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The main individual volume properties that do not work include:

• visibility: aggregate volumes do not respect the standard visibility flags for camera, transmission, and indirect visibility that you might normally expect to set on the individual volumes. To get around this limitation, PxrPathTracer allows the specification of separate volume aggregates that are used when dealing with rays of a particular type: these are the volumeAggregateCamera, volumeAggregateTransmission, and volumeAggregateIndirect parameters. These controls are not as flexible as the normal visibility flags - you cannot mix the visibility flags on the individual volumes of the aggregate - you can only operate on either the visibility of all aggregates, or specify which aggregates to use for certain types of rays.
• volume sampling flags: the parameters on PxrVolume that control sampling techniques (e.g. minSamples, maxSamples, equiangularWeight) do not apply when using aggregate volumes. Instead, the controls for volume sampling are set in the main PxrPathTracer integrator. 

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Multi-scattering optimizations are fully supported, but the relevant controls on PxrVolume are ignored on aggregate volumes. Instead, in the aggregate volume workflow, those controls are now on the light sources themselves.In the initial 24.2 release, matte aggregate volumes may not correctly hold out against non-matte volumes. We anticipate that this will be addressed in an upcoming dot release.

Mipmapping

Mipmapping is a standard technique used to improve texture aliasing and rendering performance by automatically choosing an appropriate texture resolution level from a pyramid based on viewing size and angle of the texture on screen. This technique can optionally be applied to volumetric data comprised of voxels: in much the same way that a 2D mipmap pyramid level contains texels that are averages of four finer texels, a 3D mipmap pyramid level contains voxels that are each averaged from eight finer voxels. Mipmapping a volume asset will increase its disk space significantly, and may increase memory during rendering somewhat if the finest voxels are appropriately sized to the level of detail required for the camera; however, if the voxels are much finer in detail than the camera settings require, mipmapping can save significant RAM and render time because the renderer will only load coarser averaged voxels instead. Aggregate volumes using the impl_openvdb plugin as a data source for the volume primitives can enable mipmapping by setting the filterWidth parameter to a value greater than 0; this parameter is a multiplier on the standard filterwidth computed by the renderer.

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