Gimp plugins that alter images in some way — although usually just the single active layer — are called filters.
Filters is a broad definition that can also include plugins used for other purposes, that do not alter images.
Filters and plugins are considered separate from the Gimp core, and — in theory — should not affect the core functionality of Gimp itself.
Plugins are found throughout the menus in Gimp, while there is an entire menu devoted to filters, as over 100 filters come installed with Gimp.
New filters and/or plugins can also be added, and although the Filters Menu is the default location for filters to be added, some may appear in other menus.
In fact, many plugins can already be found — by default, with the initial installation — in other menus.
Gimp plugins are available all over the web, but are not always secure or bug-free, so use them with caution and/or a bit of research beforehand.
Gimp’s plugin registry is a good place to start, although plugins from this source are as prone to issues as any other.
No, script-fu is not something I made up; its something somebody else made up!
Script-fu is scripts that allow you to do repetitive and/or complicated tasks quickly, by automating them in a script that you can recall again and again.
The word “macros”, if you are familiar with it, is what script-fu is often compared to, but script-fu is even more powerful.
Script-fu uses a language called Scheme to query functions in the Gimp database.
Script-fu scripts use the .scm file extension.
Move script-fu files to the scripts directory (see Preferences > Folders > Scripts to find the directory) and then go to Filters > Script-Fu > Refresh Scripts and your script-fu should appear in a menu.
There is a simple tutorial that teaches you how to write script-fu in the Gimp documentation.
Installing Plugins in Linux
If a plugin is any good, then hopefully it includes an install file with instructions.
But we live in the real world, so let’s not count on that.
Plugins and filters often come packaged as .zip, .rar. or .tar.gz files (etc.), but may also be single/unzipped .py (Python) files.
In any case, the method of installation depends entirely on the operating system in use, and even then may vary slightly.
The basic methods can be explained like so:
Installing Plugins in Linux
Copy the file(s) to the Gimp plugins directory, something along the lines of: /usr/lib/gimp/2.0/plug-ins/
You may need to restart Gimp (if it was open while you did this) before the plugin can be found in the menu.
See the Gimp documentation for help.
Installing Plugins in Windows
Navigate through your file system to the folder Gimp is installed in (usually somewhere in Program Files) and then find the lib\gimp\*version*\plug-ins\ directory.
Unzip (if necessary) your plugin, and put the plugin files in this directory.
The Filter Menu
There is one menu entry devoted entirely to filters. Due to constraints including time and space — ok, mainly time — I will provide a list of the default filters, but not stop to explain them.
Most are self-explanatory, or better of explored/tested than explained, anyway!
The first 3 filters I will detail just a bit, as they are better categorized as tools-related-to-filters than as actual filters.
Repeat Last will repeat the same action, with the same settings, as the last-used plugin. (Also available via the Ctrl + F keyboard shortcut.)
Reshow Last will show the dialog box for the last plugin, with the last-used settings. (Also available via the Shift + Ctrl + F keyboard shortcut.)
Reset All Filters resets all plugin settings to their default settings, wiping out any changes to may have made to setting in your session.
Selective Gaussian Blur
Red Eye Removal
Erase Every Other Row
Whirl and Pinch
Light and Shadow Filters
Difference of Gaussians
Van Gogh (LIC)
Alpha to Logo Filters
Basic I & II