Physics-Based Animation

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

Siwang Li, Jin Huang, Fernando de Goes, Xiaogang Jin, Hujun Bao, and Mathieu Desbrun

We present a novel method for elastic animation editing with spacetime constraints. In a sharp departure from previous approaches, we not only optimize control forces added to a linearized dynamic model, but also optimize material properties to better match user constraints and provide plausible and consistent motion. Our approach achieves efficiency and scalability by performing all computations in a reduced rotation-strain (RS) space constructed with both cubature and geometric reduction, leading to two orders of magnitude improvement over the original RS method. We demonstrate the utility and versatility of our method in various applications, including motion editing, pose interpolation, and estimation of material parameters from existing animation sequences.

Space-Time Editing of Elastic Motions through Material Optimization and Reduction

Here they are thus far:

• Multimaterial Mesh-Based Surface Tracking
• Blending Liquids
• Adaptive Tearing and Cracking of Thin Sheets
• Codimensional Surface Tension Flow on Simplicial Complexes
• Defending Continuous Collision Detection Against Errors
• Physics-Inspired Adaptive Fracture Refinement
• Detailed Water with Coarse Grids: Combining Surface Meshes and  Adaptive Discontinuous Galerkin
• Smoke Rings from Smoke
• Projective Dynamics: Fusing Constraint Projections for Fast Simulation
• Inverse-Foley Animation: Synchronizing rigid-body motions to sound
• Animating Deformable Objects using Sparse Spacetime Constraints 
• Simulating Articulated Subspace Self-Contact
• Robust Hair Capture Using Simulated Examples
• Augmented MPM for Phase-Change and Varied Materials
• Space-Time Editing of Elastic Motion through Material Optimization and Reduction
• Unified Particle Physics for Real-Time Applications
• Active Volumetric Musculoskeletal Systems
• From Capture to Simulation – Connecting Forward and Inverse Problems in Fluids
• Subspace Clothing Simulation Using Adaptive Bases
• Sensitivity-Optimized Rigging for Example-Based Real-Time Clothing Synthesis
• Adaptive Nonlinearity for Collisions in Complex Rod Assemblies
• A Reduced Model for Interactive Hairs

TOG:

• Deformation Embedding for Point-Based Elastoplastic Simulation
• Animation of Deformable Bodies with Quadratic Bézier Finite Elements
• Exponential Integrators for Stiff Elastodynamic Problems
• Interactive Generalized Penetration Depth Computation for Rigid and Articulate Models Using Object Norm

Fang Da, Christopher Batty, Eitan Grinspun

We present a triangle mesh-based technique for tracking the evolution of three-dimensional multimaterial interfaces undergoing complex deformations. It is the first non-manifold triangle mesh tracking method to simultaneously maintain intersection-free meshes and support the proposed broad set of multimaterial remeshing and topological operations. We represent the interface as a non-manifold triangle mesh with material labels assigned to each half-face to distinguish volumetric regions. Starting from proposed application-dependent vertex velocities, we deform the mesh, seeking a non-intersecting, watertight solution. This goal necessitates development of various collision-safe, label-aware non-manifold mesh operations: multimaterial mesh improvement; T1 and T2 processes, topological transitions arising in foam dynamics and multi-phase flows; and multimaterial merging, in which a new interface is created between colliding materials. We demonstrate the robustness and effectiveness of our approach on a range of scenarios including geometric flows and multiphase fluid animation.

Multimaterial Mesh-Based Surface Tracking

Karthik Raveendran, Chris Wojtan, Nils Thuerey, Greg Turk

We present a method for smoothly blending between existing liquid animations. We introduce a semi-automatic method for matching two existing liquid animations, which we use to create new fluid motion that plausibly interpolates the input. Our contributions include a new space-time non-rigid iterative closest point algorithm that incorporates user guidance, a subsampling technique for efficient registration of meshes with millions of vertices, and a fast surface extraction algorithm that produces 3D triangle meshes from a 4D space-time surface. Our technique can be used to instantly create hundreds of new simulations, or to interactively explore complex parameter spaces. Our method is guaranteed to produce output that does not deviate from the input animations, and it generalizes to multiple dimensions. Because our method runs at interactive rates after the initial precomputation step, it has potential applications in games and training simulations.

