In computing, a plug-in (or add-in / addin, plugin, extension or add-on / addon) is a software component that adds a specific feature to an existing software application. When an application supports plug-ins, it enables customization. The common examples are the plug-ins used in web browsers to add new features such as search-engines, virus scanners, or the ability to use a new file type such as a new video format. Well-known browser plug-ins include the Adobe Flash Player, the QuickTime Player, and the Java plug-in, which can launch a user-activated Java applet on a web page to its execution on a local Java virtual machine.
A theme or skin is a preset package containing additional or changed graphical appearance details, achieved by the use of a graphical user interface (GUI) that can be applied to specific software and websites to suit the purpose, topic, or tastes of different users to customize the look and feel of a piece of computer software or an operating system front-end GUI (and window managers).
Purpose and examples
Applications support plug-ins for many reasons. Some of the main reasons include:
- to enable third-party developers to create abilities which extend an application
- to support easily adding new features
- to reduce the size of an application
- to separate source code from an application because of incompatible software licenses.
Types of applications and why they use plug-ins:
- Audio editors use plug-ins to generate, process and/or analyse sound (Ardour, Audacity)
- Email clients use plug-ins to decrypt and encrypt email (Pretty Good Privacy)
- Graphics software use plug-ins to support file formats and process images (Adobe Photoshop, GIMP)
- Media players use plug-ins to support file formats and apply filters (foobar2000, GStreamer, Quintessential, VST, Winamp, XMMS)
- Packet sniffers use plug-ins to decode packet formats (OmniPeek)
- Remote sensing applications use plug-ins to process data from different sensor types (Opticks)
- Text editors and Integrated development environments use plug-ins to support programming languages (Eclipse, jEdit, MonoDevelop)
- Web browsers use plug-ins (often implementing the NPAPI specification) to play video and presentation formats (Flash, QuickTime, Microsoft Silverlight, 3DMLW)
As shown in the figure, the host application provides services which the plug-in can use, including a way for plug-ins to register themselves with the host application and a protocol for the exchange of data with plug-ins. Plug-ins depend on the services provided by the host application and do not usually work by themselves. Conversely, the host application operates independently of the plug-ins, making it possible for end-users to add and update plug-ins dynamically without needing to make changes to the host application.
Programmers typically implement plug-in functionality using shared libraries installed in a place prescribed by the host application. HyperCard supported a similar facility, but more commonly included the plug-in code in the HyperCard documents (called stacks) themselves. Thus the HyperCard stack became a self-contained application in its own right, distributable as a single entity that end-users could run without the need for additional installation-steps. Programs may also implement plugins by loading a directory of simple script files written in a scripting language like Python or Lua.
Compared to extensions
In the Mozilla Firefox web browser, extensions differ slightly from plug-ins. Firefox supports plug-ins using NPAPI. An example of this is the Adobe Flash plugin. When the browser encounters references to content a plug-in specializes in, the data is handed off to be processed by that plug-in. An extension is an enhancement to the browser that adds new functionality and often is presented in the form of new user interface elements such as a download manager, toolbar buttons, or enhancements specific to a particular web site.
The original impetus behind the development of Mozilla Firefox was the pursuit of a small baseline application, leaving exotic or personalized functionality to be implemented by extensions to avoid feature creep. This is in contrast to the "kitchen sink" approach in its predecessors, the Mozilla Application Suite and Netscape 6 and 7.
The same distinction between plug-ins and extensions is in use by other web browsers, such as Microsoft Internet Explorer, where a typical extension might be a new toolbar, and a plug-in might embed a video player on the page. Since plug-ins and extensions both increase the utility of the original application, Mozilla uses the term "add-on" as an inclusive category of augmentation modules that consists of plug-ins, extensions and themes.
Plug-ins appeared as early as the mid 1970s, when the EDT text editor running on the Unisys VS/9 operating system using the UNIVAC Series 90 mainframe computers provided the ability to run a program from the editor and to allow such a program to access the editor buffer, thus allowing an external program to access an edit session in memory. The plug-in program could make calls to the editor to have it perform text-editing services upon the buffer that the editor shared with the plug-in. The Waterloo Fortran compiler used this feature to allow interactive compilation of Fortran programs edited by EDT.
Very early PC software applications to incorporate plug-in functionality included HyperCard and QuarkXPress on the Macintosh, both released in 1987. In 1988, Silicon Beach Software included plug-in functionality in Digital Darkroom and SuperPaint, and Ed Bomke coined the term plug-in.
|Look up plug-in in Wiktionary, the free dictionary.|
|Look up add-on in Wiktionary, the free dictionary.|
- Shared library
- Add-ons for Firefox
- Add-on (Mozilla)
- Browser extension
- Theme (computing)
- Skin (computing)