A set of bathymetry point clouds acquired by different measurement techniques at different times,... more A set of bathymetry point clouds acquired by different measurement techniques at different times, having different accuracy and varying patterns of points, are approximated by an LR B-spline surface. The aim is to represent the sea bottom with good accuracy and at the same time reduce the data size considerably. In this process the point clouds must be cleaned by selecting the "best" points for surface generation. This cleaning process is called deconfliction, and we use a rough approximation of the combined point clouds as a reference surface to select a consistent set of points. The reference surface is updated with the selected points to create an accurate approximation. LR B-splines is the selected surface format due to its suitability for adaptive refinement and approximation, and its ability to represent local detail without a global increase in the data size of the surface.
The univariate minimal support B-spline basis (UMB) has been used in Computer Aided Design (CAD) ... more The univariate minimal support B-spline basis (UMB) has been used in Computer Aided Design (CAD) since the 1970s. Freeform curves use UMB, while sculptured surfaces are represented using a tensor product of two UMBs. The coefficients of a B-spline curve and surface are respectively represented in a vector and a rectangular grid. In CAD-intersection algorithms for UMB represented objects, a divide-and-conquer strategy is often used. Refinement by knot insertion is used to split the objects intersected into objects of the same type with a smaller geometric extent. In many cases the intersection of the resulting sub-objects has simpler topology than the original problem. The sub-objects created are represented using their parents' UMB format and deleted when the sub-problem is solved. Consequently, no global representations of the locally refined bases are needed. This is contrary to when locally refined splines are used for approximation of large point sets. As soon as a B-spline is locally refined, the regular structure of UMB objects in CAD is no longer valid. In this chapter we discuss how Locally Refined B-splines (LR B-splines) address this challenge and present the properties of LR B-splines. Keywords Locally Refined B-splines • Minimal support basis • Refinement
Performing surface approximation of geospatial point clouds with locally refined (LR) B-splines c... more Performing surface approximation of geospatial point clouds with locally refined (LR) B-splines comes with several challenges: (i) Point clouds have varying data density, (ii) outliers should be eliminated without deleting features, (iii) voids, also called holes, or data gaps should be treated specifically to avoid the drop of the approximated surface in domains without points. These factors tend to be even more challenging when point clouds acquired from different sensors having different noise characteristics are fused together. The data set becomes non-uniform and the fusing process itself involves a risk of an increased noise level. In this chapter, we provide some tools to answer those specific challenges. We will use terrain and seabed data and show didactically how to perform adaptive surface approximation with local refinement and to select customized parameters. We will further address the problem of choosing an appropriate tolerance for performing an adaptive fitting, and discuss the refinement strategies within the context of LR B-splines. The latter is shown to provide a promising framework for surface fitting of heterogeneous point clouds from various sources. Keywords Adaptive refinement • Surface fitting • Outliers • Voids • Trimming • Tolerance • Bathymetry • Data fusion • LR B-splines
ISPRS Annals of the Photogrammetry, Remote Sensing and Spatial Information Sciences
Geospatial data acquisition of terrains produces huge, noisy and scattered point clouds. An effic... more Geospatial data acquisition of terrains produces huge, noisy and scattered point clouds. An efficient use of the acquired data requires structured and compact data representations. Working directly in a point cloud is often not appealing. To face this challenge, approximation with tensor product B-spline surfaces is attractive. It reduces the point cloud description to relatively few coefficients compared to the volume of the original point cloud. However, this representation lacks the ability to adapt the resolution of the shape to local variations in the point cloud. The result is frequently that noise is approximated and that surfaces have unwanted oscillations. Locally Refined (LR) B-spline surfaces were introduced to face this challenge and provide a tool for approximating Geographic Information System point clouds. In our LR B-spline based approximation algorithm, iterative least-squares approximation is combined with a Multilevel B-spline Approximation to reduce memory consumption. We apply the approach to data sets from coastal regions in Norway and the Netherlands, and compare the obtained approximation with a raster method. We further highlight the potential of LR B-spline volumes for spatio-temporal visualisation of deformation patterns.
