Saturday, June 12, 2010


OpenGL (Open Graphics Library) is a standard specification defining a cross-language, cross-platform API for writing applications that produce 2D and 3D computer graphics. The interface consists of over 250 different function calls which can be used to draw complex three-dimensional scenes from simple primitives. OpenGL was developed by Silicon Graphics Inc. (SGI) in 1992 and is widely used in CAD, virtual reality, scientific visualization, information visualization, and flight simulation. It is also used in video games, where it competes with Direct3D on Microsoft Windows platforms (see OpenGL vs. Direct3D). OpenGL is managed by the non-profit technology consortium Khronos Group.

OpenGL serves two main purposes:
  • Hide complexities of interfacing with different 3D accelerators by presenting a single, uniform interface
  • Hide differing capabilities of hardware platforms by requiring support of full OpenGL feature set for all implementations (using software emulation if necessary).
OpenGL's basic operation is to accept primitives such as points, lines and polygons, and convert them into pixels. This is done by a graphics pipeline known as the OpenGL state machine. Most OpenGL commands either issue primitives to the graphics pipeline, or configure how the pipeline processes these primitives. Prior to the introduction of OpenGL 2.0, each stage of the pipeline performed a fixed function and was configurable only within tight limits. OpenGL 2.0 offers several stages that are fully programmable using GLSL.
OpenGL is a low-level, procedural API, requiring the programmer to dictate the exact steps required to render a scene. This contrasts with descriptive (aka scene graph or retained mode) APIs, where a programmer only needs to describe a scene and can let the library manage the details of rendering it. OpenGL's low-level design requires programmers to have a good knowledge of the graphics pipeline, but also gives a certain amount of freedom to implement novel rendering algorithms.
OpenGL has historically been influential on the development of 3D accelerators, promoting a base level of functionality that is now common in consumer-level hardware:

Simplified version of the Graphics Pipeline Process; excludes a number of features like blending, VBOs and logic ops
  • Rasterised points, lines and polygons as basic primitives
  • A transform and lighting pipeline
  • Z-buffering
  • Texture mapping
  • Alpha blending
A brief description of the process in the graphics pipeline could be:
  1. Evaluation, if necessary, of the polynomial functions which define certain inputs, like NURBS surfaces, approximating curves and the surface geometry.
  2. Vertex operations, transforming and lighting them depending on their material. Also clipping non-visible parts of the scene in order to produce the viewing volume.
  3. Rasterisation or conversion of the previous information into pixels. The polygons are represented by the appropriate colour by means of interpolation algorithms.
  4. Per-fragment operations, like updating values depending on incoming and previously stored depth values, or colour combinations, among others.
  5. Lastly, fragments are inserted into the frame buffer.
Many modern 3D accelerators provide functionality far above this baseline, but these new features are generally enhancements of this basic pipeline rather than radical revisions of it.

In the 1980s, developing software that could function with a wide range of graphics hardware was a real challenge. Software developers wrote custom interfaces and drivers for each piece of hardware. This was expensive and resulted in much duplication of effort.
By the early 1990s, Silicon Graphics (SGI) was a leader in 3D graphics for workstations. Their IRIS GL API[8] was considered the state of the art and became the de facto industry standard, overshadowing the open standards-based PHIGS. This was because IRIS GL was considered easier to use, and because it supported immediate mode rendering. By contrast, PHIGS was considered difficult to use and outdated in terms of functionality.
SGI's competitors (including Sun Microsystems, Hewlett-Packard and IBM) were also able to bring to market 3D hardware, supported by extensions made to the PHIGS standard. This in turn caused SGI market share to weaken as more 3D graphics hardware suppliers entered the market. In an effort to influence the market, SGI decided to turn the IrisGL API into an open standard.
SGI considered that the IrisGL API itself wasn't suitable for opening due to licensing and patent issues. Also, the IrisGL had API functions that were not relevant to 3D graphics. For example, it included a windowing, keyboard and mouse API, in part because it was developed before the X Window System and Sun's NeWS systems were developed.
In addition, SGI had a large number of software customers; by changing to the OpenGL API they planned to keep their customers locked onto SGI (and IBM) hardware for a few years while market support for OpenGL matured. Meanwhile, SGI would continue to try to maintain their customers tied to SGI hardware by developing the advanced and proprietary Iris Inventor and Iris Performer programming APIs.
As a result, SGI released the OpenGL standard.
The OpenGL standardised access to hardware, and pushed the development responsibility of hardware interface programs, sometimes called device drivers, to hardware manufacturers and delegated windowing functions to the underlying operating system. With so many different kinds of graphic hardware, getting them all to speak the same language in this way had a remarkable impact by giving software developers a higher level platform for 3D-software development.
In 1992, SGI led the creation of the OpenGL architectural review board (OpenGL ARB), the group of companies that would maintain and expand the OpenGL specification for years to come. OpenGL evolved from (and is very similar in style to) SGI's earlier 3D interface, IrisGL. One of the restrictions of IrisGL was that it only provided access to features supported by the underlying hardware. If the graphics hardware did not support a feature, then the application could not use it. OpenGL overcame this problem by providing support in software for features unsupported by hardware, allowing applications to use advanced graphics on relatively low-powered systems.
In 1994, SGI played with the idea of releasing something called "OpenGL++" which included elements such as a scene-graph API (presumably based on their Performer technology). The specification was circulated among a few interested parties – but never turned into a product.
Microsoft released Direct3D in 1995, which would become the main competitor of OpenGL. On December 17, 1997, Microsoft and SGI initiated the Fahrenheit project, which was a joint effort with the goal of unifying the OpenGL and Direct3D interfaces (and adding a scene-graph API too). In 1998, Hewlett-Packard joined the project. It initially showed some promise of bringing order to the world of interactive 3D computer graphics APIs, but on account of financial constraints at SGI, strategic reasons at Microsoft, and general lack of industry support, it was abandoned in 1999.
OpenGL releases are backward compatible. In general, graphics cards released after the OpenGL version release dates shown below support those version features, and all earlier features. For example the GeForce 6800, listed below, supports all features up to and including OpenGL 2.0. (Specific cards may conform to an OpenGL spec, but selectively not support certain features. For details, the GPU Caps Viewer software includes a database of cards and their supported specs)