vriphys: Workshop in Virtual Reality Interactions and Physical Simulations

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Area Preserving Strain Limiting
(The Eurographics Association, 2015) Han, Dongsoo; Fabrice Jaillet and Florence Zara and Gabriel Zachmann
In this paper we present a novel fast strain-limiting method that allows cloth to preserve its surface area efficiently. By preserving triangle area rather than edge length as other approaches do, this method does not remove the degrees of freedom of triangles and does not suffer from locking. Borrowing ideas from fluid simulation, we define pressures in each triangle and solve the global linear equation which shows a faster convergence over prior approaches which use Gauss-Seidel-like iterations. The linear equation is easy to build by using edge and normal vectors and can be solved using Conjugate Gradient solver with regularization which not only helps the solver converge fast but also allows users to have a control over the stretchiness of cloth materials. Our area preserving strain limiting (APSL) can be also used as stand-alone cloth solver with linear bending springs.
Brownian Dynamics Simulation on the GPU: Virtual Colloidal Suspensions
(The Eurographics Association, 2015) Tran, Công Tâm; Crespin, Benoît; Cerbelaud, Manuella; Videcoq, Arnaud; Fabrice Jaillet and Florence Zara and Gabriel Zachmann
Brownian Dynamics simulations are frequently used to describe and study the motion and aggregation of colloidal particles, in the field of soft matter and material science. In this paper, we focus on the problem of neighbourhood search to accelerate computations on a single GPU. Our approach for one kind of particle outperforms existing implementations by introducing a novel dynamic test. For bimodal size distributions we also introduce a new algorithm that separates computations for large and small particles, in order to avoid additional friction that is known to restrict diffusive displacements.
Continuous Collision Detection Between Points and Signed Distance Fields
(The Eurographics Association, 2014) Xu, Hongyi; Barbic, Jernej; Jan Bender and Christian Duriez and Fabrice Jaillet and Gabriel Zachmann
We present an algorithm for fast continuous collision detection between points and signed distance fields. Such robust queries are often needed in computer animation, haptics and virtual reality applications, but have so far only been investigated for polygon (triangular) geometry representations. We demonstrate how to use an octree subdivision of the distance field for fast traversal of distance field cells. We also give a method to combine octree subdivision with points organized into a tree hierarchy, for efficient culling of continuous collision detection tests. We apply our method to multibody rigid simulations, and demonstrate that our method accelerates continuous collision detection between points and distance fields by an order of magnitude.
Efficient Cloth Simulation Using an Adaptive Finite Element Method
(The Eurographics Association, 2012) Bender, Jan; Deul, Crispin; Jan Bender and Arjan Kuijper and Dieter W. Fellner and Eric Guerin
In this paper we present an efficient adaptive cloth simulation based on the sqrt-3-refinement scheme. Our adaptive cloth model can handle arbitrary triangle meshes and is not restricted to regular grid meshes which are required by other methods. Previous works on adaptive cloth simulation often use discrete cloth models like mass-spring systems in combination with a specific subdivision scheme. The problem of such models is that the simulation does not converge to the correct solution as the mesh is refined. We propose to use a cloth model which is based on continuum mechanics since continuous models do not have this problem. In order to perform an efficient simulation we use a linear elasticity model in combination with a corotational formulation. The sqrt-3-subdivision scheme has the advantage that it generates high quality meshes while the number of triangles increases only by a factor of 3 in each refinement step. However, the original scheme only defines a mesh refinement. Therefore, we introduce an extension to support the coarsening of our simulation model as well. Our proposed mesh adaption can be performed efficiently and therefore does not cause much overhead. In this paper we will show that a significant performance gain can be achieved by our adaptive method.
An Efficient Surface Reconstruction Pipeline for Particle-Based Fluids
(The Eurographics Association, 2012) Akinci, Gizem; Akinci, Nadir; Ihmsen, Markus; Teschner, Matthias; Jan Bender and Arjan Kuijper and Dieter W. Fellner and Eric Guerin
In this paper we present an efficient surface reconstruction pipeline for particle-based fluids such as smoothed particle hydrodynamics. After the scalar field computation and the marching cubes based triangulation, we post process the surface mesh by applying surface decimation and subdivision algorithms. In comparison to existing approaches, the decimation step alleviates the particle alignment related bumpiness very efficiently and reduces the number of triangles in flat regions. Later, the subdivision step ensures that the non-smooth regions are smoothed in a performance friendly way which allows our approach to run significantly faster by using lower resolution marching cubes grids. The presented pipeline is applicable to particle position data sets in a frame by frame basis. Throughout the paper, we present both visual and performance comparisons with different parameter settings, and with a state-of-the-art surface reconstruction technique. Our results demonstrate that in comparison to other approaches with comparable surface quality, our pipeline runs 15 to 20 times faster with up to 80% less memory and secondary storage consumption.
