Projective Root-Locus: An Extension of Root-Locus Plot to the Projective   Plane
  In this paper we present an extension of the classical Root-Locus (RL) method where the points are calculated in the real projective plane instead of the conventional affine real plane; we denominate this extension of the Root-Locus as "Projective Root-Locus (PjRL)". To plot the PjRL we use the concept of "Gnomonic Projection" in order to have a representation of the projective real plane as a simi-sphere of radius one in ${\mathbb R}^3$. We will see that the PjRL reduces to the RL in the affine $XY$ plane, but also we can plot the RL onto another affine component of the projective plane, like $ZY$ affine plane for instance, to obtain what we denominate complementary plots of the conventional RL. We also show that with the PjRL the points at infinity of the RL can be computed as solutions of a set algebraic equations. 
Francisco Mota
cs.SY