Core OpenGL

Core OpenGL, or CGL, is Apple Inc.'s Macintosh Quartz windowing system interface to the OS X implementation of the OpenGL specification. CGL is analogous to GLX, which is the X11 interface to OpenGL, as well as WGL, which is the Microsoft Windows interface to OpenGL.

All windowing system interfaces to OpenGL arose out of the migration of Silicon Graphics proprietary 3D graphics application programming interface (API) IrisGL to its current open standard form OpenGL. When the decision was made to make IrisGL an open standard, the primary required design change was to make this graphics standard API windowing system agnostic. All window system specific logic was therefore removed from IrisGL when moving to OpenGL. Window system logic includes any event mechanism for gathering input from devices such as keyboards and mice, as well as any window ordering or sizing logic used when drawing to a modern windowed user interface. Further, all internal management of window memory buffers, sometimes referred to as surfaces, was also removed from IrisGL to create OpenGL.

With OpenGL windowing system agnostic, companies such as Apple must shoulder the burden of configuring and managing the surfaces used as a destination for OpenGL rendering.

On OS X, CGL is the foundation layer of windowing system interfaces to OpenGL. Both AGL (Apple Graphics Library) and the Cocoa (API) (or AppKit) have interfaces to OpenGL and are logical software layers and depend on CGL for their behavior. CGL and AGL interoperate freely. CGL and Cocoa may be used together, however Cocoa classes may implicitly make changes to CGL state. Function calls from AGL and Cocoa should not be mixed.

Configuration of these surfaces is done through a pixel format selection process where different compatible layers of rendering information are combined to form a framebuffer. Examples of such layers are color buffers, transparency buffers (alpha), stencil buffers, and depth buffers. The CGL function CGLChoosePixelFormat is used to perform this buffer compatibility check. CGLChoosePixelFormat will, based on input parameters and their scoring policy, choose a pixel format that represents a compatible buffer configuration that is supported by the underlying renderer that will be used to process graphics commands. Renderers may be either hardware based, such that they correspond to graphics cards installed in the system or they may be software based, where the main CPU of the system handles all of the graphics command processing and final rasterization work.

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