Tandem Workshop
on Geometry, Numerics and Visualization

26-28 May 2003

Organized by TU-Berlin, ZIB and WIAS as part of the FZT86 research center

Overview of Talks

Philipp Beckmann (ZIB) 

Detection of Shape Features by Skeletonization
Shape matching between triangulated surfaces requires detection and identification of mutually
corresponding features on shapes.  In order to localize such features, a set of discrete
curvature operators can be applied. First we translate various types/notions of curvatures
known from differential geometry to discrete, piecewise linear surfaces,
and discuss some of the problems arising from generalizing curvature notions to triangulated
Thresholding the curvature field defines which nodes of the surface
potentially are shape features. The final step comprises the skeletonization of the feature
field, resulting in a set of feature lines.

Tobias Gaenzler (ZIB) 

Interior Point Methods: An Introduction

 The modern era of interior-point methods dates to 1984, when Karmarkar
proposed his algorithm for linear programming. In the years since then,
algorithms and software for linear programming have become
quite sophisticated, while extensions to more general classes of problems,
such as quadratic programming, nonconvex and nonlinear problems
have reached varying levels of maturity. In this talk we will introduce
interior-point methods for linear programming.
This talk is in preparation for the following talk from Martin Weiser:
Adaptive Multilevel Methods for Optimal Control Problems.

Evgeny Gladilin (ZIB) 

Biomechanical Modeling of Deformable Soft Tissues

 The realistic simulation of interactions with virtual bodies
is essential to many computer assisted medical applications.
Since soft tissues are non-rigid, any interaction with
biological structures causes their deformation. Thus, modeling
tissue deformations under the impact of external forces is
of general importance.

In this talk, a brief overview over biomechanical modeling of
deformable biological tissues with emphasis on the quasi-static
FE-approach for the long term soft tissue prediction in
the craniofacial surgery planning is presented.

Hans-Christian Hege (ZIB) 

Creating 3D Geometrical Models from 3D Image Data

3D imaging methods are widely used in various areas like medicine,
biology, material sciences and earth sciences. In this talk we focus
on biology and medicine, where 3D image data form the basis of
morphological and functional analyses, as well as of image-guided
diagnosis and computer assisted therapy.

A central task in such applications is the creation of 3D models
that faithfully represent the objects depicted in the image data.
Some problems of this task will be sketched. A set of practically
useful methods will be presented which enables researchers to segment
biomedical images and to reconstruct smooth 3D geometrical models.

This core technology provides the basis of a wealth of new quantitative
methods -- in the long term turning many biomedical fields into more
quantitative areas. Examples from various areas like medical treatment
and surgery planning, neuroanatomy and gene expression analysis will
be presented.

Klaus Hildebrandt (TU) 

Denoising and enhencement of surface features

Noise is an omnipresent artifact in 2d and 3d meshes due to resolution 
problems in mesh acquisition process. For example, meshes extracted from 
image data or supplied by laser scanning devices often carry high-frequency 
noise in the position of the vertices. Many filtering techniques have been 
suggested in recent years, among them Laplace smoothing is the most prominent 
example. We present a method for anisotropic denoising that concentrates on 
the preservation and enhencement of linear and curved surface features. The 
method havily relies on explicit measures for curvature of discrete surfaces.

Rolf Krahl (WIAS) 

Reorientation of a liquid surface - solving the Navier Stokes equations in a time-dependent domain

 The prediction of the dynamic behavior of liquids with free capillary
surfaces under various acceleration conditions is of high interest for
the construction of space vehicles using liquid propulsion.  This
behavior may be numerically simulated by solving the incompressible
Navier-Stokes equations.

Since the free surface is moving, the computional domain has to change
as well with the time during the simulation.  This talk shall present
and discuss one technique to solve PDEs in time-dependent domains.

