Getting a fully masked array for your indices happens when you are using a Grid_S and when all your points are discovered to be off-grid. This is an inconsistent bit of API. Grid_U and Grid_R do not return a masked array for the same locate_faces function.
Getting a fully masked array for your indices happens when you are using a Grid_S and when all your points are discovered to be off-grid. This is an inconsistent bit of API. Grid_U and Grid_R do not return a masked array for the same locate_faces function.