Simulation of a flowing snow avalanche using molecular dynamics

This paper presents an approach for the modeling and simulation of a flowing snow avalanche, which is formed of dry and liquefied snow that slides down a slope, using molecular dynamics and the discrete element method. A particle system is utilized as a base method for the simulation and marching cubes with real-time shaders are employed for rendering. A uniform grid-based neighbor search algorithm is used for collision detection for interparticle and particleterrain interactions. A mass-spring model of the collision resolution is employed to mimic the compressibility of the snow and particle attraction forces are put into use between the particles and terrain surface. In order to achieve greater performance, general purpose GPU language and multithreaded programming are utilized for collision detection and resolution. The results are displayed with different combinations of rendering methods for the realistic representation of the flowing avalanche. © TÜB̄TAK

Simulation of a flowing snow avalanche using molecular dynamics

Ankara : The Department of Computer Engineering and the Institute of Engineering and Science of Bilkent University, 2010.Thesis (Master's) -- Bilkent University, 2010.Includes bibliographical references leaves 45-50.This thesis presents an approach for modeling and simulation of a flowing snow avalanche, which is formed of dry and liquefied snow that slides down a slope, by using molecular dynamics and discrete element method. A particle system is utilized as a base method for the simulation and marching cubes with real-time shaders are employed for rendering. A uniform grid based neighbor search algorithm is used for collision detection for inter-particle and particle-terrain interactions. A mass-spring model of collision resolution is employed to mimic compressibility of snow and particle attraction forces are put into use between particles and terrain surface. In order to achieve greater performance, general purpose GPU language and multi-threaded program-ming is utilized for collision detection and resolution. The results are dis-played with different combinations of rendering methods for the realistic re-presentation of the flowing avalanche.Güçer, DenizhanM.S

Desarrollo de un entorno virtual para la simulación de intervenciones quirúrgicas en neurocirugía

RESUMEN: Los sistemas de neurocirugía guiada por imágenes han surgido como una herramienta útil en la formación médica para enfrentar las deficiencias de los métodos tradicionales de enseñanza. El hecho de que cada año muchas personas mueran en los hospitales por errores en los procedimientos quirúrgicos ayuda a entender el posible impacto del entrenamiento médico y revela que la insuficiencia de éste podría costar vidas humanas. Aunque en el medio existen diferentes tipos de entornos desarrollados para la aplicación de sistemas de neurocirugía guiada por imágenes, con una alta calidad en la visualización y la interacción con los modelos, en general estos sistemas presentan un costo muy elevado, por lo que su aplicación con fines educativos se ve disminuida, además, tampoco ofrecen todas las opciones de visualización existentes ya que las opciones más avanzadas requieren conocimientos mayores de procesamiento de imágenes. Por esta razón se desarrolló un entorno para el acceso a operaciones avanzadas de procesamiento de imágenes y la manipulación intuitiva de imágenes volumétricas, el cual puede manipularse por medio de un periférico de entrada diferente al mouse, mejorando y facilitando la forma de interacción usuario-sistema. El entorno fue desarrollado en 3D Slicer, que es un software de uso libre, y facilitará el acceso de los neurocirujanos en formación de la ciudad, a las técnicas de craneotomía virtual y manipulación de representaciones tridimensionales

Particle based modeling and simulation of natural phenomena

Ankara : The Department of Computer Engineering and the Institute of Engineering and Science of Bilkent University, 2010.Thesis (Ph. D.) -- Bilkent University, 2010.Includes bibliographical references leaves 92-108.This thesis is about modeling and simulation of fluids and cloth-like deformable objects by the physically-based simulation paradigm. Simulated objects are modeled with particles and their interaction with each other and the environment is defined by particle-to-particle forces. We propose several improvements over the existing particle simulation techniques. Neighbor search algorithms are crucial for the performance efficiency and robustness of a particle system. We present a sorting-based neighbor search method which operates on a uniform grid, and can be parallelizable. We improve upon the existing fluid surface generation methods so that our method captures surface details better since we consider the relative position of fluid particles to the fluid surface. We investigate several alternatives of particle interaction schema (i.e. Smoothed Particle Hydrodynamics, the Discrete Element Method, and Lennard-Jones potential) for the purpose of defining fluid-fluid, fluid-cloth, fluid-boundary interaction forces. We also propose a practical way to simulate knitwear and its interaction with fluids. We employ capillary pressure–based forces to simulate the absorption of fluid particles by knitwear. We also propose a method to simulate the flow of miscible fluids. Our particle simulation system is implement to exploit parallel computing capabilities of the commodity computers. Specifically, we implemented the proposed methods on multicore CPUs and programmable graphics boards. The experiments show that our method is computationally efficient and produces realistic results.Bayraktar, SerkanPh.D

