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Faculty / Organisational entity
Gauss Frame Offsets
(1992)
Hardware / Software Codesign
(1994)
A fundamental variance reduction technique for Monte Carlo integration in the framework of integro-approximation problems is
presented. Using the method of dependent tests a successive hierarchical function approximation algorithm is developed, which
captures discontinuities and exploits smoothness in the target function. The general mathematical scheme and its highly efficient
implementation are illustrated for image generation by ray tracing,
yielding new and much faster image synthesis algorithms.
We propose several algorithms for efficient Testing of logical Implication in the case of ground objects. Because the problem of Testing a set of propositional formulas for (un)satisfiability is \(NP\)-complete there's strong evidence that there exist examples for which every algorithm which solves the problem of testing for (un)satisfiability has a runtime that is exponential in the length of the input. So will have our algorithms. We will therefore point out classes of logic programs for which our algorithms have a lower runtime. At the end of this paper we will give an outline of an algorithm for theory refinement which is based on the algorithms described above.
We propose a framework for the synthesis of temporal logic programs which are formulated in a simple temporal logic programming language from both positive and negative examples. First we will prove that results from the theory of first order inductive logic programming carry over to the domain of temporal logic. After this we will show how programs formulated in the presented language can be generalized or specialized in order to satisfy the specification induced by the sets of examples.
In this paper, the complexity of full solution of Fredholm integral equations of the second kind with data from the Sobolev class \(W^r_2\) is studied. The exact order of information complexity is derived. The lower bound is proved using a Gelfand number technique. The upper bound is shown by providing a concrete algorithm of optimal order, based on a specific hyperbolic cross approximation of the kernel function. Numerical experiments are included, comparing the optimal algorithm with the standard Galerkin method.
Instant Radiosity
(1997)
We present a fundamental procedure for instant rendering from the radiance equation. Operating directly on the textured scene description, the very efficient and simple algorithm produces photorealistic images without any kernel or solution discretization of the underlying integral equation. Rendering rates of a few seconds are obtained by exploiting graphics hardware, the deterministic
technique of the quasi-random walk for the solution of the global illumination problem, and the new method of jittered low discrepancy sampling.
In dieser Arbeit wird eine Integration der temporallogischen Verarbeitungskonzepte
der Programmiersprache ExTeLL in die objektorientierte Wirtssprache \(C^{++}\) vorgestellt. Dabei war unser Ziel eine Schnittstelle zur komfortablen Kommunikation der Sprachkomponenten zu entwickeln, derart daß die Sprachsynthese eine homogene Gesamtsprache darstellt . Hierbei haben wir besonderen Wert auf die Nutzung der Möglichkeiten der jeweils hinzugefügten Sprachkomponente und einen syntaktisch einheitlichen Aufbau der Gesamtsprache gelegt. Dies erforderte insbesondere die Integration des Typkonzepts von \(C^{++}\) sowie der Mechanismen zur Überladung von Funktionen und Prozeduren in ExTeLL und in der zugrundeliegenden Temporallogik
EITeL.
Interactive graphics has been limited to simple direct illumination that commonly results in an artificial appearance. A more realistic appearance by simulating global illumination effects has been too costly to compute at interactive rates. In this paper we describe a new Monte Carlo-based global illumination algorithm. It achieves performance of up to 10 frames per second while arbitrary changes to the scene may be applied interactively. The performance is obtained through the effective use of a fast, distributed ray-tracing engine as well as a new interleaved sampling technique for parallel Monte Carlo simulation. A new filtering step in combination with correlated sampling avoids the disturbing noise artifacts common to Monte Carlo methods.
Interleaved Sampling
(2001)
The sampling of functions is one of the most fundamental tasks in computer graphics, and occurs in a variety of different forms. The known sampling methods can roughly be grouped in two categories. Sampling on regular grids is simple and efficient, and the algorithms are often easy to built into graphics hardware. On the down side, regular sampling is prone to aliasing artifacts that are expensive to overcome. Monte Carlo methods, on the other hand,
mask the aliasing artifacts by noise. However due to the lack of coherence, these methods are more expensive and not weil suited for hardware implementations. In this paper, we introduce a novel sampling scheme where samples from several regular grids are a combined into a single irregular sampling pattern. The relative positions of the regular grids are themselves determined by Monte Carlo methods. This generalization obtained by interleaving yields,significantly improved quality compared to traditional approaches while at the same time preserving much of the advantageous coherency of regular sampling. We demonstrate the quality of the new sampling scheme with a number of applications ranging from supersampling over motion blur simulation to volume rendering. Due to the coherence in the interleaved samples, the method is optimally suited for implementations in graphics hardware.
