If the size of a vector is known in advance, it should used since it's more efficient than push_back which will cause multiple allocations and copies.

std::vector<openvdb::Vec3s> points(totvertices);
for (unsigned int i = 0; i < totvertices; i++) {
  points[i] = openvdb::Vec3s(vertices[i * 3], vertices[i * 3 + 1], vertices[i * 3 + 2])
}

unsigned int -> size_t, it's better to use the same type as .size().

this->grid && is redundant.

Why is this passing iterations / 100.0? If it's not actually a number of iterations that we are specifying, this seems misleading.

Either remove or explain commented out code.

In this case there is no copy, just a reference, so the name is misleading.

The member function names in this class don't need an OpenVDB_ or OpenVDB_level_set_ prefix, it's redundant.

The grid pointer here should be passed by reference:

void OpenVDB_level_set_set_grid(const openvdb::FloatGrid::Ptr& grid)

Not a performance issue in this case, but copying shared pointers like these is not a cheap operation in general, so best to have the habit of passing them around more efficiently.

This goes for all the OpenVDB ::Ptr types.

This function is not implemented?

This function is not implemented?

int -> OpenVDBLevelSet_FilterBias

I see no reason for this to exist at all, seems fine to remove. But then OpenVDBIntGrid and OpenVDBVectorGrid can be removed as well?

struct is not needed here.

Commented out code should have an explaining comment, or get removed.

Add empty line above this one.

For type safetype and clarity you could instead use this:

enum OpenVDBLevelSet_FilterType filter_type;
enum OpenVDBLevelSet_FilterBias filter_bias;

Comment style: /* Parameter for gaussian, median, mean. */

Add const

Add const for both

This should be:

const openvdb::FloatGrid::Ptr &get_grid();
void set_grid(const openvdb::FloatGrid::Ptr& grid);

The const is important, we don't want callers to be able to modifier the grid pointer inside OpenVDBLevelSet, we just want to avoid copying it when it's not needed.

Add const

Add const for both

Add const

Use const openvdb::math::Transform::Ptr&