Tutorial: Converting VDB files to NanoVDB via OpenVDB in C++

VDB files are commonly used for storing volumetric density grids, such as clouds or smoke. Blender, Embergen, Houdini are a few sources of such data. More recently there is a sparse specification, called NanoVDB, storing sparse voxel octrees (SVOs) that is a more optimized representation for GPU rendering.

I'm outlining steps for Ubuntu, but these libraries are supported on Windows as well with a bit more setup.

build / install OpenVDB check here for cmake instructions
open your terminal and install libtbb
```
sudo apt install libtbb-dev
```
OpenExr is a dependency of OpenVDB, so this should already be built w/your version of OpenVDB

Here are the interesting excerpts from the OpenVDB library that you need to convert between VDB and NanoVDB:

...
#include "nanovdb/util/OpenToNanoVDB.h"
#include "nanovdb/util/IO.h"
...
    openvdb::initialize();
    openvdb::io::File file(argv[1]);
    file.open();

    openvdb::GridBase::Ptr baseGrid;
    for (openvdb::io::File::NameIterator nameIter = file.beginName();
       nameIter != file.endName(); ++nameIter) {
       // TODO: deal with other channels if you have them
       if (nameIter.gridName() == "density") {
           baseGrid = file.readGrid(nameIter.gridName());
       }
    }
    file.close();
        
    openvdb::FloatGrid::Ptr srcGrid = openvdb::gridPtrCast<openvdb::FloatGrid>(baseGrid);
    auto handle = nanovdb::openToNanoVDB(*srcGrid);

    // Write the NanoVDB grid to file
    nanovdb::io::writeGrid(argv[2], handle);
...

Find the complete source for my basic CLI example converting an input vdb file to a nanovdb file at https://github.com/bmahlbrand/openvdb2nanovdb-util

...