GIMP Vignette Plugin
About the Plugin
According to Wikipedia the definition of vignetting is
a reduction of an image's brightness or saturation at the periphery compared to the image center. It is often an undesireable effect in photography caused by camera or lens limitations but it is sometimes purposely introduced for creative effect, such as to draw attention to the center of the image and it can highlight the subject of a portrait photograph.
There are other vignetting plugins available for GIMP but I couldn't find one that worked with GIMP 2.8 so I decided to create one which would replicate the manual process that I had been using for some time.
The plugin is a Python script in a file called vignette.py, to install the plugin you will need to copy the file to one of the GIMP plugin folders on your system.
In GIMP, navigate to the "Edit/Preferences" dialog, expand "Folders" and select "Plug-Ins", the plugin folders are listed on the right hand side.
Copy the vignette.py script to one of these folders (On UNIX based systems, ensure that the script has executable permissions: chmod +x vignette.py)
Restart GIMP and you should have a new menu option at "Filters/Light and Shadow/Vignette..."
Using the Plugin
To use the plugin, select the area of the image which encompasses the main subject, you can use any of the selection tools in the toolbox, open the plugin, adjust the parameters as required and click OK. The vignette is created on a new layer so it will not affect any existing layers. The opacity of the vignette layer can also be adjusted afterwards if required. If you do not select an area before running the plugin a default selection will be created for you.
This is an example of a vignette being applied based on a selection: