### Browsing by Author "Bargteil, Adam W."

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Item Automatic Construction of Coarse, High-Quality Tetrahedralizations that Enclose and Approximate Surfaces for Animation(ACM, 2013-11-06) Stuart, David A.; Levine, Joshua A.; Jones, Ben; Bargteil, Adam W.Embedding high-resolution surface geometry in coarse control meshes is a standard approach to achieving high-quality computer animation at low computational expense. In this paper we present an effective, automatic method for generating such control meshes. The resulting high-quality, tetrahedral meshes enclose and approximate an input surface mesh, avoiding extrapolation artifacts and ensuring that the resulting coarse volumetric meshes are adequate collision proxies. Our approach comprises three steps: we begin with a tetrahedral mesh built from the body-centered cubic lattice that tessellates the bounding box of the input surface; we then perform a sculpting phase that carefully removes elements from the lattice; and finally a variational vertex adjustment phase iteratively adjusts vertex positions to more closely approximate the surface geometry. Our approach provides explicit trade-offs between mesh quality, resolution, and surface approximation. Our experiments demonstrate the technique can be used to build high-quality meshes appropriate for simulations within games.Item Basis enrichment and solid-fluid coupling for model-reduced fluid simulation(ACM, 2015-03) Gerszewski, Dan; Kavan, Ladislav; Sloan, Peter-Pike; Bargteil, Adam W.We present several enhancements to model-reduced fluid simulation that allow improved simulation bases and two way solid-fluid coupling. Specifically, we present a basis enrichment scheme that allows us to combine data driven or artistically derived bases with more general analytic bases derived from Laplacian Eigenfunctions. We handle two-way solid-fluid coupling in a time-splitting fashion— we alternately timestep the fluid and rigid body simulators, while taking into account the effects of the fluid on the rigid bodies and vice versa. We employ the vortex panel method to handle solid-fluid coupling and use dynamic pressure to compute the effect of the fluid on rigid bodies.Item Deformation Embedding for Point-Based Elastoplastic Simulation(ACM, 2014-03) Jones, Ben; Ward, Stephen; Jallepalli, Ashok; Perenia, Joseph; Bargteil, Adam W.We present a straightforward, easy-to-implement, point-based approach for animating elastoplastic materials. The core idea of our approach is the introduction of embedded space—the least-squares best fit of the material’s rest state into three dimensions. Nearest neighbor queries in the embedded space efficiently update particle neighborhoods to account for plastic flow. These queries are simpler and more efficient than remeshing strategies employed in mesh-based finite element methods. We also introduce a new estimate for the volume of a particle, allowing particle masses to vary spatially and temporally with fixed density. Our approach can handle simultaneous extreme elastic and plastic deformations. We demonstrate our approach on a variety of examples that exhibit a wide range of material behaviors.Item Enhancements to Model-reduced Fluid Simulation(ACM, 2013-11-06) Gerszewski, Dan; Kavan, Ladislav; Sloan, Peter-Pike; Bargteil, Adam W.We present several enhancements to model-reduced fluid simulation that allow improved simulation bases and two-way solid-fluid coupling. Specifically, we present a basis enrichment scheme that allows us to combine data driven or artistically derived bases with more general analytic bases derived from Laplacian Eigenfunctions. We handle two-way solid-fluid coupling in a time-splitting fashion—we alternately timestep the fluid and rigid body simulators, while taking into account the effects of the fluid on the rigid bodies and vice versa. We employ the vortex panel method to handle solid-fluid coupling and use dynamic pressure to compute the effect of the fluid on rigid bodies.Item Fast simulation of mass-spring systems(ACM, 2013-11) Liu, Tiantian; Bargteil, Adam W.; O'Brien, James F.; Kavan, LadislavWe describe a scheme for time integration of mass-spring systems that makes use of a solver based on block coordinate descent. This scheme provides a fast solution for classical linear (Hookean) springs. We express the widely used implicit Euler method as an energy minimization problem and introduce spring directions as auxiliary unknown variables. The system is globally linear in the node positions, and the non-linear terms involving the directions are strictly local. Because the global linear system does not depend on run-time state, the matrix can be pre-factored, allowing for very fast iterations. Our method converges to the same final result as would be obtained by solving the standard form of implicit Euler using Newton's method. Although the asymptotic convergence of Newton's method is faster than ours, the initial ratio of work to error reduction with our method is much faster than Newton's. For real-time visual applications, where speed and stability are more important than precision, we obtain visually acceptable results at a total cost per timestep that is only a fraction of that required for a single Newton iteration. When higher accuracy is required, our algorithm can be used to compute a good starting point for subsequent Newton's iteration.Item Fluid Simulation on Unstructured Quadrilateral Surface MeshesBhattacharya, Haimasree; Levine, Joshua A.; Bargteil, Adam W.In this paper, we present a method for fluid simulation on unstructured quadrilateral surface meshes. We solve the Navier-Stokes equations by performing the traditional steps of fluid simulation, semi-Lagrangian advection and pressure projection, directly on the surface. We include level-set based front-tracking for visualizing “liquids,” while we use densities to visualize “smoke.” We demonstrate our method on a variety of meshes and create an assortment of visual effectsItem Global Momentum Preservation for Position-based Dynamics(Association for Computing Machinery, 2019-10-28) Dahl, Alex; Bargteil, Adam W.Position-based dynamics has emerged as an exceedingly popular approach for animating soft body dynamics. Unfortunately, the basic approach suffers from artificial loss of angular momentum. We propose a simple approach to preserve global linear and angular momenta of bodies by directly tracking these quantities and adjusting velocities to ensure they are preserved. This approach entails