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Structure of OpenGL in Mac OS X

Mac OS X supports a display space that can consist of multiple dissimilar displays, each driven by different graphics cards with different capabilities. In addition, multiple OpenGL renderers can drive each graphics card. To accommodate this versatility, OpenGL for Mac OS X is segmented into three well-defined layers: a window system layer, a framework layer, and a driver layer, as shown in Figure 1-2. This segmentation allows for plug-in interfaces to both the window system layer and the framework layer. Plug-in interfaces offer flexibility in software and hardware configuration without violating the OpenGL standard.

The window system layer is what allows your OpenGL program to become a reality onscreen. You'll use the Apple-specific OpenGL APIs provided in this layer—the NSOpenGL classes and the AGL and CGL APIs—to direct where OpenGL drawing takes place and control a variety of aspects of rendering. These APIs contain functions and methods specific to the Mac OS X windowing system. (See “OpenGL APIs Specific to Mac OS X” for more information.) This layer also includes the OpenGL libraries—GL, GLU, and GLUT. (See “Apple-Implemented OpenGL Libraries” for details.)

The common OpenGL framework layer is the software interface to the graphics hardware. This layer contains Apple's implementation of the OpenGL specification.

The driver layer contains the optional GLD plug-in interface and one or more GLD plug-in drivers, which may have different software and hardware support capabilities. The GLD plug-in interface supports third-party plug-in drivers, allowing third-party hardware vendors to take advantage of newer driver technology.

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