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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 Direct3D vs. OpenGL). OpenGL is managed by the non-profit technology consortium, the Khronos Group.

Design

OpenGL serves two main purposes:
  • To hide the complexities of interfacing with different 3D accelerators, by presenting the programmer with a single, uniform API.
  • To hide the differing capabilities of hardware platforms, by requiring that all implementations support the full OpenGL feature set (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


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.

Example

This example will draw a green square on the screen. OpenGL has several ways to accomplish this task, but this is the easiest to understand. However, the reader should be aware that most of the APIs used in the code below have been deprecated in and after the OpenGL 3.0 specification.

glClear( GL_COLOR_BUFFER_BIT );
This statement clears the color buffer, so that the screen will start blank.


glMatrixMode( GL_PROJECTION ); /* Subsequent matrix commands will affect the projection matrix */glLoadIdentity(); /* Initialise the projection matrix to identity */glFrustum( -1, 1, -1, 1, 1, 1000 ); /* Apply a perspective-projection matrix */
These statements initialize the projection matrix, setting a 3d frustum matrix that represents the viewable area. This matrix transforms objects from camera-relative space to OpenGL's projection space.


glMatrixMode( GL_MODELVIEW ); /* Subsequent matrix commands will affect the modelview matrix */glLoadIdentity(); /* Initialise the modelview to identity */glTranslatef( 0, 0, -3 ); /* Translate the modelview 3 units along the Z axis */
These statements initialize the modelview matrix. This matrix defines a transform from model-relative coordinates to camera space. The combination of the modelview matrix and the projection matrix transforms objects from model-relative space to projection screen space.


glBegin( GL_POLYGON ); /* Begin issuing a polygon */glColor3f( 0, 1, 0 ); /* Set the current color to green */glVertex3f( -1, -1, 0 ); /* Issue a vertex */glVertex3f( -1, 1, 0 ); /* Issue a vertex */glVertex3f( 1, 1, 0 ); /* Issue a vertex */glVertex3f( 1, -1, 0 ); /* Issue a vertex */glEnd(); /* Finish issuing the polygon */
These commands draw a green square in the XY plane.


Documentation

OpenGL's popularity is partially due to the excellence of its official documentation. The OpenGL ARB released a series of manuals along with the specification which have been updated to track changes in the API. These are almost universally known by the colors of their covers:

  • The Red Book – OpenGL Programming Guide, 6th edition. ISBN 0-321-48100-3
  • : A readable tutorial and reference book – this is a 'must have' book for OpenGL programmers. The first edition is available online here and here.
  • The Blue Book – OpenGL Reference manual, 4th edition. ISBN 0-321-17383-X
  • : Essentially a hard-copy printout of the man pages for OpenGL.
  • : Includes a poster-sized fold-out diagram showing the structure of an idealised OpenGL implementation. It is available here.
  • The Green Book – OpenGL Programming for the X Window System. ISBN 0-201-48359-9
  • : A book about X11 interfacing and GLUT.
  • The Alpha Book (which actually has a white cover) – OpenGL Programming for Windows 95 and Windows NT. ISBN 0-201-40709-4
  • : A book about interfacing OpenGL with Microsoft Windows.


Then, for OpenGL 2.0 and beyond:

  • The Orange Book – OpenGL Shading Language, 2nd edition. ISBN 0-321-33489-2
  • : A readable tutorial and reference book for GLSL.


Extensions

The OpenGL standard allows individual vendors to provide additional functionality through extensions as new technology is created. Extensions may introduce new functions and new constants, and may relax or remove restrictions on existing OpenGL functions. Each vendor has an alphabetic abbreviation that is used in naming their new functions and constants. For example, NVIDIAmarker's abbreviation (NV) is used in defining their proprietary function glCombinerParameterfvNV() and their constant GL_NORMAL_MAP_NV.

It may happen that more than one vendor agrees to implement the same extended functionality. In that case, the abbreviation EXT is used. It may further happen that the Architecture Review Board "blesses" the extension. It then becomes known as a standard extension, and the abbreviation ARB is used. The first ARB extension was GL_ARB_multitexture, introduced in version 1.2.1. Following the official extension promotion path, multitexturing is no longer an optionally implemented ARB extension, but has been a part of the OpenGL core API since version 1.3.

