Soft Body Locomotion

Jie Tan, Greg Turk, Karen Liu

We present a physically-based system to simulate and control the locomotion of soft body characters without skeletons. We use the finite element method to simulate the deformation of the soft body, and we instrument a character with muscle fibers to allow it to actively control its shape. To perform locomotion, we use a variety of intuitive controls such as moving a point on the character, specifying the center of mass or the angular momentum, and maintaining balance. These controllers yield an objective function that is passed to our optimization solver, which handles convex quadratic program with linear complementarity constraints. This solver determines the new muscle fiber lengths, and moreover it determines whether each point of contact should remain static, slide, or lift away from the floor. Our system can automatically find an appropriate combination of muscle contractions that enables a soft character to fulfill various locomotion tasks, including walking, jumping, crawling, rolling and balancing.

Soft Body Locomotion

Tracking Surfaces with Evolving Topology

Morten Bojsen-Hansen, Hao Li, Chris Wojtan

We present a method for recovering a temporally coherent, deforming triangle mesh with arbitrarily changing topology from an incoherent sequence of static closed surfaces. We solve this problem using the surface geometry alone, without any prior information like surface templates or velocity fields. Our system combines a proven strategy for triangle mesh improvement, a robust multi-resolution non-rigid registration routine, and a reliable technique for changing surface mesh topology. We also introduce a novel topological constraint enforcement algorithm to ensure that the output and input always have similar topology. We apply our technique to a series of diverse input data from video reconstructions, physics simulations, and artistic morphs. The structured output of our algorithm allows us to efficiently track information like colors and displacement maps, recover velocity information, and solve PDEs on the mesh as a post process.

Tracking Surfaces with Evolving Topology

Parallel Surface Reconstruction for Particle-Based Fluids

Gizem Akinci, Markus Ihmsen, Nadir Akinci, Matthias Teschner

This paper presents a novel method that improves the efficiency of high-quality surface reconstructions for particle-based fluids using Marching Cubes. By constructing the scalar field only in a narrow band around the surface, the computational complexity and the memory consumption scale with the fluid surface instead of the volume. Furthermore, a parallel implementation of the method is proposed. The presented method works with various scalar field construction approaches. Experiments show that our method reconstructs high-quality surface meshes efficiently even on single-core CPUs. It scales nearly linearly on multi-core CPUs and runs up to fifty times faster on GPUs compared to the original scalar field construction approaches.

Parallel Surface Reconstruction for Particle-Based Fluids

Unified Spray, Foam, and Bubbles for Particle-Based Fluids

Markus Ihmsen, Nadir Akinci, Gizem Akinci, Matthias Teschner

We present a new model for diffuse material, i.e. water–air mixtures, that can be combined with particle-based fluids. Diffuse material is uniformly represented with particles which are classified into spray, foam and air bubbles. Physically motivated rules are employed to generate, advect and dissipate diffuse material. The approach is realized as a post-processing step which enables efficient processing and versatile handling. As interparticle forces and the influence of diffuse material onto the fluid are neglected, large numbers of diffuse particles are efficiently processed to realize highly detailed small-scale effects. The presented results show that our approach can significantly improve the visual realism of large-scale fluid simulations.

Unified Spray, Foam, and Bubbles for Particle-Based Fluids

Reflections on Simultaneous Impact

Breannan Smith, Danny Kaufman, Etienne Vouga, Rasmus Tamstorf, Eitan Grinspun

Resolving simultaneous impacts is an open and significant problem in collision response modeling. Existing algorithms in this domain fail to fulfill at least one of five physical desiderata. To address this we present a simple generalized impact model motivated by both the successes and pitfalls of two popular approaches: pair-wise propagation and linear complementarity models. Our algorithm is the first to satisfy all identified desiderata, including simultaneously guaranteeing symmetry preservation, kinetic energy conservation, and allowing break-away. Furthermore, we address the associated problem of inelastic collapse, proposing a complementary generalized restitution model that eliminates this source of nontermination. We then consider the application of our models to the synchronous time-integration of large-scale assemblies of impacting rigid bodies. To enable such simulations we formulate a consistent frictional impact model that continues to satisfy the desiderata. Finally, we validate our proposed algorithm by correctly capturing the observed characteristics of physical experiments including the phenomenon of extended patterns in vertically oscillated granular materials.