Blending Liquids

Tobias Pfaff, Rahul Narain, Juan Miguel de Joya, and James F. O’Brien

This paper presents a method for adaptive fracture propagation in thin sheets. A high-quality triangle mesh is dynamically restructured to adaptively maintain detail wherever it is required by the simulation. These requirements include refining where cracks are likely to either start or advance. Refinement ensures that the stress distribution around the crack tip is well resolved, which is vital for creating highly detailed, realistic crack paths. The dynamic meshing framework allows subsequent coarsening once areas are no longer likely to produce cracking. This coarsening allows efficient simulation by reducing the total number of active nodes and by preventing the formation of thin slivers around the crack path. A local reprojection scheme and a substepping fracture process help to ensure stability and prevent a loss of plasticity during remeshing. By including bending and stretching plasticity models, the method is able to simulate a large range of materials with very different fracture behaviors.

Adaptive Tearing and Cracking of Thin Sheets

Bo Zhu, Ed Quigley, Matthew Cong, Justin Solomon, and Ron Fedkiw

Many visually interesting natural phenomena are characterized by thin liquid sheets, long filaments, and droplets. We present a new Lagrangian-based numerical method to simulate these codimensional surface tension driven phenomena using non-manifold simplicial complexes. Tetrahedra, triangles, segments, and points are used to model the fluid volume, thin films, filaments, and droplets, respectively. We present a new method for enforcing fluid incompressibility on simplicial complexes along with a physically-guided meshing algorithm to provide temporally consistent information for interparticle forces. Our method naturally allows for transitions between codimensions, either from tetrahedra to triangles to segments to points or vice versa, regardless of the simulation resolution. We demonstrate the efficacy of this method by simulating various natural phenomena that are characterized by thin fluid sheets, filaments, and surface tension effects.

Codimensional Surface Tension Flow on Simplicial Complexes

Yun Teng, Miguel Otaduy, Theodore Kim

We present an efficient new subspace method for simulating the self-contact of articulated deformable bodies, such as characters. Self-contact is highly structured in this setting, as the limited space of possible articulations produces a predictable set of coherent collisions. Subspace methods can leverage this coherence, and have been used in the past to accelerate the collision detection stage of contact simulation. We show that these methods can be used to accelerate the entire contact computation, and allow self-contact to be resolved without looking at all of the contact points. Our analysis of the problem yields a broader insight into the types of nonlinearities that subspace methods can efficiently approximate, and leads us to design a pose-space cubature scheme. Our algorithm accelerates self-contact by up to an order of magnitude over other subspace simulations, and accelerates the overall simulation by two orders of magnitude over full-rank simulations. We demonstrate the simulation of high resolution (100K – 400K elements) meshes
in self-contact at interactive rates (5.8 – 50 FPS).

Simulating Articulated Subspace Self-Contact

Essex Edwards, Robert Bridson

We present a new adaptive fluid simulation method that captures a high resolution surface with precise dynamics, without an inefficient fine discretization of the entire fluid volume. Prior adap- tive methods using octrees or unstructured meshes carry large over- heads and implementation complexity. We instead stick with coarse regular Cartesian grids, using detailed cut cells at boundaries, and discretize the dynamics with a p-adaptive Discontinuous Galerkin (DG) method. This retains much of the data structure simplicity of regular grids, more efficiently captures smooth parts of the flow, and offers the flexibility to easily increase resolving power where needed without geometric refinement.

Detailed Water with Coarse Grids: Combining Surface Meshes and  Adaptive Discontinuous Galerkin

Christian Schulz, Christoph von Tycowicz, Hans-Peter Seidel, Klaus Hildebrandt

We propose a scheme for animating deformable objects based on spacetime optimization. The main feature is that it robustly and quickly (within a few seconds) generates interesting motion from a sparse set of spacetime constraints. Providing only partial (as opposed to full) keyframes for positions and velocities is sufficient. The computed motion satisfies the constraints and the remaining degrees of freedom are determined by physical principles using elasticity and the spacetime constraints paradigm. Our modeling of the spacetime optimization problem combines dimensional reduction, modal coordinates, wiggly splines, and rotation strain warping. Controlling the warped motion requires the derivative of the warp map. We derive a representation of the derivative that can be efficiently and robustly evaluated. Our solver is based on a theorem that characterizes the solutions of the optimization problem and allows us to restrict the optimization to very low-dimensional search spaces. This treatment of the optimization problem avoids a time discretization and the resulting method can robustly deal with sparse input and wiggly motion.

Animating Deformable Objects using Sparse Spacetime Constraints