Mesh Creation Research Articles

Transfer learning and performance enhancement techniques for deep semantic segmentation of built heritage point clouds

<p class="VARAbstract">The growing availability of three-dimensional (3D) data, such as point clouds, coming from Light Detection and Ranging (LiDAR), Mobile Mapping Systems (MMSs) or Unmanned Aerial Vehicles (UAVs), provides the opportunity to rapidly generate 3D models to support the restoration, conservation, and safeguarding activities of cultural heritage (CH). The so-called scan-to-BIM process can, in fact, benefit from such data, and they can themselves be a source for further analyses or activities on the archaeological and built heritage. There are several ways to exploit this type of data, such as Historic Building Information Modelling (HBIM), mesh creation, rasterisation, classification, and semantic segmentation. The latter, referring to point clouds, is a trending topic not only in the CH domain but also in other fields like autonomous navigation, medicine or retail. Precisely in these sectors, the task of semantic segmentation has been mainly exploited and developed with artificial intelligence techniques. In particular, machine learning (ML) algorithms, and their deep learning (DL) subset, are increasingly applied and have established a solid state-of-the-art in the last half-decade. However, applications of DL techniques on heritage point clouds are still scarce; therefore, we propose to tackle this framework within the built heritage field. Starting from some previous tests with the Dynamic Graph Convolutional Neural Network (DGCNN), in this contribution close attention is paid to: i) the investigation of fine-tuned models, used as a transfer learning technique, ii) the combination of external classifiers, such as Random Forest (RF), with the artificial neural network, and iii) the evaluation of the data augmentation results for the domain-specific ArCH dataset. Finally, after taking into account the main advantages and criticalities, considerations are made on the possibility to profit by this methodology also for non-programming or domain experts.</p><p>Highlights:</p><ul><li><p>Semantic segmentation of built heritage point clouds through deep neural networks can provide performances comparable to those of more consolidated state-of-the-art ML classifiers.</p></li><li><p>Transfer learning approaches, as fine-tuning, can considerably reduce computational time also for CH domain-specific datasets, as well as improve metrics for some challenging categories (i.e. windows or mouldings).</p></li><li><p>Data augmentation techniques do not significantly improve overall performances.</p></li></ul>

Robust Texture Mapping Using RGB-D Cameras.

The creation of a textured 3D mesh from a set of RGD-D images often results in textured meshes that yield unappealing visual artifacts. The main cause is the misalignments between the RGB-D images due to inaccurate camera pose estimations. While there are many works that focus on improving those estimates, the fact is that this is a cumbersome problem, in particular due to the accumulation of pose estimation errors. In this work, we conjecture that camera poses estimation methodologies will always display non-neglectable errors. Hence, the need for more robust texture mapping methodologies, capable of producing quality textures even in considerable camera misalignments scenarios. To this end, we argue that use of the depth data from RGB-D images can be an invaluable help to confer such robustness to the texture mapping process. Results show that the complete texture mapping procedure proposed in this paper is able to significantly improve the quality of the produced textured 3D meshes.

Creating the finite element mesh of non-periodic masonry from the measurement of its geometrical characteristics: a novel automated procedure

This paper presents an automated procedure that enables the creation of a finite element mesh directly from the image file representing the rasterized sketch of a generic masonry element. This procedure goes under the name “pixel strategy” if a 2D finite element mesh is needed, where the elements are planar and rectangular; conversely, its extension in the 3D case is named “voxel strategy”, and there the resulting finite elements are solid bricks. The finite element meshes so obtained are then used for extracting homogenized in-plane failure surfaces for historical masonry cells, which display a non-periodic arrangement of units. These surfaces are consistent with the expected results, and their shapes suggest that the behavior of such type of masonry may range between orthotropic (if bed mortar joints are clearly noticeable) and quasi-isotropic (if some units spread over two or more masonry layers).