Contact modeling and collision detection in human joints

Collision detection among virtual objects is one of the main concerns in virtual reality and computer graphics. Usually the methods developed for collision detection are for either very general cases or very specific applications. The first main goal of this thesis is to propose accurate methods for collision detection in computer graphics for rotating or sliding objects. The methods take advantage of the limitation imposed on the rotating/sliding objects in order to ignore unnecessary calculations of the general methods and speed up the processing. In addition to finding the collision, the methods can also return penetration depths in either radial or cylindrical direction, which can be useful for further applications. The second main goal is to apply the proposed collision detection methods in biomedical research related to human hip joints. In fact, during the past few years, femoroacetabular impingement (FAI) was recognized as the leading pathomechanism contributing to a significant number of so-called "primary" hip osteoarthritis. Thus, having medical simulation of hip joint can help both physicians and surgeons for better diagnosis and surgical planning. For diagnosing some of the human joint diseases, it is important to obtain the joint's range of motion. By modifying the pre-processing stage of one of the collision detection methods, a new fast method for finding maximum range of motion in human joint was proposed and tested. The method is working without doing any collision detection tests and its accuracy does not depend on the rotational steps. We also suggested a novel fast strategy for diagnosing hip diseases based on hip contact penetration depths. In this strategy, the contact penetration depths during hip movement are calculated for diagnosing hip impingements, by using the proposed collision detection methods. The strategy has been tested on pathological hip models during a daily activity. The results were found correlated with the contact stresses estimated by finite element method (FEM). By evaluating the results, the strategy proved to be capable for distinguishing among different hip pathologies (e.g. cam and pincer impingements). In orthopedic simulations, the behavior of the bones and the related tissues are usually investigated during their movements about an estimated center of rotation. We also evaluated the importance of the hip joint center of rotation in medical simulations. For this reason, different centers of rotation calculated by five different methods were applied for hip movements about different medical axes of rotation. By calculating the hip contact penetration depths of ten patients during hip movements (using the proposed collision detection methods), the sensitivity of hip simulations to hip center of rotation has been evaluated. Hip contact pressure has been a notable parameter to evaluate the physical conditions inside the hip joint. Many computational approaches estimate the pressure and contact pressures via finite element methods (FEM) by using 3D meshes of the tissues. Although this type of simulation can provide a good evaluation of hip problems, the process may be very time consuming. Also, these mechanical methods strongly depend on the movement details. We proposed and tested a fast statistical model for estimating hip contact pressures during its movement, without performing mechanical simulations and without any need for movement details. The estimation is done by evaluating geometric features extracted from 3D meshes of hip tissues, in order to link an unknown target hip model to some already mechanically evaluated training hip models

Interaktive Echtzeitsimulation deformierbarer Oberflächen für Trainingssysteme in der Augenchirurgie

Die Arbeit befasst sich mit Simulations-Algorithmen für virtuelle Augenoperationen. Sie konzentriert sich auf die Simulation von Membranen, die im Verlauf eines chirurgischen Eingriffs aus dem Auge entfernt werden müssen. Es werden Algorithmen vorgestellt, die eine realistische Interaktion zwischen Membran und chirurgischem Instrument ermöglichen, und die eine physikalisch plausible Riss-Simulation garantieren

Simulation der Phakoemulsifikation im Augenoperationssimulator Eyesi

Diese Arbeit beschäftigt sich mit der Simulation der Phakoemulsifikation zur Ausbildung von Augenchirurgen. Besonderer Wert wurde auf die Übertragung der für die reale Operation benötigten Phakomaschine in die virtuelle Realität gelegt. Damit entstand der einzige Trainingssimulator, der über eine Maschinensimulation verfügt

