vriphys: Workshop in Virtual Reality Interactions and Physical Simulations

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Kinetic Separation Lists for Continuous Collision Detection of Deformable Objects
(The Eurographics Association, 2006) Weller, Rene; Zachmann, Gabriel; Cesar Mendoza and Isabel Navazo
We present a new acceleration scheme for continuous collision detection of objects under arbitrary deformations. Both pairwise and self collision detection are presented. This scheme is facilitated by a new acceleration data structure, the kinetic separation list. The event-based approach of our kinetic separation list enables us to transform the continuous problem into a discrete one. Thus, the number of updates of the bounding volume hierarchies as well as the number of bounding volume checks can be reduced significantly. We performed a comparison of our kinetic approaches with the classical swept volume algorithm. The results show that our algorithm performs up to fifty times faster in practically relevant scenarios.
The use of Tetrahedra to Detect Collisions
(The Eurographics Association, 2006) Madera, F. A.; Day, A. M.; Laycock, S. D.; Cesar Mendoza and Isabel Navazo
We present an algorithm for collision detection by filling the free space between the objects involved. We compute the closest regions of a pair of objects, and update them during the running simulation. The algorithm is fast because the process depends on the number of neighbouring features of a vertex, the region, and not on all the features of the object. Furthermore, our algorithm is accurate because it deals directly with the features of the objects. This work deals with convex objects, and forms the first part of the general approach for deformable objects.
Local Constraint Methods for Deformable Objects
(The Eurographics Association, 2006) Gissler, Marc; Becker, Markus; Teschner, Matthias; Cesar Mendoza and Isabel Navazo
We present a local scheme to enforce non-conflicting geometric constraints for dynamically deforming objects. The approach employs information on the underlying numerical integration method and we present a unified scheme to illustrate the incorporation of a variety of integration methods. The proposed technique is very efficient in terms of memory and computing complexity. Iterative solvers and stabilization techniques are avoided. The method is not subject to numerical drift or other inaccuracies and all constraints are accurately met at each simulation step. Since the approach does not require any pre-processing, dynamically changing constraints can be handled efficiently. Experiments indicate that thousands of constraints can be processed at interactive rates. Although our approach is restricted to non-conflicting constraints, experiments illustrate the versatility of the method in the context of deformable objects.
Real Time Simulation of Phaco-emulsification for Cataract Surgery Training
(The Eurographics Association, 2006) Agus, Marco; Gobbetti, Enrico; Pintore, Giovanni; Zanetti, Gianluigi; Zorcolo, Antonio; Cesar Mendoza and Isabel Navazo
We present a real-time simulation of the phaco-emulsification task in a virtual reality training system for cataract surgery. Phaco-emulsification consists in breaking in small fragments and completely removing the eye crystalline lens by employing an ultra-sound tool called phaco-emulsificator. Our approach employs a mesh-less shape- based dynamic algorithm integrated with a simplex geometry representation in order to efficiently handle the rendering process and the continuous modifications involved by the surgical tool interaction, and with a smoothed particle hydrodynamics scheme with spatial ordering for handling fragments interactions. The complete training system also simulates other tasks involved in cataract surgery, like the corneal incision and the capsulorhexis. The simulator runs on a multiprocessing PC platform and provides realistic physically-based visual simulations of tools interactions. The current setup employs SensAble PHANToM for simulating the interaction devices, and a binocular display for presenting images to the user.
Non structured meshes for Cloth GPU simulation using FEM
(The Eurographics Association, 2006) Rodriguez-Navarro, J.; Susin, A.; Cesar Mendoza and Isabel Navazo
We present a Finite Element Method (FEM) implementation for cloth simulation on the GPU. The advantages of FEM are twofold: the realism of cloth simulations using this method is improved compared with other methods like the widely used mass-spring one, and it has a wider application rank because it can be used for general triangulated cloth meshes. We are able to detect collisions between cloth and other objects (solids or deformables) and also we deal with self cloth collisions. This is also done in the GPU using image-based collision methods. We have also improved a GPU-Gradient Conjugate method for solving the linear equation systems involved in the FEM solution. Two more methods are also implemented in the GPU to compare with the FEM method: a mass-spring model (based on rectangular meshes) and a constraint method (based on triangular meshes).
