9204420
Partitioned neighborhood spanners of minimal outdegree
A geometric spanner with vertex set P IRD is a sparse approximation of the complete Euclidean graph determined by P. We introduce the notion of partitioned neighborhood graphs (PNGs), unifying and generalizing most constructions of spanners treated in literature. Two important parameters characterizing their properties are the outdegree k IN and the stretch factor f 1 describing the ‘quality’ of approximation. PNGs have been thuroughly investigated with respect to small values of f . We, on the other hand, present in this work results about feasable values of k — so to say the other extreme. The aim of minimizing this parameter rather than the first one arises from two observations: a) It determines the amount of space required for storing PNGs. b) Many algorithms employing a (previously constructed) spanner have running times depending on its outdegree. Our results include, for fixed dimensions D as well as asymptotically, upper and lower bounds on this optimal value of k. The upper bounds are constructive and yield efficient algorithms for actually computing the corresponding graphs. 1Partially supported by EU ESPRIT Long Term Research Project 20244 (ALCOM-IT) and DFG Grants Me872/7-1 and Me872/4-1
14681345
The randomized sample tree: a data structure for interactive walkthroughs in externally stored virtual environments
We present a new data structure for rendering highly complex virtual environments of arbitrary topology. The special feature of our approach is that it allows an interactive navigation in very large scenes (30 GB/400 million polygons in our benchmark scenes) that cannot be stored in main memory, but only on a local or remote hard disk. Furthermore, it allows interactive rendering of substantially more complex scenes by instantiating objects.For the computation of an approximate image of the scene, a sampling technique is used. In the preprocessing, a so-called sample tree is built whose nodes contain randomly selected polygons from the scene. This tree only uses space that is linear in the number of polygons. In order to produce an image of the scene, the tree is traversed and polygons stored in the visited nodes are rendered. During the interactive walkthrough, parts of the sample tree are loaded from local or remote hard disk.We implemented our algorithm in a prototypical walkthrough system. Analysis and experiments show that the quality of our images is comparable to images computed by the conventional z-buffer algorithm regardless of the scene topology.
14377391
A Network Based Approach for Realtime Walkthrough of Massive Models
New dynamic search data structures developed recently guarantee constant execution time per search and update, i.e., they fulll the real-time requirements necessary for interactive walk-through in large geometric scenes. Yet, superiority or even applicability of these new methods in practice was still an open question. Their prototypical implementation presented in this work uses common libraries on standard workstations and thus represents a rst strut to bridge this gap. Indeed our experimental results give an indication on the actual performance of these theoretical ideas on real machines and possible bottlenecks in future developments. By special algorithmic enhancements, we can even avoid the otherwise essential preprocessing step.
7656106
Geometric Searching in Walkthrough Animations with Weak Spanners in Real Time
We study algorithmic aspects in the management of geometric scenes in interactive walkthrough animations. We consider arbitrarily large scenes consisting of unit size balls. For a smooth navigation in the scene we have to fulfill hard real time requirements. Therefore, we need algorithms whose running time is independent of the total number of objects in the scene and that use as small space as possible. In this work we focus on one of the basic operations in our walkthrough system: reporting the objects around the visitor within a certain distance. 
 
Previously a randomized data structure was presented that supports reporting the balls around the visitor in an output sensitive time and allows insertion and deletion of objects nearly as fast as searching. These results were achieved by exploiting the fact that the visitor moves "slowly" through the scene. A serious disadvantage of the aforementioned data structure is a big space overhead and the use of randomization. 
 
Our first result is a construction of weak spanners that leads to an improvement of the space requirement of the previously known data structures. Then we develop a deterministic data structure for the searching problem in which insertion of objects are allowed. Our incremental data structure supports O(1+k) reporting time, where k is a certain quantity close to the number of reported objects. The insertion time is similar to the reporting time and the space is linear to the total number of objects.
1648746
Experience in early and late software engineering project courses
We report on the experience we gained in different software engineering project courses at our department. A student who is specialized in software engineering has to complete, besides lectures on software engineering and related fields, two project courses: an "early" basic project course, and a "late" complex project course. For both of them we developed leveled teaching approaches considering the different knowledge and skills of beginners and "advanced" students, respectively. The paper describes, discusses and evaluates these approaches.
62681368
Lambda-calculus schemata
A lambda-calculus schema is an expression of the lambda calculus augmented by uninterpreted constant and operator symbols. It is an abstraction of programming languages such as LISP which permit functions to be passed to and returned from other functions. When given an interpretation for its constant and operator symbols, certain schemata, calledlambda abstractions, naturally define partial functions over the domain of interpretation. Two implementation strategies are considered: the retention strategy in which all variable bindings are retained until no longer needed (implying the use of some sort of garbage-collected store) and the deletion strategy, modeled after the usual stack implementation of ALGOL 60, in which variable bindings are destroyed when control leaves the procedure (or block) in which they were created. Not all lambda abstractions evaluate correctly under the deletion strategy. Nevertheless, both strategies are equally powerful in the sense that any lambda abstraction can be mechanically translated into another that evaluates correctly under the deletion strategy and defines the same partial function over the domain of interpretation as the original. Proof is by translation into continuation-passing style.
15568865
Appraising two decades of distributed computing theory research
Abstract.The field of distributed computing started around 1970 when people began to imagine a future world of multiple interconnected computers operating collectively. The theoretical challenge was to define what a computational problem would be in such a setting and to explore what could and could not be accomplished in a realistic setting in which the different computers fell under different administrative structures, operated at different speeds under the control of uncoordinated clocks, and sometimes failed in unpredictable ways. Meanwhile, the practical problem was to turn the vision into reality by building networks and networking equipment, communication protocols, and useful distributed applications. The theory of distributed computing became recognized as a distinct discipline with the holding of the first ACM Principles of Distributed Computing conference in 1982. This paper reviews some of the accomplishments of the theoretical community during the past two decades, notes an apparent disconnect between theoretical and practical concerns, and speculates on future synergy between the two.
207660233
Impossibility of distributed consensus with one faulty process
The consensus problem involves an asynchronous system of processes, some of which may be unreliable. The problem is for the reliable processes to agree on a binary value. In this paper, it is shown that every protocol for this problem has the possibility of nontermination, even with only one faulty process. By way of contrast, solutions are known for the synchronous case, the “Byzantine Generals” problem.
15236721
Bounds on secret key exchange using a random deal of cards
We present a general model for communication among a “team” of players overheard by a passive eavesdropper, Eve, in which all players including Eve are given private inputs that may be correlated. We define and explore secret key exchange in this model. Our secrecy requirements are information-theoretic and hold even if Eve is computationally unlimited. In particular, we consider the situation in which the team players are dealt hands of cards of prespecified sizes from a known deck of distinct cards. We explore when the team players can use the information contained in their hands to determine a value that each team player knows exactly but Eve cannot guess.