The goal of this paper is to enable the interactive simulation of phenomena such as animated fluid characters. While full 3D fluid solvers achieve this with control algorithms, these 3D simulations are usually too costly for real-time environments. In order to achieve our goal, we reduce the problem from a three- to a two-dimensional one, and make use of the shallow water equations to simulate surface waves that can be solved very efficiently. In addition to a low runtime cost, stability is likewise crucial for interactive applications. Hence, we make use of an implicit time integration scheme to obtain a robust solver. To ensure a low energy dissipation, we apply an Implicit Newmark time integration scheme. We propose a general formulation of the underlying equations that is tailored towards the use with an Implicit Newmark integrator. Furthermore, we gain efficiency by making use of a direct solver. Due to the generality of our formulation, the fluid simulation can be coupled interactively with arbitrary external forces, such as forces caused by inertia or collisions. We will discuss the properties of our algorithm, and demonstrate its robustness with simulations on strongly deforming meshes.
Fool Me Twice: Exploring and Exploiting Error Tolerance in Physics-Based Animation
The error tolerance of human perception offers a range of opportunities to trade numerical accuracy for performance in physics-based simulation. However, most previous approaches either focus exclusively on understanding the tolerance of the human visual system or burden the application developer with case-specific implementations. In this paper, based on a detailed set of perceptual metrics, we propose a methodology to identify the maximum error tolerance of physics simulation. Then, we apply this methodology in the evaluation of two techniques. The first is the hardware optimization technique of precision reduction which reduces the size of floating point units (FPUs), allowing more of them to occupy the same silicon area. The increased number of FPUs can significantly improve the performance of future physics accelerators. A key benefit of our approach is that it is transparent to the application developer. The
second is the software optimization of choosing the largest timestep for simulation.
Fool Me Twice: Exploring and Exploiting Error Tolerance in Physics-Based Animation
Feedback Control of Cumuliform Cloud Formation Based on Computational Fluid Dynamics
Clouds play an important role for creating realistic images of outdoor scenes. In order to generate realistic clouds, many methods have been developed for modeling and animating clouds. One of the most effective approaches for synthesizing realistic clouds is to simulate cloud formation processes based on the atmospheric fluid dynamics. Although this approach can create realistic clouds, the resulting shapes and motion depend on many simulation parameters and the initial status. Therefore, it is very difficult to adjust those parameters so that the clouds form the desired shapes. This paper addresses this problem and presents a method for controlling the simulation of cloud formation. In this paper, we focus on controlling cumuliform cloud formation. The user specifies the overall shape of the clouds. Then, our method automatically adjusts parameters during the simulation in order to generate clouds forming the specified shape. Our method can generate realistic clouds while their shapes closely match to the desired shape.
Feeback Control of Cumuliform Cloud Formation Based on Computational Fluid Dynamics
Polyhedral Finite Elements Using Harmonic Basis Functions
Finite element simulations in computer graphics are typically based on tetrahedral or hexahedral elements, which enables simple and efficient implementations, but in turn requires complicated remeshing in case of topological changes or adaptive refinement. We propose a flexible finite element method for arbitrary polyhedral elements, thereby effectively avoiding the need for remeshing. Our polyhedral finite elements are based on harmonic basis functions, which satisfy all necessary conditions for FEM simulations and seamlessly generalize both linear tetrahedral and trilinear hexahedral elements. We discretize harmonic basis functions using the method of fundamental solutions, which enables their flexible computation and efficient evaluation. The versatility of our approach is demonstrated on cutting and adaptive refinement within a simulation framework for corotated linear elasticity.
Fast Adaptive Shape Matching Deformations
We present a new shape-matching deformation model that allows for efficient handling of topological changes and dynamic adaptive selection of levels of detail. Similar to the recently presented Fast Lattice Shape Matching (FLSM), we compute the position of simulation nodes by convolution of rigid shape matching operators on many overlapping regions, but we rely instead on octree-based hierarchical sampling and an interval-based region definition. Our approach enjoys the efficiency and robustness of shape-matching deformation models, and the same algorithmic simplicity and linear cost as FLSM, but it eliminates its dense sampling requirements. Our method can handle adaptive spatial discretizations, allowing the simulation of more degrees of freedom in arbitrary regions of interest at little additional cost. The method is also versatile, as it can simulate elastic and plastic deformation, it can handle cuts interactively, and it reuses the underlying data structures for efficient handling of (self-)collisions. All this makes it especially useful for interactive applications such as videogames.