In der industriellen Praxis werden immer häufiger Verbesserungs- und Meßansätze zur Steigerung der Qualität von Software-Produkten und -Projektdurchführungen diskutiert. Dieser Artikel gibt eine Übersicht über potentielle Ansätze zur kontinuierliche Software-Qualitätsverbesserung:
QIP, CMM und AMI. Aus dem Vergleich der Verbesserungsansätze geht hervor, daß u.a. zielorientiertes Messen eine integrale Technologie zur Verbesserung ist. Deshalb wird in diesem Artikel ein Ansatz für zielorientiertes Messen, der GQM-Ansatz, detaillierter diskutiert. Insbesondere wird auf die Anwendung in der Praxis eingegangen, wobei die Erfahrungen aus realen Projekten in Form von Richtlinien vorgestellt werden. Der Artikel will Praktikern einen Einstieg in die Software Qualitätsverbesserung mittels Messen vermittlen.
Formale Beschreibungstechniken (FDTs) erlauben durch ihre formale Syntax und Semantik eine präzise Systembeschreibung und sind Grundlage für die formale Verifikation. Bei der Implementierung von Systemen wird jedoch nach wie vor von Hand implementiert, selbst wenn ausgereifte Werkzeuge zur automatischen Generierung von Kode direkt aus der formalen Spezifikation existieren. Die Ursache dafür liegt in dem Ruf dieser Werkzeuge, Kode mit extrem geringer Leistungsfähigkeit zu erzeugen. Es gibt jedoch kaum quantitative Leistungsvergleiche zwischen manuell und automatisch generierten Implementierungen, die dieses Vorurteil stützen oder widerlegen könnten. In diesem Beitrag wird ein solcher Leistungsvergleich anhand des Hochleistungsprotokolls XTP und der FDT Estelle vorgestellt. Er liefert eine Bestandsaufnahme des momentanen Entwicklungsstandes bei der automatischen Generierung von Kode aus Estelle-Spezifikationen im direkten Vergleich zu gut optimierten Handimplementierungen. Es zeigt sich, daß in dem betrachteten Fall eines komplexen Protokolls die Handimplementierung zwar merklich leistungsstärker ist. Dieser Leistungsvorteil wird jedoch durch einen sehr hohen Implementierungsaufwand sowie die Schwierigkeit, die Korrektheit bzgl. der Spezifikation sicherzustellen, erkauft. Im einzelnen Anwendungsfall kann es daher trotz der Leistungseinbußen durchaus vorteilhaft sein, automatisch Kode zu erzeugen, zumal in der Bestandsaufnahme festgestellt wurde, daß automatisch generierte Implementierungen z.T. besser abschneiden als erwartet. Zudem besteht - anders als bei der bereits umfassend optimierten Handimplementierung - noch ein erhebliches ungenutztes Potential zur Leistungsverbesserung der automatisch generierten Implementierung.
Wireless LANs operating within unlicensed frequency bands require random access schemes such as CSMA/ CA, so that wireless networks from different administrative domains (for example wireless community networks) may co-exist without central coordination, even when they happen to operate on the same radio channel. Yet, it is evident that this Jack of coordination leads to an inevitable loss in efficiency due to contention on the MAC layer. The interesting question is, which efficiency may be gained by adding coordination to existing, unrelated wireless networks, for example by self-organization. In this paper, we present a methodology based on a mathematical programming formulation to determine the
parameters (assignment of stations to access points, signal strengths and channel assignment of both access points and stations) for a scenario of co-existing CSMA/ CA-based wireless networks, such that the contention between these networks is minimized. We demonstrate how it is possible to solve this discrete, non-linear optimization problem exactly for small
problems. For larger scenarios, we present a genetic algorithm specifically tuned for finding near-optimal solutions, and compare its results to theoretical lower bounds. Overall, we provide a benchmark on the minimum contention problem for coordination mechanisms in CSMA/CA-based wireless networks.
The provision of network Quality-of-Service (network QoS) in wireless (ad-hoc) networks is a major challenge in the development of future communication systems. Before designing and implementing these systems, the network QoS requirements are to be specified. Since QoS functionalities are integrated across layers and hence QoS specifications exist on different system layers, a QoS mapping technique is needed to translate the specifications into each other. In this paper, we formalize the relationship between layers. Based on a comprehensive and holistic formalization of network QoS requirements, we define two kinds of QoS mappings. QoS domain mappings associate QoS domains of two abstraction levels. QoS scalability mappings associate utility and cost functions of two abstraction levels. We illustrate our approach by examples from the case study Wireless Video Transmission.
