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With the geometry description of a volume being relatively simple, setting the shading parameters of the volume is critical to the overall look. For volume effects like fog, smoke, and clouds, RenderMan provides a dedicated shader called PxrVolume. After performing a volumetric simulation, a typical workflow for rendering involves setting up the geometry description as above, deciding which variables in the geometry description need to be mapped either directly to inputs of PxrVolume, or remapped using intermediate Pattern nodes, and finally tweaking the settings in PxrVolume to control the overall volumetric look. Note that scattering in the volume is controlled by the Max Path Length parameter in the chosen integrator and not the individual diffuse and specular trace depths.
The "dice" "minlength" attribute should be set to -1 to provide a hint about what the minlength should be based on the voxel data. This may become the default behavior in the future.