Reflections on Simultaneous Impact

Interactive Editing of Deformable Simulations

Jernej Barbic, Funshing Sin, Eitan Grinspun

We present an interactive animation editor for complex deformable object animations. Given an existing animation, the artist directly manipulates the deformable body at any time frame, and the surrounding animation immediately adjusts in response. The automatic adjustments are designed to respect physics, preserve detail in both the input motion and geometry, respect prescribed bilateral contact constraints, and controllably and smoothly decay in spacetime. While the utility of interactive editing for rigid body and articulated figure animations is widely recognized, a corresponding approach to deformable bodies has not been technically feasible before. We achieve interactive rates by combining spacetime model reduction, rotation-strain coordinate warping, linearized elasticity, and direct manipulation. This direct editing tool can serve the final stages of animation production, which often call for detailed, direct adjustments that are otherwise tedious to realize by re-simulation or frame-by-frame editing.

Interactive Editing of Deformable Simulations

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

PolyDepth: Real-Time Penetration Depth Computation using Iterative Contact-Space Projection

Changsoo Je, Min Tang, Youngeun Lee, Minkyoung Lee, Young J. Kim

We present a real-time algorithm that finds the Penetration Depth (PD) between general polygonal models based on iterative and local optimization techniques. Given an in-collision configuration of an object in configuration space, we find an initial collision-free configuration using several methods such as centroid difference, maximally clear configuration, motion coherence, random configuration, and sampling-based search. We project this configuration on to a local contact space using a variant of continuous collision detection algorithm and construct a linear convex cone around the projected configuration. We then formulate a new projection of the in-collision configuration onto the convex cone as a Linear Complementarity Problem (LCP), which we solve using a type of Gauss-Seidel iterative algorithm. We repeat this procedure until a locally optimal PD is obtained. Our algorithm can process complicated models consisting of tens of thousands triangles at interactive rates.

PolyDepth: Real-Time Penetration Depth Computation using Iterative Contact-Space Projection

Stress Relief: Improving Structural Strength of 3D Printable Objects

Ondrej Stava, Juraj Vanek, Bedrich Benes, Nathan Carr, Radomir Mech

3D printing is a rapidly maturing area that has shown great progress over the past couple of years. It is now possible to produce 3D printed objects with exceptionally high fidelity and precision. However, while the quality of 3D printing has gone up, both the time to print and material costs have remained high. Moreover, there is no guarantee that a printed model is structurally sound. Many times, the printed product does not survive cleaning, transportation, or handling, or it even collapses under its own weight. We present a system that addresses this issue by providing automatic detection and correction of the problematic cases. The structural problems are detected by combining a lightweight structure analysis solver with 3D medial axis approximations. After areas with high structural stress are found, the model is corrected by combining three approaches: hollowing, thickening, and strut insertion. This detection and correction repeats until all problematic cases are corrected. Our process is designed to create a model that is visually similar to the original model, while possessing greater structural integrity

Stress Relief: Improving Structural Strength of 3D Printable Objects

Underwater Rigid Body Dynamics

Steffen Weissman, Ulrich Pinkall

We show that the motion of rigid bodies under water can be realistically simulated by replacing the usual inertia tensor and scalar mass by the so-called Kirchhoff tensor. This allows us to model fluid-body interaction without simulating the surrounding fluid at all. We explain some of the phenomena that arise and compare our results against real experiments. It turns out that many real scenarios (sinking bodies, balloons) can be matched using a single, hand-tuned scaling parameter. We describe how to integrate our method into an existing physics engine, which makes underwater rigid body dynamics run in real time.

Underwater Rigid Body Dynamics