Software development organizations measure their real-world processes, products, and resources to achieve the goal of improving their practices. Accurate and useful measurement relies on explicit models of the real-world processes, products, and resources. These explicit models assist with planning measurement, interpreting data, and assisting developers with their work. However, little work has been done on the joint use of measurem(int and process technologies. We hypothesize that it is possible to integrate measurement and process technologies in a way that supports automation of measurement-based feedback. Automated support for measurementbased feedback means that software developers and maintainers are provided with on-line, detailed information about their work. This type of automated support is expected to help software professionals gain intellectual control over their software projects. The dissertation offers three major contributions. First, an integrated measurement and
process modeling framework was constructed. This framework establishes the necessary foundation for integrating measurement and process technologies in a way that will permit automation. Second, a process-centered software engineering environment was developed to support measurement-based feedback. This system provides personnel with information about the tasks expected of them based on an integrated set of measurement and process views. Third, a set of assumptions and requirements about that system were examined in a controlled experiment. The experiment compared the use of different levels of automation to evaluate the acceptance and effectiveness of measurement-based feedback.
In this paper we show how Metropolis Light Transport can be extended both in the underlying theoretical framework and the algorithmic implementation to incorporate volumetric scattering.
We present a generalization of the path integral formulation thathandles anisotropic scattering in non-homogeneous media. Based on this framework we introduce a new mutation strategy that is
specifically designed for participating media. It exploits the locality of light propagation by perturbing certain interaction points within the medium. To efficiently sample inhomogeneous media a new ray marching method has been developed that avoids aliasing artefacts and is significantly faster than stratified sampling. The resulting global illumination algorithm provides a physically correct simulation of light transport in the presence of participating media that includes effects such as volume caustics and multiple volume scattering. It is not restricted to certain classes of geometry and scattering models and has minimal memory requirements. Furthermore, it is unbiased and robust, in the sense that it produces satisfactory results for a wide range of input scenes and lighting situations within acceptable time bounds. In particular, we found that it is weil suited for complex scenes with many light sources.
Mobile Agenten im Internet
(2001)
Mobile Agenten haben sich in den letzten Jahren zunehmend in der Architektur und Programmierung verteilter Systeme bewährt. Es sind Programme, die einen Internen Zustand mit sich führen, während sie verschiedene, möglicherweise auf unterschiedlichen Plattformen basierende, Systeme besuchen. Auf dem jeweiligen System nehmen sie Dienste in Anspruch, indem sie entweder lokale Bibliotheken ansprechen, oder auf durch das System bereitgestellte Dienste zugreifen. Dabei müssen mobile Agenten sowohl alle vom Programm benötigten Daten, wie auch den gesamten Code mit sich führen. Zwar sind die Daten ein wichtiger (wenn nicht sogar der entscheidende) Teil eines Agenten, trotzdem wird in der Regel nicht als wertvoller, eigenständiger Part angesehen. Dies ist jedoch nicht immer ratsam, könnten doch Agenten am aktuellen Aufenthaltsort einen „Container" zurückzulassen um ihm anderen Agenten zur Verfügung zu stellen (natürlich erst nach erfolgter Zugriffskontrolle), bzw. die Daten erst dann auf ein Migrationsziel übertragen, wenn sich durch lokale Aufrufe des Systems herausgestellt hat, dass sie dort benötigt werden. Diese Arbeit ist zweigeteilt, insofern, als dass sie sich mit den zwei verschiedenen „Ebenen" der mobilen Agenten beschäftigt. Im ersten Teil werden die für die Migration und Nutzung der Resourcen notwendigen Aspekte besprochen. Dabei wird der Schwerpunkt auf die notwendige Unterstützung durch die Umgebung gelegt, wobei nicht eine neue integrierte Umgebung entworfen, sondern vielmehr die notwendigen Blöcke aufgezeigt werden sollen. Diese können dann als Teil eines Environments oder aber als eigentständige Komponente bereitsgestellt werden. Der zweite Teil beschäftigt sich mit den durch die Interaktion verschiedener Agenten entstehenden Probleme. Stichworte hierbei sind die Kostenkontrolle (wer bezahlt auf welche Art für in Anspruch genommene Dienste), Workflow Unterstützung, sowie Sicherheit in einem offenen, verteilten System, in dem es keine zentrale Überprüfung von Rechten und Identitäten geben kann. Abgeschlossen wird diese Ausarbeitung mit einer Bewertung der auf den beiden Ebenen gefundenen Problemen und Eigenheiten, wobei dann die Frage aufgeworfen wird, ob Agenten in der heutigen Form überhaupt sinnvoll sind.
