What is a plug-in?

A plug-in is a software module that can be added to a program to extend its functionality. It's a bit like a program inside a program. There are plug-ins for various types of software, but I'm only going to be discussing plug-ins as they relate to graphics software.
You'll find plug-ins for image editing software, illustration software, and page layout software. Plug-ins are almost always developed by someone other than the publisher of the software that the plug-in is intended for.


This makes sense, since the purpose of a plug-in is to add functionality that is not inherent in the software. The most abundant type of plug-ins for graphics software are Photoshop-compatible plug-ins for image editing software.

Filters and extensions

You may have also seen the terms filter and extension in relation to graphics software and plug-ins. The term filter comes from traditional camera photography where a filter is attached to the camera lens. The "filter" is actually a piece of glass or plastic media that alters the light entering the camera to produce a special effect. When you see the term filter in relation to graphics software, it generally refers to a plug-in that produces a special effect. The type of effects you can get with plug-in filters can range from something as simple as a blurred or softened image to wild, psychedelic patterns, distortions, and artistic effects.
The term extension refers to the fact that the plug-in extends the functionality of the software. Plug-ins that are created for QuarkXPress are called XTensions.

For Macromedia software, the plug-ins are called Xtras. Whatever you call them, the premise is the same.

Many use the terms filter and plug-in interchangeably, but there is a distinction. The term plug-in refers to the entire third-party module which plugs into a host application. The complete plug-in module may consist of several individual filters which can be accessed through a common user interface. One example of a plug-in consisting of several individual filters is Xenofex from Alien Skin.

It's also worth noting that the native effects built in to Photoshop and available from the Filters menu, are not plug-ins. After all, they are built in, so they can't be plugged in! (For the trivia buffs who may be reading; this was not always the case. At one time, Photoshop's native plug-ins were actually a third-party collection called Aldus Gallery Effects. When Adobe absorbed Aldus, the Gallery Effects were incorporated into Photoshop and have lived under the Filter menu since version 4.0.)

Photoshop-compatible

Almost all of today's image editors support the Photoshop standard for plug-ins. This means that any Photoshop-compatible plug-in, for the most part, should work interchangeably in any image editor that supports the Photoshop standard for plug-ins. It should be noted that not all Photoshop-compatible plug-ins will work as intended in all image editors. If you plan to use a plug-in with software other than Photoshop, you should test a trial version or check with the publisher of the software and the plug-in for compatibility.
I mentioned that page layout and illustration software programs have their own plug-ins and extensions, however, many of these programs also support Photoshop-compatible plug-ins. You should be aware that when you are working with Photoshop-compatible plug-ins within this type of software, they usually only work on bitmap type images, or objects that have been converted to bitmaps within the software. Also, you may not be able to use some filters; for instance, those that require you to make a selection.

Filter Factory

Much of the reason for the abundance of freeware filters is due to the Filter Factory. The Filter Factory is a plug-in for Photoshop that allows users to compile their own custom filters. There are literally thousands of free filters created with Filter Factory all over the Net. This site has links to additional resources where you can learn more about Filter Factory and creating your own plug-in filters.

Do you need plug-ins and filters?

You can certainly get by without using plug-ins. Most of what can be accomplished with them can be done in your software without them. However, the productivity improvements that can be achieved with plug-ins make them very worthwhile. A plug-in can often reduce a task that would normally take several hours down to just a few mouse clicks. Plug-ins can also be an inspiration to creativity; or when you're not creating serious graphics, they are just plain fun to experiment with. Don't be surprised, however, if some of the best plug-ins turn up as native features included in the next upgrade version of your software.
As today's software becomes more and more advanced you would think the desire or need for plug-ins would diminish; but it comes as no surprise that plug-in developers just keep coming up with exciting new ideas for more elaborate effects and even better productivity improvements.