3rd CGAL User Workshop
Innovations and Applications
Robust geometric software for complex shapes

The CGAL Open Source Project and the European Project ACS are organizing the third CGAL User Workshop, to be held on Monday March 17, 2008, immediately before EuroCG2008 in Nancy, France.

This one day event around the Computational Geometry Algorithms Library CGAL will bring together CGAL users from academia and industry, and CGAL developers. The most recent advances in CGAL will be presented, as well as CGAL's cutting-edge use in research and development. In addition, there will be ample opportunity to meet in an informal atmosphere with the CGAL developers.


10:00 - 11:00

Welcome - Overview - Algorithms under Development

11:00 - 11:15

Coffee Break

11:15 - 12:45

Industrial and Academic User Talks

11:15 - 11:45

Modeling of Cell Shape In Cellular Networks
Thomas DONY, Orange Labs, FT/NSM/RD/RESA/NET/OPS Belfort

Cellular networks comprise a certain amount of transmitters whose purpose is to provide a radio access in a given area. Each transmitter is supposed to serve a certain portion of the area. Also they have to communicate each other to exchange users as they move through the network. In this talk we will describe how we can model cell shapes and neighbor relationships with Voronoï diagrams and Delaunay triangulations.

11:45 - 12:15

Structural Modeling in Geophysics
Sebastien Guillon, Total

The topic of this talk is the generation of surface and volume meshes, and the modeling of topological relationships between surfaces.

12:15 - 12:45

Improved Hydraulic Models using Airborne Laser Scanning
Johannes Otepka, Christian Doppler Laboratory “Spatial Data from Laser Scanning and Remote Sensing”, Vienna University of Technology.

Using Airborne Laser Scanning precise Digital Terrain Models (DTM) can be derived which form the basis of high quality hydraulic models. Due to restriction and limitation of Computational Fluid Dynamic Simulations DTM data have to be appropriately reduced and adapted to achieve (meaningful) results. In the DTM reduction step the CGAL implementation of the Constrained Delaunay Triangulation is used.

12:45 - 14:00


14:00 - 15:30

Industrial and Academic User Talks

14:00 - 14:30

Using CGAL for robust planar geometry processing in Agilent ADS
Michiel Dewilde, Agilent

The Agilent Advanced Design System contains the method-of-moments solver Momentum for layered planar (2.5D) electromagnetic problems. Layouts that are just fine for fabrication often contain small features that are undesirable for the EM solver as they complicate adequate mesh generation. The layout clean-up process that is being performed builds on CGAL for robust geometrical processing. We will present our interaction with CGAL's Arrangement_2 class and the acceleration offered by CGAL's filtered predicate mechanism.

14:30 - 15:00

3D/4D Image-Based Modeling Using the Delaunay Triangulation
Jean-Philippe Pons, Ecole des Ponts

The topic of this talk is the application of 3D and 4D Delaunay triangulation to difficult computer vision problems: the automatic 3D reconstruction of a static scene from photographs, and the spatio-temporal reconstruction of a moving scene from multiple video sequences.

(click here for full picture)

15:00 - 15:30

Computational Geometry for Computational Architecture
Jelle Feringa, EZCT Architecture & Design Research

EZCT has worked together in close collaboration with Marc Schoenauer to evolve an architectural envelope, based on specific daylight conditions. Rather than applying a binary matrix for its genotype, a Voronoï diagram was applied, resulting in a more powerful representation. This talk will discuss both the potential of computational geometry for architecture and will discuss the current state as well as the future of the CGAL python bindings.

(click here for full picture)

15:30 - 16:00

Wrap Up - Questions and Answers

16:00 - 16:15

Coffee Break

16:15 - 18:00

Meet the Developers - Get Demos - Let's Hack

In this session we have a couple of booths where developers give demos of their CGAL packages, some of which being under development and scheduled for the next release of the library. The selected demos cover large areas of the library and give you the opportunity to ask questions, have a closer look at code, and maybe do some rapid prototyping together.

Arrangements on Surfaces
Ophir Setter and Efi Fogel, Tel-Aviv University

In the illustration you can see an arrangement on a sphere induced by geodesic arcs. It represents the map of the world. The town of Nancy is represented as an isolated vertex in the arrangement, and is colored red, while all other vertices are colored green. Coordinates are taken from Google Earth. This arrangement, like any other, is ready for point location, overlay computation, Boolean set operations, etc.

