Properties of Ridges and Cores for Two-Dimensional Images

Pizer and Eberly introduced the “core” as the analogue of the medial axis for greyscale images. For two-dimensional images, it is obtained as the “ridge” of a “medial function” defined on 2 + 1-dimensional scale space. The medial function is defined using Gaussian blurring and measures the extent to which a point is in the center of the object measured at a scale. Numerical calculations indicate the core has properties quite different from the medial axis. In this paper we give the generic properties of ridges and cores for two-dimensional images and explain the discrepancy between core and medial axis properties. We place cores in a larger “relative critical set structure”, which coherently relates disjoint pieces of core. We also give the generic transitions which occur for sequences of images varying with a parameter such as time. The genericity implies the stability of the full structure in any compact viewing area of scale space under sufficiently small L2 perturbations of the image intensity function. We indicate consequences for finding cores and also for adding “markings” to completely determine the structure of the medial function.