add OpenVDB format

[orbeckst]

OpenVDB grids are used in Blender's volume grids. In order to make it easier to visualize biomolecular simulation density data in Blender, we may want to support export to OpenVDB.

A OpenVDB Python API exists.

There's a conda-forge package available:

• https://anaconda.org/conda-forge/openvdb
• license: Apache-2.0

[orbeckst]

See OpenVDB: Working with NumPy arrays how we can easily create the appropriate data structures and then write top a vdb file.

[orbeckst]

See Introduction to Scientific Visualization with Blender

[orbeckst]

@BradyAJohnston , how does MN handle densities? Does it use openvdb to convert eg MRC files to VDB?

[BradyAJohnston]

@orbeckst yep that's exactly the approach. Currently we do GDF.Grid() -> write a .vdb via openvdb / numpy -> import .vdb into Blender.

There is no way inside of Blender (currently) to import a volume / density in-memory, it has to first be written to disk as a .vdb that is then imported.