Before using an extension a program must first determine its availability, and then obtain pointers to any new functions the extension defines. The mechanism for doing this is platform-specific and libraries such as GLEW and GLEE exist to simplify the process.

Specifications for nearly all extensions can be found at the official extension registry.

Associated utility libraries

Several libraries are built on top of or beside OpenGL to provide features not available in OpenGL itself. Libraries such as GLU can always be found with OpenGL implementations, and others such as GLUT and SDL have grown over time and provide rudimentary cross platform windowing and mouse functionality and if unavailable can easily be downloaded and added to a development environment. Simple graphical user interface functionality can be found in libraries like GLUI or FLTK. Still other libraries like GLAux (OpenGL Auxiliary Library) are deprecated and have been superseded by functionality commonly available in more popular libraries, but code using them still exists, particularly in simple tutorials. Other libraries have been created to provide OpenGL application developers a simple means of managing OpenGL extensions and versioning. Examples of these libraries include GLEW (the OpenGL Extension Wrangler Library) and GLEE (the OpenGL Easy Extension Library).

In addition to the aforementioned simple libraries, other higher level object oriented scene graph retained mode libraries exist such as PLIB, OpenSG, OpenSceneGraph, and OpenGL Performer. These are available as cross platform free/open source or proprietary programming interfaces written on top of OpenGL and systems libraries to enable the creation of real-time visual simulation applications. Other solutions support parallel OpenGL programs for Virtual Reality, scalability or graphics clusters usage, either transparently like Chromium or through a programming interface like Equalizer.

Mesa 3D is a free/open source implementation of OpenGL. It supports pure software rendering as well as providing hardware acceleration for several 3D graphics cards under Linux. As of June 22, 2007 it implements the 2.1 standard, and provides some of its own extensions for some platforms.

Bindings

In order to emphasize its multi-language and multi-platform characteristics, various bindings and ports have been developed for OpenGL in many languages. Some languages and their bindings are:

For more information on OpenGL Language bindings, see opengl.org's Wiki.

Higher level functionality

OpenGL was designed to be graphic output-only: it provides only rendering functions. The core API has no concept of windowing systems, audio, printing to the screen, keyboard/mouse or other input devices. While this seems restrictive at first, it allows the code that does the rendering to be completely independent of the operating system it is running on, allowing cross-platform development. However, some integration with the native windowing system is required to allow clean interaction with the host system. This is performed through the following add-on APIs:



Additionally, GLUT, SDL and the GLFW libraries provide functionality for basic windowing using OpenGL, in a portable manner.

Some open source cross-platform toolkits, such as GTK+, Qt and WxWidgets, include widgets to embed OpenGL contents.

History

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 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 around 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 17 December 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 2.0

OpenGL 2.0 was conceived by 3Dlabs to address concerns that OpenGL was stagnating and lacked a strong direction. 3Dlabs proposed a number of major additions to the standard. Most of these were, at the time, rejected by the ARB or otherwise never came to fruition in the form that 3Dlabs proposed. However, their proposal for a C-style shading language was eventually completed, resulting in the current formulation of GLSL (the OpenGL Shading Language, also slang). Like the assembly-like shading languages that it was replacing, it allowed the programmer to replace the fixed-function vertex and fragment pipe with shaders, though this time written in a C-like high-level language.

The design of GLSL was notable for making relatively few concessions to the limitations of the hardware then available; this hearkened back to the earlier tradition of OpenGL setting an ambitious, forward-looking target for 3D accelerators rather than merely tracking the state of currently available hardware. The final OpenGL 2.0 specification includes support for GLSL.

OpenGL 2.1

OpenGL 2.1 was released on August 2, 2006 and is backward compatible with all prior OpenGL versions. OpenGL 2.1 incorporates the following functionality:
  • OpenGL Shading Language revision 1.20 (GLSL)
  • Commands to specify and query non-square matrix uniforms for use with the OpenGL Shading Language
  • Pixel buffer objects for efficient image transfers to and from buffer objects for commands such as glTexImage2D and glReadPixels.
  • : This functionality corresponds to the ARB_pixel_buffer_object extension.
  • sRGB texture formats.
  • : This functionality corresponds to the EXT_texture_sRGB extension.


