Publications of Wolfgang Effelsberg

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Jürgen Vogel, Martin Mauve, Volker Hilt und Wolfgang Effelsberg. 2003 Late Join Algorithms for Distributed Interactive Applications Multimedia systems / Association for Computing Machinery, 9, 2, 327-336
Distributed interactive applications such as shared whiteboards and multiplayer games often support dynamic groups where users may join and leave at any time. A participant joining an ongoing session has missed the data that have previously been exchanged by the other session members. It is therefore necessary to initialize the application instance of the latecomer with the current state. In this paper, we propose a late join algorithm for distributed interactive applications that provides such an initialization of applications. The algorithm is scalable and robust and can be easily adapted to the needs of different applications by means of late join policies. The behavior of the late join algorithm and the impact of design alternatives are investigated in detail by means of an extensive simulation study. This study also shows that an improper handling of the late join problem can cause very high application and network load.

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Dirk Farin, Peter H. N. de With und Wolfgang Effelsberg. SPIE Press, 2003 A Segmentation System with Model Assisted Completion of Video Objects Proceedings of SPIE. Bellingham, WAThis paper presents a new algorithm for video-object segmentation, which combines motion-based segmentation, high-level object-model detection, and spatial segmentation into a single framework. This joint approach overcomes the disadvantages of these algorithms when applied independently. These disadvantages include the low semantic accuracy of spatial segmentation and the inexact object boundaries obtained from object-model matching and motion segmentation. The now proposed algorithm alleviates three problems common to all motion-based segmentation algorithms. First, it completes object areas that cannot be clearly distinguished from the background because their color is near the background color. Second, parts of the object that are not considered to belong to the object since they are not moving, are still added to the object mask. Finally, when several objects are moving, of which only one is of interest, it is detected that the remaining regions do not belong to any object-model and these regions are removed from the foreground. This suppresses regions erroneously considered as moving or objects that are moving but that are completely irrelevant to the user.
Dirk Farin, Peter H. N. de With und Wolfgang Effelsberg. Werkgemeenschap voor Informatie- en Communicatietheorie, 2003 Optimal Partitioning of Video Sequences for MPEG-4 Sprite Encoding . EnschedeThis paper shows that the concept of merging several views of a non-changing scene-background into a single background sprite is not necessarily the most efficient way to transmit the background image. We have found that the counter-intuitive approach of splitting the background into several independent parts can reduce the overall amount of data. For this reason, we propose an algorithm that provides an optimal partitioning of a video sequence into independent background sprites, resulting in the minimum coding cost. Moreover, our algorithm results in background sprites with better quality by ensuring that the sprite resolution has at least the final display resolution throughout the sequence.
Dirk Farin, Peter H. N. de With und Wolfgang Effelsberg. SPIE Press, 2003 Recognition of User-Defined Video Object Models using Weighted Graph Homomorphisms Proceedings of SPIE. Bellingham, Wash.In this paper, we propose a new system for video object detection based on user-defined models. Object models are described by model graphs in which nodes represent image regions and edges denote spatial proximity. Each node is attributed with color and shape information about the corresponding image region. Model graphs are specified manually based on a sample image of the object. Object recognition starts with automatic color segmentation of the input image. For each region, the same features are extracted as specified in the model graph. Recognition is based on finding a subgraph in the image graph that matches the model graph. Evidently, it is not possible to find an isomorph subgraph, since node and edge attributes will not match exactly. Furthermore, the automatic segmentation step leads to an oversegmented image. For this reason, we employ emph{inexact} graph matching, where several nodes of the image graph may be mapped onto a single node in the model graph. We have applied our object recognition algorithm to cartoon sequences. This class of sequences is difficult to handle with current automatic segmentation algorithms because the motion estimation has difficulties arising from large homogeneous regions and because the object appearance is typically highly variable. Experiments show that our algorithm can robustly detect the specified objects and also accurately locates the object boundary.
Dirk Farin, Peter H. N. de With und Wolfgang Effelsberg. IEEE, 2003 Robust Background Estimation for Complex Video Sequences . Piscataway, NJKnowing the background image of a video scene simplifies the general video-object segmentation problem and therefore it is required by several automatic segmentation algorithms. This paper presents a new background estimation algorithm which is applicable to complex video sequences where many objects are simultaneously visible and the background is visible for a short time period only. The algorithm applies a rough segmentation of the input images into foreground and background regions to exclude the foreground objects from background synthesis. This prevents a bias of the synthesized background image towards the color of foreground objects. Experiments show that the obtained background images differ significantly less from the real background than those obtained with previous algorithms.
Hans Christian Liebig und Wolfgang Effelsberg. Sauerländer, 2003 Automatic Adaption of Exercise Problems to the Proficiency Level of the Learner . BernOnline exercise systems should be able to automatically adapt the level of difficulty of the problems to the level of proficiency of the learner. In our Communication and Tutoring Sys-tem CATS we have implemented a measurement algorithm that keeps track of the perform-ance level of a learner by measuring the percentage of correct answers at the current level, the reliability with which the learner answers the questions and the time he/she takes. From these measures we derive a unified performance parameter that controls the presentation of the next set of questions. These are then generated dynamically by the exercise applet.
Hans Christian Liebig und Wolfgang Effelsberg. IEEE Computer Society Press, 2003 Seamless Integration of Exercise Problems to the Proficiency Level of the Learner . Piscataway, NJDistance learners in traditional online exercise and tutoring systems often get stuck with questions for which they need the help of a tutor or colleague. Learning alone can also be frustrating. In our Communication And Tutoring System, CATS, we have integrated the possibility to dial up a tutor and/or to set up immediate group communication with other distance learners using Internet videoconferencing technology. To find an appropriate partner, we have implemented a measurement algorithm that keeps track of the performance level of a learner by measuring the percentage of correct answers, the reliability with which the learner answers the questions, and the time he/she takes. From these measures we derive a unified performance parameter that controls the presentation of the next set of questions. These are then generated dynamically by CATS. In this paper we explain our pedagogical approach and present the architecture and implementation of the CATS system.
Jürgen Vogel, Jörg Widmer, Dirk Farin, Martin Mauve und Wolfgang Effelsberg. ACM, 2003 Priority-Based Distribution Trees for Application-Level Multicast . New York, NYIn this paper, we propose a novel multicast routing algorithm that is based on application-level priorities and network characteristics: The application may specify an individual priority for each packet-receiver pair. The multicast distribution tree is then constructed such that the higher the priority, the more direct the path from the sender to the packet's destination and the lower the resulting end-to-end delay. This algorithm can be used to realize application-level multicast for delay-sensitive applications such as networked computer games. However, optimizing the multicast tree with respect to the end-to-end delay comes at the cost of an increase in link stress -- the more direct a path, the less likely it is that it can be integrated efficiently into an overlay distribution tree. Our algorithm takes this tradeoff into account and constructs efficient priority-based multicast trees. We demonstrate the performance and characteristics of the algorithm through simulation.

