Hegemann, Tim - Chair of Computer Science I

Lehrstuhl für Informatik I
Universität Würzburg
Am Hubland
D-97074 Würzburg

Room: 01.006, Building M4

Email: tim.hegemann@uni-wuerzburg.de

0009-0008-4770-3391

Research interests

Visualize complex industrial networks
Overlap-free placement of labels in maps and graph drawings

Projects

Storylines with a Protagonist

Visualize joint publications with your coauthors over time (open interactive version).

Graph Harvester

Extract graphs from vector drawings in pdfs. Master's project of Julius Deynet supervised by Sebastian Kempf, Alexander Wolff, and myself.

Try it yourself!

ZiNsVis

Space-efficient visualization of large industrial networks (project page).

Publines

Master's thesis project of Tim Herrmann supervised by Felix Klesen, Alexander Wolff, and myself.

See it in action!

Awards

2024: 1st place in the manual category of the Graph Drawing Live Challenge at the 32nd International Symposium on Graph Drawing and Network Visualization (GD'24) in Vienna, together with Johannes Zink
2023: 1st place in the manual category of the Graph Drawing Live Challenge at the 31st International Symposium on Graph Drawing and Network Visualization (GD'23) in Palermo, together with Florentina Voboril and Johannes Zink

Supervised Theses and Practical Projects

Jan Sauer: Chord-Diagramme mit partiellen Knotenvorgaben
(Master Project, in German), 2026

Teaching

Winter term 2025/26:

Exercises in Computational Geometry

Summer term 2022:

Exercises in Visualization of Graphs

Publications

2026[ to top ]

Morphing graph drawings in the presence of point obstacles. Oksana Firman, Tim Hegemann, Boris Klemz, Felix Klesen, Marie Diana Sieper, Alexander Wolff, and Johannes Zink. Journal of Computer and System Sciences, 157, H. Fernau; S. Gaspers; R. Klasing (eds.), p. 103755. 2026.
- [ Abstract ]
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A crossing-free morph is a continuous deformation between two graph drawings that preserves straight-line pairwise noncrossing edges. Motivated by applications in 3D morphing problems, we initiate the study of morphing graph drawings in the plane in the presence of stationary point obstacles, which need to be avoided throughout the deformation. As our main result, we prove that it is NP-hard to decide whether such an obstacle-avoiding 2D morph between two given drawings of the same graph exists. In fact, this statement remains true even in the severely restricted special case where only three vertices have to change positions. This is in sharp contrast to the classical case without obstacles, where there is an efficiently verifiable (necessary and sufficient) criterion for the existence of a morph. Further, we provide several combinatorial results related to conditions under which the existence of a morph between two drawings of a graph can or cannot be prevented by the placement of a given number of point obstacles.

@article{fhkkswz-mgppo-jcss26, abstract = {A crossing-free morph is a continuous deformation between two graph drawings that preserves straight-line pairwise noncrossing edges. Motivated by applications in 3D morphing problems, we initiate the study of morphing graph drawings in the plane in the presence of stationary point obstacles, which need to be avoided throughout the deformation. As our main result, we prove that it is NP-hard to decide whether such an obstacle-avoiding 2D morph between two given drawings of the same graph exists. In fact, this statement remains true even in the severely restricted special case where only three vertices have to change positions. This is in sharp contrast to the classical case without obstacles, where there is an efficiently verifiable (necessary and sufficient) criterion for the existence of a morph. Further, we provide several combinatorial results related to conditions under which the existence of a morph between two drawings of a graph can or cannot be prevented by the placement of a given number of point obstacles.}, author = {Firman, Oksana and Hegemann, Tim and Klemz, Boris and Klesen, Felix and Sieper, Marie Diana and Wolff, Alexander and Zink, Johannes}, editor = {Fernau, Henning and Gaspers, Serge and Klasing, Ralf}, journal = {Journal of Computer and System Sciences}, keywords = {graph-morphing}, pages = 103755, title = {Morphing graph drawings in the presence of point obstacles}, volume = 157, year = 2026 }

2025[ to top ]

