Core OpenGL, or CGL, is

Apple Inc. Apple Inc. is an American multinational technology company headquartered in Cupertino, California, United States. Apple is the largest technology company by revenue (totaling in 2021) and, as of June 2022, is the world's biggest company ...

's Macintosh Quartz

windowing system In computing, a windowing system (or window system) is software that manages separately different parts of display screens. It is a type of graphical user interface (GUI) which implements the WIMP (windows, icons, menus, pointer) paradigm for ...

interface to the

OS X macOS (; previously OS X and originally Mac OS X) is a Unix operating system developed and marketed by Apple Inc. since 2001. It is the primary operating system for Apple's Mac computers. Within the market of desktop and lapt ...

implementation of the

OpenGL OpenGL (Open Graphics Library) is a cross-language, cross-platform application programming interface (API) for rendering 2D and 3D vector graphics. The API is typically used to interact with a graphics processing unit (GPU), to achieve hardwa ...

specification. CGL is analogous to

GLX GLX (initialism for "OpenGL Extension to the X Window System") is an extension to the X Window System core protocol providing an interface between OpenGL and the X Window System as well as extensions to OpenGL itself. It enables programs wishing ...

, which is the

X11 The X Window System (X11, or simply X) is a windowing system for bitmap displays, common on Unix-like operating systems. X provides the basic framework for a GUI environment: drawing and moving windows on the display device and interacting wi ...

interface to OpenGL, as well as WGL, which is the

Microsoft Windows Windows is a group of several proprietary graphical operating system families developed and marketed by Microsoft. Each family caters to a certain sector of the computing industry. For example, Windows NT for consumers, Windows Server for serv ...

interface to OpenGL.

History

All windowing system interfaces to

arose out of the migration of

Silicon Graphics Silicon Graphics, Inc. (stylized as SiliconGraphics before 1999, later rebranded SGI, historically known as Silicon Graphics Computer Systems or SGCS) was an American high-performance computing manufacturer, producing computer hardware and soft ...

proprietary 3D graphics application programming interface (

API An application programming interface (API) is a way for two or more computer programs to communicate with each other. It is a type of software interface, offering a service to other pieces of software. A document or standard that describes how ...

) 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.

Features

Windowing system interfaces

On OS X, CGL is the foundation layer of windowing system interfaces to OpenGL. Both AGL (Apple Graphics Library) and the

Cocoa (API) Cocoa is Apple's native object-oriented application programming interface (API) for its desktop operating system macOS. Cocoa consists of the Foundation Kit, Application Kit, and Core Data frameworks, as included by the Cocoa.h header file, and ...

(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 A framebuffer (frame buffer, or sometimes framestore) is a portion of random-access memory (RAM) containing a bitmap that drives a video display. It is a memory buffer containing data representing all the pixels in a complete video frame. Modern ...

. Examples of such layers are color buffers, transparency buffers (alpha),

stencil buffer A stencil buffer is an extra data buffer, in addition to the ''color buffer'' and ''Z-buffer'', found on modern graphics hardware. The buffer is per pixel and works on integer values, usually with a depth of one byte per pixel. The Z-buffer and ...

s, and depth buffers. The CGL functio
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 In computer graphics, rasterisation (British English) or rasterization (American English) is the task of taking an image described in a vector graphics format (shapes) and converting it into a raster image (a series of pixels, dots or lines, whic ...

work.

Handling Mac OS X heterogeneity

On Mac OS X, CGL is also responsible for handling the heterogeneous nature of graphics device installations and configuration on Macintosh systems. Macintosh computers may have any number of displays and graphics cards installed in them. In these configurations, the user's desktop may be virtualized (extended) or mirrored across multiple displays which are connected to multiple graphics cards which may or may not be from the same graphics vendor.

Controlling the rendering

When users configure their Macintosh to use a virtualized desktop, and they drag windows from one display to another, CGL handles the management of OpenGL graphics state that must be shadowed between devices to provide command processing consistency between them. Dragging a window across a Macintosh desktop between two different displays that are supported by two different renderers is known as a "Virtual Screen Change". CGL also provides a mechanism to obtain information about the renderer that is currently in use. The primary data structure that maintains OpenGL state on Mac OS X is
CGLContextObj
These CGL contexts can be retrieved at any time using a call t

The ''CGLContextObj'' may then be queried for specifics about the renderer that is associated with it.

Software renderer

Also included is Apple's in-house OpenGL

software renderer Software rendering is the process of generating an image from a model by means of computer software. In the context of computer graphics rendering, software rendering refers to a rendering process that is not dependent upon graphics hardware AS ...

. Originally, this was a simple integer package, in 10.3 a new floating point one was introduce
which ultimately replaced it
The software renderer, though slow, is fast enough for basic applications an

with OS X's OpenGL implementation for development purposes.

External links

CGL reference guide on Apple website (html).
-
CGL reference guide on Apple website (PDF).
- {{OS X Application programming interfaces Graphics standards OpenGL