Position-based Dynamics Research Articles

Real-time dissection of organs via hybrid coupling of geometric metaballs and physics-centric mesh-free method

This paper systematically describes a real-time dissection approach for digital organs by strong coupling of geometric metaballs and physically correct mesh-free method. For organ geometry, we employ a novel hybrid model comprising both inner metaballs and high-resolution surface mesh with texture information. Through the use of metaballs, the organ interior is geometrically simplified via a set of overlapping spheres with different radii. As for digital organ’s physical representation, we systematically articulate a hybrid framework to interlink metaballs with physics-driven mesh-free method based on moving least squares (MLS) shape functions. MLS approach enables the direct and rapid transition from metaball geometry to local nodal formulations, which afford potential-energy-correct physical modeling and simulation over continuum domain with physical accuracy. For soft tissue dissection, the nature of our MLS-driven mesh-free method also facilitates adaptive topology modification and cutting surface reconstruction. To expedite simulation towards real-time performance, at the numerical level, we resort to position-based dynamics to simplify physical deformation to drive metaballs participating in the mesh-free formulation. Since nodal points participating in the physical process exist temporarily only in localized regions adjacent to the cutting path, our method could warrant accurate cutting surface without sacrificing real-time computational efficiency. This hybrid dissection technique has already been integrated into a VR-based laparoscopic surgery simulator with a haptic interface.

Soft tissue deformation for surgical simulation: a position-based dynamics approach

PurposeTo assist the rehearsal and planning of robot-assisted partial nephrectomy, a real-time simulation platform is presented that allows surgeons to visualise and interact with rapidly constructed patient-specific biomechanical models of the anatomical regions of interest. Coupled to a framework for volumetric deformation, the platform furthermore simulates intracorporeal 2D ultrasound image acquisition, using preoperative imaging as the data source. This not only facilitates the planning of optimal transducer trajectories and viewpoints, but can also act as a validation context for manually operated freehand 3D acquisitions and reconstructions.MethodsThe simulation platform was implemented within the GPU-accelerated NVIDIA FleX position-based dynamics framework. In order to validate the model and determine material properties and other simulation parameter values, a porcine kidney with embedded fiducial beads was CT-scanned and segmented. Acquisitions for the rest position and three different levels of probe-induced deformation were collected. Optimal values of the cluster stiffness coefficients were determined for a range of different particle radii, where the objective function comprised the mean distance error between real and simulated fiducial positions over the sequence of deformations.ResultsThe mean fiducial error at each deformation stage was found to be compatible with the level of ultrasound probe calibration error typically observed in clinical practice. Furthermore, the simulation exhibited unconditional stability on account of its use of clustered shape-matching constraints.ConclusionsA novel position-based dynamics implementation of soft tissue deformation has been shown to facilitate several desirable simulation characteristics: real-time performance, unconditional stability, rapid model construction enabling patient-specific behaviour and accuracy with respect to reference CT images.Electronic supplementary materialThe online version of this article (doi:10.1007/s11548-016-1373-8) contains supplementary material, which is available to authorized users.

A chebyshev semi-iterative approach for accelerating projective and position-based dynamics

In this paper, we study the use of the Chebyshev semi-iterative approach in projective and position-based dynamics. Although projective dynamics is fundamentally nonlinear, its convergence behavior is similar to that of an iterative method solving a linear system. Because of that, we can estimate the "spectral radius" and use it in the Chebyshev approach to accelerate the convergence by at least one order of magnitude, when the global step is handled by the direct solver, the Jacobi solver, or even the Gauss-Seidel solver. Our experiment shows that the combination of the Chebyshev approach and the direct solver runs fastest on CPU, while the combination of the Chebyshev approach and the Jacobi solver outperforms any other combination on GPU, as it is highly compatible with parallel computing. Our experiment further shows position-based dynamics can be accelerated by the Chebyshev approach as well, although the effect is less obvious for tetrahedral meshes. The whole approach is simple, fast, effective, GPU-friendly, and has a small memory cost.

Fast multiple-fluid simulation using Helmholtz free energy

Multiple-fluid interaction is an interesting and common visual phenomenon we often observe. In this paper, we present an energy-based Lagrangian method that expands the capability of existing multiple-fluid methods to handle various phenomena, such as extraction, partial dissolution, etc. Based on our user-adjusted Helmholtz free energy functions, the simulated fluid evolves from high-energy states to low-energy states, allowing flexible capture of various mixing and unmixing processes. We also extend the original Cahn-Hilliard equation to be better able to simulate complex fluid-fluid interaction and rich visual phenomena such as motion-related mixing and position based pattern. Our approach is easily integrated with existing state-of-the-art smooth particle hydrodynamic (SPH) solvers and can be further implemented on top of the position based dynamics (PBD) method, improving the stability and incompressibility of the fluid during Lagrangian simulation under large time steps. Performance analysis shows that our method is at least 4 times faster than the state-of-the-art multiple-fluid method. Examples are provided to demonstrate the new capability and effectiveness of our approach.

Algorithm of position-based dynamics and cutting simulation for soft tissue using tetrahedral mesh

In order to improve the realism and real-time performance of virtual surgery technique,a cutting algorithm for soft tissue using tetrahedral mesh and position-based dynamics was proposed. Firstly,the tetrahedral mesh was chosen as the geometrical model for soft tissue,and position-based dynamics were employed as the physical model. The external surface of soft tissue was formed by the convex hull of the tetrahedral mesh. Secondly,to support the cutting simulation,we also modified the position-based dynamics method to ensure it worked well when the topology of soft tissue model changed. Moreover,the tetrahedral mesh coupled with texture information was applied for the highly-detailed incision exposure. Finally,the haptic rendering was implemented on this dissection algorithm to enhance the realism of the surgery simulation environment.The experimental result shows that the simulator works effectively and stably with this physical model,when deformation and dissection occurs. Moreover,the algorithm has been used in the virtual surgery simulator designed by State Key Laboratory of Virtual Reality Technology and Systems in Beihang University. The laparoscopic surgeons from cooperative hospitals have given a pilot study and high evaluation has been given from the doctors for this simulator.

Metaballs-based physical modeling and deformation of organs for virtual surgery

Prior research on metaballs-based modeling solely focuses on shape geometry and its processing for organic objects. This paper takes a different approach by exploring a new metaballs-based physical modeling method for digital organs that are imperative to support virtual surgery. We propose a novel hybrid physical model comprising both surface mesh and the metaballs which occupy organs' interior. The finer surface mesh with high-precision geometric information and texture is necessary to represent the boundary structure of organs. Through the use of metaballs, the organ interior is geometrically simplified via a set of overlapping spheres with different radii. This work's novelty hinges upon the integration of metaballs and position-based dynamics (PBD) which enables metaballs-based organs to serve as physical models and participate in dynamic simulation. For the metaballs construction, we develop an adaptive approach based on Voronoi Diagram for model initialization. Using global optimization, an electrostatic attraction model is proposed to drive the metaballs to best match with the organ's boundary. Using PBD, we devise a novel metaballs-based deformation algorithm, which preserves two local shape properties via constraints on Laplacian coordinates and local volume. To retain the organ's smooth deformation, we propose a new skinning method based on distance field, and it is employed to build the mapping between the metaballs and organ boundary. This metaballs-based deformation technique has already been integrated into a VR-based laparoscopic surgery simulator.

Position‐Based Skinning for Soft Articulated Characters

AbstractIn this paper, we introduce a two‐layered approach addressing the problem of creating believable mesh‐based skin deformation. For each frame, the skin is first deformed with a classic linear blend skinning approach, which usually leads to unsightly artefacts such as the well‐known candy‐wrapper effect and volume loss. Then we enforce some geometric constraints which displace the positions of the vertices to mimic the behaviour of the skin and achieve effects like volume preservation and jiggling. We allow the artist to control the amount of jiggling and the area of the skin affected by it. The geometric constraints are solved using a position‐based dynamics (PBDs) schema. We employ a graph colouring algorithm for parallelizing the computation of the constraints. Being based on PBDs guarantees efficiency and real‐time performances while enduring robustness and unconditional stability. We demonstrate the visual quality and the performance of our approach with a variety of skeleton‐driven soft body characters.

Volume preserving viscoelastic fluids with large deformations using position-based velocity corrections

We propose a particle-based hybrid method for simulating volume preserving viscoelastic fluids with large deformations. Our method combines smoothed particle hydrodynamics (SPH) and position-based dynamics (PBD) to approximate the dynamics of viscoelastic fluids. While preserving their volumes using SPH, we exploit an idea of PBD and correct particle velocities for viscoelastic effects not to negatively affect volume preservation of materials. To correct particle velocities and simulate viscoelastic fluids, we use connections between particles which are adaptively generated and deleted based on the positional relations of the particles. Additionally, we weaken the effect of velocity corrections to address plastic deformations of materials. For one-way and two-way fluid-solid coupling, we incorporate solid boundary particles into our algorithm. Several examples demonstrate that our hybrid method can sufficiently preserve fluid volumes and robustly and plausibly generate a variety of viscoelastic behaviors, such as splitting and merging, large deformations, and Barus effect.

Unified particle physics for real-time applications

We present a unified dynamics framework for real-time visual effects. Using particles connected by constraints as our fundamental building block allows us to treat contact and collisions in a unified manner, and we show how this representation is flexible enough to model gases, liquids, deformable solids, rigid bodies and cloth with two-way interactions. We address some common problems with traditional particle-based methods and describe a parallel constraint solver based on position-based dynamics that is efficient enough for real-time applications.

