Influence of noise and flat regions: (a) a membership function from our experimental study, (b) the approximated function with spherical topology computed from the lowest to highest membership values, with δ = 0, (c) the same approximation with δ = 1E - 12, (d) the approximation with δ = 1E - 3. The propagation is started from the boundary of the image, and evolves at random through the background region if δ = 0. The smallest increase to will remove most of the critical points, and the remaining artificial structures due to background noise will also disappear with a higher δ.