OpenGL Programming Guide

Appendix CThe OpenGL Utility Library

Polygon Tessellation

The Callback Mechanism

To tessellate a polygon using the GLU, first you need to create a tessellation object, and then provide a series of callback routines to be called at appropriate times during the tessellation. After you specify the callbacks, you describe the polygon and any holes using GLU routines, which are similar to the OpenGL polygon routines. When the polygon description is complete, the tessellation facility invokes your callback routines as necessary.

The callback routines typically save the data for the triangles, triangle meshes, and triangle fans in user-defined data structures, or in OpenGL display lists (see Chapter 4 ). To render the polygons, other code traverses the data structures or calls the display lists. Although the callback routines could call OpenGL commands to display them directly, this is usually not done, as tessellation can be computationally expensive. It's a good idea to save the data if there is any chance that you want to display it again. The GLU tessellation routines are guaranteed never to return any new vertices, so interpolation of vertices, texture coordinates, or colors is never required.

OpenGL Programming Guide