The stereographic projection presents the quadric hypersurface as a rational hypersurface. The projective model identifies the conformal sphere with a certain quadric in a projective space. There are some authors who define a conic as a two-dimensional nondegenerate quadric. In more generality, the plane sections of a quadric are conic sections. This is the affine cone over the projective quadric "S". An "affine quadric" is the set of zeros of a polynomial of degree two. In fact, the hyperboloid is part of a quadric in real projective four-space. Elliptic cones are sometimes considered to be degenerate quadric surfaces as well. Similarly, quadratic polynomials with three or more variables correspond to quadric surfaces and hypersurfaces. Among quadric surfaces, an ellipsoid is characterized by either of the two following properties. For formula 14 one has two intersecting planes (reducible quadric). QCGA is capable of constructing quadric surfaces either using control points or implicit equations. For formula 16 one has two parallel planes (reducible quadric). Thus the density function is a scalar-to-scalar transformation of a quadric expression. A quadric is called non-degenerate if formula 154.