Marc Gissler

Boundary Handling and Adaptive Time-stepping for PCISPH

We present a novel boundary handling scheme for incompressible fluids based on Smoothed Particle Hydrodynamics (SPH). In combination with the predictive-corrective incompressible SPH (PCISPH) method, the boundary handling scheme allows for larger time steps compared to existing solutions. Furthermore, an adaptive time-stepping approach is proposed. The approach automatically estimates appropriate time steps independent of the scenario. Due to its adaptivity, the overall computation time of dynamic scenarios is significantly reduced compared to simulations with constant time steps.

Guiding the Generation of Manipulation Plans by Qualitative Spatial Reasoning

Abstract Manipulation planning is a complex task for robots with a manipulator arm that need to grasp objects in the environment, specifically under narrow spatial conditions restricting the workspace of the robot. A popular approach for generating motion plans is probabilistic roadmap planning. However, the sampling strategy of such planners is usually unguided, and hence may lead to motion plans that seem counterintuitive for a human observer. In this article we present an approach that generates heuristics for the probabilistic sampling strategy from spatial plans that abstract from concrete metric data. These spatial plans describe a free trajectory in the workspace of the robot on a purely qualitative level, i.e., by employing spatial relations from formalisms considered in the domain of Qualitative Spatial and Temporal Reasoning. We discuss how such formalisms and constraint-based reasoning methods can be applied to approximate geometrically feasible motions. The paper is completed by an evaluation of a hybrid planning system in different spatial settings showing that run-times are notably improved when an abstract plan is considered as a guidance heuristic.

Local Constraint Methods for Deformable Objects

We present a local scheme to enforce non-conflicting geometric constraints for dynamically deforming objects. The approach employs information on the underlying numerical integration method and we present a unified scheme to illustrate the incorporation of a variety of integration methods. The proposed technique is very efficient in terms of memory and computing complexity. Iterative solvers and stabilization techniques are avoided. The method is not subject to numerical drift or other inaccuracies and all constraints are accurately met at each simulation step. Since the approach does not require any pre-processing, dynamically changing constraints can be handled efficiently. Experiments indicate that thousands of constraints can be processed at interactive rates. Although our approach is restricted to non-conflicting constraints, experiments illustrate the versatility of the method in the context of deformable objects.

Simultaneous cutting of coupled tetrahedral and triangulated meshes and its application in orbital reconstruction.

IntroductionRecently, advances in imaging techniques for diagnostics and associated technologies have led to an improved preoperative planning for craniomaxillofacial surgeons. In particular, the application of navigation-aided procedures for orbital reconstruction has proved to be essential. Preforming orbital implants for orbital floor reconstruction and determining overcorrection with regard to the orbital floor reconstruction could be achieved using preoperative planning. It has turned out that the computation of soft tissue cuts is an essential prerequisite for the realistic placement of implants.MethodsWe propose a simulation framework that allows for the static and dynamic cutting of soft and hard tissue representations. The framework comprises components to model tissue deformation, cutting of tissue and interaction between the physical bodies. Furthermore, volume and surface representations are decoupled which allows for an independent scaling in the complexity of the representations and, therefore, in the simulation and visualisation performance. In contrast to many other cutting approaches, our algorithm handles both representations simultaneously.ConclusionThe framework is used to simulate the realistic insertion of a preformed orbital implant model through the soft tissue cut and the prediction of the postoperative eye bulb position. Experiments show that the framework can be used to determine overcorrection and to preform orbital implants.

Optimized damping for dynamic simulations

Dynamic simulations can benefit a lot from an appropriate damping approach. For example, the stability is improved and a larger time step can be chosen. Furthermore, badly shaped meshes, e. g. containing sharp angles or slivers, can be handled if a proper damping approach is used. However, it must be ensured that the damping forces do not change the global movement of the object, i. e. they have to preserve linear and angular momentum. In this paper, we present a novel damping approach that is based on iterative spring damping to further improve the stability. We show that the resulting forces can be computed directly without actually performing the iterations. The approach does not require any connectivity information about the object and therefore, it can be used for arbitrary object representations. Further, it is independent of the integration scheme and the deformation model. The approach provides a simple parameter setting and guarantees that the damping forces do not overshoot. Finally, we illustrate that our approach allows for larger time steps compared to existing damping methods.

Deformable Proximity Queries and Their Application in Mobile Manipulation Planning

We describe a proximity query algorithm for the exact minimum distance computation between arbitrarily shaped objects. Special characteristics of the Gilbert-Johnson-Keerthi (GJK) algorithm are employed in various stages of the algorithm. In the first stage, they are used to search for sub-mesh pairs whose convex hulls do not intersect. In the case of an intersection, they guide a recursive decomposition. Finally, they are used to derive lower and upper distance bounds in non-intersecting cases. These bounds are utilized in a spatial subdivision scheme to achieve a twofold culling of the domain. The algorithm does not depend on spatial or temporal coherence and is, thus, specifically suited to be applied to deformable objects. Furthermore, we describe its embedding into the geometrical part of a mobile manipulation planning system. Experiments show its usability in dynamic scenarios with deformable objects as well as in complex manipulation planning scenarios.

Constraint Sets for Topology-changing Finite Element Models

We propose constraint sets as an efficient data structure for topology-changing deformable tetrahedral meshes. Using constraint sets, data structure updates in case of topology changes are simple and efficient. The consistency of the geometric representation is maintained and elasto-mechanical properties of the object are preserved. In combination with a Finite Element model for elasto-plastic objects and a geometric constraint approach, constraint sets are applied to simulate the merging and breaking of conforming and non-conforming tetrahedral meshes. Experiments illustrate the efficiency of the data structure in interactive applications and its versatility.