Missing sets in rational parametrizations of surfaces of revolution
  Parametric representations do not cover, in general, the whole geometric object that they parametrize. This can be a problem in practical applications. In this paper we analyze the question for surfaces of revolution generated by real rational profile curves, and we describe a simple small superset of the real zone of the surface not covered by the parametrization. This superset consists, in the worst case, of the union of a circle and the mirror curve of the profile curve. 
J. Rafael Sendra, David Sevilla, Carlos Villarino
math.AG cs.SC