Simulation Of Deformable Bodies Research Articles

Orbit: A Unified Simulation Framework for Interactive Robot Learning Environments

We present <sc xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">Orbit</small> , a unified and modular framework for robot learning powered by <sc xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">Nvidia</small> Isaac Sim. It offers a modular design to easily and efficiently create robotic environments with photo-realistic scenes and high-fidelity rigid and deformable body simulation. With <sc xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">Orbit</small> , we provide a suite of benchmark tasks of varying difficulty– from single-stage cabinet opening and cloth folding to multi-stage tasks such as room reorganization. To support working with diverse observations and action spaces, we include fixed-arm and mobile manipulators with different physically-based sensors and motion generators. <sc xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">Orbit</small> allows training reinforcement learning policies and collecting large demonstration datasets from hand-crafted or expert solutions in a matter of minutes by leveraging GPU-based parallelization. In summary, we offer an open-sourced framework that readily comes with 16 robotic platforms, 4 sensor modalities, 10 motion generators, more than 20 benchmark tasks, and wrappers to 4 learning libraries. With this framework, we aim to support various research areas, including representation learning, reinforcement learning, imitation learning, and task and motion planning. We hope it helps establish interdisciplinary collaborations in these communities, and its modularity makes it easily extensible for more tasks and applications in the future. For videos, documentation, and code: <uri xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">https://isaac-orbit.github.io/</uri> .

A GPU-based multilevel additive schwarz preconditioner for cloth and deformable body simulation

In this paper, we wish to push the limit of real-time cloth and deformable body simulation to a higher level with 50K to 500K vertices, based on the development of a novel GPU-based multilevel additive Schwarz (MAS) pre-conditioner. Similar to other preconditioners under the MAS framework, our preconditioner naturally adopts multilevel and domain decomposition concepts. But contrary to previous works, we advocate the use of small, non-overlapping domains that can well explore the parallel computing power on a GPU. Based on this idea, we investigate and invent a series of algorithms for our preconditioner, including multilevel domain construction using Morton codes, low-cost matrix precomputation by one-way Gauss-Jordan elimination, and conflict-free symmetric-matrix-vector multiplication in runtime preconditioning. The experiment shows that our preconditioner is effective, fast, cheap to precompute and scalable with respect to stiffness and problem size. It is compatible with many linear and nonlinear solvers used in cloth and deformable body simulation with dynamic contacts, such as PCG, accelerated gradient descent and L-BFGS. On a GPU, our preconditioner speeds up a PCG solver by approximately a factor of four, and its CPU version outperforms a number of competitors, including ILU0 and ILUT.

An Extended Position Based Dynamics Method for Cloth Simulation

Position based dynamics is an effective method in real time deformable body simulation, which is widely used. But, as we all know, it is not suitable for greater accuracy applications. As to cloth simulation, the over-stretching problem especially affects its accuracy. Therefore, an extended simulation method based on position dynamics is proposed in this paper. The time step is subdivided effectively, and the number of iterations is controlled when cloth is deforming; We also optimize the Gauss-Seidel iterative solver in parallel using an improved grouping method which ensures the efficiency of the algorithm. This method is simple and easy to implement. Compared with the traditional method, the position error of deformation is significantly reduced while cloth is stretched, and the simulation efficiency is enhanced. Besides, it can be easily integrated into the existing position based dynamic framework.

Decomposed optimization time integrator for large-step elastodynamics

Simulation methods are rapidly advancing the accuracy, consistency and controllability of elastodynamic modeling and animation. Critical to these advances, we require efficient time step solvers that reliably solve all implicit time integration problems for elastica. While available time step solvers succeed admirably in some regimes, they become impractically slow, inaccurate, unstable, or even divergent in others --- as we show here. Towards addressing these needs we present the Decomposed Optimization Time Integrator (DOT), a new domain-decomposed optimization method for solving the per time step, nonlinear problems of implicit numerical time integration. DOT is especially suitable for large time step simulations of deformable bodies with nonlinear materials and high-speed dynamics. It is efficient, automated, and robust at large, fixed-size time steps, thus ensuring stable, continued progress of high-quality simulation output. Across a broad range of extreme and mild deformation dynamics, using frame-rate size time steps with widely varying object shapes and mesh resolutions, we show that DOT always converges to user-set tolerances, generally well-exceeding and always close to the best wall-clock times across all previous nonlinear time step solvers, irrespective of the deformation applied.

