Motion blur for custom prim adapter stopped working in Solaris for Houdini 19.5


We have some custom USD primitives with associated Hydra adapters, and these were working as expected in Solaris for Houdini 19.0 (USD 21.08). The primitive schemas derive from “Imageable”, and the adapters derive from UsdImagingPrimAdapter. However, motion blur doesn’t work under Houdini 19.5 / USD 22.05 in both the Renderman RIS and Karma delegates. We have found that the adapters’ ::SamplePrimvar method is always being invoked with maxNumSamples set to 1 (as might be expected).

We’ve played around with a few “wild guess” ideas, but nothing has worked. I have also skimmed over the USD release notes and, the only change which seems like it might be relevant is:

“Improved hdPrman’s motion blur support by adding blur scale and velocity and
acceleration motion blur implemented through a new scene index.
The instantaneousShutter setting has been deprecated in favor of

I am not really sure what to do with this information.

Does anyone know what we might be missing?

Thanks in advance