Position Based Dynamics
(The Eurographics Association, 2006) Müller, Matthias; Heidelberger, Bruno; Hennix, Marcus; Ratcliff, John; Cesar Mendoza and Isabel Navazo
The most popular approaches for the simulation of dynamic systems in computer graphics are force based. Internal and external forces are accumulated from which accelerations are computed based on Newton's second law of motion. A time integration method is then used to update the velocities and finally the positions of the object. A few simulation methods (most rigid body simulators) use impulse based dynamics and directly manipulate velocities. In this paper we present an approach which omits the velocity layer as well and immediately works on the positions. The main advantage of a position based approach is its controllability. Overshooting problems of explicit integration schemes in force based systems can be avoided. In addition, collision constraints can be handled easily and penetrations can be resolved completely by projecting points to valid locations. We have used the approach to build a real time cloth simulator which is part of a physics software library for games. This application demonstrates the strengths and benefits of the method.
Virtual Resectoscope Interface for a Surgery Simulation Training System of the Prostate
(The Eurographics Association, 2006) Padilla, Miguel A.; Monte, Felipe Altamirano del; Cosio, Fernando Arambula; Cesar Mendoza and Isabel Navazo
In this work is presented the current state of the development of a virtual resectocope interface for a surgery simulation system for training Transurethral Resection of the Prostate (TURP). The interface consists of two parts, the first part is a mechatronics device that emulates a real resectosope and allows to perform the most important movements of the surgical tool during a TURP. The second part is a software interface that consist on a collision detection mechanism that allows to calculate in real-time the interactions between the mechatronic device and the deformable tissue model of the prostate, in order to simulate tissue resection and deformation. The current prototype has five degrees of freedom, which are enough to have a realistic simulation of the surgery movements. The results show that the interface is suitable for a real-time surgery simulation training system of the prostate without force feedback.
Deformable Hybrid Approach for Haptic Interaction
(The Eurographics Association, 2006) Mero, Maximo G.; Susin, A.; Cesar Mendoza and Isabel Navazo
A new hybrid approach for deformable models is presented here and carried out in a virtual reality environment, achieving real time performance with haptic interactions. Our implementation consists in using two approaches for the deformable model. The deformation is modelled using simultaneously a Finite Element Method and a Mesh Free Method. With this Mesh Free method, particles are used to simulate large deformations in the volume region near the surface of the object. The remaining internal volume of the object can be modelled employing a coarse mesh using the Finite Element Method.
Virtual reality training system for portal selection in shoulder arthroscopy
(The Eurographics Association, 2006) Fernández-Arroyo, José Manuel Fernández; Beriso, Sofía Bayona; Moya, Carolina Gómez; Carro, Lauren García; Cesar Mendoza and Isabel Navazo
In arthroscopy, the first step is to choose the correct position and orientation of the entry points (portals) by palpating the osseous references. We have built a virtual reality training system for placement of the incisions, with which the novice can touch and feel surface anatomy in order to train, as much as desire, adequate location and orientation angle for portal performance. We present an innovative simulator for Minimal Invasive Surgery that pretends to solve the lack of most simulators, in which simulation starts with predefined entry points, making possible to train this particular skill, so critical in arthroscopic procedures.
