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The rendering of effects like fog, smoke, clouds, cloudy water, and even glass is complicated because light no longer just interacts with the surface of a material; it can be both attenuated and scattered inside and within the participating media. Volume rendering is required to handle the complex light transport.
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.
For debugging purposes, in the Renderman.ini file you can find /prman/deepcomp/flagvolumes and Option "deep" "int flagvolumes" controls now interpret the value 1 as an "auto" mode. This is on by default, but off if generating any deep images in DTEX format. Use 2 to force it always on. This only affects samples generated by volumes.