active contours coding challenge:

Modify the code spacing-stub.c (linked at website) to calculate an
internal energy term that pushes the contour point towards being
equidistant from its neighboring points.

extra challenge:

Adjust the energy calculation to yield a single local minimum instead
of a band of equal local minima.

hints:

I recommend breaking down the calculation into multiple steps.  For example:
(a) calculate the "move to" location based upon the window loops
(b) find the distance between the "move to" point and previous contour point
(c) find the distance between the "move to" point and next contour point
(d) calculate an appropriate energy from those two distances