A fast level set framework for large three-dimensional topography simulations

We present fast methods to describe the surface evolution of large three-dimensional structures. Based on the sparse field level set method and the hierarchical run-length encoding level set data structure optimal figures for the computation time and for the memory consumption are achieved. Furthermore, we introduce a new multi-level-set technique, which is able to incorporate multiple material regions, and which can also handle material specific surface speeds accurately. We also describe an optimal algorithm for the visibility check for unidirectional etching. The presented techniques are demonstrated on various examples.     

Block-structured grids in Lagrangian 3D edge plasma transport simulations

Distinct from conventional Eulerian 2D fluid solvers, applied routinely to magnetic fusion edge plasma studies, complex 3D magnetic topologies are currently treated by the geometrically more flexible Lagrangian schemes, supplemented by Monte Carlo procedures for higher order derivatives (dissipative terms due to diffusion processes) and sources. These particle based algorithms are combined with a field line reconstruction technique for dealing with partially ergodic magnetic fields, involving field aligned regular grids. A generalization to block-structured grids is carried out, which greatly enhances the applicability range of present 3D fusion plasma edge codes, in particular also to poloidally magnetic diverted configurations, as currently envisaged for the largest magnetic fusion device under construction: ITER.     

A level set embedded interface method for conjugate heat transfer simulations of low speed 2D flows

This study examines the use of a level set based embedded interface method to simulate fluid-solid heat transfer processes using Cartesian grids. The flow field is described by the incompressible 2D Navier-Stokes equations using a vorticity-streamfunction approach. A fluid-solid coupling formulation for the thermal and momentum fields is developed that is robust, computationally efficient and second-order accurate. Solutions for several example problems are presented for flow over stationary and moving cylinders to bench mark the current approach. Heat transfer for an isolated cylinder and two cylinders in series are then examined to explore the Nusselt number dependence on cylinder spacing and unsteady conjugate heat transfer processes.     

一种使用水平集的中轴提取算法

在岩心三维模型中,中轴是描述孔隙结构特征的一种重要表示方法。针对现有的拓扑细化和距离变换方法存在的中轴提取不准确和不连续的问题,提出了一种使用快速行进水平集方法进行距离变换的三维中轴提取算法。对比实验结果表明,该算法中提取的中轴在准确性和连续性上有着较好的保证,实际应用中效果良好。     

3D level set method for blastomere segmentation of preimplantation embryos in fluorescence microscopy images

Detailed and accurate characteristics of preimplantation embryos are fundamental for a deep understanding of their development. Recent studies indicate that various geometric features of cells, such as size, shape, volume, and position play a significant role in embryo growth. However, a quantitative assessment of these characteristics first needs a segmentation of the individual cells. The manual separation and labeling of cells is extremely inefficient, and an automated approach is highly desirable. This paper presents an automatic method for early stage embryo segmentation into its constituent cells and membranes using three-dimensional (3D) data. The input data consist of two Z-stacks of fluorescence microscope images containing nuclei and membranes. The method uses a 3D level set segmentation algorithm. Its evaluation is based on a dataset composed of 20 mouse embryos, each with 4–32 blastomeres. Segmentation accuracy was evaluated by calculating F-scores with ground truth obtained by manually labeling desired regions. We also compared output of our method with the one acquired with a watershed algorithm. The proposed approach was able to achieve more than \(90\%\) accuracy for embryos with 4 and 8 cells, while for embryos with higher number of cells it was lower, reaching \(75\%\) for 32-cell embryo.     

Partitioning 3D space for parallel many-particle simulations

In a common approach for parallel processing applied to simulations of many-particle systems with short-ranged interactions and uniform density, the cubic simulation box is partitioned into domains of equal shape and size, each of which is assigned to one processor. We compare the commonly used simple-cubic (SC) domain shape to domain shapes chosen as the Voronoi cells of BCC, FCC, and HCP sphere packings. The latter three are found to result in superior partitionings with respect to communication overhead. Scaling of the domain shape is used to extend the range of applicability of these partitionings to a large set of processor numbers. The higher efficiency with BCC and FCC partitionings is demonstrated in simulations of the sillium model for amorphous silicon.     