Monte Carlo & Beyond
(2002)
We study the problem of global solution of Fredholm integral equations. This means that we seek to approximate the full solution function (as opposed to the local problem, where only the value of the solution in a single point or a functional of the solution is sought). We analyze the Monte Carlo complexity, i.e. the complexity of stochastic solution of this problem. The framework for this analysis is provided by information based complexity theory. Our investigations complement previous ones on stochastic complexity of local solution and on deterministic complexity of
both local and global solution. The results show that even in the global case Monte Carlo algorithms can perform better than deterministic ones, although the difference is not as large as in the local case.
The Monte Carlo complexity of computing integrals depending on a parameter is analyzed for smooth integrands. An optimal algorithm is developed on the basis of a multigrid variance reduction technique. The complexity analysis implies that our algorithm attains a higher convergence rate than any deterministic algorithm. Moreover, because of savings due to computation on multiple grids, this rate is also higher than that of previously developed Monte Carlo algorithms for parametric integration.
Intellectual control over software development projects requires the existence of an integrated set of explicit models of the products to be developed, the processes used to develop them, the resources needed, and the productivity and quality aspects involved. In recent years the development of languages, methods and tools for modeling software processes, analyzing and enacting them has become a major emphasis of software engineering research. The majority of current process research concentrates on prescriptive modeling of small, completely formalizable processes and their execution entirely on computers. This research direction has produced process modeling languages suitable for machine rather than human consumption. The MVP project, launched at the University of Maryland and continued at Universität Kaiserslautern, emphasizes building descriptive models of large, real-world processes and their use by humans and computers for the purpose of understanding, analyzing, guiding and improving software development projects. The language MVP-L has been developed with these purposes in mind. In this paper, we
motivate the need for MVP-L, introduce the prototype language, and demonstrate its uses. We assume that further improvements to our language will be triggered by lessons learned from applications and experiments.
Vorliegender Bericht ist eine Studie für einen möglichen Immissionsdatenverbund in Österreich. Die Grundlage dieser ersten Version der Studie sind Gespräche, welche Anfang Januar 1992 im Forschungszentrum Seibersdorf und im Umweltbundesamt in Wien stattfanden. Seit einigen Jahren beschäftigt sich die von mir geleitete Gruppe Umweltinformatik an der Universität Kaiserslautern mit den besonderen Schwierigkeiten bei der Vernetzung und Integration heterogener Systeme, welche darüberhinaus unter unterschiedlichen Vollzugshoheiten stehen können. Wir haben diese Problemstellung bei der Führung verfahrenstechnischer Anlagen weitestgehend gelöst und beschäftigen uns, zum Teil in Zusammenarbeit mit Kollegen aus anderen Institutionen, nun hauptsächlich mit der Umsetzung dieser Lösungen in verteilten Systemen im Umweltschutz. Unsere derzeitigen Arbeiten haben zum Ziel, möglichst allgemeine Ansätze für die Integration in verteilten, offenen Umweltinformationssystemen (UIS) zu entwickeln. Dabei sind wir uns darüber bewußt, daß diese allgemeinen Ansätze nur aus den konkreten Gegebenheiten, Zielen und Vorstellungen abgeleitet werden können. Diese Studie soll zwei Dinge bezwecken: einerseits will ich versuchen, den Blick dafür zu öffnen, wie ein Immissionsdatenverbund aussehen könnte, welcher allen Betreibern eine hohe Funktionalität und großen Komfort bietet. Es soll auch diskutiert werden, welcher technischer und organisatorischer Aufwand unter Verwendung welcher Konzepte entsteht. Auch wenn man sich in naher Zukunft nicht dazu entschließen sollte, die von mir vorgeschlagenen oder ähnliche Wege zu gehen, so könnte man doch bei der Realisierung auf
niedrigerem funktionalen Niveau zukünftige Möglichkeiten schon heute berücksichtigen und damit zukünftige Entwicklungen begünstigen. Ich hoffe, daß die Leser dieser Studie in dieser Hinsicht von meinen Erfahrungen profitieren. Zum zweiten ist diese Studie für meine Arbeitsgruppe ein Einstieg in die konkreten Problemstellungen großer verteilter UIS. Meßnetze sind inhärente Komponenten solcher UIS und weisen aufgrund ihrer technischen Orientierung interessante Merkmale auf. Daher erhoffen wir uns, hier wichtige Erkenntnisse auch für unsere Arbeiten zu gewinnen. Im Prinzip weiß heute noch niemand, wie man einen großen Umweltdatenverbund organisieren könnte. Ein Teil eines solchen Verbundes sind die Meßnetze. Die damit verbundenen Probleme alleine technischer Art sind riesig und es gibt bisher nur wenige Personen, die in der Umweltinformatik sich überhaupt mit diesen Themen beschäftigen. Diese Studie versteht sich daher hochgradig als Diskussionpapier. Jegliche geäußerten Ideen und Konzepte sollen von Lesern kritisch bewertet, notfalls angegriffen und vernichtend geschlagen werden - sofern sie dies verdienen. Diese Diskussion ist notwendig, damit wir überhaupt einmal eine Ahnung davon bekommen, wohin die Umweltinformatik der verteilten Systeme gehen kann.
