Peter J. Giblin | Searcharxiv

arXiv2005

Vertices and inflexions of plane sections of surfaces in R^3

We investigate the behaviour of vertices and inflexions on 1-parameter families of curves on smooth surfaces in the 3-space, which include a singular member. In particular, we discuss the context where the curves evolve as sections of a smooth surface by parallel planes. More precisely we will trace the patterns of inflexions and vertices (maxima and minima of curvature) on the sections of a surface as the section passes through a tangential point. We also keep track on the evolution of the curvature of the curves at vertices and control its limit when the vertices collapse at singular points. In particular, we cover all the generic cases, namely when the tangential points are elliptic (A1), umbilic (A1), hyperbolic(A1), parabolic (A2) and cusp of Gauss (A3) points. This has some applications in Computer vision and is also related to interesting mathematical problems such as Legendrian collapse, foliations of surfaces, the 4-vertex Theorem or in general the behaviour of vertices and inflexions in parameter families of curves etc.

math.DG↗

arXiv2005

Pre-Symmetry Sets of 3D shapes

The investigation of 3D euclidean symmetry sets (SS) and medial axis is an important area, due in particular to their various important applications. The pre-symmetry set of a surface M in 3-space (resp. smooth closed curve in 2D) is the set of pairs of points which contribute to the symmetry set, that is, the closure of the set of pairs of distinct points p and q in M, for which there exists a sphere (resp. a circle) tangent to M at p and at q. The aim of this paper is to address problems related to the smoothness and the singularities of the pre-symmetry sets of 3D shapes. We show that the pre-symmetry set of a smooth surface in 3-space has locally the structure of the graph of a function from R^2 to R^2, in many cases of interest.

math.DG↗