Evaluation of Surface Tension Models for SPH-Based Fluid Animations Using a Benchmark Test
(The Eurographics Association, 2015) Huber, Markus; Reinhardt, Stefan; Weiskopf, Daniel; Eberhardt, Bernhard; Fabrice Jaillet and Florence Zara and Gabriel Zachmann
We evaluate surface tension models in particle-based fluid simulation systems using smoothed particle hydrodynamics (SPH) with a benchmark test. Our benchmark consists of three experiments and a set of analysis methods that are useful for the comparison of surface tension models. Although visual quality is of major interest and is considered as well, we suggest quantification methods for the properties of these models. The goal is to identify if a certain model is suitable for a given scenario and to be able to control the results in the creation of animations. We apply the proposed evaluation methods to three existing surface tension models in combination with different SPH techniques (WCSPH, PCISPH, and IISPH) and perform systematic tests to show the influence of different settings and parameter choices. The surface tension models are chosen from different classes: a pure inter-particle force model, a model based on surface curvature, and a model using a combination of these. Additionally, we present a simple modification to improve the quality of inter-particle force models.
Generic Spine Model with Simple Physics for Life-Like Quadrupeds and Reptiles
(The Eurographics Association, 2012) Karim, Ahmad Abdul; Meyer, Alexandre; Gaudin, Thibaut; Buendia, Axel; Bouakaz, Saida; Jan Bender and Arjan Kuijper and Dieter W. Fellner and Eric Guerin
We propose a pseudo-physics system and a spine model that can be coupled to generate life-like locomotion animations of quadrupeds and reptiles. The pseudo-physics system uses minimalist particle-based physics and values of the gait pattern to generate the sinusoidal-like ballistic movement of the pelvis observed in nature. While the spine model uses simple geometry-based calculations and 3D Hermite curves to generate a flexible spine model, giving the animated creatures more agility. Our final system is totally controllable by the user in order to generate any desired style.
High-Resolution Simulation of Granular Material with SPH
(The Eurographics Association, 2012) Ihmsen, Markus; Wahl, Arthur; Teschner, Matthias; Jan Bender and Arjan Kuijper and Dieter W. Fellner and Eric Guerin
We present an efficient framework for simulating granular material with high visual detail. Our model solves the computationally and numerically critical forces on a coarsely sampled particle simulation. We incorporate a new frictional boundary force into an existing continuum-based method which enables realistic interactions and a more robust simulation. Visual realism is achieved by coupling a set of highly resolved particles with the base simulation at low computational costs. Thereby, visual details can be added which are not resolved by the base simulation.
Implicit Incompressible SPH on the GPU
(The Eurographics Association, 2015) Goswami, Prashant; Eliasson, André; Franzén, Pontus; Fabrice Jaillet and Florence Zara and Gabriel Zachmann
This paper presents CUDA-based parallelization of implicit incompressible SPH (IISPH) on the GPU. Along with the detailed exposition of our implementation, we analyze various components involved for their costs. We show that our CUDA version achieves near linear scaling with the number of particles and is faster than the multi-core parallelized IISPH on the CPU. We also present a basic comparison of IISPH with the standard SPH on GPU.
Interleaved Cloth Simulation
(The Eurographics Association, 2015) Han, Dongsoo; Fabrice Jaillet and Florence Zara and Gabriel Zachmann
Implicit integration is a standard for stiff spring-based cloth simulation because of its stability. However constraints are useful to simulate various physical behaviors such as contact collisions or interaction with rigid bodies. Modified Conjugate Gradient (MCG) could support constraints as a part of implicit integration but constraints could not be added or removed during integration and they were limited to vertex nodes. Normally, a contact constraint has one or two frictional constraints and act inside of triangle or edge rather than vertex node. Also its inequality property makes it harder to be included in MCG. For this reason, constraints are typically applied after implicit integration as a separate step or replaced with springs. In this paper, we propose a novel method to interleave various constraints with stiff springs so that we can take advantages from both sides. Also our Jacobian-free and matrix-free implicit integration allows us to use various nonlinear forces such as pressure or none vertex-centered forces. Interleaving collision constraints into integration step can eliminate unpleasant local deformation.