NURBS (Non Uniform Rational B-Spline) curves and surfaces have been used extensively in Computer ... more NURBS (Non Uniform Rational B-Spline) curves and surfaces have been used extensively in Computer Aided Design (CAD) in the last two decades but not in Finite Element Analysis (FEA). Isogeometric analysis [6], introduced in 2005 by Hughes [9] proposes to replace traditional Finite Elements with volumetric NURBS. Although mathematically this seems to be a minor adjustment, it will drastically change the model life-cycle in FEA
The surface approximation obtained with adaptive strategies using locally refined (LR) B-splines ... more The surface approximation obtained with adaptive strategies using locally refined (LR) B-splines depends on the degrees of freedom of the spline space, the tolerance from which the refinement is performed, the noise level of the scattered observations, the refinement strategy and the bidegree of the spline space. The choice of the best model is a challenging task that can be partially answered with statistical criteria, such as the Akaike Information Criterion (AIC). Here we relax the assumption that the approximation error should be normally distributed and with equal variance and propose the use of the student distribution to compute the AIC. We apply the AIC to decide which tolerance, refinement level, or polynomial bidegree are the most adequate for an optimal fitting. We highlight how the resulting AIC can be combined with more usual criteria to judge the goodness of fit of the surface approximation. Keywords Information criterion • AIC • Surface approximation • t-distribution • Locally Refined B-splines • Local refinement
A locally refined (LR) B-spline surface is a piecewise polynomial surface for which the distribut... more A locally refined (LR) B-spline surface is a piecewise polynomial surface for which the distribution of the surface coefficients can be locally adapted. Such a mathematical representation is interesting for fitting scattered and noisy data, as the local behaviour of a real point cloud may require more degrees of freedom only locally. The number of redundant surface coefficients is minimized, which avoids the fitting of the point cloud's noise. The surface approximation is performed iteratively either by solving a least squares system or by a local approximation method. This procedure allows for mesh refinement in domains where the distance between a current surface and the point cloud exceeds a prescribed tolerance. In this way, parts of the LR B-spline surface obtained at previous steps may be kept unchanged. This chapter aims at explaining the adaptive fitting using local refinement with LR B-splines. We present two examples with simulated point clouds to illustrate the methodology. Keywords LR B-splines surface • Adaptive refinement • Surface fitting • Local refinement • Least-squares • Multilevel B-spline Approximation (MBA)
ISPRS Annals of Photogrammetry, Remote Sensing and Spatial Information Sciences, 2015
Digital environmental data are becoming commonplace and the amount of information they provide is... more Digital environmental data are becoming commonplace and the amount of information they provide is huge, yet complex to process, due to the size, variety, and dynamic nature of the data captured by the available sensing devices. Making use of the data largely relies on the availability of efficient methods to extract meaningful information, and requires to process the environmental events at the speed data are acquired. This paper focuses on the evaluation of methods to approximate observed rain data, in real conditions of sparsity of the observations. The novelty stands in the selection of a particularly complex area, Liguria region, located in the north-west of Italy, where the orography and the closeness to the sea causes complex hydro-meteorological events. Approximation results are compared on a fine granularity in terms of cumulated rain interval used, gathered from two different rain gauge networks, with different characteristics and spatial distribution. Moreover, beside trad...