OpenGL 3.0

Revision 3.0 of the OpenGL API was released on August 11, 2008 and is backward compatible with all prior OpenGL versions, though a deprecation mechanism has been introduced to simplify the API in future revisions.
  • OpenGL Shading Language revision 1.30 (GLSL)
  • Vertex Array Objects
  • More flexible Framebuffer Objects
  • 32-bit (single precision) floating-point textures and render buffers
  • 16-bit (half precision) floating-point vertex and pixel data
  • Ability to render vertex transformations into a buffer
  • Texture arrays
  • 32-bit (single precision) floating point depth buffer support


Full use of OpenGL 3.0 requires the same level of hardware as is required for DirectX 10 support. Unlike DirectX 10, OpenGL 3.0 does not require Windows Vista and can be used on any OS for which the appropriate drivers are provided.

Longs Peak and OpenGL 3.0 controversy

Prior to the release of OpenGL 3.0 the new revision was known as the codename Longs Peakmarker. At the time of its original announcement Longs Peak was presented as the first major API revision in OpenGL's lifetime. It consisted of an overhaul to the way that OpenGL works, calling for fundamental changes to the API.

The draft introduced a change to object management. The GL 2.1 object model was built upon the state-based design of OpenGL. That is, in order to modify an object or to use it, one needs to bind the object to the state system, then make modifications to the state or perform function calls that use the bound object.

Because of OpenGL's use of a state system, objects must be mutable. That is, the basic structure of an object can change at any time, even if the rendering pipeline is asynchronously using that object. A texture object can be redefined from 2D to 3D. This requires any OpenGL implementations to add a degree of complexity to internal object management.

Under the Longs Peak API object creation would become atomic, using templates to define the properties of an object which would be created with a single function call. The object could then be used immediately across multiple threads. Objects would also be immutable; however, they could have their contents changed and updated. For example, a texture could change its image, but its size and format could not be changed.

To support backwards compatibility the old state based API would still be available, but no new functionality would be exposed via the old API in later versions of OpenGL. This would have allowed legacy code bases, such as the majority of CAD products, to continue to run while other software could be written against or ported to the new API.

Longs Peak was initially due to be finalized in September 2007 under the name OpenGL 3.0, but the Khronos group announced October 30 that it had run into several issues that it wished to address before releasing the specification. As a result the spec was delayed, and the Khronos group went into a media blackout until the release of the final OpenGL 3.0 spec.

The final specification proved far less revolutionary than the Longs Peak proposal. Instead of removing all immediate mode and fixed functionality (non-shader mode), the spec included them as deprecated features. The proposed object model was not included, and no plans have been announced to include it in any future revisions. As a result the API remained largely the same with a few existing extensions being promoted to core functionality.

Among some developer groups this decision caused something of an uproar , with many developers professing that they would switch to DirectX in protest. Most complaints revolved around the lack of communication by Khronos to the development community and multiple features being discarded that were viewed favorably by many. Other frustrations included the requirement of DirectX 10 level hardware in order to use OpenGL 3.0 and the absence of geometry shaders and instanced rendering as core features.

Other sources reported that the community reaction wasn't quite as severe as originally presented , with many vendors showing support for the update.

OpenGL 3.1

Revision 3.1 was released on March 24 2009 and introduces a range of features to make the API more convenient to use, in addition to performance oriented features:

  • OpenGL Shading Language revision 1.40 (GLSL)
  • Texture Buffer Objects - a new texture type that holds a one-dimensional array of texels
  • Uniform Buffer Objects for fast data share/update
  • Signed normalized textures (±1.0 range)
  • A minimum of 16 texture units accessible by the vertex shader
  • Primitive restart
  • Instancing - drawing of objects multiple times through the re-use of vertex data
  • CopyBuffer API for fast data copy; used in conjunction with OpenCL


With the release of the OpenGL 3.1 specification, a compatibility extension that enables developers to access the OpenGL 1.X/2.X functionality removed in OpenGL 3.1 was also released. Notably, legacy functionality for wide line support is retained.

