Active contour model

Active Contour, also called Snakes, are a framework for delineating an object outline from a possibly noisy 2D image.

In this framework, we attempt to minimize an energy associated to the current contour as a sum of an internal and external energy:

The external energy is supposed minimal when the snake is at the object boundary position: the most straightforward approach consists in giving low values when the regularized gradient around the contour position reaches its peak value.

The internal energy is supposed minimal when the snake has a shape which is supposed to be relevant considering the shape of the seeked object. The most straightforward approach grants high energy to elongated contours (elastic force) and to bended/high curvature contours (rigid force), considering the shape should be as regular and smooth as possible.

This model is highly popular in Computer Vision, and led to several developments in 2D and 3D. Namely, in two dimensions, the Active Shape Model represents a discrete version of this approach taking advantage of the Point Distribution Model to restrict the shape range to an explicit domain learned from a training set.