In this article we present a method to generate random objects from a large variety of combinatorial classes according to a given distribution. Given a description of the combinatorial class and a set of sample data our method will provide an algorithm that generates objects of size n in worst-case runtime O(n^2) (O(n log(n)) can be achieved at the cost of a higher average-case runtime), with the generated objects following a distribution that closely matches the distribution of the sample data.
The CAD/CAM-based design of free-form surfaces is the beginning of a chain of operations, which ends with the numerically controlled (NC-) production of the designed object. During this process the shape control is an important step to amount efficiency. Several surface interrogation methods already exist to analyze curvature and continuity behaviour of the shape. This paper deals with a new aspect of shape control: the stability of surfaces with respect to infnitesimal bendings. Each inEnitesimal bending of a surface determines a so called instability surface, which is used for the stability investigations. The kinematic meaning of this instability surface will be discussed and we present algorithms to calculate it.
On Abstract Shapes of RNA
(2008)
As any RNA sequence can be folded in many different ways, there are lots of different possible secondary structures for a given sequence. Most computational prediction methods based on free energy minimization compute a number of suboptimal foldings and we have to identify the native structures among all these possible secondary structures. For this reason, much effort has been made to develop approaches for identifying good predictions of RNA secondary structure. Using the abstract shapes approach as introduced by Giegerich et al., each class of similar secondary structures is represented by one shape and the native structures can be found among the top shape representatives. In this article, we derive some interesting results answering enumeration problems for abstract shapes and secondary structures of RNA. We start by computing symptotical representations for the number of shape representations of length n. Our main goal is to find out how much the search space can be reduced by using the concept of abstract shapes. To reach this goal, we analyze the number of secondary structures and shapes compatible with an RNA sequence of length n under the assumption that base pairing is allowed between arbitrary pairs of bases analytically and compare their exponential growths. Additionally, we analyze the number of secondary structures compatible with an RNA sequence of length n under the assumptions that base pairing is allowed only between certain pairs of bases and that the structures meet some appropriate conditions. The exponential growth factors of the resulting asymptotics are compared to the corresponding experimentally obtained value as given by Giegerich et al.
Guaranteeing correctness of compilation is a ma jor precondition for correct software. Code generation can be one of the most error-prone tasks in a compiler. One way to achieve trusted compilation is certifying compilation. A certifying compiler generates for each run a proof that it has performed the compilation run correctly. The proof is checked in a separate theorem prover. If the theorem prover is content with the proof, one can be sure that the compiler produced correct code. This paper presents a certifying code generation phase for a compiler translating an intermediate language into assembler code. The time spent for checking the proofs is the bottleneck of certifying compilation. We exhibit an improved framework for certifying compilation and considerable advances to overcome this bottleneck. We compare our implementation featuring the Coq theorem prover to an older implementation. Our current implementation is feasible for medium to large sized programs.
On the Complexity of the Uncapacitated Single Allocation p-Hub Median Problem with Equal Weights
(2007)
The Super-Peer Selection Problem is an optimization problem in network topology construction. It may be cast as a special case of a Hub Location Problem, more exactly an Uncapacitated Single Allocation p-Hub Median Problem with equal weights. We show that this problem is still NP-hard by reduction from Max Clique.
Approximating illumination by point light sources, as done in many professional applications, suffers from the problem of the weak singularity: Numerical exceptions caused by the division by the squared distance between the point light source and the point to be illuminated must be avoided. Multiple importance sampling overcomes these problems by combining multiple sampling techniques by weights. Such a set of weights is called a heuristic. So far the estimators resulting from a heuristic only have been analyzed for variance. Since the cost of sampling is not at all constant for different sampling techniques, it is possible to find more efficient heuristics, even though they may hove higher variance. Based on our new stratification heuristic, we present a robust and unbiased global illumination algorithm. By numerical examples, we show that it is more efficient than previous heuristics. The algorithm is as simple as a path tracer, but elegantly avoids the problem of the weak singularity.
This article focuses on the analytical analysis of the free energy in a realistic model for RNA secondary structures. In fact, the free energy in a stochastic model derived from a database of small and large subunit ribosomal RNA (SSU and LSU rRNA) data is studied. A common thermody-namic model for computing the free energy of a given RNA secondary structure, as well as stochastic context-free grammars and generating functions are used to derive the desired results. These results include asymptotics for the expected free energy and for the corresponding variance of a random RNA secondary structure. The quality of our model is judged by comparing the derived results to the used database of SSU and LSU rRNA data. At the end of this article, it is discussed how our results could be used to help on identifying good predictions of RNA secondary structure.
