OpenGL Programming Guide

Chapter 3Viewing

Projection Transformations

Viewing Volume Clipping

After the vertices of the objects in the scene have been transformed by the modelview and projection matrices, any vertices that lie outside the viewing volume are clipped. The six clipping planes used are those that define the sides and ends of the viewing volume. You can specify additional clipping planes and locate them wherever you choose; this relatively advanced topic is discussed in "Additional Clipping Planes." Keep in mind that OpenGL reconstructs the edges of polygons that get clipped.

OpenGL Programming Guide