha channel. This parameter has an effect if the Alpha Channel is enabled in the Kernel settings.

Thin wall - When enabled, the geometry becomes very thin, so the ray bounce exits the material immediately, rather than entering the medium.

Bump Pin - Creates fine details on the material’s surface using a Procedural or Image texture. When you connect a grayscale texture to this parameter, light areas of the texture give the appearance of protruding bumps, and dark areas create the appearance of indentation. You can adjust the bump map strength by setting the Power or Gamma values on the Image texture node. These attributes are covered in more detail under the Texture Overview section.

Normal Pin - Creates the look of fine detail on the surface. A normal map is a special type of image texture that uses red, green, and blue color values to perturb the normals of the surface at render time, thus giving the appearance of added detail. They can be more accurate than Bump maps, but require specific software such as ZBrush®, Mudbox®, Substance Designer, XnormalTM, or others to generate.

Displacement Pin - Allows adjustment for the height of points on a surface based on an image value. Displacement differs from Bump or Normal mapping by providing true displacement of an objects surface. Displacement mapping is covered in more detail under the Displacement Overview section.

Smooth - Refers to normal smoothing, which smooths the normals of all meshes sharing that material. When disabled, the materials are faceted and polygonal.

Smooth Shadow Terminator - If enabled, self-intersecting shadows for low polygon objects is smoothed according to the polygon's curvature.

Round Edges Pin - This creates a shader effect at render time that rounds the sharp edges of objects without modifying and reloading the geometry. Higher values will round the edges more. This is useful to bevel hard edges during render time, like when using low-polygon models. See the Round Edges section for more information.  

Priority - Used to resolve the ambiguity in overlapping surfaces, the surface priority control allows artists to control the order of preference for surfaces. A higher number suggests a higher priority for the surface material, which means it is preferred over a lower priority surface material if a ray enters a higher priority surface and then intersects a lower priority surface while inside the higher priority surface medium.

Custom AOV - Writes a mask to the specified custom AOV.

Custom AOV Channel - Determines whether the custom AOV is written to a specific color channel (R, G, or B) or to all the color channels. 

Layer Pin - Adds a Material Layer above the base material. See the Material Layers section in this manual for more details.

Compatibility Version The Octane version that the behavior of this node should match. 

• Latest (2024.1) - Default.
• 2023.1.1 - Highlights for transmission through materials with fake shadows and roughness are doubled.
• 2023.1 - The slope of bump maps is calculated slightly differently, making it more sensitive to the orientation of the UV mapping.
• 2022.1 - Legacy behavior for bump map strength is active and bump map height is ignored. This applies in addition to 2023.1 compatibility mode behavior.