Animating Shapes at Arbitrary Resolution with Non-Uniform Stiffness
(The Eurographics Association, 2006) Nesme, Matthieu; Payan, Yohan; Faure, François; Cesar Mendoza and Isabel Navazo
We present a new method for physically animating deformable shapes using finite element models (FEM). Contrary to commonly used methods based on tetrahedra, our finite elements are the bounding voxels of a given shape at arbitrary resolution. This alleviates the complexities and limitations of tetrahedral volume meshing and results in regular, well-conditionned meshes. We show how to build the voxels and how to set the masses and stiffnesses in order to model the physical properties as accurately as possible at any given resolution. Additionally, we extend a fast and robust tetrahedron-FEM approach to the case of hexahedral elements. This permits simulation of arbitrarily complex shapes at interactive rates in a manner that takes into account the distribution of material within the elements.
Partitioning Meshes into Strips using the Enhanced Tunnelling Algorithm (ETA)
(The Eurographics Association, 2006) Porcu, Massimiliano B.; Scateni, Riccardo; Cesar Mendoza and Isabel Navazo
Triangle meshes are the most used representations for three-dimensional objects, and triangle strips are the organization of triangles mostly used for efficient rendering. Since the problem of optimal strip decomposition of a given mesh is NP-complete, many different heuristics have been proposed; the quality of the stripification is usually evaluated using standard indicators as the total number of strips, the number of isolated triangles, the cache coherence, the number of swap vertices. In this paper we present the Enhanced Tunnelling Algorithm (ETA), a stripification method working on the dual graph of a mesh. The method uses a sophisticated mechanism of dynamical update of identifiers, guided by a localization procedure. The algorithm adopts a modified search approach in the dual graph that accelerated the convergence speed of the algorithm. The ETA results efficient and robust, able to deal with datasets of any dimension. The quality of the stripification is remarkable: very few strips (not seldom just one), no isolated triangles, good cache coherence (ACMR value), good number of vertex per triangle.
Simulating Prostate Surgical Procedures with a Discrete Soft Tissue Model
(The Eurographics Association, 2006) Marchal, Maud; Promayon, Emmanuel; Troccaz, Jocelyne; Cesar Mendoza and Isabel Navazo
Simulating surgical procedures is still a complex challenge. Either modeling methods or simulators have to take into account specific geometries and properties of the patient organs. In this paper, a new soft tissue modeling method dedicated to prostate surgical interventions is presented. The chosen medical application requires the modeling of a complex anatomical environment including intricate interactions between organs and surgical instruments. We present a discrete model that has been specifically developed to fulfill these requirements. The influence of two particular instruments is studied: needles and ultrasound probe, according to two surgical interventions: biopsy and brachytherapy.
A study of the Manipulability of the PHANToM OMNI Haptic Interface
(The Eurographics Association, 2006) Martín, José San; Triviño, Gracián; Cesar Mendoza and Isabel Navazo
When a haptic interface is going to be used as a part of a computational system a design criteria consist of maximizing the coincidence between the application 3D space and the physical volume where the haptic device provides the maximum performance. A well known parameter to evaluate the performance of a mechanical manipulator is the manipulability. This paper explains in detail the analysis of manipulability of the Phantom Omni haptic interface including the study of the manipulability distribution into its real workspace boundaries.
Stable Interactive Cutting of Deformable Objects
(The Eurographics Association, 2006) Jerábková, L.; Jerábek, J.; Chudoba, R.; Kuhlen, T.; Cesar Mendoza and Isabel Navazo
In this paper we present a novel approach for stable interactive cutting of deformable objects in virtual environments. Our method is based on the extended finite elements method, allowing for a modeling of discontinuities without remeshing. As no new elements are created, the impact on performance of the simulation is minimized.
Fast Dynamic Simulation of Multi-Body Systems Using Impulses
(The Eurographics Association, 2006) Bender, Jan; Schmitt, Alfred A.; Cesar Mendoza and Isabel Navazo
A dynamic simulation method for multi-body systems is presented in this paper. The special feature of this method is that it satisfies all given constraints by computing impulses. In each simulation step the joint states after the step are predicted. In order to obtain valid states after the simulation step, impulses are computed and applied to the connected bodies. Since a valid joint state is targeted exactly, there is no drift as the simulation proceeds in time and so no additional stabilisation is required. In previous approaches the impulses for a multi-body system were computed iteratively. Since dependencies between joints were not taken into account, the simulation of complex models was slow. A novel method is presented that uses a system of linear equations to describe these dependencies. By solving this typically sparse system the required impulses are determined. This method allows a very fast simulation of complex multi-body systems.