Universal Quality Metrics for Graph Drawings: Which Graphs Excite Us Most?. Gavin J. Mooney, Tim Hegemann, Alexander Wolff, Michael Wybrow, and Helen C. Purchase. In Graph Drawing and Network Visualization (GD), Vol. 357 of LIPIcs, F. Montecchiani, V. Dujmović (eds.), pp. 30:1–30:20. Schloss Dagstuhl – Leibniz-Zentrum für Informatik, 2025.
- [ Abstract ]
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Graphs are drawn for various purposes, and drawings are meant to display various features of a graph (such as planarity, Hamiltonicity). Still, there is a long history in measuring the quality of a graph drawing. Most of the metrics that have been implemented and used in large studies assume that graphs are drawn straight-line. Most of the studies use randomly generated graphs or one of very few existing benchmark sets that consist of graphs with a specific technical background (e.g., telecommunication networks). In this paper, we extend ten commonly used metrics to node-link diagrams where edges can be curves or polygonal chains. We implement these measures and use them to evaluate a new collection of graph drawings that we have extracted from 27 proceedings of the Graph Drawing conference using an automated pipeline. We compare the "metrics landscape" of our new benchmark set, the GD-collection-v1, which seems to mostly contain manually drawn graphs, to the metric landscape of a benchmark set with randomly generated graphs and computer-generated straight-line drawings that has been used in a recent study [Mooney et al.; PacificVis 2024]. Comparing the GD-collection-v1 with the Mooney at al. dataset reveals a distinct metrics landscape: GD drawings come from much smaller graphs (median vertex number 11 vs. 48) and therefore attain higher medians on most readability metrics. For example, Neighbourhood Preservation (0.5 vs. 0.239) is markedly higher in the GD-collection-v1. We also find that a large proportion of extracted drawings contain curved and/or polygonal edges (57%), motivating the extended metric definitions.

@inproceedings{10.4230/lipics.gd.2025.30, abstract = {Graphs are drawn for various purposes, and drawings are meant to display various features of a graph (such as planarity, Hamiltonicity). Still, there is a long history in measuring the quality of a graph drawing. Most of the metrics that have been implemented and used in large studies assume that graphs are drawn straight-line. Most of the studies use randomly generated graphs or one of very few existing benchmark sets that consist of graphs with a specific technical background (e.g., telecommunication networks). In this paper, we extend ten commonly used metrics to node-link diagrams where edges can be curves or polygonal chains. We implement these measures and use them to evaluate a new collection of graph drawings that we have extracted from 27 proceedings of the Graph Drawing conference using an automated pipeline. We compare the "metrics landscape" of our new benchmark set, the GD-collection-v1, which seems to mostly contain manually drawn graphs, to the metric landscape of a benchmark set with randomly generated graphs and computer-generated straight-line drawings that has been used in a recent study [Mooney et al.; PacificVis 2024]. Comparing the GD-collection-v1 with the Mooney at al. dataset reveals a distinct metrics landscape: GD drawings come from much smaller graphs (median vertex number 11 vs. 48) and therefore attain higher medians on most readability metrics. For example, Neighbourhood Preservation (0.5 vs. 0.239) is markedly higher in the GD-collection-v1. We also find that a large proportion of extracted drawings contain curved and/or polygonal edges (57%), motivating the extended metric definitions.}, author = {Mooney, Gavin J. and Hegemann, Tim and Wolff, Alexander and Wybrow, Michael and Purchase, Helen C.}, booktitle = {Graph Drawing and Network Visualization (GD)}, editor = {Montecchiani, Fabrizio and Dujmović, Vida}, keywords = {myown}, pages = {30:1-30:20}, publisher = {Schloss Dagstuhl – Leibniz-Zentrum für Informatik}, series = {LIPIcs}, title = {Universal Quality Metrics for Graph Drawings: Which Graphs Excite Us Most?}, volume = 357, year = 2025 }
Optimizing Wiggle in Storylines. Alexander Dobler, Tim Hegemann, Martin Nöllenburg, and Alexander Wolff. In Graph Drawing and Network Visualization (GD), Vol. 357 of LIPIcs, F. Montecchiani, V. Dujmović (eds.), pp. 39:1–39:17. Schloss Dagstuhl – Leibniz-Zentrum für Informatik, 2025.
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A storyline visualization shows interactions between characters over time. Each character is represented by an x-monotone curve. Time is mapped to the x-axis, and groups of characters that interact at a particular point t in time must be ordered consecutively in the y-dimension at x = t. The predominant objective in storyline optimization so far has been the minimization of crossings between (blocks of) characters. Building on this work, we investigate another important, but less studied quality criterion, namely the minimization of wiggle, i.e., the amount of vertical movement of the characters over time. Given a storyline instance together with an ordering of the characters at any point in time, we show that wiggle count minimization is NP-complete. In contrast, we provide algorithms based on mathematical programming to solve linear wiggle height minimization and quadratic wiggle height minimization efficiently. Finally, we introduce a new method for routing character curves that focuses on keeping distances between neighboring curves constant as long as they run in parallel. We have implemented our algorithms, and we conduct a case study that explores the differences between the three optimization objectives. We use existing benchmark data, but we also present a new use case for storylines, namely the visualization of rolling stock schedules in railway operation.

@inproceedings{lipics.gd.2025.39, abstract = {A storyline visualization shows interactions between characters over time. Each character is represented by an x-monotone curve. Time is mapped to the x-axis, and groups of characters that interact at a particular point t in time must be ordered consecutively in the y-dimension at x = t. The predominant objective in storyline optimization so far has been the minimization of crossings between (blocks of) characters. Building on this work, we investigate another important, but less studied quality criterion, namely the minimization of wiggle, i.e., the amount of vertical movement of the characters over time. Given a storyline instance together with an ordering of the characters at any point in time, we show that wiggle count minimization is NP-complete. In contrast, we provide algorithms based on mathematical programming to solve linear wiggle height minimization and quadratic wiggle height minimization efficiently. Finally, we introduce a new method for routing character curves that focuses on keeping distances between neighboring curves constant as long as they run in parallel. We have implemented our algorithms, and we conduct a case study that explores the differences between the three optimization objectives. We use existing benchmark data, but we also present a new use case for storylines, namely the visualization of rolling stock schedules in railway operation.}, author = {Dobler, Alexander and Hegemann, Tim and Nöllenburg, Martin and Wolff, Alexander}, booktitle = {Graph Drawing and Network Visualization (GD)}, editor = {Montecchiani, Fabrizio and Dujmović, Vida}, keywords = {myown}, pages = {39:1-39:17}, publisher = {Schloss Dagstuhl – Leibniz-Zentrum für Informatik}, series = {LIPIcs}, title = {Optimizing Wiggle in Storylines}, volume = 357, year = 2025 }

2024[ to top ]

Morphing Graph Drawings in the Presence of Point Obstacles. Oksana Firman, Tim Hegemann, Boris Klemz, Felix Klesen, Marie Diana Sieper, Alexander Wolff, and Johannes Zink. In Proc. 50th Int. Conf. Current Trends Theory & Practice Comput. Sci. (SOFSEM’24), Vol. 14519 of Lecture Notes in Computer Science, H. Fernau, S. Gaspers, R. Klasing (eds.), pp. 240–254. Springer-Verlag, 2024.
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@inproceedings{fhkkswz-mgdppo-SOFSEM24, author = {Firman, Oksana and Hegemann, Tim and Klemz, Boris and Klesen, Felix and Sieper, Marie Diana and Wolff, Alexander and Zink, Johannes}, booktitle = {Proc. 50th Int. Conf. Current Trends Theory & Practice Comput. Sci. (SOFSEM'24)}, editor = {Fernau, Henning and Gaspers, Serge and Klasing, Ralf}, keywords = {myown}, pages = {240–254}, publisher = {Springer-Verlag}, series = {Lecture Notes in Computer Science}, title = {Morphing Graph Drawings in the Presence of Point Obstacles}, volume = 14519, year = 2024 }
Storylines with a Protagonist. Tim Hegemann and Alexander Wolff. In Proc. 32nd Int. Symp. Graph Drawing & Network Vis. (GD’24), Vol. 320 of LIPIcs, pp. 26:1–22. Schloss Dagstuhl – Leibniz-Zentrum für Informatik, 2024.
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@inproceedings{https://doi.org/10.4230/lipics.gd.2024.26, author = {Hegemann, Tim and Wolff, Alexander}, booktitle = {Proc. 32nd Int. Symp. Graph Drawing & Network Vis. (GD'24)}, keywords = {myown}, pages = {26:1–22}, publisher = {Schloss Dagstuhl – Leibniz-Zentrum für Informatik}, series = {LIPIcs}, title = {Storylines with a Protagonist}, volume = 320, year = 2024 }
Graph Harvester (Software Abstract). Julius Deynet, Tim Hegemann, Sebastian Kempf, and Alexander Wolff. In Proc. 32nd Int. Symp. Graph Drawing & Network Vis. (GD’24), Vol. 320 of LIPIcs, pp. 58:1–58:3. Schloss Dagstuhl – Leibniz-Zentrum für Informatik, 2024.
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@inproceedings{https://doi.org/10.4230/lipics.gd.2024.58, author = {Deynet, Julius and Hegemann, Tim and Kempf, Sebastian and Wolff, Alexander}, booktitle = {Proc. 32nd Int. Symp. Graph Drawing & Network Vis. (GD'24)}, keywords = {myown}, pages = {58:1–58:3}, publisher = {Schloss Dagstuhl – Leibniz-Zentrum für Informatik}, series = {LIPIcs}, title = {Graph Harvester (Software Abstract)}, volume = 320, year = 2024 }

