active contours coding challenge #2:

Modify the code balloon-stub.c (linked at website) to calculate an
internal energy term that pushes the contour point outwards, similar
to inflating a balloon.

extra challenge:

Normalize the energies to the range [0...1] to prepare this energy for
weighted combination with other energies.

hints:

(a) calculate the centroid of the set of contour points before the loops
(b) calculate a "move to" location based upon the window loops
(b) find the distance between the "move to" point and centroid
(d) calculate an appropriate energy using this distance