Figure 5. Local shape analysis of smooth surfaces: The intersection of a smooth surface with an almost-tangent plane generically approaches a conic called Dupin’s indicatrix which is an ellipse in case of positive Gauss curvature (a), and a hyperbola in case of negative Gauss curvature (b). In the latter case, the intersection with a tangent plane yields two smooth curves whose tangents define the asymptotic directions T1, T2 in the point under consideration (c). Right: approximate asymptotic directions of the Cour Visconti surface (Fig. 1), computed with the jet fit method of (Cazals and Pouget, 2003).