Fast and Realistic Reflections Using Screen Space and GPU Ray Tracing—A Case Study on Rigid and Deformable Body Simulations

The realistic representation of light within a computational domain is not a trivial task. Although several rendering approaches exist, the ray tracing technique is highly regarded as robust and realistic; however, its computational cost is still prohibitive for real-time games and other 3D applications. A modern tradeoff is to pair the ray tracing with the rasterization step, the former being responsible for generating complex lighting interactions, such as reflections, while the latter is used to generate less demanding visual effects, such as diffuse lighting and shadows. The stated framework has been studied by several authors, but it has not been shown to work both efficiently and accurately for highly dynamic scenes with deformable geometry. Stepping in this direction, this work presents a case study whose goal is to generate fast and realistic reflections on rigid and deformable body simulations using a hybrid approach that brings together the Screen Space technique with the GPU ray tracing algorithm and their respective main capabilities. The results show that not only realistic reflections can be generated at interactive rates, but also that the hybrid approach allows to achieve a certain level of scalability with respect to the number of triangles updated on every frame during the simulations.

Deformable Model-Based Methods for Shape Control of a Haptic Jamming Surface.

Haptic Jamming, the approach of simultaneously controlling mechanical properties and surface deformation of a tactile display via particle jamming and pneumatics, shows promise as a tangible, shape-changing human-computer interface. Previous research introduced device design and described the force-displacement interactions for individual jamming cells. The work in this article analyzes the shape output capabilities of a multi-cell array. A spring-mass deformable body simulation combines models of the three actuation inputs of a Haptic Jamming surface: node pinning, chamber pressurization, and cell jamming. Surface measurements of a 12-cell prototype from a depth camera fit the mass and stiffness parameters to the device during pressurization tests and validate the accuracy of the model for various actuation sequences. The simulator is used to develop an algorithm that generates a sequence of actuation inputs for a Haptic Jamming array of any size in order to match a desired surface output shape. Data extracted from topographical maps and three-dimensional solid object models are used to evaluate the shape-matching algorithm and assess the utility of increasing array size and resolution. Results show that a discrete Laplace operator applied to the input is a suitable predictor of the correlation coefficient between the desired shape and the device output.

A unified approach for subspace simulation of deformable bodies in multiple domains

Multi-domain subspace simulation can efficiently and conveniently simulate the deformation of a large deformable body, by constraining the deformation of each domain into a different subspace. The key challenge in implementing this method is how to handle the coupling among multiple deformable domains, so that the overall effect is free of gap or locking issues. In this paper, we present a new domain decomposition framework that connects two disjoint domains through coupling elements. Under this framework, we present a unified simulation system that solves subspace deformations and rigid motions of all of the domains by a single linear solve. Since the coupling elements are part of the deformable body, their elastic properties are the same as the rest of the body and our system does not need stiffness parameter tuning. To quickly evaluate the reduced elastic forces and their Jacobian matrices caused by the coupling elements, we further develop two cubature optimization schemes using uniform and non-uniform cubature weights. Our experiment shows that the whole system can efficiently handle large and complex scenes, many of which cannot be easily simulated by previous techniques without limitations.

Subspace condensation

Subspace deformable body simulations can be very fast, but can behave unrealistically when behaviors outside the prescribed subspace such as novel external collisions, are encountered. We address this limitation by presenting a fast, flexible new method that allows full space computation to be activated in the neighborhood of novel events while the rest of the body still computes in a subspace. We achieve this using a method we call subspace condensation , a variant on the classic static condensation precomputation. However, instead of a precomputation, we use the speed of subspace methods to perform the condensation at every frame. This approach allows the full space regions to be specified arbitrarily at runtime, and forms a natural two-way coupling with the subspace regions. While condensation is usually only applicable to linear materials, the speed of our technique enables its application to non-linear materials as well. We show the effectiveness of our approach by applying it to a variety of articulated character scenarios.

Simultaneous Estimation of Elasticity for Multiple Deformable Bodies.

Material property has great importance in deformable body simulation and medical robotics. The elasticity parameters, such as Young's modulus of the deformable bodies, are important to make realistic animations. Further in medical applications the (recovered) elasticity parameters can assist surgeons to perform better pre-op surgical planning and enable medical robots to carry out personalized surgical procedures. Previous elasticity parameters estimation methods are limited to recover one elasticity parameter of one deformable body at a time. In this paper, we propose a novel elasticity parameter estimation algorithm that can recover the elasticity parameters of multiple deformable bodies or multiple regions of one deformable body simultaneously from (at least two sets of) images. We validate our algorithm with both synthetic test cases and real patient CT images.