Técnicas de aceleración para el método de radiosidad jerárquica

[Resumen] Uno de los métodos que mejor modelan el comportamiento real de la luz en la búsqueda del realismo visual en imágenes construidas de forma sintética es el método de radiosidad. Este método presenta, sin embargo, el inconveniente de un alto coste computacional, tanto en tiempo de cálculo como en almacenamiento. Entre las numerosas variantes surgidas con el objetivo de rebajar la complejidad del método clásico destaca el método de radiosidad jerárquica, basado en la aplicación de una subdivisión adaptativa de la escena. El método de radiosidad jerárquica mantiene, no obstante, todavía una elevada complejidad que dificulta su explotación en escenas de gran tamaño. En este trabajo se han tratado de desarrollar nuevas soluciones para algunos de los diversos problemas que el método jerárquico de radiosidad plantea. El primer punto en el que se centra el trabajo es en la determinación de la visibilidad entre los distintos objetos de una escena (principal cuello de botella en un algoritmo de iluminación), analizando las principales soluciones existentes y proponiendo una nueva aproximación al problema, basada en aprovechar el principio de localidad en el espacio de direcciones de los rayos lanzados durante el proceso. Otro aspecto desarrollado en la tesis es la utilización de modelos geométricos de diferentes complejidades que permitan el tratamiento de escenas grandes con objetos detallados, independizando la correcta simulación de la distribución de la energía en la escena de la complejidad geométrica de los objetos que la componen. A este respecto se presenta una propuesta para el cálculo de la radiosidad jerárquica basada en el uso de esquemas de subdivisión de superficies. Por último, en esta tesis se propone una solución paralela para el aprovechamiento de sistemas distribuidos en la aplicación del método de radiosidad jerárquica en escenas de gran tamaño, realizando una distribución real de la geometría de la escena entre todas las memorias del sistema y con una aproximación multi-hilo para la ejecución, lo que va a permitir un mejor ajuste de la granularidad utilizada en la paralelización de las tareas.[Resumo] Uns dos métodos que mellor modelan o comportamento real da luz na búsqueda do realismo visual en imaxes construidas de forma sintética é o método de radiosidade. Este método presenta, sen embargo, a desvantaxe dun alto coste computacional, tanto en tempo de cálculo coma en almacenamento. Entre as numerosas variantes xurdidas co obxectivo de rebaixar a complexidade do método clásico sobresae o método de radiosidade xerárquica, baseado na aplicación dunha subdivisión adaptativa na escea. O método de radiosidade xerárquica mantén todavía, así a todo, unha elevada complexidade que dificulta a súa explotación en esceas de gran tamaño. Neste traballo tratáronse de desenvolver novas solucións para algúns dos diversos problemas plantexados polo método de radiosidade xerárquica. O primeiro punto ao que se presta atención no traballo é á determinación de visibilidade entre os distintos obxectos dunha escea (principal colo de botella nun algoritmo de iluminación), analizando as principais solucións existentes e propondo unha nova aproximación ao problema baseada no aproveitamento do principio de localidade no espazo de direccións dos raios lanzados durante o proceso. Outro aspecto desenvolvido na tese é a utilización de modelos xeométricos de diferente complexidad que permitan o tratamento de esceas grandes con obxectos moi detallados, independizando a correcta simulación da distribución da enerxía na escea da complexidade xeométrica dos obxectos que a compoñen. Ao respecto preséntase unha proposta para o cálculo da radiosidade xerárquica baseada no uso de esquemas de subdivisión de superficies. Por último, nesta tese proponse unha solución paralela para o aproveitamento de sistemas distribuidos na aplicación do método de radiosidade xerárquica en esceas de gran tamaño, facendo unha distribución real da xeometría da escea entre todas as memorias do sistema e cunha aproximación multi-fío na execución, o que vai permitir un mellor axuste da granularidade empregada na paralelización das tarefas.[Absract] Radiosity is one of the best methods in modelling the physical behaviour of light in a synthetic scene. However, the main drawback is the high requirements in terms of computational and storage costs. Hierarchical radiosity stands out among the different alternatives to reduce complexity in classic radiosity, applying an adaptive subdivision on scene. Hierarchical radiosity still presents, anyway, a high complexity that difficults to process really large scenes. In this work we have developed new solutions for several of the most common bottenecks presented in hierarchical radiosity. Our first goal is to accelerate visibility determination (most consuming task in global illumination), analysing the main existing solutions and proposing a new method based in taking advantage of directional coherence for the rays casted during process. Other aspect we have touched in the thesis is the use of multiresolution models that allow to work with very complex geometrical models in our input scene, isolating geometry detail and illumination detail. Specifically, we have developed a new method to compute hierarchical radiosity based on surface subdivision. Finally, a new parallel solution for computing hierarchical radiosity on multiprocessor systems, allowing huge input scenes is presented. The scene is totally distributed (geometrically and computationally) among the processors in our proposal, and a multi-thread implementation improves the flexibility in the granularity of the parallel execution

