The first operation which may be encountered is texturing, where a texel (texture element) is generated from texture memory for each fragment and applied to the fragment. Then fog calculations may be applied, followed by the scissor test, the alpha test, the stencil test, and the depth-buffer test (the depth buffer is for hidden-surface removal). Failing an enabled test may end the continued processing of a fragment's square. Then, blending, dithering, logical operation, and masking by a bitmask may be performed. (See Chapter 6, "Blending, Antialiasing, Fog, and Polygon.Offset" and Chapter 10, "The Framebuffer.") Finally, the thoroughly processed fragment is drawn into the appropriate buffer, where it has finally advanced to be a pixel and achieved its final resting place. 

C)penGL-Related Libraries 

OpenGL provides a powerful but primitive set of rendering commands, and all higher-level drawing must be done in terms of these commands. Also, OpenGL programs have to use the underlying mechanisms of the windowing system. A number of libraries exist to allow you to simplify your programming tasks, including the following: 

•   The OpenGL Utility Library (GLU) contains several routines that use lower-level OpenGL commands to perform such tasks as setting up matrices for specific viewing orientations and projections, performing polygon tessellation, and rendering surfaces. This library is provided as part of every OpenGL implementation. Portions of the GLU are described in the OpenGL Reference Manual. The more useful GLU routines are described in this guide, where they're relevant to the topic being discussed, such as in all of Chapter 1 1 and in the section "The GLU NURBS Interface" on page 455. GLU routines use the prefix glu. 

•   For every window system, there is a library that extends the functionality of that window system to support OpenGL rendering. For machines that use the X Window System, the OpenGL Extension to the X Window System (GLX) is provided as an adjunct to OpenGL. GLX routines use the prefix glX. For Microsoft Windows, the WGL routines provide the Windows to OpenGL interface. All WGL routines use the prefix wgi. For IBM OS/2, the PGL is the Presentation Manager to OpenGL interface, and its routines use the prefix pgl. 
All these window system extension libraries are described in more detail in both Appendix C. In addition, the GLX routines are also described in the OpenGL Reference Manual.