Source PostScript (points3dalignes.pps)

%% syntaxe : A B C points3dalignes -> bool
/points3dalignes {
3 dict begin
   /C defpoint3d
   /B defpoint3d
   /A defpoint3d
   A B vecteur3d /u defpoint3d
   A C vecteur3d /v defpoint3d
   u v vectprod3d norme3d 1E-7 lt
end
} def

%% syntaxe : M A B point3dsursegment --> true si M in [AB], false sinon
/point3dsursegment {
3 dict begin
   /B defpoint3d
   /A defpoint3d
   /M defpoint3d
   M A B points3dalignes {
      M A vecteur3d
      M B vecteur3d
      scalprod3d 0 lt {
         true
      } {
         false
      } ifelse
   } {
      false
   } ifelse
end
} def