Rolf Krause (FU)

 Mathematische Modellierung von Zweikörperkontakt (I)

 This talk consists of two parts.  Part I deals with the
contact problem, which is also known as the
Signorini problem.  Its aim is to simulate the mechanical
behaviour of an elastic body in the presence of a rigid
obstacle.  We explain the standard discretizations
used to tackle this problem and, if time permits, introduce
the audience to fast multigrid solvers for the
Signorini problem. For Part II see Oliver Sander.

Hans Lamecker (ZIB) 

3d Statistical Shape Models for Image Segmentation

 Image segmentation is a prerequisite for computer assisted
medical diagnosis and therapy planning. Since manual image
segmentation is rather time consuming, automatic and robust
segmentation techniques are of great practical importance.
For this, a-priori knowledge about the anatomy has to be
considered. Statistical shape models have proven
to be effective yet difficult to construct in 3D,
because of the problem of identifying corresponding
points that are densely distributed on two shapes.

In this talk the mathematical problems involved with
shape matching are sketched. A solution for matching
geometrical objects of arbitrary topology based on their
representation as triangular meshes is presented. It will
then be shown how to construct statistical shape models
and how to use them in medical image segmentation.

Carsten Lange (TU) 

Anisotropic fairing of point sets 

The use of point sets instead of meshes became more popular during the
years. We present a new method for anisotropic fairing of a point
sampled surface using an anisotropic geometric mean curvature flow.
The main advantage of our approach is that the evolution removes noise
from a point set while it detects and enhances geometric features of
the surface such as edges and corners. We derive a shape operator,
curvature properties of a point set, and an anisotropic
Laplacian of the surface. This anisotropic Laplacian reflects curvature
properties which can be understood as the point set analogue of
Taubin's curvature-tensor for polyhedral surfaces. We combine these
discrete tools with techniques from geometric diffusion and image
processing. Several applications demonstrate the efficiency and
accuracy of our method.

Jean-Marie Morvan (INRIA) 

On the approximation of geometric invariants of a smooth surface

 If S is a smooth surface approximated by a triangulation T, we
shall study the relationship between the geometry of S and the
geometry of T. In particular, we shall compare the area and the
curvatures of S and T. Moreover, we shall explain why these kind of
problems appear in different fields of science, and we
shall deal with an example in structural geology.

Matthias Nieser (TU)

Simulation of particle systems
In this talk I will present an algorithm to simulate a system of particles 
or rigid objects interacting on each other. Given such a set of rigid 
Next define some forces applied on them. They could for example 
attract themselves. The simulation should now calculate what happens 
to the objects in real time. With this algorithm it is possible to visualize 
physical processes, like a spinning gyro, as mathematical ones, like the 
moving of a flexible Steffen Sphere.

Konrad Polthier (TU)

Conformal Maps and Non-conforming Meshes

In non-conforming simplicial meshes the continuity
of adjacent triangles is required at edge midpoints only rather
than along whole edges. This relaxation provides additional freedom
which allows us to successfully study some optimization problems for
regular triangle meshes including minimization of the conformal energy.

Eike Preuss (TU) 

Recognition of 2D Vector Field Singularities Using a Discrete Hodge Decomposition

of vector fields are among the most important features of
flows. Vortices can influence the flying abilities of aircrafts, higher
order singularities often appear in magnetic fields. One approach for
the detection and analyzation of 2d singularities is to decompose the
vector field into a rotation free, a divergence free and a harmonic
component. The potential respectively co-potential of the first two
components offer an easy way to get some insight into the vector field

In this talk a discrete Hodge method is presented and an overview of
vector field singularities and their decomposition and detection is given.