Automatic Crack Tip Meshing Approach for Constraint-Based Fracture Mechanics Application

Finite element analysis is a popular method used to characterize crack tip stress and strain fields in fracture mechanics problems. In a constraint-based elastic-plastic fracture mechanics, the crack tip stress and strains are essentially dependent on the accurate crack tip finite element configuration. Typical finite element software does not provide control of node and elemental numbering order and sequence which caused difficulty to examine different crack configuration state of stress–strain using automated post-processing codes. A method to automate the creation of finite element mesh of arbitrary lengths and shapes of standard and nonstandard fracture mechanics geometries using a geometric progression technique is described. A list of equations was proposed to develop nodal and elemental model data that represented a defined region of a cracked geometry. Assembly of the regions produced a finite element mesh representing a fracture mechanics cracked geometry which can be incorporated into a typical finite element program. The approach can eliminate the modeling processes of finite element of cracked specimens and offers the ability to control the nodal and elemental numbering system to allow systematic stress and strain elucidation for large degrees of freedom mesh and history-dependent incremental plasticity crack tip analysis. The approach used to develop the finite element mesh was discussed, and examples of the finite element mesh generated by the approach were compared to established two-parameter elastic-plastic fracture mechanics results. Finally, the algorithm codes used to develop the cracked models are supplied as supplementary files to interested users.

A semi-automated finite difference mesh creation method for use with immersed boundary software IB2d and IBAMR

Numerous fluid-structure interaction problems in biology have been investigated using the immersed boundary method. The advantage of this method is that complex geometries, e.g., internal or external morphology, can easily be handled without the need to generate matching grids for both the fluid and the structure. Consequently, the difficulty of modeling the structure lies often in discretizing the boundary of the complex geometry (morphology). Both commercial and open source mesh generators for finite element methods have long been established; however, the traditional immersed boundary method is based on a finite difference discretization of the structure. Here we present a software library for obtaining finite difference discretizations of boundaries for direct use in the 2D immersed boundary method. This library provides tools for extracting such boundaries as discrete mesh points from digital images. We give several examples of how the method can be applied that include passing flow through the veins of insect wings, within lymphatic capillaries, and around starfish using open-source immersed boundary software.

Stress analysis of the discs of axial-flow microturbines

The article discusses the mesh creation techniques for models of discs of axial-flow microturbines. A universal method of optimization of such devices, in terms of their strength improvement, has been proposed. The research focused on microturbines that can operate in combination with ORC systems, especially the ones whose discs have many structural components such as pins or chamfers. Calculations were done using the commercial software ANSYS Workbench. Both tetrahedral and hexahedral grids were used in the analysed models. The calculation time needed for the grid preparation was regarded as an important parameter. Therefore, the reference model was created using the disc slice method. The results obtained for the models that included the full complex geometry of the disc were compared with the results obtained for the reference model. The mesh size coefficient was defined. It enabled to simplify the strength optimisation method for discs of axial-flow microturbine and also made it more universal. After carrying out all analyses and computations, it was possible to develop a scheme of conduct during the optimization of the aforementioned expansion devices.

Mesh generation for thin layered domains and its application to parallel multigrid simulation of groundwater flow

The generation of detailed three dimensional meshes for the simulation of groundwater flow in thin layered domains is crucial to capture important properties of the underlying domains and to reach a satisfying accuracy. At the same time, this level of detail poses high demands both on suitable hardware and numerical solver efficiency. Parallel multigrid methods have been shown to exhibit near optimal weak scalability for massively parallel computations of density driven flow. A fully automated parameterized algorithm for prism based meshing of coarse grids from height data of individual layers is presented. Special structures like pinch outs of individual layers are preserved. The resulting grid is used as a starting point for parallel mesh and hierarchy creation through interweaved projected refinement and redistribution. Efficiency and applicability of the proposed approach are demonstrated for a parallel multigrid based simulation of a realistic sample problem.

Fully-automated adaptive mesh refinement for media embedding complex heterogeneities: application to poroelastic fluid pressure diffusion

