OpenAL

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Image:OpenAL logo.png OpenAL (Open Audio Library) is a cross-platform audio API. It is designed for efficient rendering of multichannel three dimensional positional audio. Its API style and conventions deliberately resemble that of OpenGL.

1 History
2 API Structure and Functionality
3 Portability
4 Applications
5 See also
6 External links

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History

OpenAL was originally developed by Loki Software in order to help them in their business of porting Windows games to Linux. After the demise of Loki, the project was maintained for a while by the open source community - but it is now hosted (and largely developed) by Creative Labs with on-going support from Apple and open source enthusiasts.

Whilst the OpenAL charter says that there will be an "Architecture Review Board" (ARB) modelled on the OpenGL ARB, no such organisation has ever been formed and the OpenAL specification is generally handled and discussed via e-mail on its public mailing list.

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API Structure and Functionality

Unlike the OpenGL specification, the OpenAL specification includes two subsections of the API; The core consisting of the actual OpenAL function calls, and the ALC API which is used to manage rendering contexts, resource usage and locking in a cross platform manner. There is also an ALUT library that provides higher level 'convenience' functions - exactly analogous to OpenGL's 'GLUT'.

In order to provide additional functionality in the future OpenAL utilises an extension mechanism. Individual vendors are thereby able to include their own extensions into distributions of OpenAL, commonly for the purpose of exposing additional functionality on their proprietary hardware. Extensions can be promoted to ARB (Architecture Review Board) status, indicating a standard extension which will be maintained for backwards compatibility. ARB extensions have the prospect of being added to the core API after a period of time.

The general functionality of OpenAL is encoded in source objects, audio buffers and a single listener. A source object contains a pointer to a buffer, the velocity, position and direction of the sound, and the intensity of the sound. The listener object contains the velocity, position and direction of the listener, and the general gain applied to all sound. Buffers contain audio data in PCM format, either 8- or 16-bit, in either monaural or stereo format. The rendering engine performs all necessary calculations as far as distance attenuation, doppler, etc.

The net result of all of this for the end user is that in a properly written OpenAL application, sounds behave quite naturally as the user moves through the three-dimensional space of the virtual world. The beauty of OpenAL from a programmers perspective is that very little additional work is required to make this happen in an existing OpenGL-based 3D graphical application.

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