azul: A fast and efficient 3D city model viewer for macOS

3D city models are an important research topic within geographic information, but there is still a lack of good tools to work with them in practice. In an attempt to alleviate this problem and to help with our own research, we have developed azul, a free and open‐source macOS 3D viewer that was especially engineered for the visualization of 3D city models. The aim of this article is, first of all, to describe the inner workings of azul as a complete methodology to efficiently visualize 3D city models, and which can be also applied to other hierarchically structured data models, which are becoming more prevalent in geographic information. In addition, the article has three ancillary goals: (a) to present other general‐purpose methods that are useful to efficiently process 3D city models (e.g., robust error‐correcting parsers and data models that can be used with multiple formats); (b) to describe technical issues and problematic aspects related to current 3D city model formats that are known by developers but are not properly documented in the scientific literature; and (c) to foster an open discussion about the best data structures and algorithms to process 3D city models in practice.