GLdouble *location,
                                      GLvoid* data )

PARAMETERS
       tess      Specifies the tessellation object (created with gluNewTess).

       location  Specifies the location of the vertex.

       data      Specifies  an  opaque  pointer passed back to the program with
                 the vertex callback (as specified by gluTessCallback).

DESCRIPTION
       gluTessVertex describes a vertex on a polygon that the program  defines.
       Successive  gluTessVertex  calls describe a closed contour. For example,
       to describe a quadrilateral gluTessVertex should be called  four  times.
       gluTessVertex   can  only  be  called  between  gluTessBeginContour  and
       gluTessEndContour.

       data normally points to a structure containing the vertex  location,  as
       well  as  other  per-vertex  attributes  such as color and normal.  This
       pointer is passed back  to  the  user  through  the  GLU_TESS_VERTEX  or
       GLU_TESS_VERTEX_DATA    callback    after    tessellation    (see    the
       gluTessCallback reference page).

EXAMPLE
       A quadrilateral with a triangular hole in it can be  described  as  fol‐
       lows:

       gluTessBeginPolygon(tobj, NULL);
        gluTessBeginContour(tobj);
          gluTessVertex(tobj, v1, v1);
          gluTessVertex(tobj, v2, v2);
          gluTessVertex(tobj, v3, v3);
          gluTessVertex(tobj, v4, v4);
        gluTessEndContour(tobj);
        gluTessBeginContour(tobj);
          gluTessVertex(tobj, v5, v5);
          gluTessVertex(tobj, v6, v6);
          gluTessVertex(tobj, v7, v7);
        gluTessEndContour(tobj); gluTessEndPolygon(tobj);

NOTES
       It  is  a  common error to use a local variable for location or data and
       store values into it as part of a loop.  For example: for (i =  0;  i  <
       NVERTICES; ++i) {
         GLdouble data[3];
         data[0] = vertex[i][0];
         data[1] = vertex[i][1];
         data[2] = vertex[i][2];
         gluTessVertex(tobj, data, data);
         }