OpenGL Programming Guide

Chapter 6Lighting

Selecting a Lighting Model

Two-sided Lighting

Lighting calculations are performed for all polygons, whether they're front-facing or back-facing. Since you usually set up lighting conditions with the front-facing polygons in mind, however, the back-facing ones typically aren't correctly illuminated. In Example 6-1 where the object is a sphere, only the front faces are ever seen, since they're the ones on the outside of the sphere. So, in this case, it doesn't matter what the back-facing polygons look like. If the sphere was going to be cut away so that its inside surface would be visible, however, you might want to have the inside surface be fully lit according to the lighting conditions you've defined; you might also want to supply a different material description for the back faces. When you turn on two-sided lighting, as follows

glLightModeli(LIGHT_MODEL_TWO_SIDE, GL_TRUE);

OpenGL reverses the surface normals for back-facing polygons; typically, this means that the surface normals of visible back- and front-facing polygons face the viewer, rather than pointing away. As a result, all polygons are illumnated correctly.

To turn two-sided lighting off, pass in GL_FALSE as the argument in the preceding call. See "Defining Material Properties" for information about how to supply material properties for both faces. You can also control which faces OpenGL considers to be front-facing with the command glFrontFace(). See "Reversing and Culling Polygon Faces" for more information about this command.

OpenGL Programming Guide