Removed legacy functionality includes:

  • All fixed-function options
  • Direct mode
  • Color index mode, i.e. pixel formats with color palettes


OpenGL 3.2

Revision 3.2 was released on August 3, 2009.It includes the following features:

  • OpenGL Shading Language revision 1.50 (GLSL)
  • Geometry Shader support
  • BGRA vertex component ordering
  • Shader Fragment coordinate convention control
  • Seamless cube map filtering
  • Fragment depth clamping
  • Multisampled textures and texture samples for specific sample locations
  • Sync and Fence objects


Sample renderings

Image:JOGL Refrection Demo Screenshot.png|Refraction using programmable vertex shadersImage:Perl OpenGL fob2.jpg|Animated textures using framebuffer objects and vertex/fragment program extensionsImage:Synth_video_game_screenshot_C.png|Procedural artwork using display lists


See also



OpenGL support libraries

  • GLUT – The OpenGL utility toolkit.
  • SDL – The Simple DirectMedia Layer.
  • GLU – Some additional functions for OpenGL programs.
  • GLee - The OpenGL Easy Extension library.
  • GLEW – The OpenGL Extension Wrangler Library.
  • GLUI - a GUI toolkit made with GLUT
  • GLFW - a portable framework for OpenGL application development.
  • GLM - C++ mathematics toolkit for OpenGL based on the GLSL specification.
  • SFML - Simple and Fast Multimedia Library.
  • Glux - The OpenGL Utility & Auxiliary Library.


Other 3D graphics APIs



Other 2D graphics APIs

  • Cairo - a cross platform graphical vectorial draw and text toolkit.
  • GTK+ - a cross platform graphical widget toolkit.
  • Java2d - a cross platform API for drawing two-dimensional graphics.
  • Qt - a cross platform graphical widget toolkit.
  • wxWidgets - a cross platform graphical widget toolkit.
All can incorporate OpenGL 3D objects. Cairo, Java2D and Qt can use OpenGL as backend.

Benchmarks



References

  1. http://www.opengl.org/registry/
  2. http://www.taoframework.com/
  3. http://pike.ida.liu.se/about/pike/modules.xml
  4. http://www.opengl.org/documentation/specs/version2.0/glspec20.pdf
  5. http://www.opengl.org/registry/doc/glspec30.20080811.pdf
  6. http://tech.slashdot.org/article.pl?sid=08/08/11/2135259
  7. http://feeds.feedburner.com/~r/opengl/zOLP/~3/365627409/
  8. http://feeds.feedburner.com/~r/opengl/zOLP/~3/368325680/
  9. http://feeds.feedburner.com/~r/opengl/zOLP/~3/364953172/


Books

  • Richard S. Wright, Benjamin Lipchak, Nicholas Haemel: OpenGL SuperBible: Comprehensive Tutorial and Reference, 4th Edition, Addison-Wesley, June 18 2007, ISBN 0-321-49882-8
  • Dave Shreiner, Mason Woo, Jackie Neider, Tom Davis: OpenGL Programming Guide: The Official Guide to Learning OpenGL, Version 2.1, 6th Edition, Addison-Wesley, July 30 2007, ISBN 0-321-48100-3
  • Fosner, Ron: OpenGL Programming for Windows 95 and Windows NT, Addison Wesley, ISBN 0-201-40709-4
  • Kilgard, Mark: OpenGL for the X Window System, Addison-Wesley, ISBN 0-201-48359-9
  • Lengyel, Eric: The OpenGL Extensions Guide, Charles River Media, ISBN 1-58450-294-0
  • OpenGL Architecture Review Board, et al.: OpenGL Reference Manual: The Official Reference Document to OpenGL, Version 1.4, Addison-Wesley, ISBN 0-321-17383-X
  • OpenGL Architecture Review Board, et al.: OpenGL Programming Guide: The Official Guide to Learning OpenGL, Version 2, Fifth Edition, Addison-Wesley, ISBN 0-321-33573-2
  • Rost, Randi J.: OpenGL Shading Language, Addison-Wesley, ISBN 0-321-19789-5


External links




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