...

Here is an example OpScript showing how to set an arbitrary constant color primvar:

Interface
          .
          SetAttr
          (
          "
          geometry.arbitrary.myColor.scope"

, StringAttribute

(

"

primitive"

))

 Interface

.

SetAttr

(

 "

geometry.arbitrary.myColor.value"

, FloatAttribute({

0.0

, 

1.0

, 

 0.0

}

 ))

 Interface

.

SetAttr

(

 "

geometry.arbitrary.myColor.inputType"

 , StringAttribute

(

"

color3"

))

Using Primvars

Typically, production workflows will export these arbitrary attributes as a part of the alembic or USD asset, so no set up inside of Katana is required.  Otherwise, you can use an OpScript to set the geometry.arbitrary attribute directly on a geometry location - the attribute is not inheritable.

Primvars are read by various shading plugins. PxrPrimvar, PxrSeExpr, PxrVariable, and PxrVary are some plugins distributed with RenderMan that use arbitrary primvars.