  The calculus of variations applied to the image processing requires some
numerical models able to perform the variations of images and the extremization
of appropriate actions. To produce the variations of images, there are several
possibilities based on the brightness maps. Before a numerical model, I propose
an experimental approach, based on a tool of Gimp, GNU Image Manipulation
Program, in order to visualize how the image variations can be. After the
discussion of this tool, which is able to strongly increase the visibility of
images, the variations and a possible functional for the visibility are
proposed in the framework of a numerical model. The visibility functional is
analogous to the fringe visibility of the optical interference.
