Year: Author:

Jan Bender, Dan Koschier
IEEE Transactions on Visualization and Computer Graphics

In this paper we present a novel Smoothed Particle Hydrodynamics (SPH) method for the efficient and stable simulation of incompressible fluids. The most efficient SPH-based approaches enforce incompressibility either on position or velocity level. However, the continuity equation for incompressible flow demands to maintain a constant density and a divergence-free velocity field. We propose a combination of two novel implicit pressure solvers enforcing both a low volume compression as well as a divergence-free velocity field. While a compression-free fluid is essential for realistic physical behavior, a divergence-free velocity field drastically reduces the number of required solver iterations and increases the stability of the simulation significantly. Thanks to the improved stability, our method can handle larger time steps than previous approaches. This results in a substantial performance gain since the computationally expensive neighborhood search has to be performed less frequently. Moreover, we introduce a third optional implicit solver to simulate highly viscous fluids which seamlessly integrates into our solver framework. Our implicit viscosity solver produces realistic results while introducing almost no numerical damping. We demonstrate the efficiency, robustness and scalability of our method in a variety of complex simulations including scenarios with millions of turbulent particles or highly viscous materials.

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@article{Bender2016, author = {Jan Bender and Dan Koschier}, title = {Divergence-Free SPH for Incompressible and Viscous Fluids}, year = {2016}, journal = {IEEE Transactions on Visualization and Computer Graphics}, publisher = {IEEE}, keywords={Smoothed Particle Hydrodynamics;divergence-free fluids;fluid simulation;implicit integration;incompressibility;viscous fluids}, doi={10.1109/TVCG.2016.2578335}, ISSN={1077-2626} }

Tassilo Kugelstadt, Elmar Schoemer
ACM SIGGRAPH / EUROGRAPHICS Symposium on Computer Animation

We present a novel method to simulate bending and torsion of elastic rods within the position-based dynamics (PBD) framework. The main challenge is that torsion effects of Cosserat rods are described in terms of material frames which are attached to the centerline of the rod. But frames or orientations do not fit into the classical position-based dynamics formulation. To solve this problem we introduce new types of constraints to couple orientations which are represented by unit quaternions. For constraint projection quaternions are treated in the exact same way as positions. Unit length is enforced with an additional constraint. This allows us to use the strain measures form Cosserat theory directly as constraints in PBD. It leads to very simple algebraic expressions for the correction displacements which only contain quaternion products and additions. Our results show that our method is very robust and accurately produces the complex bending and torsion effects of rods. Due to its simplicity our method is very efficient and more than one order of magnitude faster than existing position-based rod simulation methods. It even achieves the same performance as position-based simulations without torsion effects.

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@INPROCEEDINGS{Kugelstadt2016, author = {Tassilo Kugelstadt and Elmar Schoemer}, title = {Position and Orientation Based Cosserat Rods}, booktitle = {Proceedings of the 2016 ACM SIGGRAPH/Eurographics Symposium on Computer Animation}, year = {2016}, publisher = {Eurographics Association}, location = {Zurich, Switzerland} }

Dan Koschier, Crispin Deul, Jan Bender
ACM SIGGRAPH / EUROGRAPHICS Symposium on Computer Animation

In this paper we propose a novel method to construct hierarchical $hp$-adaptive Signed Distance Fields (SDFs). We discretize the signed distance function of an input mesh using piecewise polynomials on an axis-aligned hexahedral grid. Besides spatial refinement based on octree subdivision to refine the cell size (h), we hierarchically increase each cell's polynomial degree (p) in order to construct a very accurate but memory-efficient representation. Presenting a novel criterion to decide whether to apply h- or p-refinement, we demonstrate that our method is able to construct more accurate SDFs at significantly lower memory consumption than previous approaches. Finally, we demonstrate the usage of our representation as collision detector for geometrically highly complex solid objects in the application area of physically-based simulation.

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@INPROCEEDINGS{Koschier2016, author = {Dan Koschier and Crispin Deul and Jan Bender}, title = {Hierarchical hp-Adaptive Signed Distance Fields}, booktitle = {Proceedings of the 2016 ACM SIGGRAPH/Eurographics Symposium on Computer Animation}, year = {2016}, publisher = {Eurographics Association}, location = {Zurich, Switzerland} }

Martin Knuth, Jan Bender, Michael Goesele, Arjan Kuijper
IEEE Computer Graphics and Applications

We introduce deferred warping, a novel approach for real-time deformation of 3D objects attached to an animated or manipulated surface. Our target application is virtual prototyping of garments where 2D pattern modeling is combined with 3D garment simulation which allows an immediate validation of the design. The technique works in two steps: First, the surface deformation of the target object is determined and the resulting transformation field is stored as a matrix texture. Then the matrix texture is used as look-up table to transform a given geometry onto a deformed surface. Splitting the process in two steps yields a large flexibility since different attachment types can be realized by simply defining specific mapping functions. Our technique can directly handle complex topology changes within the surface. We demonstrate a fast implementation in the vertex shading stage allowing the use of highly decorated surfaces with millions of triangles in real-time.

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@ARTICLE{Knuth2016, author={Martin Knuth and Jan Bender and Michael Goesele and Arjan Kuijper}, journal={IEEE Computer Graphics and Applications}, title={Deferred Warping}, year={2016}, doi={10.1109/MCG.2016.41}, ISSN={0272-1716} }

Marcel Weiler, Dan Koschier, Jan Bender
ACM SIGGRAPH Motion in Games

We present a new method for particle based fluid simulation, using a combination of Projective Dynamics and Smoothed Particle Hydrodynamics (SPH). The Projective Dynamics framework allows the fast simulation of a wide range of constraints. It offers great stability through its implicit time integration scheme and is parallelizable in large parts, so that it can make use of modern multi core CPUs. Yet existing work only uses Projective Dynamics to simulate various kinds of soft bodies and cloth. We are the first ones to incorporate fluid simulation into the Projective Dynamics framework. Our proposed fluid constraints are derived from SPH and seamlessly integrate into the existing method. Furthermore, we adapt the solver to handle the constantly changing constraints that appear in fluid simulation. We employ a highly parallel matrix-free conjugate gradient solver, and thus do not require expensive matrix factorizations.

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@inproceedings{Weiler2016, author = {Marcel Weiler and Dan Koschier and Jan Bender}, title = {Projective Fluids}, booktitle = {Proceedings of ACM SIGGRAPH Conference on Motion in Games}, series = {MIG '16}, year = {2016}, publisher = {ACM} }

Matthias Müller, Jan Bender, Nuttapong Chentanez, Miles Macklin
ACM SIGGRAPH Motion in Games

We present a novel algorithm to extract the rotational part of an arbitrary 3x3 matrix. This problem lies at the core of two popular simulation methods in computer graphics, the co-rotational Finite Element Method and Shape Matching techniques. In contrast to the traditional method based on polar decomposition, degenerate configurations and inversions are handled robustly and do not have to be treated in a special way. In addition, our method can be implemented with only a few lines of code without branches which makes it particularly well suited for GPU-based applications. We demonstrate the robustness, coherence and efficiency of our method by comparing it to stabilized polar decomposition in several simulation scenarios.

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@inproceedings{Mueller2016, author = {Matthias M\"{u}ller and Jan Bender and Nuttapong Chentanez and Miles Macklin}, title = {A Robust Method to Extract the Rotational Part of Deformations}, booktitle = {Proceedings of ACM SIGGRAPH Conference on Motion in Games}, series = {MIG '16}, year = {2016}, publisher = {ACM} }

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