Abstraction is intensively used in the verification of large, complex or infinite-state systems. With abstractions getting more complex it is often difficult to see whether they are valid. However, for using abstraction in model checking it has to be ensured that properties are preserved. In this paper, we use a translation validation approach to verify property preservation of system abstractions. We formulate a correctness criterion based on simulation between concrete and abstract system for a property to be verified. For each distinct run of the abstraction procedure the correctness is verified in the theorem prover Isabelle/HOL. This technique is applied in the verification of embedded adaptive systems. This paper is an extended version a previously published work.
We survey old and new results about optimal algorithms for summation of finite sequences and for integration of functions from Hölder or Sobolev spaces. First we discuss optimal deterministic and randornized algorithms. Then we add a new aspect, which has not been covered before on conferences
about (quasi-) Monte Carlo methods: quantum computation. We give a short introduction into this setting and present recent results of the authors on optimal quantum algorithms for summation and integration. We discuss comparisons between the three settings. The most interesting case for Monte
Carlo and quantum integration is that of moderate smoothness \(k\) and large dimension \(d\) which, in fact, occurs in a number of important applied problems. In that case the deterministic exponent is negligible, so the \(n^{-1/2}\) Monte Carlo and the \(n^{-1}\) quantum speedup essentially constitute the entire convergence rate.
In den Modellierungssystemen des CAD/CAM werden oft unterschiedliche Methoden zur mathematischen Beschreibung von Freiformkurven und -flächen eingesetzt. Als Basisfunktionen können sowohl Monome, Bernstein-Polynome, B-Spline-Basisfunktionen als auch nicht lineare Funktionen auftreten. In den einzelnen CAD-Systemen kann der maximal zulässige Grad dieser Basisfunktionen variieren. Müssen nun Daten zwischen verschiedenen CAD-Systemen ausgetauscht werden, so muß u. U. eine Basistransformation
und/oder eine Gradanpassung durchgeführt werden. Diese Transformationen sind i.a. nicht exakt möglich. Hier sind geeignete, möglichst optimale Approximationen nötig. Bisher wurden verschiedene Verfahren entwickelt. Das älteste geht zurück auf Forrest [Forr72]. Farin [FAR90] invertiert den Prozeß der Graderhöhung. Watkins und Worsey [Wat88] sowie Lachance [Lach88] reduzieren den Polynomgrad in der Tschebyscheff-Basis. Hoschek et al. [Hos89] sowie Plass und Stone [Plas83] approximieren die Kurve bzw. Fläche punktweise. Dadurch lassen sich alle Kurven- und Flächenrepräsentationen durch eine Bézier-Darstellung approximieren. Ein Approximationsfehler kann jedoch auch nur punktweise garantiert werden. Durch einen anschließenden Parameteriterationsprozeß läßt sich eine weitere Approximationsverbesserung erzielen. Eine solche Parameterkorrektur ist jedoch nur dann sinnvoll, wenn die Parametrisierung der Approximationskurve bzw. -fläche frei gewählt werden kann. In Fällen, in denen die Funktionswerte dei; zu approximierenden Flächen bzgl. ihrer Parameterwerte mit anderen Flächen korrespondieren, darf keine Parameteränderung durchgeführt werden, wie z.B. bei der Approximation sogenannter Eigenschaftsflächen, die eine bestimmte Eigenschaft einer anderen Fläche, wie etwa die Gausskrümmung oder die Normalenrichtung darstellen. In dieser Arbeit wird ein Verfahren zur optimalen Gradreduktion von Bézierkurven und -flächen vorgestellt. Damit eine \(C^0\)-stetige Approximation innerhalb einer vom Benutzer vorgegebenen Fehlertoleranz durchgeführt werden kann, muß die Approximation mindestens eine Berührordnung ersten Grades mit der Originalkurve bzw. -fläche aufweisen. Mit Hilfe arithmetischer Operationen auf Bézierdarstellungen [Faro88], [Schr92] werden lineare Gleichungssysteme für eine optimale Belegung der freien Parameter aufgestellt, sowie eine Fehlerkurve bzw. -fläche in Bézierform berechnet, um die Einhaltung einer Fehlertoleranz zu gewährleisten.
Optimization of Projection Methods for Solving ill-posed Problems. In this paper we propose a modification of the projection scheme for solving ill-posed problems. We show that this modification allows to obtain the best possible order of accuracy of Tikhonov Regularization using an amount of information which is far less than for the standard projection technique.
