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If you already have a .vdb file, RenderMan will read it with blobbydso:impl_openvdb procedural without converting the .vdb file to volume. Before Houdini 16.0.655, the only way is to create a box SOP and manually specify the Post include to inject it to the RIB.
Using Post Include
Make sure $RMANTREE/lib/plugins is in your Procedural Path. This is required for finding the impl_openvdb.so.
Create a box OBJ. In the box's Render/Geometry's Obj post-include, include the RiVolume that will be emitted to the RIB.
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