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98
Position Based Dynamics
, 2006
"... 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 final ..."
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Cited by 47 (1 self)
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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.
Fast proximity computation among deformable models using discrete Voronoi diagrams
 ACM Trans. Graph. (Proc ACM SIGGRAPH
, 2006
"... Figure 1: Multiple deformable models simulation: This sequence shows the positions of the objects at three time instances in a simulation. The environment initially consists of 10 deforming objects represented using 5.5K triangles. As the simulation proceeds, the objects break into 25 subobjects. O ..."
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Cited by 32 (5 self)
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Figure 1: Multiple deformable models simulation: This sequence shows the positions of the objects at three time instances in a simulation. The environment initially consists of 10 deforming objects represented using 5.5K triangles. As the simulation proceeds, the objects break into 25 subobjects. Our algorithm is able to perform collision and separation distance computations, including selfcollisions, among dynamically generated objects within 120 ms on a highend PC. We present novel algorithms to perform collision and distance queries among multiple deformable models in dynamic environments. These include interobject queries between different objects as well as intraobject queries. We describe a unified approach to compute these queries based on Nbody distance computation and use properties of the 2 nd order discrete Voronoi diagram to perform Nbody culling. Our algorithms involve no preprocessing and also work well on models with changing topologies. We can perform all proximity queries among complex deformable models consisting of thousands of triangles in a fraction of a second on a highend PC. Moreover, our Voronoibased culling algorithm can improve the performance of separation distance and penetration queries by an order of magnitude.
ExampleBased Skeleton Extraction
, 2007
"... We present a method for extracting a hierarchical, rigid skeleton from a set of example poses. We then use this skeleton to not only reproduce the example poses, but create new deformations in the same style as the examples. Since rigid skeletons are used by most 3D modeling software, this skeleton ..."
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Cited by 23 (0 self)
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We present a method for extracting a hierarchical, rigid skeleton from a set of example poses. We then use this skeleton to not only reproduce the example poses, but create new deformations in the same style as the examples. Since rigid skeletons are used by most 3D modeling software, this skeleton and the corresponding vertex weights can be inserted directly into existing production pipelines. To create the skeleton, we first estimate the rigid transformations of the bones using a fast, face clustering approach. We present an efficient method for clustering by providing a Rigid Error Function that finds the best rigid transformation from a set of points in a robust, space efficient manner and supports fast clustering operations. Next, we solve for the vertex weights and enforce locality in the resulting weight distributions. Finally, we use these weights to determine the connectivity and joint locations of the skeleton.
Fast viscoelastic behavior with thin features
 ACM Trans. Graph
, 2008
"... We introduce a method for efficiently animating a wide range of deformable materials. We combine a high resolution surface mesh with a tetrahedral finite element simulator that makes use of frequent remeshing. This combination allows for fast and detailed simulations of complex elastic and plastic ..."
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Cited by 21 (2 self)
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We introduce a method for efficiently animating a wide range of deformable materials. We combine a high resolution surface mesh with a tetrahedral finite element simulator that makes use of frequent remeshing. This combination allows for fast and detailed simulations of complex elastic and plastic behavior. We significantly expand the range of physical parameters that can be simulated with a single technique, and the results are free from common artifacts such as volumeloss, smoothing, popping, and the absence of thin features like strands and sheets. Our decision to couple a high resolution surface with lowresolution physics leads to efficient simulation and detailed surface features, and our approach to creating the tetrahedral mesh leads to an orderofmagnitude speedup over previous techniques in the time spent remeshing. We compute masses, collisions, and surface tension forces on the scale of the fine mesh, which helps avoid visual artifacts due to the differing mesh resolutions. The result is a method that can simulate a large array of different material behaviors with high resolution features in a short amount of time.
An examplebased procedural system for element arrangement
 COMPUT. GRAPH. FORUM
, 2008
"... We present a method for synthesizing two dimensional (2D) element arrangements from an example. The main idea is to combine texture synthesis techniques basedon a local neighborhood comparison and procedural modeling systems basedon local growth. Given a userspecified reference pattern, our syste ..."
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Cited by 19 (1 self)
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We present a method for synthesizing two dimensional (2D) element arrangements from an example. The main idea is to combine texture synthesis techniques basedon a local neighborhood comparison and procedural modeling systems basedon local growth. Given a userspecified reference pattern, our system analyzes neighborhood information of each element by constructing connectivity. Our synthesis process starts with a single seed and progressively places elements one by one by searching a reference element which has local features that are the most similar to the target place of the synthesized pattern. To support creative design activities, we introduce three types of interaction for controlling global features of the resulting pattern, namely a spray tool, a flow field tool, and a boundary tool. We also introduce a global optimization process that helps to avoid local error concentrations. We illustrate the feasibility of our method by creating several types of 2D patterns.
Deforming Meshes that Split and Merge
"... Figure 1: Dropping viscoelastic balls in an Eulerian fluid simulation. Invisible geometry is quickly deleted, while the visible surfaces retain their details even after translating through the air and splashing on the ground. We present a method for accurately tracking the moving surface of deformab ..."
