%% syntaxe : vect1 vect2 vect3 angle3d
%% vect3 est la normale au plan (vect1, vect2)
/angle3doriente {
4 dict begin
   normalize3d /vect3 defpoint3d
   normalize3d /vect2 defpoint3d
   normalize3d /vect1 defpoint3d
   /cosalpha vect1 vect2 scalprod3d def
   /sinalpha vect1 vect2 vectprod3d vect3 scalprod3d def
   sinalpha cosalpha atan
end
} def