1991-2006
Silicon Graphics, Inc.
gluTessVertex
3G

# 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 when the data 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

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

## Copyright

Copyright 1991-2006 Silicon Graphics, Inc. This document is licensed under the SGI Free Software B License. For details, see http://oss.sgi.com/projects/FreeB/.