Application of the level set method for the visual representation of continuous cellular automata oriented to anisotropic wet etching

Atomistic models are a very valuable simulation tool in the field of material science. Among them are the continuous cellular automata (CCA), which can simulate accurately the process of chemical etching used in micro-electro-mechanical-systems (MEMS) micromachining. Due to the CCA intrinsic atomistic nature, simulation results are obtained in the form of a cloud of points, so data visualization has been usually problematic. When using these models as a part of a computer aided design tool, good data visualization is very important. In this paper, a minimum energy model implemented with the level set (LS) method for improving the visual representation of simulated MEMS is presented. Additionally, the sparse field method has been applied to reduce the high computational cost of the original LS. Finally, some reconstructed surfaces with completely different topologies are presented, proving the effectiveness of our implementation and the fact that it is capable of producing any real surface, flat and smooth ones.     

A level set method for oil slick segmentation in SAR images

Despite much effort and significant progress in recent years, image segmentation remains a challenging problem in image processing, especially for the low contrast, noisy synthetic aperture radar (SAR) images. This paper explores the segmentation of oil slicks using a partial differential equation (PDE)‐based level set method, which represents the slick surface as an implicit propagation interface. Starting from an initial estimation with priori information, the level set method creates a set of speed functions to detect the position of the propagation interface. Specifically, the image intensity gradient and the curvature flow are utilized together to determine the speed and direction of the propagation. This allows the front interface to propagate naturally with topological changes, significant protrusions and narrow regions, giving rise to stable and smooth boundaries that discriminate oil slicks from the surrounding water. As the speckles are removed concurrently while the front interface propagates, the pre‐filtering of noise is saved. The proposed method has been illustrated by experiments on oil slick segmentation using the ERS‐2 SAR images. Its advantages over the traditional image segmentation approaches have also been demonstrated.     

A novel vision-based method for 3D profile extraction of wire harness in robotized assembly process

Automating stages for deformable objects in the production line, in which assembling a wire harness into a predefined position is a complex task owing to the specialized characteristics of the objects. Besides a few automatized systems proposed in the other studies to implement this task under simplified setup conditions, a significant portion of this process remains to be completed manually in industrial environments. To construct an automatic wire harness assembly system, the development of a method that can automatically detect the wire harness profile in a 3D environment and, consequently, guide robot arms to implement assembly tasks is indispensable. Therefore, this study presents an approach that satisfies this requirement, which not only proposes a deep learning-based system to detect the wire profile, but also improves the accuracy of the detected results through a correction method according to the depth values of contiguous areas. The verification of the approach in a robot system that highlights its usefulness and practicality demonstrates the potential of the proposed method to replace people and consequently, reduce labour costs in factory environments.     

三维可视化建模方法在位场模拟中的应用

根据重磁位场正演模拟的要求,提出了一种基于截面的三维地质体可视化建模方法。该方法利用改进的最短对角线法构建截面间的地质体表面,改进的最短对角线法弥补了最短对角线法不能有效寻找起始点的缺陷,并能够处理任意角点数的截面之间的地质体表面构建。将其应用于重磁位场正演模拟可以使可视化数据结构和计算数据结构一致,简化了算法并提高了计算精度,因而提出的方法是一种有效实用的方法。     

Multipoint potential field method for path planning of autonomous underwater vehicles in 3D space

A multipoint potential field method (MPPF) for path planning of autonomous underwater vehicles (AUV) in 3D space is presented in this paper. The algorithm is developed based on potential field method by incorporating a directed search method for sampling the potential field. In this approach, the analytical gradient of the total potential function is not computed, as it is not essentially required for moving the vehicle to the next position. Rather, a hemispherical region in the direction of motion around the AUV’s bow is discretized into equiangular points with center as the current position. By determining the point at which the minimum potential exists, the vehicle can be moved towards that point in 3D space. This method is very simple and applicable for real-time implementation. The problem of local minima is also analyzed and found that the local minima in 2D space can be easily overcome with the MPPF. A simple strategy to avoid the local minima in 3D space is also proposed. The proposed method reduces the burden of fine-tuning the positive scaling factors of potential functions to avoid local minimum. The algorithm development and the simulation results are presented.     

