The “Volume Quality” setting is no longer in the Mantra > Rendering > Sampling interface by default, and isn't listed in the render properties in “Edit Parameter Interface”.
If I add it as a float (vm_volumequality) it doesn't appear to do anything when I change the value.
Is that parameter still doing anything? It's still in the documentation for 16.5

http://www.sidefx.com/docs/houdini/props/mantra [www.sidefx.com]

Thanks,
~Mike D.

This was renamed to vm_volumesteprate in a late build of 16.0

16.5 introduced a new property ‘vm_volumequality’ that has a different effect – controlling indirect sample quality for volume rays. You won't see any change if volume limit is 0, as no indirect sampling takes place.

The documentation in that link is correct for 16.5:

Volume Quality

Houdini render property vm_volumequality
IFD property object:volumequality
Controls the quality of indirect volume sampling (for information on the difference between direct and indirect rays, see sampling and noise). Often, indirect sources of light (such as the surfaces of other objects, and light scattered inside of a volume) will be a significant cause of noise in your renders. Turning this up should decrease this type of noise, at the cost of slowing down rendering.

This parameter acts as a multiplier on Min Ray Samples and Max Ray Samples and also as a divisor for Noise Level. For example, if you have Min Ray Samples set to 1, May Ray Samples set to 8 and your Noise Level to 0.1, then set Volume Quality to 2, Mantra will send between 2 and 16 secondary volume ray samples based on a Noise Level of 0.05. Remember these numbers apply only to the indirect samples. Mantra uses the original values for all direct sampling.

To find how much noise is in your indirect volume component, add the “Indirect Lighting (per-component)” image plane in the Extra Image Planes tab. This lets you check each indirect component individually. For this parameter, you should check the Indirect Volume component.

Thank you so much!