Category: Rigid bodies

Parallel Particles (P^2): A Parallel Position Based Approach for Fast and Stable Simulation of Granular Materials

Daniel Holz

Granular materials exhibit a large number of diverse physical phenomena which makes their numerical simulation challenging. When set in motion they flow almost like a fluid, while they can present high shear strength when at rest. Those macroscopic effects result from the material’s microstructure: a particle skeleton with interlocking particles which stick to and slide across each other, producing soil cohesion and friction. For the purpose of Earthmoving equipment operator training, we developed Parallel Particles (P2), a fast and stable position based granular material simulator which models inter-particle friction and adhesion and captures the physical nature of soil to an extend sufficient for training. Our parallel solver makes the approach scalable and applicable to modern multi-core architectures yielding the simulation speed required in this application. Using a regularization procedure, we successfully model visco-elastic particle interactions on the position level which provides real, physical parameters allowing for intuitive tuning. We employ the proposed technique in an Excavator training simulator and demonstrate that it yields physically plausible results at interactive to real-time simulation rates.

Parallel Particles (P^2): A Parallel Position Based Approach for Fast and Stable Simulation of Granular Materials

Efficient Denting and Bending of Rigid Bodies

Saket Patkar, Mridul Aanjaneya, Aric Bartle, Minjae Lee, Ronald Fedkiw

We present a novel method for the efficient denting and bending of rigid bodies without the need for expensive finite element simulations. Denting is achieved by deforming the triangulated surface of the target body based on a dent map computed on-the-fly from the projectile body using a Z-buffer algorithm with varying degrees of smoothing. Our method accounts for the angle of impact, is applicable to arbitrary shapes, readily scales to thousands of rigid bodies, is amenable to artist control, and also works well in combination with prescoring algorithms for fracture. Bending is addressed by augmenting a rigid body with an articulated skeleton which is used to drive skinning weights for the bending deformation. The articulated skeleton is simulated to include the effects of both elasticity and plasticity. Furthermore, we allow joints to be added dynamically so that bending can occur in a nonpredetermined way and/or as dictated by the artist. Conversely, we present an articulation condensation method that greatly simplifies large unneeded branches and chains on-the-fly for increased efficiency.

Efficient Denting and Bending of Rigid Bodies

Adaptive Tetrahedral Meshes for Brittle Fracture Simulation

Dan Koschier, Sebastian Lipponer, Jan Bender

We present a method for the adaptive simulation of brittle fracture of solid objects based on a novel reversible tetrahedral mesh refinement scheme. The refinement scheme preserves the quality of the input mesh to a large extent, it is solely based on topological operations, and does not alter the boundary, i.e. any geometric feature. Our fracture algorithm successively performs a stress analysis and increases the resolution of the input mesh in regions of high tensile stress. This results in an accurate location of crack origins without the need of a general high resolution mesh which would cause high computational costs throughout the whole simulation. A crack is initiated when the maximum tensile stress exceeds the material strength. The introduced algorithm then proceeds by iteratively recomputing the changed stress state and creating further cracks. Our approach can generate multiple cracks from a single impact but effectively avoids shattering artifacts. Once the tensile stress decreases, the mesh refinement is reversed to increase the performance of the simulation. We demonstrate that our adaptive method is robust, scalable and computes highly realistic fracture results.

Adaptive Tetrahedral Meshes for Brittle Fracture Simulation

Unified Particle Physics for Real-Time Applications

Miles Macklin, Matthias Müller, Nuttapong Chentanez, and Tae-Yong Kim

We present a unified dynamics framework for real-time visual effects. Using particles connected by constraints as our fundamental building block allows us to treat contact and collisions in a unified manner, and we show how this representation is flexible enough to model gases, liquids, deformable solids, rigid bodies and clothing with two-way interactions. We address some common problems with traditional particle based methods and describe a parallel constraint solver based on position based dynamics that is efficient enough for real-time applications.

Unified Particle Physics for Real-Time Applications

Inverse-Foley Animation: Synchronizing rigid-body motions to sound

Timothy R. Langlois and Doug L. James

In this paper, we introduce Inverse-Foley Animation, a technique for optimizing rigid-body animations so that contact events are synchronized with input sound events. A precomputed database of randomly sampled rigid-body contact events is used to build a contact-event graph, which can be searched to determine a plausible sequence of contact events synchronized with the input sound’s events. To more easily find motions with matching contact times, we allow transitions between simulated contact events using a motion blending formulation based on modified contact impulses. We fine tune synchronization by slightly retiming ballistic motions. Given a sound, our system can synthesize synchronized motions using graphs built with hundreds of thousands of precomputed motions, and millions of contact events. Our system is easy to use, and has been used to plan motions for hundreds of sounds, and dozens of rigid-body models.

