8. Plugins

8.1. Introduction

One of the nicest things about Gimp is how easily its functionality can be extended, by using plugins. Gimp plugins are external programs that run under the control of the main Gimp application and interact with it very closely. Plugins can manipulate images in almost any way that users can. Their advantage is that it is much easier to add a capability to Gimp by writing a small plugin than by modifying the huge mass of complex code that makes up the Gimp core. Many valuable plugins have C source code that only comes to 100-200 lines or so.

Several dozen plugins are included in the main Gimp distribution, and installed automatically along with Gimp. Most of them can be accessed through the Filters menu (in fact, everything in that menu is a plugin), but a number are located in other menus. In many cases you can use one without ever realizing that it is a plugin: for example, the "Normalize" function for automatic color correction is actually a plugin, although there is nothing about the way it works that would tell you this.

In addition to the plugins included with Gimp, many more are available on the net. A large number can be found at the Gimp Plugin Registry, a web site whose purpose is to provide a central repository for plugins. Creators of plugins can upload them there; users in search of plugins for a specific purpose can search the site in a variety of ways.

Anybody in the world can write a Gimp plugin and make it available over the web, either via the Registry or a personal web site, and many very valuable plugins can be obtained in this way ­ some are described elsewhere in the User's Manual. With this freedom from constraint comes a certain degree of risk, though: the fact that anybody can do it means that there is no effective quality control. The plugins distributed with Gimp have all been tested and tuned by the developers, but many that you can download were just hacked together in a few hours and then tossed to the winds. Some plugin creators just don't care about robustness, and even for those who do, their ability to test on a variety of systems in a variety of situations is often quite limited. Basically, when you download a plugin, you are getting something for free, and sometimes you get exactly what you pay for. This is not said in an attempt to discourage you, just to make sure you understand reality.

[Warning] Warning

Plugins, being full-fledged executable programs, can do any of the things that any other program can do, including install back-doors on your system or otherwise compromise its security. Don't install a plugin unless it comes from a trusted source.

These caveats apply as much to the Plugin Registry as to any other source of plugins. The Registry is available to any plugin creator who wants to use it: there is no systematic oversight. Obviously if the maintainers became aware that something evil was there, they would remove it. (That hasn't happened yet.) There is, however, for Gimp and its plugins the same warranty as for any other free software: namely, none.

[Caution] Caution

Plugins have been a feature of Gimp for many versions. However, plugins written for one version of Gimp can hardly ever be used successfully with other versions. They need to be ported: sometimes this is easy, sometimes not. Many plugins are already available in several versions. Bottom line: before trying to install a plugin, make sure that it is written for your version of Gimp.