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This type of volumetrics can currently load an OpenVDB file (.vdb). This file format can efficiently store very detailed and large volumetrics based on a 3D grid of voxels instead of individual point particles, which makes it more efficient for storing and rendering detailed volumetrics where otherwise billions and billions of individual particles would have been needed. With the 3.1 update, Maxwell Render can now read and render these files directly. Any Maxwell material can be applied to the volumetric object (except an emitter material), although in most cases a simple lambert is enough to create a very convincing volumetric that reacts very realistically to all lighting in the scene. Once you've set the "Field type" parameter to "Volume file based" you can load an .vdb file in the filename parameter.
OpenVDB parameters
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