The prior energy

  Up is the prior term of the energy. It takes into account the geometrical constraints of the network: slow curvature and good crossing points between the segments. As described in [13,14], the network structure is obtained by penalizing segments which are not connected. The curvature constraint is optimized by quality functions with respect to the connection angles and the orientation between the segments. The overlaps between segments are forbidden in order to have neat crossing points.