SIGGRAPH 2008 papers list
The 2008 edition of the annual unofficial list of SIGGRAPH papers is up.
It’s looking like a big year for physics – just over 14% so far…
- Fast Viscoelastic Behavior with Thin Features
- Backward Steps in Rigid Body Simulations
- Bubbles Alive
- Porous Flow in Particle-Based Fluid Simulations
- Animating developable surfaces using nonconforming elements
- Spline Joints for Multibody Dynamics
- Feedback Control of Cumuliform Cloud Formation based on Computational Fluid Dynamics
- Discrete Elastic Rods
- Robust Treatment of Simultaneous Collisions
- Two-way Coupling of Fluids to Rigid and Deformable Solids and Shells
- A Mass Spring Model for Hair Simulation
- Wavelet Turbulence for Fluid Simulation
- Simulating Knitted Cloth at the Yarn Level
SCA 2008 Papers List
The list of papers accepted to the 2008 Symposium on Computer Animation is up here.
The physics-oriented subset of those papers:
- Evolving Sub-Grid Turbulence for Smoke Animation
- Low Viscosity Flow Simulations for Animation
- Visual Simulation of Shockwaves
- Fast Adaptive Shape Matching Deformations
- Two-way Coupling of Rigid and Deformable Bodies
- Flexible Simulation of Deformable Models Using Discontinuous Galerkin FEM
- Globally Coupled Impulse-Based Collision Handling for Cloth Simulation
- Image-based Collision Detection and Response between Arbitrary Volumetric Objects
- Interactive Terrain Modeling Using Hydraulic Erosion
- Density Contrast SPH Interfaces
- Accurate Viscous Free Surfaces for Buckling, Coiling and Rotating Liquids
Low Viscosity Flow Simulations for Animation
We present a combination of techniques to simulate turbulent fluid flows in 3D. Flow in a complex domain is modeled using a regular rectilinear grid with a finite-difference solution to the incompressible Navier-Stokes equations. We propose the use of the QUICK advection algorithm over a globally high resolution grid. To calculate pressure over the grid, we introduce the Iterated Orthogonal Projection (IOP) framework. In IOP a series of orthogonal projections ensures that multiple conditions such as non-divergence and boundary conditions arising through complex domains shapes or moving objects will be satisfied simultaneously to specified accuracy. This framework allows us to use a simple and highly efficient multigrid method to enforce non-divergence in combination with complex domain boundary conditions. IOP is amenable to GPU implementation, resulting in over an order of magnitude improvement over a CPU-based solver. We analyze the impact of these algorithms on the turbulent energy cascade in simulated fluid flows and the resulting visual quality.
Density Contrast SPH Interfaces
To simulate multiple fluids realistically many important interaction effects have to be captured accurately.
Smoothed Particle Hydrodynamics (SPH) has shown to be a simple, yet flexible method to cope with many fluid simulation problems in a robust way. Unfortunately, the results obtained when using SPH to simulate miscible fluids are severely affected, especially if density ratios become large. The undesirable effects reach from unphysical density and pressure variations to spurious and unnatural interface tensions, as well as severe numerical instabilities. In this work, we present a formulation based on SPH which can handle density discontinuities at interfaces between multiple fluids correctly without increasing the computational costs compared to standard SPH. The basic idea is to replace the density computation in SPH by a measure of particle densities and consequently derive new formulations for pressure and viscous forces. The new method enables the user to select the desired amount of interface tension according to the simulation problem at hand. We succeed to stably simulate multiple fluids with high density contrasts without the above described artifacts apparent in standard SPH simulations.
Two-way Coupling of Rigid and Deformable Bodies
We propose a framework for the full two-way coupling of rigid and deformable bodies, which is achieved with both a unified time integration scheme as well as individual two-way coupled algorithms at each point of that scheme. As our algorithm is two-way coupled in every fashion, we do not require ad hoc methods for dealing with stability issues or interleaving parts of the simulation. We maintain the ability to treat the key desirable aspects of rigid bodies (e.g. contact, collision, stacking, and friction) and deformable bodies (e.g. arbitrary constitutive models, thin shells, and self-collisions). In addition, our simulation framework supports more advanced features such as proportional derivative controlled articulation between rigid bodies. This not only allows for the robust simulation of a number of new phenomena, but also directly lends itself to the design of deformable creatures with proportional derivative controlled articulated rigid skeletons that interact in a life-like way with their environment.