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The following diagram illustrates several fundamental builtin variables: the surface position P, the normalized shading normal Nn, and the parametric derivatives dPdu and dPdv, as well as Vlen and Vn which provide information about the incoming ray.

Primitive Variables

Primitive variables (often referred to as primvars) represent values which may be optionally present on the geometry. These primvars are uniquely named and may be supplied with the geometry definition. Shaders are usually written knowing the name and data type of certain primvars in advance, such as texture coordinates. However, since the primvars are optional, shaders are expected to fail gracefully if the primvars are missing.

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