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Primitive variables (often referred as "primvar") are data attached to the geometry.  This is useful when a shader needs to reference custom data from the geometry such as foam in an ocean shader.

Mapping Houdini to RenderMan Primvar

When the primvar is already available in Houdini such as from an alembic file, we can add Attribute Rename inside your object to map it to RenderMan primvar.


We then specify the variable name on the Houdini side and the variable name on the RenderMan side under RiName as well as its RiType.


Inside the RIS Network, we create a PxrPrimvar to read this primvar.


For example, we wire the resultF of PxrPrimvar to a PxrSurface's Diffuse Gain.