Denis Steinemann

Implicit Contact Handling for Deformable Objects

We present an algorithm for robust and efficient contact handling of deformable objects. By being aware of the internal dynamics of the colliding objects, our algorithm provides smooth rolling and sliding, stable stacking, robust impact handling, and seamless coupling of heterogeneous objects, all in a unified manner. We achieve dynamicsawareness through a constrained dynamics formulation with implicit complementarity constraints, and we present two major contributions that enable an efficient solution of the constrained dynamics problem: a time stepping algorithm that robustly ensures non‐penetration and progressively refines the formulation of constrained dynamics, and a new solver for large mixed linear complementarity problems, based on iterative constraint anticipation. We show the application of our algorithm in challenging scenarios such as multi‐layered cloth moving at high velocities, or colliding deformable solids simulated with large time steps.

Hybrid Cutting of Deformable Solids

A central training objective of virtual reality based surgical simulation is the removal of pathologic tissue. This necessitates stable, real-time updates of the underlying mesh representation. Within the framework of a hysteroscopy simulator, we have developed a hybrid cutting approach for tetrahedral meshes. It combines the topological update by subdivision with adjustments of the existing topology. Moreover, the mechanical and the visual model are decoupled, thus allowing different resolutions for the underlying mesh representations. With our method, we can closely approximate an arbitrary, user-defined cut surface while avoiding the creation of small or badly shaped elements, thus strongly reducing stability problems in the subsequent deformation computation. The presented approach has been integrated into a virtual reality training system for hysteroscopic interventions. The performance of the algorithm is demonstrated by examples of intra-uterine tumor ablations.

Fast arbitrary splitting of deforming objects

We present a novel algorithm for efficiently splitting deformable solids along arbitrary piecewise linear crack surfaces in cutting and fracture simulations. We propose the use of a meshless discretization of the deformation field, and a novel visibility graph for fast update of shape functions in meshless discretizations. We decompose the splitting operation into a first step where we synthesize crack surfaces as triangle meshes, and a second step where we use the newly synthesized surfaces to update the visibility graph, and thus the meshless discretization of the deformation field. The separation of the splitting operation into two steps, along with our novel visibility graph, enables high flexibility and control over the splitting trajectories, provides fast dynamic update of the meshless discretization, and facilitates an easy implementation, making our algorithm scalable, versatile, and suitable for a large range of applications, from computer animation to interactive medical simulation.

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.

Efficient Animation of Point‐Sampled Thin Shells

We present a novel framework for the efficient simulation and animation of discrete thin shells. Our method takes a point sampled surface as input and performs all necessary computations without intermediate triangulation. We discretize the thin shell functional using so-called fibers. Such fibers are locally embedded parametric curves crisscrossing individual point samples. In combination, they create a dense mesh representing the surface structure and connectivity for the shell computations. In particular, we utilize the fibers to approximate the differential surface operators of the thin shell functional. The polynomials underlying the fiber representation allow for a robust and fast simulation of thin shell behavior. Our method supports both elastic and plastic deformations as well as fracturing and tearing of the material. To compute surfaces with rich surface detail, we designed a multiresolution representation which maps a high-resolution surface onto a fiber network of lower resolution. This makes it possible to animate densely sampled models of very high surface complexity. While being tuned for point sampled objects, the presented framework is versatile and can also take triangle meshes or triangle soups as input.

Balanced Hierarchies for Collision Detection between Fracturing Objects

The simulation of fracture leads to collision-intensive situations that call for efficient collision detection algorithms and data structures. Bounding volume hierarchies (BVHs) are a popular approach for accelerating collision detection, but they rarely see application in fracture simulations, due to the dynamic creation and deletion of geometric primitives. We propose the use of balanced trees for storing BVHs, as well as novel algorithms for dynamically restructuring them in the presence of progressive or instantaneous fracture. By paying a small loss of fitting quality compared with complete reconstruction, we achieve more than one order of magnitude speedup in the update of BVHs

Robust interactive cutting based on an adaptive octree simulation mesh

We present an adaptive octree based approach for interactive cutting of deformable objects. Our technique relies on efficient refine- and node split-operations. These are sufficient to robustly represent cuts in the mechanical simulation mesh. A high-resolution surface embedded into the octree is employed to represent a cut visually. Model modification is performed in the rest state of the object, which is accomplished by back-transformation of the blade geometry. This results in an improved robustness of our approach. Further, an efficient update of the correspondences between simulation elements and surface vertices is proposed. The robustness and efficiency of our approach is underlined in test examples as well as by integrating it into a prototype surgical simulator.

Splitting meshless deforming objects with explicit surface tracking

We present a novel algorithm for efficiently splitting deformable solids along arbitrary piecewise linear crack surfaces in cutting and fracture simulations. The algorithm combines a meshless discretization of the deformation field with explicit surface tracking using a triangle mesh. We decompose the splitting operation into a first step where we synthesize crack surfaces, and a second step where we use the newly synthesized surfaces to update the meshless discretization of the deformation field. We present a novel visibility graph for facilitating fast update of shape functions in the meshless discretization. The separation of the splitting operation into two steps, along with our novel visibility graph, enables high flexibility and control over the splitting trajectories, provides fast dynamic update of the meshless discretization, and allows for an easy implementation. As a result, our algorithm is scalable, versatile, and suitable for a large range of applications, from computer animation to interactive medical simulation.

Virtual reality based simulation of hysteroscopic interventions

Virtual reality based simulation is an appealing option to supplement traditional clinical education. However, the formal integration of training simulators into the medical curriculum is still lacking. Especially, the lack of a reasonable level of realism supposedly hinders the widespread use of this technology. Therefore, we try to tackle this situation with a reference surgical simulator of the highest possible fidelity for procedural training. This overview describes all elements that have been combined into our training system as well as first results of simulator validation. Our framework allows the rehearsal of several aspects of hysteroscopyfor instance, correct fluid management, handling of excessive bleeding, appropriate removal of intra-uterine tumors, or the use of the surgical instrument.

A hybrid cutting approach for hysteroscopy simulation

An integral element of every surgical simulator is the ability to interactively cut tissue. A number of approaches have been suggested in the past, the most important being mesh subdivision by introducing new elements and mesh adaptation by adjusting existing topology. In this paper we combine these two methods and optimize them for our training system of hysteroscopic interventions. The basic methodology is introduced in 2D, a first extension to 3D is presented and finally the integration into the simulator described.