Simulation of forced deformable bodies interacting with two-dimensional incompressible flows: Application to fish-like swimming

We present an efficient algorithm for simulation of deformable bodies interacting with two-dimensional incompressible fluid flows. The temporal and spatial discretizations of the Navier–Stokes equations in vorticity stream-function formulation are based on classical fourth-order Runge–Kutta scheme and compact finite differences, respectively. Using a uniform Cartesian grid we benefit from the advantage of a new fourth-order direct solver for the Poisson equation to ensure the incompressibility constraint down to machine zero over an optimal grid. For introducing a deformable body in fluid flow, the volume penalization method is used. A Lagrangian structured grid with prescribed motion covers the deformable body which is interacting with the surrounding fluid due to the hydrodynamic forces and the torque calculated on the Eulerian reference grid. An efficient law for controlling the curvature of an anguilliform fish, swimming toward a prescribed goal, is proposed which is based on the geometrically exact theory of nonlinear beams and quaternions. Validation of the developed method shows the efficiency and expected accuracy of the algorithm for fish-like swimming and also for a variety of fluid/solid interaction problems.

DESIGN AND IMPLEMENTATION OF REAL-TIME PHYSICS SIMULATION ENGINE FOR e-ENTERTAINMENT

We present our recent research results regarding the designing and implementation of real-time physics simulation engines, which aim at developing physics-inspired e-entertainment such as computer games, mobile applications, interactive TV and other smart media in Korea. Our real-time physics engine consists of three functional components: rigid body dynamics simulation, deformable body simulation, and data-driven physics simulation. The core simulation techniques to realize these simulation components include real-time collision detection and response, large-scale model simulation, and character model control. In this paper, we highlight these features and demonstrate their performances. We also showcase some of the gaming applications that we have integrated our physics engine into.

Efficient collision detection for composite finite element simulation of cuts in deformable bodies

Composite finite elements (CFEs) based on a hexahedral discretization of the simulation domain have recently shown their effectiveness in physically based simulation of deformable bodies with chang...

Energy Conservation for the Simulation of Deformable Bodies

We propose a novel technique that allows one to conserve energy using the time integration scheme of one's choice. Traditionally, the time integration methods that deal with energy conservation, such as symplectic, geometric, and variational integrators, have aimed to include damping in a manner independent of the size of the time step, stating that this gives more control over the look and feel of the simulation. Generally speaking, damping adds to the overall aesthetics and appeal of a numerical simulation, especially since it damps out the high frequency oscillations that occur on the level of the discretization mesh. We propose an alternative technique that allows one to use damping as a material parameter to obtain the desired look and feel of a numerical simulation, while still exactly conserving the total energy-in stark contrast to previous methods in which adding damping effects necessarily removes energy from the mesh. This allows, for example, a deformable bouncing ball with aesthetically pleasing damping (and even undergoing collision) to collide with the ground and return to its original height exactly conserving energy, as shown in Fig. 2. Furthermore, since our method works with any time integration scheme, the user can choose their favorite time integration method with regards to aesthetics and simply apply our method as a postprocess to conserve all or as much of the energy as desired.

Haptic Interaction with Elastic Volumetric Structures

Considerable efforts have been done to produce realistic results when simulating interaction with elastic materials. Many applications such as surgery planning, medical training, or virtual sculpting would benefit from a plausible simulation scenario. However, even though many works have proposed very satisfactory results, realistic simulation of deformable bodies is still an open issue. One of the challenges when designing a realistic elastic body simulation is the huge amount of data that needs to be processed. For the inner properties of the material are crucial when it comes to reproduce the elastic problem, the simulation naturally calls for volumetric information. In this paper the authors propose a technique to interactively deform 3D images, such as those acquired by a CT scanner. While producing a physically plausible haptic feedback, deformation and visualization algorithms produce an efficient and natural feeling. Using a free form deformation structure as a wrapper, it is possible to deform complex structures at high frame rates, independently of the size of the volume.

GPU-based physical cut in interactive haptic simulations

Interactive, physics based, simulations of deformable bodies are a growing research area with possible applications to computer-aided surgery. Their aim is to create virtual environments where surgeons are free to practice. To ensure the needed realism, the simulations must be performed with deformable bodies. The goal of this paper is to describe the approach to the development of a physics-based surgical simulator with haptic feedback. The main development issue is the representation of the organ behavior at the high rates required by haptic realism. Since even high-end computers have inadequate performance, our approach exploits the parallelism of modern Graphics Processing Units (GPU). Particular attention is paid to the simulation of cuts because of their great importance in the surgical practice and the difficulty in handling topological changes in real time. To prove the correctness of our approach, we simulated an interactive, physically based, virtual abdomen. The simulation allows the user to interact with deformable models. Deformable models are updated in real time, thus allowing the rendering of force feedback to the user. The method is optimized to handle high quality scenes: we report results of interactive simulation of two virtual tools interacting with a complex model. The integration of physics-based deformable models in simulations greatly increases the realism of the virtual environment, taking into account real tissue properties and allowing the user to feel the actual forces exerted by organs on virtual tools. Our method proves the feasibility of exploiting GPU to simulate deformable models in interactive virtual environments.

