The fxImage function allows you to define almost any possible image manipulation by defining the operation to perform as a string composed of a large selection of variables.
For the list of possible variables please see the FX reference sheet
. For a list of examples and another description of how it can be used please see the ImageMagick Examples page
And yes, it is very, very slow. You should only use it if you need an image generating that can't be generated any other way, or if you wish to generate a
Evaluate expression for each pixel in the image. Consult The Fx Special Effects Image
Operator for more information.