Ulrich Reitebuch (TU) 

Smoothing subdivision of discrete surfaces using the Rivara algorithm

As the Rivara algorithm itself does not smooth surfaces, it is combined
with the Butterfly subdivision scheme to a smoothing local subdivision
algorithm. The Butterfly subdivison scheme works usually with a 4-to-1
split of triangles. A great disadvantage of the 4-to-1 split is the need
of temporary refinements to hold the conformity of the triangulaiton;
these must be removed again, if the subdivision process shall be
continued later. In contrast a Rivara refined triangulation is
conformized in a way, that does not require to distinguish between
triangles refined to improve the geometrical properties of the surface
and triangles refined to gain conformity. Using Rivara refinement and
Butterfly scheme together combines the good smoothing properties of the
Butterfly scheme with the very pleasant triangulation handling of the
Rivara algorithm. Thus the Rivara refinement becomes an interesting
algorithm for a local subdivision not only for two-dimensional
triangulations, for what it is mainly used up to now, but as well for
triangulaitons of surfaces in higher-dimensional space.

Oliver Sander (FU) 

Mathematische Modellierung von Zweikörperkontakt (II)

 This talk consists of two parts.  For Part I see Rolf Krause.
Part II treats the behaviour of two elastic bodies
in contact with each other.  This two-body contact problem
is considerably less well understood.  We discuss several ways
of  formalizing and discretizing the contact conditions and
explain some of the problems encountered on the way.
We will further show where two-body contact fits into the framework
of human-gait simulation.

Alfred Schmidt (Uni Bremen)

Adaptive finite element methods for phase transition problems

During solidification of an undercooled melt, anisotropic interface
effects lead to a geometrical law of motion for the rapidly moving
front between liquid and solid material, the Gibbs-Thomson equation
which couples velocity, anisotropic curvature and temperature.
The interface motion is coupled to heat conduction in the solid and
liquid material.
Different models can be used for the moving interface:
- a parametric surface representing a sharp interface,
- a level set approach,
- a diffuse interface modeled by a phase variable.
All those models lead to degenerate parabolic partial differential
equations for the interface motion.
After introducing the models, we present adaptive finite element
methods for sharp and diffuse interfaces.

The talk presents joint work with E. Bänsch (Berlin), Z. Chen (Beijing),
G. Dziuk (Freiburg), D. Kessler, R.H. Nochetto (UMD College Park), and
K.G. Siebert (Augsburg).

Hang Si (WIAS) 

TetGen, a 3D tetrahedral mesh generator based on Delaunay method

TetGen is a C++ program for three-dimensional tetrahedral mesh
generation. It generates boundary constrained meshes and quality meshes
for 3D piecewise linear domains. It uses Delaunay based mesh generation
algorithms. It's a public domain program and runs almost all platforms
with a C++ compiler.

This talk discusses the mesh problems TetGen solves and the algorithms
TetGen used. Some main features of TetGen are introduced by simple

Max Wardetzky (TU)

Symmetric Differential Operators on Polyhedral Manifolds

Whereas the traces of differential geometric operators in the discrete
setting (e.g. Laplacian and mean curvature) are well understood, the
underlying operators themselves are not as thoroughly studied.

We present a novel simple approach towards  the discrete Hessian and
Weingarten operators by considering a weak (integral) version of
vector-valued normal curvatures on  piecewise linear models.
In contrast to the smooth setting where these operators are a priori
symmetric this is not the case in the discrete world. Hence we briefly
discuss their symmetrization using a certain 1-parameter family of
convolution operators and show that the traces as well as the principal
directions remain unchanged throughout this symmetrizing family.
In particular, the traces (Laplacian and mean curvature) agree
with what has been well established for polyhedral surfaces. We will
shortly discuss how to  recover the symmetrization at vertices from  the
symmetrization at incident edges.

Martin Weiser (ZIB) 

Adaptive Multilevel Methods for Optimal Control Problems

A function space oriented approach to solving optimal control problems
with interior point methods is presented. The algorithm combines
advantages from both direct and indirect methods by obtaining accurate
solutions without analytical preparation.

Convergence theory and the algorithmic realization via inexact Newton
methods and adaptive refinement are sketched, and ODE examples are given.
Finally, the prospective extension to PDEs with application to
hyperthermia treatment planning is discussed.

Last updated on 23/05/2003 09:21