It seems using a volumesamplefile VOP in an attributeVOP and pointing the geometry file to a SOP using something like op:/obj/myvolume/vdbfrompolygons works just fine to sample the volume. But when the same VOP is used inside a material it fails to sample the volume. It seems in the material context it will only sample the volume correctly if it is read from an actual file on disk.

Is this true? I thought the op:/ syntax should work almost anywhere ?

Mantra doesn't have access to houdini network, only to whatever geo is currently in ifd

You can use op: syntax to point to any SOP that has Render flag on whose Object is included in render

Typical workflow is to make such object not renderable, but still in Forced Objects, just to get it to .ifd

Oh wow I've been working with for Houdini for 7 years and I just would frequently have issues with the op: syntax. I never knew you needed to have the object mentioned in the render node >.<

You just helped me with the XYZ Distance node. Adding this comment for some SEO.