10. Glossary#

Attribute state: The set of variables that determines all the properties (other than shape) of a geometric primitive — such as its local transformation matrix, the surface, displacement, and volume shaders to be used, which light sources illuminate objects that share the attribute state, whether surfaces are one-sided or two-sided, etc. Basically all of the options that determine the behavior and appearance of the primitives, that are not determined by the shape itself or the code of the shaders. A single attribute state may be shared among multiple geometric primitives. Also sometimes called graphics state.
BSDF: Bidirectional scattering distribution function, a function that describes light scattering properties of a surface.
Built-in function: A function callable from within a shader, where the implementation of the function is provided by the renderer (as opposed to a function that the shader author writes in OSL itself).
Closure: A symbolic representation of a function to be called, and values for its parameters, that are packaged up to be evaluated at a later time to yield a final numeric value.
Connection: A routing of the value of an output parameter of one shader layer to an input parameter of another shader layer within the same shader group.
Default parameter value: The initial value of a shader parameter, if the renderer does not override it with an instance value, an interpolated primitive variable, or a connection to an output parameter of another layer within the group. The default value of a shader parameter is explicitly given in the code for that shader, and may either be a constant or a computed expression.
EDF: Emission distribution function, a function that describes the distribution of light emitted by a light source.
Geometric primitive: A single shape, such as a NURBS patch, a polygon or subdivision mesh, a hair primitive, etc.
Global variables: The set of “built-in” variables describing the common renderer inputs to all shaders (as opposed to shader-specific parameters). These include position (P), surface normal (N), surface tangents (dPdu, dPdv), as well as standard radiance output (Ci). Different shader types support different subsets of the global variables.
Graphics state: See attribute state.
Group: See shader group.
Input parameter: A read-only shader parameter that provides a value to control a shader’s behavior. Can also refer to a read-only parameter to a shader function.
Instance value: A constant value that overrides a default parameter value for a particular shader instance. Each instance of a shader may have a completely different set of instance values for its parameters.
Layer: See shader layer.
Output parameter: A read/write shader parameter allows a shader to provide outputs beyond the global variables such as Ci. Can also refer to a read/write parameter to a shader function, allowing a function to provide more outputs than a simple return value.
Primitive: Usually refers to a geometric primitive.
Primitive variable: A named variable, and values, attached to an individual geometric primitive. Primitive variables may have one of several interpolation methods — such as a single value for the whole primitive, a value for each piece or face of the primitive, or per-vertex values that are smoothly interpolated across the surface.

Shader: A small self-contained program written in OSL, used to extend the functionality of a renderer with custom behavior of materials and lights. A particular shader may have multiple shader instances within a scene, each of which has its unique instance parameters, transformation, etc.
Shader function: A function written in OSL that may be called from within a shader.
Shader group: An ordered collection of shader instances (individually called the layers of a group) that are executed to collectively determine material properties or displacement of a geometric primitive, emission of a light source, or scattering properties of a volume. In addition to executing sequentially, layers within a group may optionally have any of their input parameters explicitly connected to output parameters of other layers within the group in an acyclic manner (thus, sometimes being referred to as a shader network).
Shader instance: A particular reference to a shader, with a unique set of instance values, transformation, and potentially other attributes. Each shader instance is a separate entity, despite their sharing executable code.
Shader network: See shader group.
Shader layer: An individual shader instance within a shader group.
Shader parameter: A named input or output variable of a shader. Input parameters provide “knobs” that control the behavior of a shader; output parameters additional provide a way for shaders to produce additional output beyond the usual global variables.
Shading: The computations within a renderer that implement the behavior and visual appearance of materials and lights.
VDF: Volumetric distribution function, a function that describes the scattering properties of a volume.