Winding Numbers on Discrete Surfaces

In the plane, the winding number is the number of times a curve wraps around a given point. Winding numbers are a basic component of geometric algorithms such as point-in-polygon tests, and their generalization to data with noise or topological errors has proven valuable for geometry processing tasks ranging from surface reconstruction to mesh booleans. However, standard definitions do not immediately apply on surfaces, where not all curves bound regions. We develop a meaningful generalization, starting with the well-known relationship between winding numbers and harmonic functions. By processing the derivatives of such functions, we can robustly filter out components of the input that do not bound any region. Ultimately, our algorithm yields (i) a closed, completed version of the input curves, (ii) integer labels for regions that are meaningfully bounded by these curves, and (iii) the complementary curves that do not bound any region. The main computational cost is solving a standard Poisson equation, or for surfaces with nontrivial topology, a sparse linear program. We also introduce special basis functions to represent singularities that naturally occur at endpoints of open curves.

Automated summary

The paper introduces a meaningful generalization of winding numbers on surfaces, based on the relationship between winding numbers and harmonic functions. By processing the derivatives of these functions, the algorithm filters out components that do not bound any region, resulting in closed, completed input curves, integer labels for bounded regions, and complementary curves that do not bound any region. The main computational cost is solving a standard Poisson equation or sparse linear program for surfaces with nontrivial topology. Special basis functions are also introduced to represent singularities at endpoints of open curves.