Relating the attenuation and velocity dispersion of seismic waves to fluid pressure diffusion (FDP) by means of numerical simulations is essential for constraining the mechanical and hydraulic properties of heterogeneous porous rocks. This, in turn, is of significant importance for a wide range of prominent applications throughout the Earth, environmental, and engineering sciences, such as, for example, geothermal energy production, hydrocarbon exploration, nuclear waste disposal, and CO2 storage. In order to assess the effects of wave-induced FDP in heterogeneous porous rocks, we simulate time-harmonic oscillatory tests based on a finite element (FE) discretization of Biot’s equations in the time-frequency domain for representative elementary volumes (REVs) of the considered rock masses. The major challenge for these types of simulations is the creation of adequate computational meshes, which resolve the numerous and complex interfaces between the heterogeneities and the embedding background. To this end, we have developed a novel method based on adaptive mesh refinement (AMR), which allows for the fully automatic creation of meshes for strongly heterogenous media. The key concept of the proposed method is to start from an initially uniform coarse mesh and then to gradually refine elements which have non-empty overlaps with the embedded heterogeneities. This results in a hierarchy of non-uniform meshes with a large number of elements close to the interfaces, which do, however, not need to be explicitly resolved. This dramatically simplifies and accelerates the laborious and time-consuming process of meshing strongly heterogeneous poroelastic media, thus enabling the efficient simulation of REVs containing heterogeneities of quasi-arbitrary complexity. After a detailed description of the methodological foundations, we proceed to demonstrate that the FE discretization with low-order FE has a unique solution and hence does not present spurious modes. We assess the practical effectiveness and accuracy of the proposed method by means of four case studies of increasing complexity.

On the estimation of unsteady aerodynamic forces and wall spectral content with immersed boundary conditions

Immersed boundary conditions (IBC) have become a practical tool to simplify the meshing process for the simulation of complex geometries in CFD. This approach has reached a sufficient level of maturity to allow the simulation of compressible high Reynolds number flows. However, the access of physical quantities at the immersed wall is far from being straightforward. This paper provides two methods for the reconstruction of fluctuating wall quantities relying on the creation of explicit watertight surface meshes of the immersed boundary. These surface meshes are used for the investigation of highly unsteady compressible flows of two generic space launcher afterbody configurations using Zonal Detached Eddy Simulation (ZDES). Since the flows are massively separated, the side load arising from the pressure is mainly responsible for the global load unsteadiness. Therefore, in the present study the focus is put on the accuracy of the wall pressure reconstructed on immersed boundaries and compared to validated numerical simulations using a classical body-fitted approach and experimental data. The numerical results demonstrate the ability of the present approaches to accurately capture the global load fluctuation around both afterbody configurations. Moreover, the IBC surface meshes simplify the overall post-processing operations and allow the extraction of wall quantities for unsteady simulation at low computational cost. This last feature has been used for the spectral analysis on IBC surfaces which reproduced successfully the location and the intensity of the pressure fluctuation.

Non-invasive methods: The applicability of unmanned aerial vehicle (UAV) technology for recording fossilised footprints

Hominin fossilised trackways are commonly used to reconstruct locomotory behaviour and to characterise track-maker biometrics. They are the most direct representation of hominin locomotion available, yet the fossil sites are susceptible to extreme cases of erosion and weathering, often resulting in the destruction of the fossil beds, as was the case at two prehistoric footprint sites: Formby Point and Happisburgh, UK. Footprints can also be found in easily deformable sediments where the excavator can unintentionally trample and destroy the prints, as occurred at Formby Point, UK. There is the need for non-destructive methods to be identified that can rapidly record the fossil area before further damage can occur to the fossil beds. Because current methods of 3D data capture (laser scanning or photogrammetry) can be invasive (i.e., the excavator is often required to trample the sediment layer during data capture), non-invasive methods need to be explored to circumvent issues in advertently destroying fossil material.In this project, a series of experiments using non-invasive methods tested the applicability of Unmanned Aerial Vehicle (UAV) technology combined with photogrammetry to rapidly and accurately record footprints before further damage to the fossil interface occurred. Various flight paths, UAVs, cameras and capture types (video versus camera stills) were incorporated to test the accuracy in minute depth reconstruction and subsequent 3D mesh creation of small, intricate detail.Shape and size, the consequence of poorly reconstructed depth dimensionality of surfaces, were found to be affected by flight path and by the height of the UAV, indicating that the most optimal method of recording tracks is to use a handheld DSLR camera following a circular or rastered flight path. Despite attempts to utilise UAV technology, inaccurate reconstructions of footprint topography were produced.Although UAV technology produced unreliable reconstructions, UAVs remain a technological solution when sites may be at immediate risk of destruction, as was the case at Happisburgh, UK. The deployment of UAV technology will permit the digital preservation of fossil material which would otherwise be lost.

