A decade of CGAL

The Computational Geometry Algorithms Library CGAL is the largest and most influential collection of algorithms and data structures for geometric computing that is available today. It started off in 1996 as a European research project with a small number of partners. Over the last ten years, it has grown tremendously, now being a major open source project with a lot of infrastructure (see www.cgal.org). Contributors come from the original partner sites, from sites involved in subsequent research projects, but increasingly also from many other sites worldwide. In these ten years, CGAL has fostered a new computational geometry research culture, and it has substantially shortened the time that it takes to turn academic results into industrial-strength software. For the editors of this special issue (both being with CGAL from day one), it is a great pleasure to see how the computational geometry community has widely adopted CGAL as a platform that helps in teaching, testing ideas, writing research papers, and building complex software. The papers of this special issue are a living proof for this adoption. The earned reward for the community is that people in many application domains can now successfully use the clever and ingenious techniques developed by computational geometers. This was not always the case. Back in the early nineties, the field of computational geometry was going through an adolescence crisis. Since the early eighties—when the field had come into existence as a distinguished subfield of algorithms design—many of the classical problems had been resolved, with much enthusiasm and many brilliant new ideas. This resulted in a computational geometry theory, a necessary body of fundamental results. In building this theory, its potential relevance in applications was never in doubt, but actual claims of applicability were scarcely substantiated. Computational geometry algorithms were designed for an idealized model of computation (the real RAM), and they often disregarded certain special situations as “degenerate” and irrelevant for the asymptotic results. On the one hand, only these abstractions had enabled the field to advance so quickly, but on the other hand, they now proved to be a serious obstacle toward practical implementations: computers simply did not work with infinite precision numbers, and real world problem instances were full of exactly those special situations that one never saw in testing with randomly generated synthetic data. In order not to become irrelevant in a world of vastly growing application challenges, the computational geometry community had to reconsider the basis of its theory. Within the European research project ALCOM, the software library LEDA was developed. LEDA also contained a number of geometric algorithms, designed for actual computers, and prepared to deal with all the situations that might arise. Also, a first version of the CGAL kernel was developed by then. The time was ripe for a fully-fledged library of computational geometry algorithms, and the European project CGAL took up the challenge. Quoting from the project proposal: “The goal of the CGAL project is to make the large body of geometric algorithms, developed within the field of computational geometry, available for industrial applications. . . Even though for most problems we want to solve in the library, correct and efficient algorithms have been published, these cannot immediately be turned into robust and efficient code.” The CGAL project, funded by the European Union, started in 1996. The fact that the project was on the right track was confirmed when the Computational Geometry Impact Task Force published its report Application Challenges to Computational Geometry in the same year. This task force, installed as a reaction to the computational geometry crisis, issued concrete recommendations for the future of the field. Here are two quotes from the report: “An on-line library of geometric code available through the Internet would be a useful starting point.” and “Building novel geometric