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A PxrMaterialBuilder is just a container for RIS VOPs. It is required for Houdini to work with RIS materials. Important: A material is represented by a PxrMaterialBuilder VOP with a BxDF output_collect VOP input. To add a material, you can add a PxrSurface from the RenderMan Shelf, which automatically creates the nodes, or select another BxDF from the RenderMan Menu to wire into the output_collect VOP inside of a PxrMaterialBuilder. These tools also automatically create an RIS Network if it does not exist as well as the selected VOP.
Assigning Material
To assign the material to your object, simply drag the PxrMaterialBuilder VOP you just created to the Material parameter of your object.
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