Efficient Updates of Bounding Sphere Hierarchies for Geometrically Deformable Models
(The Eurographics Association, 2006) Spillmann, J.; Becker, M.; Teschner, M.; Cesar Mendoza and Isabel Navazo
We present a new approach for efficient collision handling of meshless objects undergoing geometric deformation. The presented technique is based on bounding sphere hierarchies. We show that information of the geometric deformation model can be used to significantly accelerate the hierarchy update. The cost of the presented hierarchy update depends on the number of primitives in close proximity, but not on the total number of primitives. Further, the hierarchical collision detection is combined with a level-of-detail response scheme. Since the collision response can be performed on any level of the hierarchy, it allows for balancing accuracy and efficiency. Thus, the collision handling scheme is particularly useful for time-critical applications.
Collision Detection between a Complex Solid and a Particle Cloud assisted by Programmable GPU
(The Eurographics Association, 2006) Jiménez, Juan J.; Ogáyar, Carlos J.; Segura, Rafael J.; Feito, Francisco R.; Cesar Mendoza and Isabel Navazo
In this work the problem of collision detection (CD) between a Complex Solid and a Particle cloud with autonomous movement is studied. In order to do this, some algorithms and data have been adapted to suit new extensions of the new generations of programmable graphics cards. These types of graphics cards allow more flexible programming in order to solve problems not-related to visualization process. We use a representation based on simplicial coverings as well as a structure named Tetra-Tree in order to represent and classify the simplices of complex objects. With this type of representation the operations carried out in CD are more robust and efficient than those used in classic algorithms, so it is not necessary to decompose the complex object into more simple pieces. We also propose some implementation alternatives and give a study of their performance.
Explicit Integrators Analysis for a Simulation Engine
(The Eurographics Association, 2006) García, Marcos; Espadero, José Miguel; Rodríguez, Angel; Cesar Mendoza and Isabel Navazo
One of the most significant elements when dealing with particle based object modeling in virtual reality applications is the integrator used to compute the state of the particles, because it grants the deformation stability of the scene objects. This paper presents a study of the response achieved by a set of integrators applied to a multiresolution mass-spring model used in a virtual reality environment. The dynamic of the animation engine has been adapted to each one of the tested integrators, comparing the different responses obtained in each case taking into account the efficiency and the stability of the object dynamics.
Stable Collision Response Method in Complex Interactions
(The Eurographics Association, 2006) Hernantes, J.; Diaz, I.; Alvarez, H.; Borro, D.; Cesar Mendoza and Isabel Navazo
Haptic technology is quite recent and therefore in many cases it is difficult to simulate real contacts or interactions with a high sensation of realism. Collision response methods that calculate the force-feedback tend to cause haptic instabilities when the normal direction changes abruptly. In consequence, collision events in sharp corners are often difficult to render properly by means of haptic devices. This paper describes a collision response method that first distinguishes the nature of the corners -concave or convex- and then provides an adequate solution in each case to avoid haptic instabilities.
A Real-time Implementation of the Dynamic Particle Coating Method on a GPU Architecture
(The Eurographics Association, 2007) Sillam, Kevin; Evrard, Matthieu; Luciani, Annie; John Dingliana and Fabio Ganovelli
This paper deals with a real-time implementation on graphic processor of the dynamic particle coating method (DPCM) first proposed by [HL02] and founded on a mass-interaction formalism. When this method was proposed, it was too much time-consuming to be inserted in an interactive application. This paper solves this major drawback. Our real-time implementation allows inserting this method in a real-time simulation chain composed of a haptic device, an upstream mass-interaction model that interacts with the user through this device and the new real-time implementation of the DPCM method to visualize this model

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