Materials

A material determines the response of a surface to illumination. Materials in mental ray consist of a material name and one mandatory and several optional shaders, each of which can be a standard shader or a user-provided C/C++ shader:

The primary shader is the material shader itself. This is the only one that may not be omitted in case no BRDF is specified. The material shader determines the color of a point on an object, based on its parameters which may include object colors, textures, lists of light sources, and other arbitrary parameters.
An optional volume shader controls rays passing through the inside of the object. This is functionally equivalent to atmosphere calculations, but takes place inside objects, not outside.
An optional photon shader determines how the material interacts with indirect illumination if caustics or global illumination are enabled, much like the main material shader determines how the material interacts with direct illumination.
An optional photon volume shader determines how the inside of the object interacts with indirect illumination if caustics or global illumination are enabled, much like the volume shader determines how the volume interacts with direct illumination.
An optional environment shader provides an environment map for non-raytraced reflections.
An optional displacement shader can be named that displaces any type of surface geometry at each point in the direction of the normal. Displacement maps affect the triangles resulting from the tessellation of free-form surfaces, polygonal meshes, and subdivision surfaces.
An optional shadow shader determines the way shadow rays pass through the object. This can be used for calculating effects like colored shadows.
An optional contour shader specifies how contours should be drawn in and around the object that this material is applied to, if contour rendering is enabled.
An optional lightmap shader causes the object to be sampled to create a light map, which collects arbitrary information (usually illumination) about an object that can later be used during rendering.
An optional BRDF shader as a replacement or fallback for any material or photon shader in the material. This allows advanced rendering algorithms to take advantage of known shading properties provided by a single BRDF.

The shading function may be either a user written function linked at run time, or it may be one of the standard functions. Shaders may define parameters that control their behavior. Shaders are completely free to define any set of parameters required for their function, but there is a set of commonly used terms and parameter names that will be found in many parameter lists.

Parameters have names and values. The declaration of a shader, which is provided by the author of the shader, defines the list of possible parameters, including names and types (such as "color" or "vector"). The definition of a shader is done in a scene file, and provides the values. For example, a shader might have a color parameter named "diffuse" with the value 1.0 1.0 0.0, which specifies a yellow color. Parameter values can be given in any order. Parameters can also be omitted; mental ray will substitute null values. Shaders are typically programmed to provide reasonable defaults in this case.

BRDF/BSDF

A Bidirectional Reflection Distribution Function (BRDF) is a common model in computer graphics to describe illumination characteristics of a material. A Bidirectional Scattering Distribution Function (BSDF) is an extension of a BRDF with transmission properties like refraction added to the model, and used in mental ray as the basis for its internal representation.

A BRDF can provide additional information to the renderer compared to conventional shaders normally used in materials. As a benefit, these functions typically adhere to physical or mathematical constraints. This knowledge can be essential to improve rendering efficiency. On the other hand, a BRDF does not offer the same flexibility as a material shader. However, this flexibility is not needed for the majority of rendering applications, but the performance costs associated with "black box" shading can be avoided in those cases.

The current implementation allows rendering algorithms to take advantage of specified BRDFs as needed, or still support fully programmable shading. mental ray provides a set of built-in BRDF shaders for common shading models. They are exposed as built-in shaders which can be attached to materials slots like regular material or photon shaders. For advanced rendering algorithms like irradiance particles a BRDF should be attached to the bsdf slot of a material description.

MDL Materials 3.13

The NVIDIA Material Definition Language, or MDL, is a formal specification to describe material designs that comprise all properties relevant for a realistic visualization and physically accurate appearance. That includes the base appearance like color and texture of the surface, the reflection and refraction behavior, absorption and scattering effects of volumetric parts, emissive properties, down to geometric detail from bump, normal, or displacement maps. The underlying shading models are strictly based on BSDF descriptions, and any combinations of them that do not break physics.

The MDL materials are typically provided in files of type .mdl. Often, they are part of a larger catalog, which delivers many variations of materials. A final MDL material description for use in the renderer may be built from other base MDL components within the same catalog, which are ultimately constructed from standard MDL functions and BSDF models implemented in the MDL-capable rendering engine. A renderer like mental ray can take advantage of that knowledge about shading properties by applying certain optimizations, like importance sampling.

The usage and application of MDL materials in a scene is straight forward. The .mdl files can be included into a scene like regular shader declarations. mental ray will create material nodes in the internal scene database that can be referenced by name in objects and instances in the usual way. See known limitations for details.