Grid_strain and grid_strain3: Software packages for strain field computation in 2D and 3D environments

Two Matlab™ software packages for strain field computation, starting from displacements of experimental points (EPs), are here presented. In particular, grid_strain estimates the strain on the nodes of a regular planar grid, whereas grid_strain3 operates on the points of a digital terrain model (DTM). In both cases, the computations are performed in a modified least-square approach, emphasizing the effects of nearest points. This approach allows users to operate at different scales of analysis by introducing a scale factor to reduce or also exclude points too far from grid nodes. The input data are displacements (or velocities) that can be provided by several techniques (e.g. GPS, total topographical station, terrestrial laser scanner). The analysis can be applied to both regional- and local-scale phenomena, to study tectonic crustal deformations (strain ≈10⁻⁸−10⁻⁶) or rapid landslide collapses (10⁻⁴−10²), and to characterize the kinematics of the studied system. Errors on strains and geometric significance of the results are also provided.     

VENUS-LEVIS and its spline-Fourier interpolation of 3D toroidal magnetic field representation for guiding-centre and full-orbit simulations of charged energetic particles

Curvilinear guiding-centre drift and full-orbit equations of motion are presented as implemented in the VENUS-LEVIS code. A dedicated interpolation scheme based on Fourier reconstruction in the toroidal and poloidal directions and cubic spline in the radial direction of flux coordinate systems is detailed. This interpolation method exactly preserves the order of the RK4 integrating scheme which is crucial for the investigation of fast particle trajectories in 3D magnetic structures such as helical saturated tokamak plasma states, stellarator geometry and resonant magnetic perturbations (RMP). The initialisation of particles with respect to the guiding-centre is discussed. Two approaches to implement RMPs in orbit simulations are presented, one where the vacuum field is added to the 2D equilibrium, creating islands and stochastic regions, the other considering 3D nested flux-surfaces equilibrium including the RMPs.     

一种有效的三维人脸恒定识别特征构造方法*

为消除表情变化对人脸识别结果的影响,构造相对恒定的人脸识别特征,提出了一种恒定特征构造方法：在人脸曲面上分割出基本不受表情变化影响的区域;对该区域原始数据进行统一规格的二次采样,保证数据精度的一致性;分析特征区域的数据,得到原始特征向量;变换、组合特征向量,构造此区域的恒定特征向量;将此特征用于人脸的分类识别,以克服表情变化的影响。通过在实际三维人脸数据库上的实验,验证了该方法的有效性。     

基于立体视觉的人脸三维空间位置定位方法*

针对基于姿势的自然人机交互接口设计中头部定位问题,提出了一种快速的人脸三维空间位置定位算法。首先采用级联肤色分类器与类Haar特征分类器的人脸检测器对左右路输入的图像进行快速的人脸区域定位;然后利用仿射模型匹配局部收敛性好、速度快的特点实现了左右路图像的人脸区域对齐;最后通过立体视觉原理恢复人脸的三维空间坐标。实验结果证明,提出的人脸三维空间位置定位方法速度快、定位精度较高。     

Path planning in complex 3D environments using a probabilistic roadmap method

This paper presents a 3D path planning algorithm for an unmanned aerial vehicle （UAV） in complex environments. In this algorithm, the environments are divided into voxels by octree algorithm. In order to satisfy the safety requirement of the UAV, free space is represented by free voxels, which have enough space margin for the UAV to pass through. A bounding box array is created in the whole 3D space to evaluate the free voxel connectivity. The probabilistic roadmap method （PRM） is improved by random sampling in the bounding box array to ensure a more efficient distribution of roadmap nodes in 3D space. According to the connectivity evaluation, the roadmap is used to plan a feasible path by using A＊ algorithm. Experimental results indicate that the proposed algorithm is valid in complex 3D environments.