Optimal Surface Fitting of Point Clouds Using Local Refinement
Geospatial data acquisition of terrains with contact-free sensors such as Terrestrial or Airbone ... more Geospatial data acquisition of terrains with contact-free sensors such as Terrestrial or Airbone Laser Scanners generates scattered and noisy point clouds. Performing a surface approximation is an efficient way to reduce and structure the recorded point clouds. To that end, LR B-splines are attractive as they allow a local refinement, on the contrary to the tensor product B-spline and raster surfaces. By comparing the approximation error with a given tolerance, a local refinement is performed. We apply this adaptive refinement strategy to landslides data sets from Alpine terrain in Austria. We show how different epochs of the point clouds can be analyzed with LR B-spline volumes for spatio-temporal visualisation of deformation. We highlight the potential of a time-differenced LR B-splines volume for analysing geomorphological changes. A further application of this method is the drawing of contour lines.
Locally refined B-spline (LRB) surfaces provide a representation that is well suited to scattered... more Locally refined B-spline (LRB) surfaces provide a representation that is well suited to scattered data approximation. When a data set has local details in some areas and is largely smooth elsewhere, LR B-splines allow the spatial distribution of degrees of freedom to follow the variations of the data set. An LRB surface approximating a data set is refined in areas where the accuracy does not meet a required tolerance. In this paper we address, in a systematic study, different LRB refinement strategies and polynomial degrees for surface approximation. We study their influence on the resulting data volume and accuracy when applied to geospatial data sets with different structural behaviour. The relative performance of the refinement strategies is reasonably coherent for the different data sets and this paper concludes with some recommendations. An overall evaluation indicates that bi-quadratic LRB are preferable for the use cases tested, and that the strategies we denote as “full span...
� Based on the accepted approach for the integration of CAD and FEA � FEM well established in 197... more � Based on the accepted approach for the integration of CAD and FEA � FEM well established in 1970 � Volumetric CAD research in 1970s, established in the 1990s � From the patch work of CADsurface establish a FEA type surface mesh � Approximate shape, watertight � Add volumetric mesh � Define boundary conditions � Simplify shape
New data acquisition techniques are emerging and are providing a fast and efficient means for mul... more New data acquisition techniques are emerging and are providing a fast and efficient means for multidimensional spatial data collection. Single and multi-beam echo-sounders, airborne LIDAR, SAR satellites and mobile mapping systems are increasingly used for the digital reconstruction of the environment. All these systems provide point clouds, often enriched with other sensor data providing extremely high volumes of raw data. With these acquisition approaches, a great deal of data is collected, but it often requires harmonisation and integration before reaching its maximum use potential. For use in modelling waves and flow in seas and oceans, collections of surveys of water depth are such an example of a potentially ‘big data’ source structured as a point cloud. Individual surveys vary both spatially and temporally and can overlap with many other similar surveys. Where measurements of water depth differ greatly between surveys a strategy needs to be employed to determine how to create...
In the industrial practice, additive manufacturing processes are often followed by post-processin... more In the industrial practice, additive manufacturing processes are often followed by post-processing operations such as subtractive machining, milling, etc. to achieve the desired surface quality and dimensional accuracy. Hence, a given part must be 3D printed with extra material to enable such finishing phase. This combined additive/subtractive technique can be optimized to reduce manufacturing costs by saving printing time and reducing material and energy usage. In this work, a numerical methodology based on parametric shape optimization is proposed for optimizing the thickness of the extra material, allowing for minimal machining operations while ensuring the finishing requirements. Moreover, the proposed approach is complemented by a novel algorithm for generating inner structures leading to reduced distortion and improved weight reduction. The computational effort induced by classical constrained optimization methods is alleviated by replacing both the objective and constraint functions by their sparse-grid surrogates. Numerical results showcase the effectiveness of the proposed approach.
Computers & Mathematics with Applications, 2018
Digital representations targeting design and simulation for Additive Manufacturing (AM) are addre... more Digital representations targeting design and simulation for Additive Manufacturing (AM) are addressed from the perspective of Computer Aided Geometric Design. We discuss the feasibility for multi-material AM for Brep based CAD, STL, sculptured triangles as well as trimmed and blockstructured trivariate locally refined spline representations. The trivariate spline representations support Isogeometric Analysis (IGA), and topology structures supporting these for CAD, IGA and AM are outlined. The ideas of (Truncated) Hierarchical B-splines, T-splines and LR B-splines are outlined and the approaches are compared. An example from the EC H2020 Factories of the Future Research and Innovation Actions CAxMan illustrates both trimmed and block-structured spline representations for IGA and AM.