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Cited by 15 (3 self)
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Figure 1: Dropping viscoelastic balls in an Eulerian fluid simulation. Invisible geometry is quickly deleted, while the visible surfaces retain their details even after translating through the air and splashing on the ground. We present a method for accurately tracking the moving surface of deformable materials in a manner that gracefully handles topological changes. We employ a Lagrangian surface tracking method, and we use a triangle mesh for our surface representation so that fine features can be retained. We make topological changes to the mesh by first identifying merging or splitting events at a particular grid resolution, and then locally creating new pieces of the mesh in the affected cells using a standard isosurface creation method. We stitch the new, topologically simplified portion of the mesh to the rest of the mesh at the cell boundaries. Our method detects and treats topological events with an emphasis on the preservation of detailed features, while simultaneously simplifying those portions of the material that are not visible. Our surface tracker is not tied to a particular method for simulating deformable materials. In particular, we show results from two significantly different simulators: a Lagrangian FEM simulator with tetrahedral elements, and an Eulerian gridbased fluid simulator. Although our surface tracking method is generic, it is particularly wellsuited for simulations that exhibit fine surface details and numerous topological events. Highlights of our results include merging of viscoplastic materials with complex geometry, a taffypulling animation with many fold and merge events, and stretching and slicing of stiff plastic material.
Gradient Domain Editing of Deforming Mesh Sequences
"... Figure 1: A straight run is adapted to a curved path on an uneven terrain. The original deforming mesh sequence moves along a straight line on a plane. We first make the HORSE move along a curve using path editing, and then adapt the sequence onto the terrain using footprint editing. Many graphics a ..."
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Cited by 15 (2 self)
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Figure 1: A straight run is adapted to a curved path on an uneven terrain. The original deforming mesh sequence moves along a straight line on a plane. We first make the HORSE move along a curve using path editing, and then adapt the sequence onto the terrain using footprint editing. Many graphics applications, including computer games and 3D animated films, make heavy use of deforming mesh sequences. In this paper, we generalize gradient domain editing to deforming mesh sequences. Our framework is keyframe based. Given sparse and irregularly distributed constraints at unevenly spaced keyframes, our solution first adjusts the meshes at the keyframes to satisfy these constraints, and then smoothly propagate the constraints and deformations at keyframes to the whole sequence to generate new deforming mesh sequence. To achieve convenient keyframe editing, we have developed an efficient alternating leastsquares method. It harnesses the power of subspace deformation and twopass linear methods to achieve highquality deformations. We have also developed an effective algorithm to define boundary conditions for all frames using handle trajectory editing. Our deforming mesh editing framework has been successfully applied to a number of editing scenarios with increasing complexity, including footprint editing, path editing, temporal filtering, handlebased deformation mixing, and spacetime morphing. † This work was done while Qifeng Tan was an intern at Microsoft Research Asia.
Efficient raytracing of deforming pointsampled surfaces
 Computer Graphics Forum
, 2005
"... We present efficient data structures and caching schemes to accelerate raysurface intersections for deforming pointsampled surfaces. By exploiting spatial and temporal coherence of the deformation during the animation, we are able to improve rendering performance by a factor of two to three compar ..."
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Cited by 14 (3 self)
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We present efficient data structures and caching schemes to accelerate raysurface intersections for deforming pointsampled surfaces. By exploiting spatial and temporal coherence of the deformation during the animation, we are able to improve rendering performance by a factor of two to three compared to existing techniques. Starting from a tight bounding sphere hierarchy for the undeformed object, we use a lazy updating scheme to adapt the hierarchy to the deformed surface in each animation step. In addition, we achieve a significant speedup for raysurface intersections by caching perray intersection points. We also present a technique for rendering sharp edges and corners in pointsampled models by introducing a novel surface clipping algorithm.
Fracturing Rigid Materials
, 2007
"... We propose a novel approach to fracturing (and denting) brittle materials. To avoid the computational burden imposed by the stringent time step restrictions of explicit methods or with solving nonlinear systems of equations for implicit methods, we treat the material as a fully rigid body in the lim ..."
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Cited by 13 (2 self)
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We propose a novel approach to fracturing (and denting) brittle materials. To avoid the computational burden imposed by the stringent time step restrictions of explicit methods or with solving nonlinear systems of equations for implicit methods, we treat the material as a fully rigid body in the limit of infinite stiffness. In addition to a triangulated surface mesh and level set volume for collisions, each rigid body is outfitted with a tetrahedral mesh upon which finite element analysis can be carried out to provide a stress map for fracture criteria. We demonstrate that the commonly used stress criteria can lead to arbitrary fracture (especially for stiff materials) and instead propose the notion of a time averaged stress directly into the FEM analysis. When objects fracture, the virtual node algorithm provides new triangle and tetrahedral meshes in a straightforward and robust fashion. Although each new rigid body can be rasterized to obtain a new level set, small shards can be difficult to accurately resolve. Therefore, we propose a novel collision handling technique for treating both rigid bodies and rigid body thin shells represented by only a triangle mesh.
Fast Musculoskeletal Registration Based on Shape Matching
"... Abstract. This paper presents a new method for computing elastic and plastic deformations in the context of discrete deformable modelbased registration. Internal forces are estimated by averaging local transforms between reference and current particle positions. Our technique can accommodate large ..."
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Cited by 10 (5 self)
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Abstract. This paper presents a new method for computing elastic and plastic deformations in the context of discrete deformable modelbased registration. Internal forces are estimated by averaging local transforms between reference and current particle positions. Our technique can accommodate large nonlinear deformations, and is unconditionally stable. Moreover, it is simple to implement and versatile. We show how to tune model stiffness and computational cost, which is important for efficient registration, and demonstrate our technique in the complex problem of interpatient musculoskeletal registration. 1