Other variations arise from different definitions of the internal or external energy. In this case the coefficient of the term containing the first derivative would indicate the elasticity of the contour, and the coefficient of the second derivative would determine its stiffness. There are also many ways to define external energy. In the active nets approach the deformable model is thought of as a grid of nodes, connected by elastic strings. When a string is stretched beyond a certain limit it may be allowed to break, thus changing the topology of the model.
Deformable models are extremely flexible by their very nature.
They can be applied to a very wide range of tasks on various images. In most implementations of deformable models, the contour is attracted to edges. There is a clear analogy with the force of gravity, which is proportional to the object s mass. However, there is nothing magical about this particular analogy. This choice is motivated by the simple fact that this model is well understood, and by the fact that there are problems for which this approach works.