Closed surfaces with different shapes that are indistinguishable by the SRNF

TL;DR

This paper investigates the limitations of the Square Root Normal Field (SRNF) representation for surfaces, showing it cannot distinguish certain different shapes, but it reliably differentiates spheres and convex surfaces.

Contribution

The paper demonstrates the degeneracy of SRNF for some surfaces and proves its effectiveness in distinguishing spheres and convex shapes.

Findings

SRNF cannot distinguish some different surfaces with the same SRNF

SRNF uniquely identifies the shape of a sphere

SRNF distinguishes between shapes of convex surfaces

Abstract

The Square Root Normal Field (SRNF), introduced by Jermyn et al. in [3], provides a way of representing immersed surfaces in $\mathbb R^3$, and equipping the set of these immersions with a "distance function" (to be precise, a pseudometric) that is easy to compute. Importantly, this distance function is invariant under reparametrizations (i.e., under self-diffeomorphisms of the domain surface) and under rigid motions of $\mathbb R^3$. Thus, it induces a distance function on the shape space of immersions, i.e., the space of immersions modulo reparametrizations and rigid motions of $\mathbb R^3$. In this paper, we give examples of the degeneracy of this distance function, i.e., examples of immersed surfaces (some closed and some open) that have the same SRNF, but are not the same up to reparametrization and rigid motions. We also prove that the SRNF does distinguish the shape of a standard sphere from the shape of any other immersed surface, and does distinguish between the shapes of any two embedded strictly convex surfaces.