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Deformation motion blurred volumes don't currently work with densityFloatPrimVar or densityColorPrimVar. You will need to use a PxrPrimVar node connected to densityFloat and densityColor instead
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.