A set of bathymetry point clouds acquired by different measurement techniques at different times,... more A set of bathymetry point clouds acquired by different measurement techniques at different times, having different accuracy and varying patterns of points, are approximated by an LR B-spline surface. The aim is to represent the sea bottom with good accuracy and at the same time reduce the data size considerably. In this process the point clouds must be cleaned by selecting the "best" points for surface generation. This cleaning process is called deconfliction, and we use a rough approximation of the combined point clouds as a reference surface to select a consistent set of points. The reference surface is updated with the selected points to create an accurate approximation. LR B-splines is the selected surface format due to its suitability for adaptive refinement and approximation, and its ability to represent local detail without a global increase in the data size of the surface.
The univariate minimal support B-spline basis (UMB) has been used in Computer Aided Design (CAD) ... more The univariate minimal support B-spline basis (UMB) has been used in Computer Aided Design (CAD) since the 1970s. Freeform curves use UMB, while sculptured surfaces are represented using a tensor product of two UMBs. The coefficients of a B-spline curve and surface are respectively represented in a vector and a rectangular grid. In CAD-intersection algorithms for UMB represented objects, a divide-and-conquer strategy is often used. Refinement by knot insertion is used to split the objects intersected into objects of the same type with a smaller geometric extent. In many cases the intersection of the resulting sub-objects has simpler topology than the original problem. The sub-objects created are represented using their parents' UMB format and deleted when the sub-problem is solved. Consequently, no global representations of the locally refined bases are needed. This is contrary to when locally refined splines are used for approximation of large point sets. As soon as a B-spline is locally refined, the regular structure of UMB objects in CAD is no longer valid. In this chapter we discuss how Locally Refined B-splines (LR B-splines) address this challenge and present the properties of LR B-splines. Keywords Locally Refined B-splines • Minimal support basis • Refinement
Performing surface approximation of geospatial point clouds with locally refined (LR) B-splines c... more Performing surface approximation of geospatial point clouds with locally refined (LR) B-splines comes with several challenges: (i) Point clouds have varying data density, (ii) outliers should be eliminated without deleting features, (iii) voids, also called holes, or data gaps should be treated specifically to avoid the drop of the approximated surface in domains without points. These factors tend to be even more challenging when point clouds acquired from different sensors having different noise characteristics are fused together. The data set becomes non-uniform and the fusing process itself involves a risk of an increased noise level. In this chapter, we provide some tools to answer those specific challenges. We will use terrain and seabed data and show didactically how to perform adaptive surface approximation with local refinement and to select customized parameters. We will further address the problem of choosing an appropriate tolerance for performing an adaptive fitting, and discuss the refinement strategies within the context of LR B-splines. The latter is shown to provide a promising framework for surface fitting of heterogeneous point clouds from various sources. Keywords Adaptive refinement • Surface fitting • Outliers • Voids • Trimming • Tolerance • Bathymetry • Data fusion • LR B-splines
ISPRS Annals of the Photogrammetry, Remote Sensing and Spatial Information Sciences
Geospatial data acquisition of terrains produces huge, noisy and scattered point clouds. An effic... more Geospatial data acquisition of terrains produces huge, noisy and scattered point clouds. An efficient use of the acquired data requires structured and compact data representations. Working directly in a point cloud is often not appealing. To face this challenge, approximation with tensor product B-spline surfaces is attractive. It reduces the point cloud description to relatively few coefficients compared to the volume of the original point cloud. However, this representation lacks the ability to adapt the resolution of the shape to local variations in the point cloud. The result is frequently that noise is approximated and that surfaces have unwanted oscillations. Locally Refined (LR) B-spline surfaces were introduced to face this challenge and provide a tool for approximating Geographic Information System point clouds. In our LR B-spline based approximation algorithm, iterative least-squares approximation is combined with a Multilevel B-spline Approximation to reduce memory consumption. We apply the approach to data sets from coastal regions in Norway and the Netherlands, and compare the obtained approximation with a raster method. We further highlight the potential of LR B-spline volumes for spatio-temporal visualisation of deformation patterns.
