How to use advection?

[alessandroAM]

Hello, after following a partial example here: [https://www.openvdb.org/documentation/doxygen/codeExamples.html#sModifyingGrids] (Advecting Points section), I've been trying to implement advection using a gravity velocity field. The resulting vdb is just the initial one, and I can't understand what I'm doing wrong. There is really little information around, anyone can help me? Here is the code I've so far:

openvdb::points::PointDataTree::Ptr pointTree(new openvdb::points::PointDataTree(grid->tree(), 0, openvdb::TopologyCopy())); openvdb::points::PointDataGrid::Ptr points = openvdb::points::PointDataGrid::create(pointTree); // get the PointDataGrid from the initial FloatGrid auto gravity = openvdb::Vec3SGrid::create(openvdb::Vec3s(0, -9.81, 0)); // Create an empty velocity grid with gravity as background value openvdb::points::advectPoints(*points->create(), *gravity, 4, 1.0 / 24.0, 1000); // Advect points in-place using gravity velocity grid

Thanks for any suggestion.

[danrbailey]

The code you're sharing here doesn't appear to add any points to the points grid - are you doing this outside of this snippet?

I suggest starting out looking at the "Hello World" example in the docs if you haven't already - https://www.openvdb.org/documentation/doxygen/codeExamples.html#sPointsHelloWorld that creates a handful of points based on world space position. You should also not use the create() method when calling that method as that will create a new (empty) point grid to pass in to the advection method.

openvdb::points::advectPoints(*points, *gravity, /*integrationOrder=*/4, /*dt=*/1.0/24.0, /*timeSteps=*/1);