  This paper introduces a new approach toward characterizing local structural
features of two-dimensional particle systems. The approach can accurately
identify and characterize defects in high-temperature crystals, distinguish a
wide range of nominally disordered systems, and robustly describe complex
structures such as grain boundaries. This paper also introduces two-dimensional
functionality into the open-source software program VoroTop which automates
this analysis. This software package is built on a recently-introduced
multithreaded version of VORO++, enabling the analysis of systems with billions
of particles on high-performance computer architectures.