Triangulations in Periodic Spaces
Manuel Caroli and Monique Teillaud, INRIA

The goal of our work is to extend the CGAL 3D triangulation package to compute triangulations in 3-dimensional spaces other than R3. For example, in simulation one is typically interested in having no boundaries. In this case, it makes sense to compute triangulations in the periodic space T3.

Surface and Volume Mesh Generation
Laurent Rineau, GeometryFactory, Mariette Yvinec, INRIA

We present the latest features of the Surface mesh generator which is already a package in CGAL 3.3, as well as a first version of a tetrahedral mesh generator, which allows to simultaneously generate iso surface meshes in a grey level image and the volume between these surfaces.

In the picture on the right, you see the Visible Human. The input voxel data were already segmented.

(click here for full image)

Visualizing Arrangements of Algebraic Plane Curves
Pavel Emeliyanenko and Michael Kerber, MPI

We present a online accessible web page that visualizes planar arrangements induced by arbitrary algebraic curves. Our tool is exact in the sense that it computes the exact topology of the arrangement, and the plot is always correct for the chosen resolution. The website also provides zooming functions, partial selection of curve segments, and export into a png-file.

Click on the example on the right to see more.

(Click here to get to the web site)

3D Boolean Operations
Peter Hachenberger, TU Eindhoven

The definition of Nef polyhedra is the most general polyhedron definition there is. In contrast to other polyhedron definitions, Nef polyhedra are closed under Boolean and topological operations. Consequently, Nef polyhedra can model non-manifold situations, open and closed boundaries, and lower dimensional features. We provide an exact and efficient implementation of 3D Nef polyhedra together with Boolean and topological operations. They have a great number of application, like computer-aided design, the Minkowski sum, and the visual hull.

The picture to the right shows a hanging tree from which a part subtracted, such that it looks cracked. (image courtesy of The Moving Picture Company).

Voronoi Diagrams of Ellipses
George Tzoumas, University of Athens

Following the generic programming paradigm, we applied the Apollonius diagram algorithm in order to compute the Voronoi diagram of ellipses by supplying the appropriate predicates. This requires adequate algebraic support for dealing with algebraic numbers of degree 184 or computing resultants of trivariate systems.

(click here to get to the website)

Variational Meshing
Jane Tournois and Pierre Alliez, INRIA

We address the problem of generating high quality simplicial meshes through optimization techniques.
In 2D, our algorithm interleaves Delaunay refinement and mesh optimization using Lloyd's iteration. Our experiments improve over other methods by producing higher quality meshes with fewer Steiner points.
In 3D, we are currently extending the optimal Delaunay triangulations technique [Chen04] so as to handle domains bounded by piecewise smooth surfaces.

(click here for full size image)

The Python Language Binding and Visualization with VPython
Christodoulos Fragoudakis, University of Athens

The CGAL-Python bindings provide robust geometric classes in Python, which is the language of choice when low level details about algorithm implementations have to be abstracted.. Our py.CGAL.visual initiative extends the existing Python-CGAL bindings and adds visualization. Our main contributions are new bindings for polygons and kd-trees as well as a custom visualization library on top of VPython.

The CGAL Project
Andreas Fabri, GeometryFactory

Learn more about how the CGAL project works and how it is organized internally: The Editorial Board, internal releases, the nightly testsuite, the developer manual, CGAL developer meetings, etc.

Registration and Accommodation

You can register for the CGAL User Workshop when you register for EuroCG'08. There is no separate registration system. Should you wish to participate only in the workshop, please check the appropriate box on the workshop registration form.

There will be no registration fee for the CGAL User Workshop. However, you are encouraged to register in order to guarantee your participation. In particular, there will be complimentary lunch for registered participants only.


This event is an initiative of the European project Algorithms for Complex Shapes (Project Fact Sheet for IST-006413 - ACS). The project partners are ETH Zurich (Switzerland), Groningen University (The Netherlands), Freie Universitaet Berlin (Germany), INRIA Sophia-Antipolis (France), Max-Planck-Institute Saarbruecken (Germany), Tel-Aviv University (Israel), GeometryFactory (France) and University of Athens (Greece), it started in June 2005 and has a duration of 36 months. CGAL has been originally funded by European Union's information technologies programme Esprit, by Project 21957 - CGAL, which was followed by Project 28155 - GALIA andProject IST-2000-26473 - ECG. Additionnaly, the Aim@Shape European Network of Excellence provided some ressources to support the development of CGAL.

Special thanks to Sylvain Lazard, Anne-Lise Charbonnier, and Mirsada Tihic from Loria, who took care of all local organizational issues.

Organization committee: