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Altenhofen, Christian; Schuwirth, Felix; Stork, André; Fellner, Dieter W.

Implicit Mesh Generation Using Volumetric Subdivision

2017

Jaillet, Fabrice (Ed.) et al.: VRIPHYS 17: 13th Workshop in Virtual Reality Interactions and Physical Simulations. Goslar: Eurographics Association, 2017, pp. 9-19

International Workshop in Virtual Reality Interaction and Physical Simulations (VRIPHYS) <13, 2017, Lyon, France>

In this paper, we present a novel approach for a tighter integration of 3D modeling and physically-based simulation. Instead of modeling 3D objects as surface models, we use a volumetric subdivision representation. Volumetric modeling operations allow designing 3D objects in similar ways as with surface-based modeling tools. Encoding the volumetric information already in the design mesh drastically simplifies and speeds up the mesh generation process for simulation. The transition between design, simulation and back to design is consistent and computationally cheap. Since the subdivision and mesh generation can be expressed as a precomputable matrix-vector multiplication, iteration times can be greatly reduced compared to common modeling and simulation setups. Therefore, this approach is especially well suited for early-stage modeling or optimization use cases, where many geometric changes are made in a short time and their physical effect on the model has to be evaluated frequently. To test our approach, we created, simulated and adapted several 3D models. Additionally, we measured and evaluated the timings for generating and applying the matrices for different subdivision levels. For comparison, we also measured the tetrahedral meshing functionality offered by CGAL for similar numbers of elements. For changing topology, our implicit meshing approach proves to be up to 70 times faster than creating the tetrahedral mesh only based on the outer surface. Without changing the topology and by precomputing the matrices, we achieve a speed-up of up to 2800.

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Altenhofen, Christian; Dietrich, Andreas; Stork, André; Fellner, Dieter W.

Rixels: Towards Secure Interactive 3D Graphics in Engineering Clouds

2017

Lukas, Uwe von (Ed.) et al.: Go-3D 2017: Mit 3D Richtung Maritim 4.0 : Tagungsband zur Konferenz Go-3D 2017. Stuttgart: Fraunhofer Verlag, 2017, pp. 25-43

Go-3D <8, 2017, Rostock, Germany>

Cloud computing rekindles old and imposes new challenges on remote visualization especially for interactive 3D graphics applications, e.g., in engineering and/or in entertainment. In this paper we present and discuss an approach entitled 'rich pixels' (short 'rixels') that balances the requirements concerning security and interactivity with the possibilities of hardware accelerated post-processing and rendering, both on the server side as well as on the client side using WebGL.

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Mueller-Roemer, Johannes; Altenhofen, Christian; Stork, André

Ternary Sparse Matrix Representation for Volumetric Mesh Subdivision and Processing on GPUs

2017

Computer Graphics Forum, Vol.36 (2017), 5, pp. 59-69

Eurographics Symposium on Geometry Processing (SGP) <15, 2017, London, UK>

In this paper, we present a novel volumetric mesh representation suited for parallel computing on modern GPU architectures. The data structure is based on a compact, ternary sparse matrix storage of boundary operators. Boundary operators correspond to the first-order top-down relations of k-faces to their (k-1)-face facets. The compact, ternary matrix storage format is based on compressed sparse row matrices with signed indices and allows for efficient parallel computation of indirect and bottomup relations. This representation is then used in the implementation of several parallel volumetric mesh algorithms including Laplacian smoothing and volumetric Catmull-Clark subdivision. We compare these algorithms with their counterparts based on OpenVolumeMesh and achieve speedups from 3x to 531x, for sufficiently large meshes, while reducing memory consumption by up to 36%.

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Mueller-Roemer, Johannes; Altenhofen, Christian

JIT-Compilation for Interactive Scientific Visualization

2016

Skala, Vaclav (Ed.): WSCG 2016. Short Papers Proceedings : 24th International Conference in Central Europe on Computer Graphics, Visualization and Computer Vision. [cited 04 January 2017] Available from http://wscg.zcu.cz/DL/wscg DL.htm: University of West Bohemia, 2016. (Computer Science Research Notes (CSRN) 2602), pp. 197-206

International Conference in Central Europe on Computer Graphics, Visualization and Computer Vision (WSCG) <24, 2016, Plzen, Czech Republic>

Due to the proliferation of mobile devices and cloud computing, remote simulation and visualization have become increasingly important. In order to reduce bandwidth and (de)serialization costs, and to improve mobile battery life, we examine the performance and bandwidth benefits of using an optimizing query compiler for remote postprocessing of interactive and in-situ simulations. We conduct a detailed analysis of streaming performance for interactive simulations. By evaluating pre-compiled expressions and only sending one calculated field instead of the raw simulation results, we reduce the amount of data transmitted over the network by up to 2/3 for our test cases. A CPU and a GPU version of the query compiler are implemented and evaluated. The latter is used to additionally reduce PCIe bus bandwidth costs and provides an improvement of over 70% relative to the CPU implementation when using a GPU-based simulation back-end.

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Altenhofen, Christian; Dietrich, Andreas; Stork, André; Fellner, Dieter W.

Rixels: Towards Secure Interactive 3D Graphics in Engineering Clouds

2016

The IPSI BgD Transactions on Internet Research, Vol.12 (2016), 1, pp. 31-38

Cloud computing rekindles old and imposes new challenges on remote visualization especially for interactive 3D graphics applications, e.g., in engineering and/or in entertainment. In this paper we present and discuss an approach entitled 'rich pixels' (short 'rixels') that balances the requirements concerning security and interactivity with the possibilities of hardware accelerated post-processing and rendering, both on the server side as well as on the client side using WebGL.

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Müller, Joel; Stork, André (Betreuer); Altenhofen, Christian (Betreuer)

A Subdivision-Based Approach to the Heat Equation for Simulation-Based Modeling

2015

Darmstadt, TU, Bachelor Thesis, 2015

In this thesis a subdivision-based method is presented for calculating numerical solutions to differential equations on the basis of a geometric representation that is also well suited for modeling. While modern CAD programs mainly use continuous representations, like B-splines or NURBS, numerical methods like FEM require a discrete mesh to perform the calculation on. The conversion between these two representations can become a hugely time consuming process. Utilizing the same representation for modeling and simulating objects speeds up the whole engineering process, as the need for mesh generation is eliminated. This also reduces the error made by approximating the geometry. As subdivision schemes are intuitive and efficient to use for modeling and visualizing complex geometries, they serve well as a basis for this method. The presented method is based on Chaikin's algorithm for one-dimensional objects and utilizes Catmull-Clark surfaces to represent two-dimensional objects. On the basis of these two subdivision schemes, solutions to the heat equation are generated, demonstrating the applicability of the approach. The exactness of this solution and the performance of the algorithm are compared to a traditional FEM approach to the heat equation.

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Weber, Daniel; Mueller-Roemer, Johannes; Altenhofen, Christian; Stork, André; Fellner, Dieter W.

Deformation Simulation using Cubic Finite Elements and Efficient p-multigrid Methods

2015

Computers & Graphics, Vol.53 (2015), PART B, pp. 185-195

We present a novel p-multigrid method for efficient simulation of corotational elasticity with higher-order finite elements. In contrast to other multigrid methods proposed for volumetric deformation, the resolution hierarchy is realized by varying polynomial degrees on a tetrahedral mesh. The multigrid approach can be either used as a direct method or as a preconditioner for a conjugate gradient algorithm. We demonstrate the efficiency of our approach and compare it to commonly used direct sparse solvers and preconditioned conjugate gradient methods. As the polynomial representation is defined w.r.t. the same mesh, the update of the matrix hierarchy necessary for corotational elasticity can be computed efficiently. We introduce the use of cubic finite elements for volumetric deformation and investigate different combinations of polynomial degrees for the hierarchy. We analyze the applicability of cubic finite elements for deformation simulation by comparing analytical results in a static and dynamic scenario and demonstrate our algorithm in dynamic simulations with quadratic and cubic elements. Applying our method to quadratic and cubic finite elements results in a speed-up of up to a factor of 7 for solving the linear system.

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Stahl, Christian; Altenhofen, Christian; Bellos, Efstratios; Hjelmervik, Jon

Flexible Integration of Cloud-based Engineering Services using Semantic Technologies

2015

Institute of Electrical and Electronics Engineers (IEEE): 2015 IEEE International Conference on Industrial Technology : ICIT 2015. Los Alamitos, Calif.: IEEE Computer Society, 2015, pp. 1520-1525

IEEE International Conference on Industrial Technology (ICIT) <2015, Seville, Spain>

Cloud-based engineering services and applications can enable a more flexible, dynamic and integrated usage of software solutions such as CAD, CAM, CFD and PLM. On demand availability of extensive computing power can lead to significantly improved products, especially for small- and medium-sized companies. In addition to these technical advantages, offering such integrated solutions in the Cloud opens the door to new business models for the software vendors, who can e.g. sell their software on a pay-per-use basis, thus attracting more possible customers. However, to exploit these benefits, such Cloud-based services must be easily accessible and combinable with minimal manual effort - even if they are provided by different software vendors. In this work, we propose a workflow (which is typically a chain of single services) description and execution system based on semantic technologies which abstracts from vendor-specific interfaces and thus allows an effective usage of Cloud-based engineering services in common workflows.

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Berz, Florian; Stork, André (Betreuer); Altenhofen, Christian (Betreuer)

Intuitive 3D-Interaktion für Design und Modellierung von volumetrischen Strukturen

2015

Darmstadt, TU, Master Thesis, 2015

Creating an intuitive way of modeling and designing different structures is a task of high relevance with a wide range of applications in computer graphics. Various options will have to be considered with respect to new developments. For instance, the following options are addressed in this thesis: visualization and interaction. The availability of modern and low-cost 3D hardware provides the users with new possibilities. Visualization and interactions can be performed in 3D and new approaches to applying the interactions are required. In this regard, a combination of zSpace tablet and Leap Motion is proven to fulfill these requirements. Interactions with the mesh can be implemented with the zSpace tablet and Leap Motion is used to control the navigation in scene and menu. Overall, some relevant and new 3D interactions are developed which, amongst other things, allow for the manipulation of inner structures. Those 3D interactions are meaningful, useful and intuitive and therefore, this thesis contributes to the current developments.

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Rücker, Marko; Altenhofen, Christian (Betreuer); Stork, André (Betreuer)

Schnelle und wasserdichte Triangulierung von Punktwolken für die interaktive Simulation: Fast and Watertight Triangulation of Point Clouds for Interactive Simulation

2015

Darmstadt, TU, Bachelor Thesis, 2015

In der frühen Designphase eines realen Objektes (z.B. mittels Lehmmodell) ist eine schnelle Beurteilung darüber notwendig, ob das erstellte Objekt vorher definierten Anforderungen (z.B. Strömungs-, Stabilitätseigenschaften) gerecht wird. Es gibt traditionell zwei Ansätze: Einmal die Strömungsmessung in einem Windkanal oder die aufwendige manuelle Digitalisierung von Objekten mit einer anschließenden Simulation des virtuellen Modells. Diese Arbeit beschäftigt sich mit einigen Aspekten, die beiden Ansätze miteinander zu verbinden. Es sollen Vorteile der Modellierung an einem realen Objekt und der schnellen Beurteilung von Eigenschaften eines virtuellen Objekts durch Simulation verbunden werden. Dabei wird ein Ansatz gezeigt, bei dem die Triangulierung einer Punktewolke von ca. 68.000 Punkten weniger als zehn Sekunden benötigt. Weiterhin bietet dieser Ansatz eine qualitativ bessere Triangulierung bei gleichbleibender bzw. halber Rechenzeit mit vergleichbaren Algorithmen, die sich mit wasserdichten 2- mannigfaltigen Dreiecksnetzen beschäftigen.

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Weber, Daniel; Mueller-Roemer, Johannes; Altenhofen, Christian; Stork, André; Fellner, Dieter W.

A p-Multigrid Algorithm using Cubic Finite Elements for Efficient Deformation Simulation

2014

Bender, Jan (Ed.) et al.: VRIPHYS 14: 11th Workshop in Virtual Reality Interactions and Physical Simulations. Goslar: Eurographics Association, 2014, pp. 49-58

International Workshop in Virtual Reality Interaction and Physical Simulations (VRIPHYS) <11, 2014, Bremen, Germany>

We present a novel p-multigrid method for efficient simulation of co-rotational elasticity with higher-order finite elements. In contrast to other multigrid methods proposed for volumetric deformation, the resolution hierarchy is realized by varying polynomial degrees on a tetrahedral mesh. We demonstrate the efficiency of our approach and compare it to commonly used direct sparse solvers and preconditioned conjugate gradient methods. As the polynomial representation is defined w.r.t. the same mesh, the update of the matrix hierarchy necessary for co-rotational elasticity can be computed efficiently. We introduce the use of cubic finite elements for volumetric deformation and investigate different combinations of polynomial degrees for the hierarchy. We analyze the applicability of cubic finite elements for deformation simulation by comparing analytical results in a static scenario and demonstrate our algorithm in dynamic simulations with quadratic and cubic elements. Applying our method to quadratic and cubic finite elements results in speed up of up to a factor of 7 for solving the linear system.

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Knuth, Martin; Altenhofen, Christian; Kuijper, Arjan; Bender, Jan

Efficient Self-Shadowing Using Image-Based Lighting on Glossy Surfaces

2014

Bender, Jan (Ed.) et al.: VMV 2014 : Vision, Modeling, and Visualization. Goslar: Eurographics Association, 2014, pp.159-166

Workshop on Vision, Modeling, and Visualization (VMV) <19, 2014, Darmstadt, Germany>

In this paper we present a novel natural illumination approach for real-time rasterization-based rendering with environment map-based high dynamic range lighting. Our approach allows to use all kinds of glossiness values for surfaces, ranging continuously from completely diffuse up to mirror-like glossiness. This is achieved by combining cosine-based diffuse, glossy and mirror reflection models in one single lighting model. We approximate this model by filter functions, which are applied to the environment map. This results in a fast, image-based lookup for the different glossiness values which gives our technique the high performance that is necessary for real-time rendering. In contrast to existing real-time rasterization-based natural illumination techniques, our method has the capability of handling high gloss surfaces with directional self-occlusion. While previous works exchange the environment map by virtual point light sources in the whole lighting and shadow computation, we keep the full image information of the environment map in the lighting process and only use virtual point light sources for the shadow computation. Our technique was developed for the usage in real-time virtual prototyping systems for garments since here typically a small scene is lit by a large environment which fulfills the requirements for imagebased lighting. In this application area high performance rendering techniques for dynamic scenes are essential since a physical simulation is usually running in parallel on the same machine. However, also other applications can benefit from our approach.

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Altenhofen, Christian; Kuijper, Arjan (Betreuer); Knuth, Martin (Betreuer)

Effiziente Selbstschattierung in Szenen mit bildbasierter Beleuchtungsinformation und glänzenden Materialien

2012

Darmstadt, TU, Master Thesis, 2012

Beim Design und Virtual Prototyping von Produkten werden heutzutage oftmals interaktive Computersimulationen eingesetzt. Um die Vorstellungen des Designers möglichst realitätsnah wiedergeben zu können, müssen diese Simulatoren über eine qualitativ hochwertige und plausible Beleuchtung verfügen. Hinzu kommt eine möglichst hohe Flexibilität bezüglich der Gestaltung der Oberflächen und der Wahl der verwendeten Materialien. In den meisten Fällen ermöglichen die verwendeten Simulatoren zusätzlich die direkte Manipulation der Geometrie oder Anordnung der Objekte durch den Benutzer. Es gibt bereits Verfahren, die solche dynamischen Szenen mit bildbasierten Beleuchtungsinformationen in Echtzeit beleuchten und Schattenwürfe berechnen können; die so genannten "Image Based Directional Occlusion" Verfahren (IBDO). Allerdings sind sie bezüglich der Materialien der zu beleuchtenden Objekte sehr eingeschränkt: Sie unterstützen nur diffuse und leicht glänzende Materialien; für hoch glänzende und spiegelnde Oberflächen sind sie nicht geeignet. In dieser Arbeit wird ein neues Beleuchtungssystem vorgestellt, das ähnlich wie die IBDO Algorithmen viele Lichtquellen aus einer Environment Map erzeugt, diese dann jedoch nicht direkt zur Beleuchtung der Szene, sondern lediglich zum Sammeln der Verdeckungsinformationen verwendet. Für die eigentliche Beleuchtung werden wie beim Environment Mapping die Farbwerte über einen oberflächenabhängigen Sample- Vektor aus der Environment Map (EM) ausgelesen. Die über die Lichtquellen gesammelten Verdeckungsinformationen in Form von Variance Shadow Maps (VSMs) werden dazu eingesetzt, nicht sichtbare Teile der EM dynamisch auszublenden. Die geforderte Flexibilität in der Materialwahl wird durch verschiedene MipMap Stufen der EM gewährleistet. Das Beleuchtungssystem verbindet somit die freie Materialwahl des Environment Mappings mit der bildbasierten Schattenberechnung der IBDO Verfahren. Die verwendeten VSMs bieten einen weichen Schattenwurf bei vergleichsweise geringem Rechenaufwand, verlieren aber bei niedrigen Auflösungen wichtige Details bei der Selbstschattierung. Die Steuerung der Reflektivität der Objekte über MipMaps erlaubt einen fließenden Übergang von 100% spiegelnd bis 100% diffus. Zusätzlich können Algorithmen zur Manipulation der Oberflächengeometrie, wie beispielsweise Bump-, Normalund Displacement-Mapping sowie Tesselierung, problemlos vorgelagert werden; das Beleuchtungssystem ist kompatibel. Der entscheidende Faktor für die Performance des Verfahrens ist die Zahl der verwendeten Lichtquellen. Für angemessene Mengen (100 bis 200) liefert es auf aktueller Mittelklasse-Hardware (z.B. AMD Radeon HD6850 oder NVIDIA Geforce GTX 280) interaktive Bildraten. Die hohe Zahl der benötigten Texturen und die damit verbundene Menge an Grafikspeicher sowie die zwanghafte Aufteilung des Renderings in mehrere Render-Passes stellen jedoch einen Nachteil gegenüber den oben erwähnten IBDO Verfahren dar.