ACTIVE MODELS

This segmentation method implements several variants of the active contours (or Snakes) methodology.

There are two relevant parameters, Field and Energy settings.

The Field selection determines how the energy is calculated and has the choices:

• Gradient: calculates a simple image gradient using pixel values differences;
• Sobel: calculates the image gradient using the Sobel digital filter;
• GVF: gradient vector flow algorithm, which calculates image energy iteratively;
• GGVF: generalized gradient vector flow algorithm, with better convergence than GVF.

Each of the Field selections has its own parameters. For GVF and GGVF:

• Iterations: Number of iterations.
• mu: Smoothness of the energy field.
• time step: Speed of the algorithm convergence. Values bigger than one can cause algorithm instability.

The Energy Settings are

• Vertices per contour: Number of contour nodes.
• Number of iterations: Number of contour modifications before the algorithm stops.
• Kappa: Convergence parameter in 3d energy equations (should be <1).
• Alfa: Hooke's constant of contour tension.
• Beta: Contour rigidity/stiffness against bending.
• Gamma: Impact of the external force field.

Reference: Mark A. Heidekker, Advanced Biomedical Image Processing. John Wiley & Sons, 29 mar 2011 - 528, Chapter 6: Active models.