Kernel Projection of Latent Structures Regression for Facial Animation Retargeting
(The Eurographics Association, 2017) Ouzounis, Christos; Kilias, Alex; Mousas, Christos; Fabrice Jaillet and Florence Zara
Inspired by kernel methods that have been used extensively in achieving efficient facial animation retargeting, this paper presents a solution to retargeting facial animation in virtual character's face model based on the kernel projection of latent structure (KPLS) regression between semantically similar facial expressions. Specifically, a given number of corresponding semantically similar facial expressions are projected into the latent space. By using the Nonlinear Iterative Partial Least Square method, decomposition of the latent variables is achieved. Finally, the KPLS is achieved by solving a kernalized version of the eigenvalue problem. By evaluating our methodology with other kernel-based solutions, the efficiency of the presented methodology in transferring facial animation to face models with different morphological variations is demonstrated.
A More Efficient Parallel Method For Neighbour Search Using CUDA
(The Eurographics Association, 2015) Morillo, Daniel; Carmona, Ricardo; Perea, Juan J.; Cordero, Juan M.; Fabrice Jaillet and Florence Zara and Gabriel Zachmann
In particle systems simulation, the procedure of neighbour searching is usually a bottleneck in terms of computational cost. Several techniques have been developed to solve this problem; one of particular interest is the cell-based spatial division, where each cell is tagged by a hash function. One of the most useful features of this technique is that it can be easily parallelized to reduce computational costs. However, the parallelizing process has some drawbacks associated to data memory management. Also, when parallelizing neighbour search, the location of neighbouring particles between adjacent cells is also costly. To solve these shortcomings we have developed a method that reduces the search space by considering the relative position of each particles in its own cell. This method, parallelized using CUDA, shows improvements in processing time and memory management over other ''standard'' spatial division techniques.
Multilevel Cloth Simulation using GPU Surface Sampling
(The Eurographics Association, 2013) Schmitt, Nikolas; Knuth, Martin; Bender, Jan; Kuijper, Arjan; Jan Bender and Jeremie Dequidt and Christian Duriez and Gabriel Zachmann
Today most cloth simulation systems use triangular mesh models. However, regular grids allow many optimizations as connectivity is implicit, warp and weft directions of the cloth are aligned to grid edges and distances between particles are equal. In this paper we introduce a cloth simulation that combines both model types. All operations that are performed on the CPU use a low-resolution triangle mesh while GPU-based methods are performed efficiently on a high-resolution grid representation. Both models are coupled by a sampling operation which renders triangle vertex data into a texture and by a corresponding projection of texel data onto a mesh. The presented scheme is very flexible and allows individual components to be performed on different architectures, data representations and detail levels. The results are combined using shader programs which causes a negligible overhead. We have implemented CPU-based collision handling and a GPU-based hierarchical constraint solver to simulate systems with more than 230k particles in real-time.
A New Force Model for Controllable Breaking Waves
(The Eurographics Association, 2015) Brousset, Mathias; Darles, Emmanuelle; Meneveaux, Daniel; Poulin, Pierre; Crespin, Benoît; Fabrice Jaillet and Florence Zara and Gabriel Zachmann
This paper presents a new method for controlling swells and breaking waves using fluid solvers. With conventional approaches that generate waves by pushing particles with oscillating planes, the resulting waves cannot be controlled easily, and breaking waves are even more difficult to obtain in practice. Instead, we propose to use a new wave model that physically describes the behavior of wave forces. We show that mapping those forces to particles produces various types of waves that can be controlled by the user with only a few parameters. Our method is based on a 2D representation that describes wave speed, width, and height. It handles many swell and wave configurations, with various breaking situations.
A p-Multigrid Algorithm using Cubic Finite Elements for Efficient Deformation Simulation
(The Eurographics Association, 2014) Weber, Daniel; Mueller-Roemer, Johannes; Altenhofen, Christian; Stork, Andre; Fellner, Dieter W.; Jan Bender and Christian Duriez and Fabrice Jaillet and Gabriel Zachmann
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.
A Parallel Architecture for IISPH Fluids
(The Eurographics Association, 2014) Thaler, Felix; Solenthaler, Barbara; Gross, Markus; Jan Bender and Christian Duriez and Fabrice Jaillet and Gabriel Zachmann
We present an architecture for parallel computation of incompressible IISPH simulations on distributed memory systems. We use orthogonal recursive bisection for domain decomposition and present a stable and fast converging load balancing controller. The neighbor search data structure is derived such that it optimally fits into the parallel pipeline. We further show how symmetry aspects of the simulation can be integrated into the architecture. Simultaneous communication and computation are used to minimize parallelization overhead. The seamless integration of these parallel concepts into IISPH results in near linear scaling for large-scale simulations.