Skipping steps in deformable simulation with online model reduction

Finite element simulations of nonlinear deformable models are computationally costly, routinely taking hours or days to compute the motion of detailed meshes. Dimensional model reduction can make simulations orders of magnitude faster, but is unsuitable for general deformable body simulations because it requires expensive precomputations, and it can suppress motion that lies outside the span of a pre-specified low-rank basis. We present an online model reduction method that does not have these limitations. In lieu of precomputation, we analyze the motion of the full model as the simulation progresses, incrementally building a reduced-order nonlinear model, and detecting when our reduced model is capable of performing the next timestep. For these subspace steps, full-model computation is "skipped" and replaced with a very fast (on the order of milliseconds) reduced order step. We present algorithms for both dynamic and quasistatic simulations, and a "throttle" parameter that allows a user to trade off between faster, approximate previews and slower, more conservative results. For detailed meshes undergoing low-rank motion, we have observed speedups of over an order of magnitude with our method.

Stable, circulation-preserving, simplicial fluids

Visual quality, low computational cost, and numerical stability are foremost goals in computer animation. An important ingredient in achieving these goals is the conservation of fundamental motion invariants. For example, rigid and deformable body simulation benefits greatly from the conservation of linear and angular momenta. In the case of fluids, however, none of the current techniques focuses on conserving invariants, and consequently, often introduce a visually disturbing numerical diffusion of vorticity . Just as important visually is the resolution of complex simulation domains. Doing so with regular (even if adaptive) grid techniques can be computationally delicate. In this article, we propose a novel technique for the simulation of fluid flows. It is designed to respect the defining differential properties, that is, the conservation of circulation along arbitrary loops as they are transported by the flow. Consequently, our method offers several new and desirable properties: Arbitrary simplicial meshes (triangles in 2D, tetrahedra in 3D) can be used to define the fluid domain; the computations involved in the update procedure are efficient due to discrete operators with small support; and it preserves discrete circulation , avoiding numerical diffusion of vorticity.

A multigrid framework for real-time simulation of deformable bodies

In this paper, we present a multigrid framework for constructing implicit, yet interactive solvers for the governing equations of motion of deformable volumetric bodies. We have integrated linearized, corotational linearized and non-linear Green strain into this framework. Based on a 3D finite element hierarchy, this approach enables realistic simulation of objects exhibiting an elastic modulus with a dynamic range of several orders of magnitude. Using the linearized strain measure, we can simulate 50 thousand tetrahedral elements with 20 fps on a single processor CPU. By using corotational linearized and non-linear Green strain, we can still simulate five thousand and two thousand elements, respectively, at the same rates.

Adaptive physics based tetrahedral mesh generation using level sets

We present a tetrahedral mesh generation algorithm designed for the Lagrangian simulation of deformable bodies. The algorithm’s input is a level set (i.e., a signed distance function on a Cartesian grid or octree). First a bounding box of the object is covered with a uniform lattice of subdivision-invariant tetrahedra. The level set is then used to guide a red green adaptive subdivision procedure that is based on both the local curvature and the proximity to the object boundary. The final topology is carefully chosen so that the connectivity is suitable for large deformation and the mesh approximates the desired shape. Finally, this candidate mesh is compressed to match the object boundary. To maintain element quality during this compression phase we relax the positions of the nodes using finite elements, masses and springs, or an optimization procedure. The resulting mesh is well suited for simulation since it is highly structured, has topology chosen specifically for large deformations, and is readily refined if required during subsequent simulation. We then use this algorithm to generate meshes for the simulation of skeletal muscle from level set representations of the anatomy. The geometric complexity of biological materials makes it very difficult to generate these models procedurally and as a result we obtain most if not all data from an actual human subject. Our current method involves using voxelized data from the Visible Male [1] to create level set representations of muscle and bone geometries. Given this representation, we use simple level set operations to rebuild and repair errors in the segmented data as well as to smooth aliasing inherent in the voxelized data.