GIMP Plugin Registry

This plug-in enables you to use the vast number of existing filters made for Photoshop's Filter Factory as well as create new effects without having to go through the process of learning how to write plug-ins for The Gimp first.
Originally written by Jens Restemeier for Gimp 1.x back in 1997, this plug-in has now been updated for use with Gimp 2.x.
Project homepage is at http://gimpuserfilter.sf.net/
You can find lots of free FF filters that can be imported into User Filter at http://www.photoshop-filters.com/  read more »

This simple plugin allows you to change the values of each color channel of each pixel of a layer by using mathematical expressions.
It is able to handle RGB* and Gray* images. Available operators are: +, -, *, /, , and %.
There are many functions available, such as sin, cos, sqrt, abs, min, max.
A few constants are provided (pi, e, j), and a lot of pre-defined variables are usable (w for image's width, h for image's height, x and y for cartesian coordinates in the image, etc.).