Return 3D normals for polygons with vertices XYZ. If NXYZ is specified, XYZ should be sum(nxyz)-by-3, with NXYZ being the list of numbers of vertices for each polygon (as for the plfp function; see page 39). If NXYZ is not specified, XYZ should be a quadrilateral mesh, ni-by-nj-by-3 (as for the plf function; see page 35). In the first case, the return value is len(NXYZ)-by-3; in the second case, the return value is (ni-1)-by-(nj-1)-by-3.
The normals are constructed from the cross product of the lines joining the midpoints of two edges which as nearly quarter the polygon as possible (the medians for a quadrilateral). No check is made that these not be parallel; the returned ``normal'' is [0,0,0] in that case. Also, if the polygon vertices are not coplanar, the ``normal'' has no precisely definable meaning.