Convert 3D vectors to directions.
A direction (x, y, z) given by the components of a vector will be represented by [x, y, z, 0] in homogeneous coordinates to which we can apply a transformation.
- Varray-like, shape (n_directions, 3)
Each row is a 3D vector that contains x, y, and z
- Parray-like, shape (n_directions, 4)
Each row is a direction vector with 0 as last element