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
	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
	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.

For the most part you can use a plugin like any other GIMP tool, without needing to be aware that it is a plugin. But there are a few things about plugins that are useful to understand.

One is that plugins are generally not as robust as the GIMP core. When GIMP crashes, it is considered a very serious thing: it can cost the user a lot of trouble and headache. When a plugin crashes, the consequences are usually not so serious. In most cases you can just continuing working without worrying about it.

	Note
	Because plugins are separate programs, they communicate with the GIMP core in a special way: The GIMP developers call it “talking over a wire”. When a plugin crashes, the communication breaks down, and you will see an error message about a “wire read error”.

Tip

When a plugin crashes, GIMP gives you a very ominous-looking message telling you that the plugin may have left GIMP in a corrupted state, and you should consider saving your images and exiting. Strictly speaking, this is quite correct, because plugins have the power to alter almost anything in GIMP, but for practical purposes, experience has shown that corruption is actually quite rare, and many users just continue working and don't worry about it. Our advice is that you simply think about how much trouble it would cause you if something went wrong, and weigh it against the odds.

Because of the way plugins communicate with GIMP, they do not have any mechanism for being informed about changes you make to an image after the plugin has been started. If you start a plugin, and then alter the image using some other tool, the plugin will often crash, and when it doesn't will usually give a bogus result. You should avoid running more than one plugin at a time on an image, and avoid doing anything to the image until the plugin has finished working on it. If you ignore this advice, not only will you probably screw up the image, you will probably screw up the undo system as well, so that you won't even be able to recover from your foolishness.

The plugins that are distributed with GIMP don't require any special installation. Plugins that you download yourself do. There are several scenarios, depending on what OS you are using and how the plugin is structured. In Linux it is usually pretty easy to install a new plugin; in Windows, it is either easy or very hard. In any case, the two are best considered separately.

If you want to learn how to write a plugin, you can find plenty of help at the GIMP Developers web site, developer.gimp.org. GIMP is a complex program, but the development team has made strenuous efforts to flatten the learning curve for plugin writing: there are good instructions and examples, and the main library that plugins use to interface with GIMP (called “libgimp”) has a well-documented API. Good programmers, learning by modifying existing plugins, are often able to accomplish interesting things after just a couple of days of work.