Our primary lighting consists of a directional light that points in the direction of the negative z-axis. We also add a dark gray ambient light (the color darkGray is defined in RGB.h). The use of ambient light allows us to see, although dimly, those facets not illuminated by our directional light. We enable here the directional light (GL_LIGHT1) but do not enable lighting in general since it is used only when rendering the surface of revolution but not the checker board (see our display function).

Last, we enable two-sided lighting with the call

     glLightModeli(GL_LIGHT_MODEL_TWO_SIDE, GL_TRUE);

We do this since inside faces of our surface of revolution are visible and, in turn, we want those faces to be illuminated by the directional light.