Inverse-Foley Animation: Synchronizing Rigid-Body Motions to Sound

Implicit Multibody Penalty-based Distributed Contact

Hongyi Xu, Yili Zhao, and Jernej Barbic

The penalty method is a simple and popular approach to resolving contact in computer graphics and robotics. Penalty-based contact, however, suffers from stability problems due to the highly variable and unpredictable net stiffness, and this is particularly pronounced in simulations with time-varying distributed geometrically complex contact. We employ semi-implicit integration, exact analytical contact gradients, symbolic Gaussian elimination and a SVD solver to simulate stable penalty-based frictional contact with large, time-varying contact
areas, involving many rigid objects and articulated rigid objects in complex conforming contact and self-contact. We also derive implicit proportional-derivative control forces for real-time control of articulated structures with loops. We present challenging contact scenarios such as screwing a hexbolt into a hole, bowls stacked in perfectly conforming configurations, and manipulating many objects using actively controlled articulated mechanisms in real time.

Implicit Multibody Penalty-based Distributed Contact

Efficient Enforcement of Hard Articulation Constraints in the Presence of Closed Loops and Contacts

Robin Tomcin, Dominik Sibbing, Leif Kobbelt

In rigid body simulation, one must distinguish between contacts (so-called unilateral constraints) and articulations (bilateral constraints). For contacts and friction, iterative solution methods have proven most useful for interactive applications, often in combination with Shock-Propagation in cases with strong interactions between contacts (such as stacks), prioritizing performance and plausibility over accuracy. For articulation constraints, direct solution methods are preferred, because one can rely on a factorization with linear time complexity for tree-like systems, even in ill-conditioned cases caused by large mass-ratios or high complexity. Despite recent advances, combining the advantages of direct and iterative solution methods wrt. performance has proven difficult and the intricacy of articulations in interactive applications is often limited by the convergence speed of the iterative solution method in the presence of closed kinematic loops (i.e. auxiliary constraints) and contacts. We identify common performance bottlenecks in the dynamic simulation of unilateral and bilateral constraints and are able to present a simulation method, that scales well in the number of constraints even in ill-conditioned cases with frictional contacts, collisions and closed loops in the kinematic graph. For cases where many joints are connected to a single body, we propose a technique to increase the sparsity of the positive definite linear system. A solution to these bottlenecks is presented in this paper to make the simulation of a wider range of mechanisms possible in real-time without extensive parameter tuning.

Efficient Enforcement of Hard Articulation Constraints in the Presence of Closed Loops and Contacts

Enhancements to Model-Reduced Fluid Simulation

Dan Gerszewski, Ladislav Kavan, Peter-Pike Sloan, Adam W. Bargteil

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.

Enhancements to Model-Reduced Fluid Simulation

Object-Centric Parallel Rigid Body Simulation with Timewarp

John Koenig, Ioannis Karamouzas, Stephen J. Guy

We present an object-centric formulation for parallel rigid body simulation that supports variable length integration time steps through rollbacks. We combine our object-centric simulation
framework with a novel spatiotemporal data structure to reduce global synchronization and achieve interactive, real-time simulations which scale across many CPU cores. Additionally, we provide proofs that both our proposed data structure and our object-centric formulation are deadlock-free. We implement our approach with the functional programming language Erlang, and test the performance and scalability of our method over several scenarios consisting of hundreds of interacting objects.

Object-Centric Parallel Rigid Body Simulation with Timewarp

Simulation and Control of Skeleton-Driven Soft Body Characters

Libin Liu, KangKang Yin, Bin Wang, Baining Guo

In this paper we present a physics-based framework for simulation and control of human-like skeleton-driven soft body characters. We couple the skeleton dynamics and the soft body dynamics to enable two-way interactions between the skeleton, the skin geometry, and the environment. We propose a novel pose-based plasticity model that extends the corotated linear elasticity model to achieve large skin deformation around joints. We further reconstruct controls from reference trajectories captured from human subjects by augmenting a sampling-based algorithm. We demonstrate the effectiveness of our framework by results not attainable with a simple combination of previous methods.

Simulation and Control of Skeleton-Driven Soft Body Characters