Enhanced Power and Ground Mesh Structure for IR-Drop Reduction

New enhanced power/ground mesh structure is presented for voltage drop (IR drop) reduction on power distribution network in integrated circuits (ICs). The new structure focuses on the length of the overall power/ground mesh wire pattern, reducing it by using a complex dotted line wire pattern instead of the usual mesh straps, which brings to wire length reduction on almost 40%, while via count on changed metal layer remained the same. Measurements of the voltage drop prove that with proposed mesh structure IR drop reduced about 76% for power net compared to standard mesh structure but runtime for mesh creation is increased due to more complex patterns. Proposed structure can be used in advanced technology nodes where IR drop phenomenon has become critical for performance of the ICs.

Results of Studying the Yield Strength of the Spatial Structure Joint Using CAE Systems

Numerical experiments of a of the spatial structure joint are presented. Studied the spatial structure joint created by the authors. The joint of spatial structure is protected by a patent of the Russian Federation. The studies were performed in four software systems: Lira (DNDIASB, Kiev), SCAD (SCAD Group, Kiev), CosmosWorks (Structural Research & Analysis Corp., Los Angeles), ANSYS Design Space (ANSYS, USA). Calculations purpose – evaluate the performance of each settlement complex. The evaluation of each computational complex was carried out according to the model creation time, computation time, FE mesh quality and ease of use of the program complex. The results of the evaluation of the accuracy and time of creation of the FE mesh, the accuracy and time of solving the problem were obtained.

Clogging process and related pressure drops in wire-wound filters: laboratory evidence.

The present work is aimed at analyzing the performance of the commercial filters used for the mechanical filtration of suspended solid particles. In particular, it is intended to evaluate the head losses due to the presence of the filter in the plant and how these losses vary depending on the circulating flow rate, the nominal porosity of the filter, and its clogging degree. Filters with interstices of different sizes, from 1 to 50 μm, were compared for the performance analysis. The polypropylene wire, in facts, wrapped around a windowed cylinder of the same material, allows the creation of meshes with the desired degree of compactness. The experiments were carried out at the Laboratory of Environmental and Maritime Hydraulics (LIDAM) in a hydraulic circuit reproducing a domestic piping system, where different water demand scenarios were analyzed. The clogging of the cartridge was made with the aid of sand particles, attached, with the help of a natural glue, to the external surface of the filter. More than 200 measurements were carried out, which showed a clear increasing trend of the head losses at the filter as the clogging degree and the circulating flow rate increase. In all cases, the trend of the pressure drop at the filter has a more than linear trend. Also, it was observed that head losses did not exhibit a marked dependence on the nominal porosity. Best performances in terms of minor head losses were obtained by the 50-μm filter.

3D-Printed Whole Prostate Models with Tumor Hotspots Using Dual-Extruder Printer.

3D printing has emerged as a popular technology in various biomedical applications. Physical models of anatomical structures concretize the digital representations and can be used for teaching and analysis. In this study we combine 3D histology with 3D printing, creating realistic physical models of tissues with hotspots of interest. As an example we use mouse prostates containing tumors. Surface meshes are created from binary masks of HE-stained serial sections of mouse prostates and manually annotated tumor areas. Sections are interpolated to expand sparse image stacks for smoother results. Fiji, Meshlab and Tinkercad are used for mesh creation and processing. Objects are printed with Prusa-based dual-extruder printer enabling different colors for tumors and the surrounding prostate tissue. Our 3D-printed mouse prostates appear realistic and tumors located at the edges of the organ are clearly visible. When transparent filament is used, the tumor hotspots are visible even when they are inside the prostate.

Low-Cost Three-Dimensional Modeling of Crop Plants.

Plant modeling can provide a more detailed overview regarding the basis of plant development throughout the life cycle. Three-dimensional processing algorithms are rapidly expanding in plant phenotyping programmes and in decision-making for agronomic management. Several methods have already been tested, but for practical implementations the trade-off between equipment cost, computational resources needed and the fidelity and accuracy in the reconstruction of the end-details needs to be assessed and quantified. This study examined the suitability of two low-cost systems for plant reconstruction. A low-cost Structure from Motion (SfM) technique was used to create 3D models for plant crop reconstruction. In the second method, an acquisition and reconstruction algorithm using an RGB-Depth Kinect v2 sensor was tested following a similar image acquisition procedure. The information was processed to create a dense point cloud, which allowed the creation of a 3D-polygon mesh representing every scanned plant. The selected crop plants corresponded to three different crops (maize, sugar beet and sunflower) that have structural and biological differences. The parameters measured from the model were validated with ground truth data of plant height, leaf area index and plant dry biomass using regression methods. The results showed strong consistency with good correlations between the calculated values in the models and the ground truth information. Although, the values obtained were always accurately estimated, differences between the methods and among the crops were found. The SfM method showed a slightly better result with regard to the reconstruction the end-details and the accuracy of the height estimation. Although the use of the processing algorithm is relatively fast, the use of RGB-D information is faster during the creation of the 3D models. Thus, both methods demonstrated robust results and provided great potential for use in both for indoor and outdoor scenarios. Consequently, these low-cost systems for 3D modeling are suitable for several situations where there is a need for model generation and also provide a favourable time-cost relationship.

