Category: Shells

Embedded Thin Shells for Wrinkle Simulation

Olivier Remillard, Paul G. Kry

We present a new technique for simulating high resolution surface wrinkling deformations of composite objects consisting of a soft interior and a harder skin. We combine high resolution thin shells with coarse finite element lattices and define frequency based constraints that allow the formation of wrinkles with properties matching those predicted by the physical parameters of the composite object. Our two-way coupled model produces the expected wrinkling behavior without the computational expense of a large number of volumetric elements to model deformations under the surface. We use C1 quadratic shape functions for the interior deformations, allowing very coarse resolutions to model the overall global deformation efficiently, while avoiding visual artifacts of wrinkling at discretization boundaries. We demonstrate that our model produces wrinkle wavelengths that match both theoretical predictions and high resolution volumetric simulations. We also show example applications in simulating wrinkles on passive objects, such as furniture, and for wrinkles on faces in character animation.

Embedded Thin Shells for Wrinkle Simulation

Adaptive Fracture Simulation of Multi-Layered Thin Plates

Oleksiy Busaryev, Tamal K. Dey, Huamin Wang

The fractures of thin plates often exhibit complex physical behaviors in the real world. In particular, fractures caused by tearing are different from fractures caused by in-plane motions. In this paper, we study how to make thin-plate fracture animations more realistic from three perspectives. We propose a stress relaxation method, which is applied to avoid shattering artifacts after generating each fracture cut. We formulate a fracture-aware remeshing scheme based on constrained Delaunay triangulation, to adaptively provide more fracture details. Finally, we use our multi-layered model to simulate complex fracture behaviors across thin layers. Our experiment shows that the system can efficiently and realistically simulate the fractures of multi-layered thin plates.

Adaptive Fracture Simulation of Multi-Layered Thin Plates

Modeling Friction and Air Effects between Cloth and Deformable Bodies

Zhili Chen, Renguo Feng, Huamin Wang

Real-world cloth exhibits complex behaviors when it contacts deformable bodies. In this paper, we study how to improve the simulation of cloth-body interactions from three perspectives: collision, friction, and air pressure. We propose an efficient and robust algorithm to detect the collisions between cloth and deformable bodies, using the surface traversal technique. We develop a friction measurement device and we use it to capture frictional data from real-world experiments. The derived friction model can realistically handle complex friction properties of cloth, including anisotropy and nonlinearity. To produce pressure effects caused by the air between cloth and deformable bodies, we define an air mass field on the cloth layer and we use real-world air permeability data to animate it over time. Our results demonstrate the efficiency and accuracy of our system in simulating objects with a three-layer structure (i.e., a cloth layer, an air layer, and an inner body layer), such as pillows, comforters, down jackets, and stuffed toys.

Modeling Friction and Air Effects between Cloth and Deformable Bodies

Folding and Crumpling Adaptive Sheets

Rahul Narain, Tobias Pfaff, and James F. O’Brien

We present a technique for simulating plastic deformation in sheets of thin materials, such as crumpled paper, dented metal, and wrinkled cloth. Our simulation uses a framework of adaptive mesh refinement to dynamically align mesh edges with folds and creases. This framework allows efficient modeling of sharp features and avoids bend locking that would be otherwise caused by stiff in-plane behavior. By using an explicit plastic embedding space we prevent remeshing from causing shape diffusion. We include several examples demonstrating that the resulting method realistically simulates the behavior of thin sheets as they fold and crumple.

Folding and Crumpling Adaptive Sheets

Interpenetration Free Simulation of Thin Shell Rigid Bodies

R. Elliot English, Michael Lentine, Ron Fedkiw

We propose a new algorithm for rigid body simulation that guarantees each body is in an interpenetration free state, both increasing the accuracy and robustness of the simulation as well as alleviating the need for ad hoc methods to separate bodies for subsequent simulation and rendering. We cleanly separate collision and contact resolution such that objects move and collide in the first step, with resting contact handled in the second step. The first step of our algorithm guarantees that each time step produces geometry that does not intersect or overlap by using an approximation to the continuous collision detection (and response) problem and thus is amenable to thin shells and degenerately flat objects moving at high speeds. In addition we introduce a novel failsafe which allows us to resolve all interpenetration without iterating to convergence. Since the first step guarantees a non-interfering state for the geometry, in the second step we propose a contact model for handling thin shells in proximity considering only the instantaneous locations at the ends of the time step.

