We propose a new fast and robust method to simulate various types of solid including rigid, plastic and soft bodies as well as one, two and three dimensional structures such as ropes, cloth and volumetric objects. The underlying idea is to use oriented particles that store rotation and spin, along with the usual linear attributes, i.e. position and velocity. This additional information adds substantially to traditional particle methods. First, particles can be represented by anisotropic shapes such as ellipsoids, which approximate surfaces more accurately than spheres. Second, shape matching becomes robust for sparse structures such as chains of particles or even single particles because the undefined degrees of freedom are captured in the rotational states of the particles. Third, the full transformation stored in the particles, including translation and rotation, can be used for robust skinning of graphical meshes and for transforming plastic deformations back into the rest state.
Category: Shells
Data-Driven Elastic Models for Cloth: Modeling and Measurement
Cloth often has complicated nonlinear, anisotropic elastic behavior due to its woven pattern and fiber properties. However, most current cloth simulation techniques simply use linear and isotropic elastic models with manually selected stiffness parameters. Such simple simulations do not allow differentiating the behavior of distinct cloth materials such as silk or denim, and they cannot model most materials with fidelity to their real-world counterparts. In this paper, we present a data-driven technique to more realistically animate cloth. We propose a piecewise linear elastic model that is a good approximation to nonlinear, anisotropic stretching and bending behaviors of various materials. We develop new measurement techniques for studying the elastic deformations for both stretching and bending in real cloth samples. Our setup is easy and inexpensive to construct, and the parameters of our model can be fit to observed data with a well-posed optimization procedure. We have measured a database of ten different cloth materials, each of which exhibits distinctive elastic behaviors. These measurements can be used in most cloth simulation systems to create natural and realistic clothing wrinkles and shapes, for a range of different materials.
Data-Driven Elastic Models for Cloth: Modeling and Measurement
Underwater Cloth Simulation with Fractional Derivatives
We introduce the use of fractional differentiation for simulating cloth de formations underwater. The proposed approach is able to achieve realistic underwater deformations without simulating the Eulerian body of water in which the cloth is immersed. Instead, we propose a particle-based cloth model where half-derivative viscoelastic elements are included for describing both the internal and external dynamics of the cloth. These elements model the cloth responses to fluid stresses and are also able to emulate the memory-laden behavior of particles in a viscous fluid. As a result, we obtain fractional clothes, which are able to correctly depict the dynamics of the immersed cloth interacting with the fluid even though the fluid is not simulated. The proposed approach produces realistic underwater cloth deformations and has obvious advantages in simplicity and speed of computation in comparison to volumetric fluid simulation approaches.
Unified Simulation of Elastic Rods, Shells, and Solids
We develop an accurate, unified treatment of elastica. Following the method of resultant-based formulation to its logical extreme, we derive a higher-order integration rule, or elaston, measuring stretching, shearing, bending, and twisting along any axis. The theory and accompanying implementation do not distinguish between forms of different dimension (solids, shells, rods), nor between manifold regions and non-manifold junctions. Consequently, a single code accurately models a diverse range of elastoplastic behaviors, including buckling, writhing, cutting and merging. Emphasis on convergence to the continuum sets us apart from early unification efforts.
Anisotropic Friction for Deformable Surfaces and Solids
This paper presents a method for simulating anisotropic friction for deforming surfaces and solids. Frictional contact is a complex phenomenon that fuels research in mechanical engineering, computational contact mechanics, composite material design and rigid body dynamics, to name just a few. Many real-world materials have anisotropic surface properties. As an example, most textile materials exhibit direction-dependent frictional behavior, but despite its tremendous impact on visual appearance, only simple isotropic models have been considered for cloth and solid simulation so far.
In this work, we propose a simple, application-oriented but physically sound model that extends existing methods to account for anisotropic friction.
The sliding properties of surfaces are encoded in friction tensors, which allows us to model frictional resistance freely along arbitrary directions. We also consider heterogeneous and asymmetric surface roughness and demonstrate the increased simulation quality on a number of two- and three-dimensional examples. Our method is computationally efficient and can easily be integrated into existing systems.