Fast simulation of viscous fluids with elasticity and thermal conductivity using position-based dynamics

Viscous fluids are ubiquitous, and reproducing their damped motions has been in demand for many applications. The most prevalent approach to simulating viscous fluids is based on the Navier–Stokes equations and necessitates viscosity integration. However, to simulate viscous fluids in a numerically stable manner, using explicit viscosity integration severely restricts time steps and requires an excessively long period for computation. In this paper, we propose a novel particle-based Lagrangian method for efficiently simulating viscous fluids by adopting position-based constraints. Our method uses the geometric configuration of particles for the positional constraints to approximate the dynamics of viscous fluids using position-based dynamics; thus the method can plausibly generate their motions while allowing for the use of much larger time steps than those previously adopted in the viscous fluid simulations. We also propose an associated boundary-handling scheme for position-based fluids to precisely specify boundary conditions for the constraints. Additionally, we reproduce elastic deformations of materials by controlling the constraints and incorporate thermal conduction into our framework to simulate resultant changes in particle properties and phase transition in the materials. By adjusting parameters, our method can encompass complex motions of fluids with different properties in a unified framework. Several examples demonstrate the effectiveness as well as versatility of our method.

A GPU-Based Implementation of Position Based Dynamics for Interactive Deformable Bodies

Position Based Dynamics (PBD) is a popular approach used for animating constrained particle systems representing soft bodies, rigid bodies, and fluids. In this article, we present a massively parallel implementation of PBD for fast, interactive animation of deformable bodies. We divide the set of constraints in independent partitions using a fast, greedy coloring graph algorithm. Then, during the animation, the constraints belonging to each partition are solved in parallel on the GPU. We employ an efficient simulation pipeline using a memory layout that favors both the memory access time for computation and batching for visualization. Our experiments show that the achieved performance speed-up is several orders of magnitude faster than its serial counterpart.

Editorial Issue 24.1

This issue contains five regular papers. Ahmad Abdul Karim, Thibaut Gaudin, Alexandre Meyer, Axel Buendia and Saïda Bouakaz, from Université Lyon 1 and Spir.Ops in France, present a fully procedural method capable of generating in real time a wide range of locomotion for multi-legged characters in a dynamic environment, without using any motion data. Their system consists of several independent blocks: a character controller, a gait/tempo manager, a 3D path constructor and a footprints planner. The four modules work cooperatively to calculate in real time the footprints and the 3D trajectories of the feet and the pelvis. The system can animate dozens of creatures using dedicated level of detail techniques and is totally controllable allowing the user to design a multitude of locomotion styles through a user-friendly interface. In the second paper, Amit Kumar and Aparajita Ojha, from PDPM Indian Institute of Information Technology in Jabalpur, in India, propose a new approach to natural path planning by adding wavelet noise to a path generated using subdivision-based corridor map method. Because wavelet noise is almost perfectly band-limited and provides good details with minimal aliasing effects, the resulting path becomes smoother and more natural. Moreover, by appropriately choosing the levels of down/up sampling in the wavelet noise generation algorithm, frequency of wavelet noise can be adjusted. This serves as an effective tool in bringing variations in path as per the requirement. The third paper by Yanzhen Wang, Yueshan Xiong, Kai Xu and Dong Liu, from National University of Defense Technology in Changsha, Hunan, in China, describes a surgical procedure simulation system for the training of arthroscopic ACL reconstruction involving operations such as puncturing, probing, incision and drilling. In this system, the authors employ a linear elastic finite element method and position-based dynamics for deformable modelling. Simplified vertex duplicating method and an implementation of real-time Boolean operations are proposed for the topological change of tissue models involved in the incision simulation and tunnel construction. Two specially designed force-feedback models are introduced for the haptic rendering of probing and drilling operations. In the fourth paper by Siddharth Hegde, Christos Gatzidis and Feng Tian, from Bournemouth University, in UK, look at the different methods presented over the past few decades that attempted to recreate digital paintings. Whereas previous surveys concentrate on the broader subject of non-photorealistic rendering, the focus of this paper is firmly placed on painterly rendering techniques. The authors compare different methods used to produce different output painting styles such as abstract, colour pencil, watercolour, oriental, oil and pastel. Although some methods demand a high level of interaction using a skilled artist, others require simple parameters provided by a user with little or no artistic experience. The last paper by Llyr ap Cenydd and Bill Teahan, from Bangor University, in UK, describes a system for dynamically animating the locomotive behaviour of arthropods in real time, facilitating realistic and autonomous traversal across an arbitrary environment. By combining a de-centralized reactive behavioural model with a hybrid approach to motion that utilizes the comparative advantages of physical simulation and kinematic control, the system is capable of automatically generating complex organic motion over a wide range of surface features, independent of structural complexity. The reactive embodiment of the creature, combined with the physical simulation of the virtual world enables emergent behaviours to form that are entirely based on circumstance, including rigid-body interaction, grip recovery and adaptive wall climbing.