_C_ License Applicability. Except to the extent portions of this file are
_C_ made subject to an alternative license as permitted in the SGI Free
_C_ Software License B, Version 1.1 (the "License"), the contents of this
_C_ file are subject only to the provisions of the License. You may not use
_C_ this file except in compliance with the License. You may obtain a copy
_C_ of the License at Silicon Graphics, Inc., attn: Legal Services, 1600
_C_ Amphitheatre Parkway, Mountain View, CA 94043-1351, or at:
_C_ 
_C_ http://oss.sgi.com/projects/FreeB
_C_ 
_C_ Note that, as provided in the License, the Software is distributed on an
_C_ "AS IS" basis, with ALL EXPRESS AND IMPLIED WARRANTIES AND CONDITIONS
_C_ DISCLAIMED, INCLUDING, WITHOUT LIMITATION, ANY IMPLIED WARRANTIES AND
_C_ CONDITIONS OF MERCHANTABILITY, SATISFACTORY QUALITY, FITNESS FOR A
_C_ PARTICULAR PURPOSE, AND NON-INFRINGEMENT.
_C_ 
_C_ Original Code. The Original Code is: OpenGL Sample Implementation,
_C_ Version 1.2.1, released January 26, 2000, developed by Silicon Graphics,
_C_ Inc. The Original Code is Copyright (c) 1991-2000 Silicon Graphics, Inc.
_C_ Copyright in any portions created by third parties is as indicated
_C_ elsewhere herein. All Rights Reserved.
_C_ 
_C_ Additional Notice Provisions: The application programming interfaces
_C_ established by SGI in conjunction with the Original Code are The
_C_ OpenGL(R) Graphics System: A Specification (Version 1.2.1), released
_C_ April 1, 1999; The OpenGL(R) Graphics System Utility Library (Version
_C_ 1.3), released November 4, 1998; and OpenGL(R) Graphics with the X
_C_ Window System(R) (Version 1.3), released October 19, 1998. This software
_C_ was created using the OpenGL(R) version 1.2.1 Sample Implementation
_C_ published by SGI, but has not been independently verified as being
_C_ compliant with the OpenGL(R) version 1.2.1 Specification.
_C_
_C_ $Date: 2004/12/14 16:20:37 $ $Revision: 1.1.1.1 $
_C_ The first character in this file must be an '_'!
_C_ Anything on a line after _C_ is ignored
_define(_filters,eqn)_C_
_C_      eqn is automatically replaced with neqn for nroff
_header(TessVertex, specify a vertex on a polygon)
_names(TessVertex)
.EQ
delim $$
.EN
.SH PARAMETERS
_phead(_param1)
Specifies the tessellation object (created with _cmnd(NewTess)).
_phead(_param2)
Specifies the location of the vertex.
_phead(_param3)
Specifies an opaque pointer passed back to the program with the vertex callback
(as specified by _cmnd(TessCallback)).
.SH DESCRIPTION
_cmnd describes a vertex on a polygon that the program defines. Successive
_cmnd calls describe a closed contour. For example, 
to describe a quadrilateral _cmnd should be called four times.
_cmnd can only be called between _cmnd(TessBeginContour) and 
_cmnd(TessEndContour).
.P
_param3 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 _gluconst(TESS_VERTEX)
or _gluconst(TESS_VERTEX_DATA) callback after tessellation 
(see the _cmnd(TessCallback) reference page).
.SH EXAMPLE
A quadrilateral with a triangular hole in it can be described as follows:
.sp
.Ex
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);
.Ee
.sp
.SH NOTES
It is a common error to use a local variable for _param2 or _param3 and store
values into it as part of a loop.
For example:
.Ex
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);
  }
.Ee
.P
This doesn't work.
Because the pointers specified by _param2 and _param3 might not be 
dereferenced until _cmnd(TessEndPolygon) is executed,
all the vertex coordinates but the very last set could be overwritten
before tessellation begins.
.P
Two common symptoms of this problem are consists of a single point
(when a local variable is used for _param3) and a
_gluconst(TESS_NEED_COMBINE_CALLBACK) error (when a local variable is
used for _param2).
.SH SEE ALSO
_cmnd(TessBeginPolygon), _cmnd(NewTess), _cmnd(TessBeginContour), 
_cmnd(TessCallback), 
_cmnd(TessProperty), _cmnd(TessNormal),
_cmnd(TessEndPolygon)