2023[ to top ]

A Simple Pipeline for Orthogonal Graph Drawing. Tim Hegemann and Alexander Wolff. In Proc. 31st Int. Symp. Graph Drawing & Network Vis. (GD’23), of Lecture Notes in Computer Science, M. Bekos, M. Chimani (eds.). Springer-Verlag, 2023.
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@inproceedings{hw-spogd-GD23, author = {Hegemann, Tim and Wolff, Alexander}, booktitle = {Proc. 31st Int. Symp. Graph Drawing & Network Vis. (GD'23)}, editor = {Bekos, Michael and Chimani, Markus}, keywords = {myown}, publisher = {Springer-Verlag}, series = {Lecture Notes in Computer Science}, title = {A Simple Pipeline for Orthogonal Graph Drawing}, year = 2023 }

2021[ to top ]

Aligning Polylines to Line Features on Bitmap Images of Historical Maps. Technical Report (Master thesis), . Tim Hegemann. Master thesis. University of Würzburg, Institut für Informatik, 2021.
- [ BibTeX ]
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@mastersthesis{hegemann2021aligning, address = {Institut für Informatik}, author = {Hegemann, Tim}, keywords = {myown}, school = {University of Würzburg}, title = {Aligning Polylines to Line Features on Bitmap Images of Historical Maps}, year = 2021 }

2019[ to top ]

Relevanz der Codequalität in einem Praktikum mit automatisch getesteten Programmierabgaben. Andre Greubel, Tim Hegemann, Marianus Ifland, and Martin Hennecke. In Proceedings of the Fourth Workshop "Automatische Bewertung von Programmieraufgaben" (ABP 2019), pp. 27–34. Gesellschaft für Informatik e.V., 2019.
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Systeme zur automatisierten Bewertung von Programmieraufgaben bewerten üblicherweise nicht die Codequalität. In dieser Arbeit wird an einem Praktikum mit 212 Studierenden empirisch untersucht, welche Relevanz diese der Codequalität beimessen und welche Relevanz verschiedene Qualitätsmetriken auf das Bestehen des Praktikums haben. Des Weiteren wird analysiert, ob diese Metriken von externen Faktoren wie dem Studiengang oder der Praktikumswiederholung beeinflusst werden.

@inproceedings{https://doi.org/10.18420/abp2019-4, abstract = {Systeme zur automatisierten Bewertung von Programmieraufgaben bewerten üblicherweise nicht die Codequalität. In dieser Arbeit wird an einem Praktikum mit 212 Studierenden empirisch untersucht, welche Relevanz diese der Codequalität beimessen und welche Relevanz verschiedene Qualitätsmetriken auf das Bestehen des Praktikums haben. Des Weiteren wird analysiert, ob diese Metriken von externen Faktoren wie dem Studiengang oder der Praktikumswiederholung beeinflusst werden.}, author = {Greubel, Andre and Hegemann, Tim and Ifland, Marianus and Hennecke, Martin}, booktitle = {Proceedings of the Fourth Workshop "Automatische Bewertung von Programmieraufgaben" (ABP 2019)}, keywords = {myown}, pages = {27-34}, publisher = {Gesellschaft für Informatik e.V.}, title = {Relevanz der Codequalität in einem Praktikum mit automatisch getesteten Programmierabgaben}, year = 2019 }

2018[ to top ]

Cross-Platform Evaluation of Performance Event Trigger. Tim Hegemann. Institut für Informatik, 2018, June.
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@misc{HegemannCrossPlatformPET2018, address = {Institut für Informatik}, author = {Hegemann, Tim}, keywords = {myown}, month = {06}, school = {University of Würzburg}, title = {Cross-Platform Evaluation of Performance Event Trigger}, type = {Bachelor Thesis}, year = 2018 }

Tim Hegemann, M.Sc.