To add a user attribute to an object, we add it via the Edit Parameter Interface.

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This is a dynamic array. Click on the + icon to add a user attribute. This is an example of setting up a MatteID0 user attribute.

Add a parameter to the User tab under either the Attributes or Primvars section in the RenderMan tab. The parameter must be located within the User folder and must be prefixed "ri_user_" to be read. 

Setting up MatteID

Create a color attribute called "ri_user_MatteID0". 

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This will emit the following to the RIB.

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