Interpenetration Free Simulation of Thin Shell Rigid Bodies

SIGGRAPH Course: Data-Driven Simulation Methods in Computer Graphics: Cloth, Tissue, and Faces

Miguel Otaduy, Bernd Bickel, Derek Bradley, Huamin Wang

In recent years, the field of computer animation has witnessed the invention of multiple simulation methods that exploit pre-recorded data to improve the performance and/or realism of dynamic deformations. Various methods have been presented concurrently, and they present differences, but also similarities, that have not yet been analyzed or discussed. This course focuses on the application of data-driven methods to three areas of computer animation, namely dynamic deformation of faces, soft volumetric tissue, and cloth. The course describes the particular challenges tackled in a data-driven manner, classifies the various methods, and also shares insights for the application to other settings.

The explosion of data-driven animation methods and the success of their results make this course extremely timely. Up till now, the proposed methods have remained familiar only at the research context, and have not made their way through computer graphics industry. This course aims to fit two main purposes. First, present a common theory and understanding of data-driven methods for dynamic deformations that may inspire the development of novel solutions, and second, bridge the gap with industry, by making data-driven approaches accessible. The course targets an audience consisting of both researchers and programmers in computer animation.

Data-Driven Simulation Methods in Computer Graphics: Cloth, Tissue, and Faces

Interactive Space-Time Control of Deformable Objects

Klaus Hildebrandt, Christian Schulz, Christoph von Tycowicz, Konrad Polthier

Creating motions of objects or characters that are physically plausible and follow an animator’s intent is a key task in computer animation. The spacetime constraints paradigm is a valuable approach to this problem, but it suffers from high computational costs. Based on spacetime constraints, we propose a framework for controlling the motion of deformable objects that offers interactive response times. This is achieved by a model reduction of the underlying variational problem, which combines dimension reduction, multipoint linearization, and decoupling of ODEs. After a preprocess, the cost for creating or editing a motion is reduced to solving a number of one-dimensional spacetime problems, whose solutions are the wiggly splines introduced by Kass and Anderson [2008]. We achieve interactive response times through a new fast and robust numerical scheme for solving the one-dimensional problems that is based on a closed-form representation of the wiggly splines.

Interactive Space-Time Control of Deformable Objects

Discrete Viscous Sheets

Christopher Batty, Andres Uribe, Basile Audoly, Eitan Grinspun

We present the first reduced-dimensional technique to simulate the dynamics of thin sheets of viscous incompressible liquid in three dimensions. Beginning from a discrete Lagrangian model for elastic thin shells, we apply the Stokes-Rayleigh analogy to derive a simple yet consistent model for viscous forces. We incorporate nonlinear surface tension forces with a formulation based on minimizing discrete surface area, and preserve the quality of triangular mesh elements through local remeshing operations. Simultaneously, we track and evolve the thickness of each triangle to exactly conserve liquid volume. This approach enables the simulation of extremely thin sheets of viscous liquids, which are difficult to animate with existing volumetric approaches. We demonstrate our method with examples of several characteristic viscous sheet behaviors, including stretching, buckling, sagging, and wrinkling.

Discrete Viscous Sheets

Computational Design of Rubber Balloons

Melina Skouras, Bernhard Thomaszewski, Bernd Bickel, Markus Gross

This paper presents an automatic process for fabrication-oriented design of custom-shaped rubber balloons. We cast computational balloon design as an inverse problem: given a target shape, we compute an optimal balloon that, when inflated, approximates the target as closely as possible. To solve this problem numerically, we propose a novel physics-driven shape optimization method, which combines physical simulation of inflatable elastic membranes with a dedicated constrained optimization algorithm. We validate our approach by fabricating balloons designed with our method and comparing their inflated shapes to the results predicted by simulation. An extensive set of manufactured sample balloons demonstrates the shape diversity that can be achieved by our method.

Computational Design of Rubber Balloons

Sensitive Couture for Interactive Garment Editing and Modeling

Nobuyuki Umetani, Danny M. Kaufman, Takeo Igarashi, Eitan Grinspun

We present a novel interactive tool for garment design that enables, for the first time, interactive bidirectional editing between 2D patterns and 3D high-fidelity simulated draped forms. This provides a continuous, interactive, and natural design modality in which 2D and 3D representations are simultaneously visible and seamlessly maintain correspondence. Artists can now interactively edit 2D pattern designs and immediately obtain stable accurate feedback online, thus enabling rapid prototyping and an intuitive understanding of complex drape form.

Sensitive Couture for Interactive Garment Editing and Modeling