Skelettbasierte implizite Flächen haben aufgrund ihrer Fähigkeit, durch automatisches Verschmelzen aus wenigen, einfachen Primitiven komplexe Strukturen zu formen, für Modellierung, Visualisierung und Animation zunehmend an Bedeutung gewonnen. Eine wesentliche Schwierigkeit beim Einsatz impliziter Flächen ist nach wie vor eine effiziente Visualisierung der resultierenden Objekte. In der vorliegenden
Arbeit werden die grundlegenden Ideen einer Methode zur partikelgestützten Triangulierung skelettbasierter impliziter Flächen beschrieben, die die Vorteile einer partikelgestützten Abtastung
impliziter Flächen mit der polygonalen Darstellung durch Dreiecke kombiniert. Der Algorithmus ist in der Lage, effizient auf dynamische Veränderungen der Gestalt sowie das Auseinanderreißen nicht allzu
komplexer implizit gegebener Objekte zu reagieren. Zusätzlich besteht die Möglichkeit, die Triangulierung krümmungsadaptiv zu gestalten, um bei gleichbleibender Darstellungsqualität eine Reduktion der Dreiecksanzahl zu erreichen.
Partitioned chain grammars
(1979)
This paper introduces a new class of grammars, the partitioned chain grammars, for which efficient parsers can be automatically generated. Besides being efficiently parsable these grammars possess a number of other properties, which make them very attractive for the use in parser-generators. They for instance form a large grammarclass and describe all deterministic context-free languages. Main advantage of the partitioned chain grammars however is, that given a language it is usually easier to describe it by a partitioned chain grammar than to construct a grammar of some other type commonly used in parser-generators for it.
The use of non-volatile semiconductor memory within an extended storage hierarchy promises significant performance improvements for transaction processing. Although page-addressable semiconductor memories like extended memory, solid-state disks and disk caches are commercially available since several years, no detailed investigation of their use for transaction processing has been performed so far. We present a comprehensive simulation study that compares the performance of these storage types and of different usage forms. The following usage forms are considered: allocation of entire log and database files in non-volatile semiconductor memory, using a so-called write buffer to perform disk writes asynchronously, and caching of database pages at intermediate storage levels (in addition to main memory caching). Our simulations are conducted with both synthetically generated workloads and traces from real-life database applications. In particular, simulation results will be presented for the debit-credit workload frequently used in transaction processing benchmarks. As expected, the greatest performance improvements (but at the highest cost) can be achieved by storing log and database files completely in non-volatile semiconductor memory. For update-intensive
workloads, a limited amount of non-volatile memory used as a write buffer also proved to be very effective. To reduce the number of disk reads; caching of database pages in addition to main memory is best supported by an extended memory buffer. In this respect, disk caches are found to be less effective as they are designed for one-level caching. Different storage costs suggest that it may be cost-effective to use two or even three of the intermediate storage types together. The performance improvements obtainable by the use of non-volatile semiconductor memory is also found to reduce the need for sophisticated DBMS buffer management in order to achieve high transaction processing performance.
Web-based authentication is a popular mechanism implemented by Wireless Internet Service Providers (WISPs) because it allows a simple registration and authentication of customers, while avoiding the high resource requirements of the new IEEE 802.11i security standard and the backward compatibility issues of legacy devices. In this work we demonstrate two different and novel attacks against web-based authentication. One attack exploits operational anomalies of low- and middle-priced devices in order to hijack wireless clients, while the other exploits an already known vulnerability within wired-networks, which in dynamic wireless environments turns out to be even harder to detect and protect against.
The problem of constructing a geometric model of an existing object from a set of boundary points arises in many areas of industry. In this paper we present a new solution to this problem which is an extension of Boissonnat's method [2]. Our approach uses the well known Delaunay triangulation of the data points as an intermediate step. Starting with this structure, we eliminate tetrahedra until we get an appropriate approximation of the desired shape. The method proposed in this paper is capable of reconstructing objects with arbitrary genus and can cope with different point densities in different regions of the object. The
problems which arise during the elimination process, i.e. which tetrahedra can be eliminated, which order has to be used to control the process and finally, how to stop the elimination procedure at the right time, are discussed in detail. Several examples are given to show the validity of the method.
