The link to the PBR is broken for me too unfortunately so I don’t know if that is related to the JSON material infrastructure in VTK/Rendering/RayTracing.
That one is just the way I came up with back in the day for VTK to drive OSPRay’s API for specifying materials. It has since been extended to cover PBR, drive OptiX via MDL internally, and also optionally with input coming from the ancient .obj/.mtl file format instead of json.
That parser is VTK not OSPRay.
It could be promoted out of the RayTracing module. That would probably help bring VTK’s user interface to PBR materials in the GL and RayTraced backends closer together which would be a good thing.
However, I am with @andras and would rather see VTK’s json material interface(s?) deprecated and replaced with something that is both open and widely adopted. glTF seems to be the leading contender today.