gluTessVertex

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 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).