Many applications dealing with geometry acquisition and processing produce polygonal meshes that carry artifacts like discretization noise. While there are many approaches to remove the artifacts by smoothing or filtering the mesh, they are not tailored to any specific application subject to·certain restrictive objectives. We show how to incorporate smoothing schemes based on the general Laplacian approximation to satsify all those objectives at
the same time for the results of flow simulation in the application field of car manufacturing. In the presented application setting the major restrictions come from the bounding volume of the flow simulation, the so-called installation space. In particular, clean mesh regions (without noise) should not be smoothed while at the same time the installation space must not be violated by the smoothing of the noisy mesh regions. Additionally, aliasing effects at the boundary between clean and noisy mesh regions must be prevented. To address the fact that the meshes come from flow simulation, the presented method is versatile enough to preserve their exact volume and to apply anisotropic filters using the flow information.
Although the paper focuses on the results of a specific application, most of its findings can be transferred to different settings as well.
The provision of quality-of-service (QoS) on the network layer is a major challenge in communication networks. This applies particularly to mobile ad-hoc networks (MANETs) in the area of Ambient Intelligence (AmI), especially with the increasing use of delay and bandwidth sensitive applications. The focus of this survey lies on the classification and analysis of selected QoS routing protocols in the domain of mobile ad-hoc networks. Each protocol is briefly described and assessed, and the results are summarized in multiple tables.
Software-Projekte bestehen aus einer Vielzahl von Teilaufgaben, die durch komplexe Wechselbeziehungen miteinander verknüpft sind. Systematische Unterstützung bei der Durchführung von Software-Projekten erfordert deshalb nicht nur die isolierte Unterstützung einzelner Teilaufgaben, sondern insbesondere der Wechselbeziehungen. Außerdem müssen Aktivitäten des Messens und Bewertens durchgeführt werden, um quantitative Aussagen über Produkte und Prozesse ableiten zu können. Ziel des MVP-Projekts (Multi-View Process modeling) ist es, derartige integrierte Unterstützung auf der Basis meßbarer Projektpläne zur Verfügung zu stellen. Projektpläne setzen sich dabei unter anderem aus Prozeß-, Produkt-, Ressourcen- und Qualitätsmodellen zusammen. Meßansätze werden nicht nur zur systematischen Unterstützung von Projekten, sondern auch zur Verbesserung existierender Prozeß-, Produkt-, Ressource- und Qualitätsmodelle aufgrund 'gemessener' Erfahrungswerte verwendet. Die Benutzer des MVP-Entwicklungssystems (MVP-S) werden durch ihre Rollen im Rahmen eines Projekts charakterisiert werden können. Es wird beschrieben, wie Rollen das MVP-System nutzen können. Dies geschieht entweder durch direkte Repräsentation ihrer Aufgaben als Prozesse oder indem die im Projektplan repräsentierte Information ausgewertet und präsentiert wird; entsprechend bezeichnen wir eine Rolle als "zustandsverändernd" oder als "zustandserfragend". Um diese Rollen zu unterstützen, existieren unterschiedliche Möglichkeiten abhängig vom Grad der Automatisierung. Es werden beispielhaft drei Stufen aufgezeigt. Anschließend wird die Realisierung einer prototypischen, qualitätsorientierten, prozeßsensitiven Software-Entwicklungsumgebung diskutiert. Zum Abschluß wird auf gegenwärtige und zukünftige Forschungsfragen im Rahmen des MVP-Projekts eingegangen.
We study high dimensional integration in the quantum model of computation. We develop quantum algorithms for integration of functions from Sobolev classes \(W^r_p [0,1]^d\) and analyze their convergence rates. We also prove lower bounds which show that the proposed algorithms are, in many cases, optimal within the setting of quantum computing. This extends recent results of Novak on integration of functions from Hölder classes.
We study summation of sequences and integration in the quantum model of computation. We develop quantum algorithms for computing the mean of sequences which satisfy a \(p\)-summability condition and for integration of functions from Lebesgue spaces \(L_p([0,1]^d)\) and analyze their convergence rates. We also prove lower bounds which show that the proposed algorithms are, in many cases, optimal within the setting of quantum computing. This extends recent results of Brassard, Høyer, Mosca, and Tapp (2000) on computing the mean for bounded sequences and complements results of Novak (2001) on integration of functions from Hölder classes.
Monte Carlo integration is often used for antialiasing in rendering processes.
Due to low sampling rates only expected error estimates can be stated, and the variance can be high. In this article quasi-Monte Carlo methods are presented, achieving a guaranteed upper error bound and a convergence rate essentially as fast as usual Monte Carlo.
The radiance equation, which describes the global illumination problem in computer graphics, is a high dimensional integral equation. Estimates of the solution are usually computed on the basis of Monte Carlo methods. In this paper we propose and investigate quasi-Monte Carlo methods, which means that we replace (pseudo-) random samples by low discrepancy sequences, yielding deterministic algorithms. We carry out a comparative numerical study between Monte Carlo and quasi-Monte Carlo methods. Our results show that quasi-Monte Carlo converges considerably faster.