Figure 2

Approximation of vessel inclination by many straight lines and approximation of vessel inclination by the value of angle of tangent inclination α at given point. The approximation of a vessel by many lines increases the accuracy, but the computational complexity increases. In an extreme case the vessel’s position is approximated by lines 1 pixel long. So it is possible to assume that each pixel is a value of inclination angle of the tangent to the vessel at the given point.