OSPRay volume rendering


I would like to use OSPRay to do volume rendering in two scenarios: (1) high resolution always, or (2) low resolution during rotation/zoom, high resolution as result. For CPU and GPU, VTK does support such scenario. However, VTK does not support this in the case of OSPRay since there is no SetLockSampleDistanceToInputSpacing for OSPRay rendering mode, do I miss something? I notice VTK 9.1 removed volume rendering sample adjustments so I use VTK 9.0 with OSPRay v1.8.5 or v1.7.3 but the scenario (1) cannot be done, also, it seems there is no way to change low resolution.
My code sneppit is shown below:

vtkSmartVolumeMapper * smapper = vtkSmartVolumeMapper::New();

Any help is very appreciated!