e frame evaluated at 'trgepc'.  (This is
                         the frame relative to which 'spoint' is given.)

                         [3,1] = size(surf(i).obspos)
                         double = class(surf(i).obspos)

                         'obspos' is returned to simplify various related
                         computations that would otherwise be cumbersome. For
                         example, the vector 'xvec' from the observer to
                         'spoint' can be calculated via

    