Forschungs Papier

Hans Christian Liebig und Wolfgang Effelsberg. , Technical Reports. 2003 Seamless Integration of Group Communication into an Adaptive Online Exercise System Mannheim, . 03-003
Distance learners in traditional online exercise and tutoring systems often get stuck with questions for which they need the help of a tutor or colleague. Learning alone can also be frustrating. In our Communication And Tutoring System CATS we have integrated the possibility to dial up a tutor and/or to setup an immediate group communication with other distance learners using Internet videoconferencing technology. To find the appropriate partner, we have implemented a measurement algorithm that keeps track of the performance level of a learner by measuring the percentage of correct answers at the current level, the reliability with which the learner answers the questions and the time he/she takes. From these measures we derive a unified performance parameter that controls the presentation of the next set of questions. These are then generated dynamically by the exercise applet. The CATS system automatically selects the most appropriate communica-tion partner(s) bas! ed on the exercises the learners are currently working on, and on their skill levels. We motivate this approach from a pedagogical point of view and present the architecture and implementation of the CATS system.

Buch Kapitel

Wolfgang Effelsberg. 2003 Netztechnik und AV-Geräte Waxmann E-Learning für die Hochschule : erfolgreiche Ansätze für ein flexibleres Studium Münster [u.a.], 57-79
Dirk Farin, Thomas Haenselmann, Stephan Kopf, Gerald Kühne und Wolfgang Effelsberg. 2003 Segmentation and Classification of Moving Video Objects CRC Press Handbook of Video Databases: Design and Applications Boca Raton, Fla., 561-591
Thomas Haenselmann, Dirk Farin, Christoph Kuhmünch und Wolfgang Effelsberg. 2003 Netzwerktechnik für TeleTeaching Bertelsmann E-Learning-Services im Spannungsfeld von Pädagogik, Ökonomie und Technologie. L3 - Lebenslanges Lernen im Bildungsnetzwerk der Zukunft Bielefeld, 185-198

Publications of Wolfgang Effelsberg

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