Physically-Based Character Skinning
(The Eurographics Association, 2013) Deul, Crispin; Bender, Jan; Jan Bender and Jeremie Dequidt and Christian Duriez and Gabriel Zachmann
In this paper we present a novel multi-layer model for physically-based character skinning. In contrast to geometric approaches which are commonly used in the field of character skinning, physically-based methods can simulate secondary motion effects. Furthermore, these methods can handle collisions and preserve the volume of the model without the need of an additional post-process. Physically-based approaches are computationally more expensive than geometric methods but they provide more realistic results. Recent works in this area use finite element simulations to model the elastic behavior of skin. These methods require the generation of a volumetric mesh for the skin shape in a pre-processing step. It is not easy for an artist to model the different elastic behaviors of muscles, fat and skin using a volumetric mesh since there is no clear assignment between volume elements and tissue types. For our novel multi-layer model the mesh generation is very simple and can be performed automatically. Furthermore, the model contains a layer for each kind of tissue. Therefore, the artist can easily control the elastic behavior by adjusting the stiffness parameters for muscles, fat and skin. We use shape matching with oriented particles and a fast summation technique to simulate the elastic behavior of our skin model and a position-based constraint enforcement to handle collisions, volume conservation and the coupling of the skeleton with the deformable model. Position-based methods have the advantage that they are fast, unconditionally stable, controllable and provide visually plausible results.
Physics-based Human Neck Simulation
(The Eurographics Association, 2013) Luo, Zhiping; Pronost, Nicolas; Egges, Arjan; Jan Bender and Jeremie Dequidt and Christian Duriez and Gabriel Zachmann
In deformable character animation, the skin deformation of the neck is important to reproduce believable facial animation. The neck also plays an important role in supporting the head in balance while generating the controlled head movements that are essential to many aspects of human behavior. However, neck animation is largely overlooked both in computer graphics and animation due to the complexity of the cervical anatomy. This paper presents a physical human neck model based on biomechanical modeling. Relevant anatomical structures part of a 3D model of the human musculoskeletal system are modeled as deformable or linked rigid bodies. We couple the soft-hard bodies using soft constraints via elastic springs and form a Lagrangian dynamic system. The simulation of dynamic skin deformation is achieved by automatically binding the skin vertices to underlying bodies in an anatomically correct manner. Experimental results are provided and show the high level of realism that our model offers. In addition, the simulation runs at interactive rates on a modern computer.
Policies for Goal Directed Multi-Finger Manipulation
(The Eurographics Association, 2012) Andrews, Sheldon; Kry, Paul G.; Jan Bender and Arjan Kuijper and Dieter W. Fellner and Eric Guerin
We present a method for one-handed task based manipulation of objects. Our approach uses a mid-level multiphase approach to break the problem into three parts, providing an appropriate control strategy for each phase and resulting in cyclic finger motions that accomplish the task. All motion is physically based, and guided by a policy computed for a particular task. The exact trajectory is never specified as the goal of our different tasks are concerned with the final orientation and position of the object. The offline simulations used to learn the policy are effective solutions for the task, but an important aspect of our work is that the policy is general enough to be used online in real time. We present two manipulation tasks and discuss their performance along with limitations.
Real-time Hair Simulation with Efficient Hair Style Preservation
(The Eurographics Association, 2012) Han, Dongsoo; Harada, Takahiro; Jan Bender and Arjan Kuijper and Dieter W. Fellner and Eric Guerin
Hair can be a prominent feature of characters in real-time games. In this paper, we propose hair simulation with efficient preservation of various hair styles. Bending and twisting effects are crucial to simulate curly or wavy hair. We propose local and global shape constraints and parallel methods to update local and global transforms to find goal positions. All three methods show good visual quality and take only a small fraction of rendering time. This simulation runs on the GPU and works smoothly as a part of rendering pipeline. Simulating around 20,000 strands composed of total 0.22 million vertices takes less than 1 millisecond. Simulation parameters such as stiffness or number of iterations for shape constraints can be manipulated by users interactively. It helps designers choose the right parameters for various hair styles and conditions. Also the simulation can handle various situations interactively.

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