A kinematic comparison of meshfree and mesh-based Lagrangian approximations using manufactured extreme deformation fields

Meshfree methods for solid mechanics have been in development since the early 1990’s. Initial motivations included alleviation of the burden of mesh creation and the desire to overcome the limitations of traditional mesh-based discretizations for extreme deformation applications. Here, the accuracy and robustness of both meshfree and mesh-based Lagrangian discretizations are compared using manufactured extreme deformation fields. For the meshfree discretizations, both moving least squares and maximum entropy are considered. Quantitative error and convergence results are presented for the best approximation in the $$H^1$$ norm.

Numerical simulation of 2D electrodynamic problems with unstructured triangular meshes

We present a generalization of standard leap-frog plus Yee mesh approach for Cauchy problem in electrodynamics simulations on unstructured triangulated mesh. The presented approach still inherits from finite-difference time-domain and do not use techniques developed in finite-volume time-domain approach. In the paper the whole flow from mesh creation to actual simulation is presented. The proposed computation flow is parallel ready and can be implemented for distributed systems (computation servers, graphical processing units, etc.). We studied the influence of non-regular triangulation on stability and dispersion properties of numerical solution.

The Speedy CTV: Creation of an Adaptable Spine CTV Mesh to Decrease Contouring Times

The Speedy CTV: Creation of an Adaptable Spine CTV Mesh to Decrease Contouring Times

Accuracy of triangular meshes of stone models created from DICOM cone beam CT data

BackgroundThe aim of this study was to assess the theory that CBCT scanners can be used for a subsequent triangular mesh generation which accurately represents the actual stone model.Ten, recently acquired stone models, were used in the present study. The stone models were initially scanned with the Dental Wings 7Series dental scanner. Each stone model was then scanned using a 150-μm voxel resolution in a Planmeca Mid CBCT device with 2 sets of exposure parameters and in a Newtom VG device. The DICOM files were initially imported in Blue Sky Plan implant surgery software, segmented and then imported for computational manipulation in CloudCompare, a dedicated mesh handling software.ResultsFor all CBCTs and for all exposure parameters, the mean (SD) difference was 0.052 (0.011) mm ranging from 0.032 to 0.070 mm with a 95% CI for the population mean of 0.052 ± 0.004 mm. Specifically, the mean (SD) difference for each device/exposure parameter tested was (1) Newtom VG = 0.040 (0.006) mm, (2) Planmeca Mid 90 = 0.057 (0.0066) mm, and (3) Planmeca Mid 80 = 0.059 (0.0063) mm.ConclusionsThere are differences amongst the CBCT models, whilst different exposure parameters of the same model do not seem to offer a significant advantage. The interaction between the threshold value and the imaging modality as far as the errors are concerned necessitates the careful selection of the right threshold value for the triangular mesh creation.

2D Contour Reconstruction for Industrial Computed Tomography using Crease Cluster

One task of industrial computed tomography in dimensional metrology is to provide surface models of examined objects. Currently, in the standard approach polygonal surface meshes from reconstructed tomograms are extracted. Precise surface meshes require a high resolution tomogram, leading to time-consuming reconstruction algorithms. In order to overcome this problem, we propose a method to approximate the contour by a point cloud of folds, which resemble the most characteristic parts of the object. These creases are reconstructed from filtered projection images directly in Cartesian space. This cloud of creases will serve as a basis for the creation of meshes. Initial results of a 2D phantom study indicate that the method reaches reconstruction accuracies comparable to standard methods while the number of necessary projections is reduced significantly, leading to a notable decrease in scan time.