class TGLParametricPlot: public TGLPlotPainter

 A parametric surface is a surface defined by a parametric equation, involving
 two parameters (u, v):

 S(u, v) = (x(u, v), y(u, v), z(u, v)).
 For example, "limpet torus" surface can be defined as:
    x = cos(u) / (sqrt(2) + sin(v))
    y = sin(u) / (sqrt(2) + sin(v))
    z = 1 / (sqrt(2) + cos(v)),
 where -pi <= u <= pi, -pi <= v <= pi.


 TGLParametricEquation * eq =
    new TGLParametricEquation("Limpet_torus", "cos(u) / (sqrt(2.) + sin(v))",
                              "sin(u) / (sqrt(2.) + sin(v))",
                              "1 / (sqrt(2) + cos(v))");

 $ROOTSYS/tutorials/gl/glparametric.C contains more examples.

 Parametric equations can be specified:
    1. by string expressions, as with TF2, but with 'u' instead of 'x' and
       'v' instead of 'y'.
    2. by function - see ParametricEquation_t declaration.