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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
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