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
window panes have rapid changes in color. By blending these features in
a narrow zone, you cannot 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 must be blended across a wide transition
zone because any sudden change in color is immediately noticeable.