Accurate Tangential Velocities for Solid-Fluid Coupling
We propose a novel method for obtaining more accurate tangential velocities for solid fluid coupling. Our method works for both rigid and deformable objects as well as both volumetric objects and thin shells. The fluid can be either one phase such as smoke or two phase such as water with a free surface. The coupling between the solid and the fluid can either be one-way with kinematic solids or fully two-way coupled. The only previous scheme that was general enough to handle both two-way coupling and thin shells required a mass lumping strategy that did not allow for freely flowing tangential velocities. Similar to that previous work, our method prevents leaking of fluid across a thin shell, however unlike that work our method does not couple the tangential velocities in any fashion, allowing for the proper slip independently on each side of the body. Moreover, since it accurately and directly treats the tangential velocity, it does not rely on grid refinement to obtain a reasonable solution. Therefore, it gives a highly improved result on coarse meshes.
Triangular Springs for Modeling Nonlinear Membranes
This paper provides a formal connexion between springs and continuum mechanics in the context of onedimensional
and two-dimensional elasticity. In a first stage, the equivalence between tensile springs and the finite element
discretization of stretching energy on planar curves is established. Furthermore, when considering a quadratic strain function of stretch, we introduce a new type of springs called tensile biquadratic springs. In a second stage, we extend this equivalence to non-linear membranes (St Venant-Kirchhoff materials) on triangular meshes leading to triangular biquadratic and quadratic springs. Those tensile and angular springs produce isotropic deformations parameterized by Young modulus and Poisson ratios on unstructured meshes in an efficient and simple way. For a specific choice of the Poisson ratio, 0.3, we show that regular spring-mass models may be used realistically to simulate a membrane behavior. Finally, the different spring formulations are tested in pure traction and cloth simulation experiments.
Continuum-based Strain Limiting
We present Continuum-based Strain Limiting (CSL) – a new method for limiting deformations in physically-based cloth simulations. Recent developments have led to methods which excel at simulating nearly inextensible materials, but the efficient simulation of general biphasic textiles and their anisotropic behavior remains challenging. Other approaches use softer materials and enforce limits on edge elongations, leading to discretization-dependent behavior. Moreover, they offer no explicit control over shearing and stretching unless specifically aligned meshes are used, which makes them less attractive for practical animation of anisotropic textiles. Based on a continuum deformation measure, our method allows accurate deformation control using individual thresholds for all types of strain. We impose deformation limits element-wise and cast the problem as a system of linear equations. We show how to further improve efficiency using an approximate formulation. CSL can be combined with any type of cloth simulator and, as a velocity filter, integrates seamlessly into standard collision handling frameworks.
Parallel Simulation of Inextensible Cloth
This paper presents an efficient simulation method for parallel cloth simulation. The presented method uses an impulse-based approach for the simulation. Cloth simulation has many application areas like computer animation, computer games or virtual reality. Simulation methods often make the assumption that cloth is an elastic material. In this way the simulation can be performed very efficiently by using spring forces. These methods disregard the fact that many textiles cannot be stretched significantly. The simulation of inextensible textiles with methods based on spring forces leads to stiff differential equations which cause a loss of performance. In contrast to that, in this paper a method is presented that simulates cloth by using impulses. The mesh of a cloth model is subdivided into strips of constraints. The impulses for each strip can be computed in linear time. The strips that have no common particle are independent from each other and can be solved in parallel. The impulse-based method allows the realistic simulation of inextensible textiles in real-time.
A Geometric Deformation Model for Stable Cloth Simulation
We propose an adapted shape-matching approach for the efficient and robust simulation of clothing. A combination of two different cluster types is employed to account for high stretching and shearing, and low bending resistance. Due to the inherent handling of overshooting issues, the proposed shape-matching deformation model is robust. The proposed cluster types allow for a computationally efficient handling of bending. The geometric deformation model is combined with a novel collision handling approach. The technique employs spatial subdivision to detect collisions and self-collisions. The response scheme is derived from an existing approach for elastic rods. To illustrate the physically plausible dynamics of our approach, it is compared to a traditional physically-based deformation model. Experiments indicate that similar cloth properties can be reproduced with both models. The computational efficiency of the proposed scheme enables the interactive animation of clothing and shells.