lp://staging/~ubuntustudio-dev/ubuntustudio/enblend

 Enblend is a tool for compositing images using a Burt & Adelson multiresolution
 spline. This technique tries to make the seams between the input images
 invisible. The basic idea is that image features should be blended across a
 transition zone proportional in size to the spatial frequency of the features.
 For example, objects like trees and windowpanes have rapid changes in color.
 By blending these features in a narrow zone, you will not be able to see the
 seam because the eye already expects to see color changes at the edge of these
 features. Clouds and sky are the opposite. These features have to be blended
 across a wide transition zone because any sudden change in color will be
 immediately noticeable.