Dynamic Plane Shifting BSP Traversal

Interactive 3D applications require fast detection of objects colliding with the environment. One popular method for fast collision detection is to offset the geometry of the environment according to the dimensions of the object, and then represent the object as a point (and the object's movement as a line segment). Previously, this geometry offset has been done in a preprocessing step and therefore requires knowledge of the object's dimensions before runtime. Furthermore, an extra copy of the environment's geometry is required for each shape used in the application. This paper presents a variation of the BSP tree collision algorithm that shifts the planes in order to offset the geometry of the environment at runtime. To prevent unwanted cases where offset geometry protrudes too much, extra plane equations, which bevel solid cells of space during expansion, are added by simply inserting extra nodes at the bottom of the tree. A simple line segment check can be used for collision detection of a moving object of any size against the environment. Only one BSP tree is needed by the application. Successful usage within commercial entertainment software is also discussed

A biomechanics-based articulation model for medical applications

Computer Graphics came into the medical world especially after the arrival of 3D medical imaging. Computer Graphics techniques are already integrated in the diagnosis procedure by means of the visual tridimensional analysis of computer tomography, magnetic resonance and even ultrasound data. The representations they provide, nevertheless, are static pictures of the patients' body, lacking in functional information. We believe that the next step in computer assisted diagnosis and surgery planning depends on the development of functional 3D models of human body. It is in this context that we propose a model of articulations based on biomechanics. Such model is able to simulate the joint functionality in order to allow for a number of medical applications. It was developed focusing on the following requirements: it must be at the same time simple enough to be implemented on computer, and realistic enough to allow for medical applications; it must be visual in order for applications to be able to explore the joint in a 3D simulation environment. Then, we propose to combine kinematical motion for the parts that can be considered as rigid, such as bones, and physical simulation of the soft tissues. We also deal with the interaction between the different elements of the joint, and for that we propose a specific contact management model. Our kinematical skeleton is based on anatomy. Special considerations have been taken to include anatomical features like axis displacements, range of motion control, and joints coupling. Once a 3D model of the skeleton is built, it can be simulated by data coming from motion capture or can be specified by a specialist, a clinician for instance. Our deformation model is an extension of the classical mass-spring systems. A spherical volume is considered around mass points, and mechanical properties of real materials can be used to parameterize the model. Viscoelasticity, anisotropy and non-linearity of the tissues are simulated. We particularly proposed a method to configure the mass-spring matrix such that the objects behave according to a predefined Young's modulus. A contact management model is also proposed to deal with the geometric interactions between the elements inside the joint. After having tested several approaches, we proposed a new method for collision detection which measures in constant time the signed distance to the closest point for each point of two meshes subject to collide. We also proposed a method for collision response which acts directly on the surfaces geometry, in a way that the physical behavior relies on the propagation of reaction forces produced inside the tissue. Finally, we proposed a 3D model of a joint combining the three elements: anatomical skeleton motion, biomechanical soft tissues deformation, and contact management. On the top of that we built a virtual hip joint and implemented a set of medical applications prototypes. Such applications allow for assessment of stress distribution on the articular surfaces, range of motion estimation based on ligament constraint, ligament elasticity estimation from clinically measured range of motion, and pre- and post-operative evaluation of stress distribution. Although our model provides physicians with a number of useful variables for diagnosis and surgery planning, it should be improved for effective clinical use. Validation has been done partially. However, a global clinical validation is necessary. Patient specific data are still difficult to obtain, especially individualized mechanical properties of tissues. The characterization of material properties in our soft tissues model can also be improved by including control over the shear modulus