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OpenVDB is an open source library developed by DreamWorks Animation and can store voxel data. The format can fully replace RealFlow's native GFC and it is possible to resume from interrupted simulations or create secondary elements from VDB files. RealFlow's fields have to be converted into VDB-compatible data and therefore it takes longer to write VDB files. OpenVDB supports compression. Since the format is an open source C++ library, TDs are able to write their own import tools. The RealFlow-specific meta data and many additional information can be found here. Please bear in mind that RealFlow's OpenVDB implementation is tied to Houdini's axis setup.
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