NAME
	  gluTessVertex	- specify a vertex on a	polygon


     C SPECIFICATION
	  void gluTessVertex( GLUtesselator* tess,
			      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 follows:

	  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);
	    }

	  This doesn't work.  Because the pointers specified by
	  location and data might not be dereferenced until
	  gluTessEndPolygon is executed, all the vertex	coordinates
	  but the very last set	could be overwritten before
	  tessellation begins.

	  Two common symptoms of this problem are consists of a	single
	  point	(when a	local variable is used for data) and a
	  GLU_TESS_NEED_COMBINE_CALLBACK error (when a local variable
	  is used for location).

     SEE ALSO
	  gluTessBeginPolygon, gluNewTess, gluTessBeginContour,
	  gluTessCallback, gluTessProperty, gluTessNormal,
	  gluTessEndPolygon