negligible computational cost, requires less than 25 lines of code, and exactly preserves global linear and angular momenta.Item A Level-set Method for Skinning Animated Particle Data(IEEE, 2014-10-09) Bhattacharya, Haimasree; Gao, Yue; Bargteil, Adam W.In this paper, we present a straightforward, easy to implement method for particle skinning—generating surfaces from animated particle data. We cast the problem in terms of constrained optimization and solve the optimization using a level-set approach. The optimization seeks to minimize the thin-plate energy of the surface, while staying between surfaces defined by the union of spheres centered at the particles. Our approach skins each frame independently while preserving the temporal coherence of the underlying particle animation. Thus, it is well-suited for environments where particle skinning is treated as a post-process, with each frame generated in parallel. We demonstrate our method on data generated by a variety of fluid simulation techniques and simple particle systems.Item A Point-based Method for Animating Elastoplastic Solids(ACM, 2009-08-01) Gerszewski, Dan; Bhattacharya, Haimasree; Bargteil, Adam W.In this paper we describe a point-based approach for animating elastoplastic materials. Our primary contribution is a simple method for computing the deformation gradient for each particle in the simulation. The deformation gradient is computed for each particle by finding the affine transformation that best approximates the motion of neighboring particles over a single timestep. These transformations are then composed to compute the total deformation gradient that describes the deformation around a particle over the course of the simulation. Given the deformation gradient we can apply arbitrary constitutive models and compute the resulting elastic forces. Our method has two primary advantages: we do not store or compare to an initial rest configuration and we work directly with the deformation gradient. The first advantage avoids poor numerical conditioning and the second naturally leads to a multiplicative model of deformation appropriate for finite deformations. We demonstrate our approach on a number of examples that exhibit a wide range of material behaviors.Item A Point-based Method for Animating Incompressible Flow(ACM, 2009-08-01) Sin, Funshing; Bargteil, Adam W.; Hodgins, Jessica K.In this paper, we present a point-based method for animating incompressible flow. The advection term is handled by moving the sample points through the flow in a Lagrangian fashion. However, unlike most previous approaches, the pressure term is handled by performing a projection onto a divergence-free field. To perform the pressure projection, we compute a Voronoi diagram with the sample points as input. Borrowing from Finite Volume Methods, we then invoke the divergence theorem and ensure that each Voronoi cell is divergence free. To handle complex boundary conditions, Voronoi cells are clipped against obstacle boundaries and free surfaces. The method is stable, flexible and combines many of the desirable features of point-based and grid-based methods. We demonstrate our approach on several examples of splashing and streaming liquid and swirling smoke.Item A semi-Lagrangian contouring method for fluid simulation(ACM, 2005-08-04) Bargteil, Adam W.; Goktekin, Tolga G.; O’Brien, James F.; Strain, John A.In this sketch we present a semi-Lagrangian surface tracking method for use with fluid simulations. Our method maintains an explicit polygonal mesh that defines the surface, and an octree data structure that provides both a spatial index for the mesh and an efficient means for evaluating the signeddistance function away from the surface. At each time step the surface is reconstructed from an implicit function defined by the composition of backward advection and the previous signed-distance function. One of the primary advantages of this formulation is that it enables tracking of surface characteristics, such as color or texture coordinates, at negligible additional cost. We include several examples demonstrating that the method can be used as part of a fluid simulation to effectively animate complex and interesting fluid behaviors.Item Strain limiting for clustered shape matching(ACM, 2014-11-06) Bargteil, Adam W.; Jones, BenIn this paper, we advocate explicit symplectic Euler integration and strain limiting in a shape matching simulation framework. The resulting approach resembles not only previous work on shape matching and strain limiting, but also the recently popular position-based dynamics. However, unlike this previous work, our approach reduces to explicit integration under small strains, but remains stable in the presence of non-linearities.Item A Texture Synthesis Method for Liquid Animations(ACM, 2006-08-03) Bargteil, Adam W.; Sin, Funshing; Michaels, Jonathan E.; Goktekin, Tolga G.; O’Brien, James F.Liquid simulation techniques have become a standard tool in production environments, producing extremely realistic liquid motion in a variety of films, commercials, and video games. Surface texturing is an essential computer graphics tool, which gives artists additional control over their results by allowing them to stylize surfaces or add detail to a lowresolution simulations. For example, an artist could use texturing techniques to add the appearance of foam to a wave, bubbles to beer, or fat globules to soup. Unfortunately, texturing liquid surfaces is difficult because the surfaces have no inherent parameterization. Creating a temporally consistent parameterization is extremely difficult for two primary reasons. First, liquid simulations are characterized by their complex and frequent topological changes. These topological changes result in significant discontinuities in any parameter tracked on the surface. Second, liquid surfaces tend to stretch and compress dramatically over the course of a simulation. Similarly, an advected parameterization will also stretch and compress. For these reasons, advected texture coordinates are often unsuitable for texturing liquid surfaces. In this sketch we present a method for generating textures on animated liquid surfaces. Rather than advecting texture coordinates on the surface, we synthesize a new texture at every frame. We initialize the texture with color values advected from the surface at the previous frame. We then run an optimization procedure which attempts to match the surface texture to an input sample texture and, for temporal coherence, the advected colors.