NURBS (Non Uniform Rational B-Spline) curves and surfaces have been used extensively in Computer ... more NURBS (Non Uniform Rational B-Spline) curves and surfaces have been used extensively in Computer Aided Design (CAD) in the last two decades but not in Finite Element Analysis (FEA). Isogeometric analysis [6], introduced in 2005 by Hughes [9] proposes to replace traditional Finite Elements with volumetric NURBS. Although mathematically this seems to be a minor adjustment, it will drastically change the model life-cycle in FEA
The surface approximation obtained with adaptive strategies using locally refined (LR) B-splines ... more The surface approximation obtained with adaptive strategies using locally refined (LR) B-splines depends on the degrees of freedom of the spline space, the tolerance from which the refinement is performed, the noise level of the scattered observations, the refinement strategy and the bidegree of the spline space. The choice of the best model is a challenging task that can be partially answered with statistical criteria, such as the Akaike Information Criterion (AIC). Here we relax the assumption that the approximation error should be normally distributed and with equal variance and propose the use of the student distribution to compute the AIC. We apply the AIC to decide which tolerance, refinement level, or polynomial bidegree are the most adequate for an optimal fitting. We highlight how the resulting AIC can be combined with more usual criteria to judge the goodness of fit of the surface approximation. Keywords Information criterion • AIC • Surface approximation • t-distribution • Locally Refined B-splines • Local refinement
A locally refined (LR) B-spline surface is a piecewise polynomial surface for which the distribut... more A locally refined (LR) B-spline surface is a piecewise polynomial surface for which the distribution of the surface coefficients can be locally adapted. Such a mathematical representation is interesting for fitting scattered and noisy data, as the local behaviour of a real point cloud may require more degrees of freedom only locally. The number of redundant surface coefficients is minimized, which avoids the fitting of the point cloud's noise. The surface approximation is performed iteratively either by solving a least squares system or by a local approximation method. This procedure allows for mesh refinement in domains where the distance between a current surface and the point cloud exceeds a prescribed tolerance. In this way, parts of the LR B-spline surface obtained at previous steps may be kept unchanged. This chapter aims at explaining the adaptive fitting using local refinement with LR B-splines. We present two examples with simulated point clouds to illustrate the methodology. Keywords LR B-splines surface • Adaptive refinement • Surface fitting • Local refinement • Least-squares • Multilevel B-spline Approximation (MBA)
ISPRS Annals of Photogrammetry, Remote Sensing and Spatial Information Sciences, 2015
Digital environmental data are becoming commonplace and the amount of information they provide is... more Digital environmental data are becoming commonplace and the amount of information they provide is huge, yet complex to process, due to the size, variety, and dynamic nature of the data captured by the available sensing devices. Making use of the data largely relies on the availability of efficient methods to extract meaningful information, and requires to process the environmental events at the speed data are acquired. This paper focuses on the evaluation of methods to approximate observed rain data, in real conditions of sparsity of the observations. The novelty stands in the selection of a particularly complex area, Liguria region, located in the north-west of Italy, where the orography and the closeness to the sea causes complex hydro-meteorological events. Approximation results are compared on a fine granularity in terms of cumulated rain interval used, gathered from two different rain gauge networks, with different characteristics and spatial distribution. Moreover, beside trad...
Optimal Surface Fitting of Point Clouds Using Local Refinement
Geospatial data acquisition of terrains with contact-free sensors such as Terrestrial or Airbone ... more Geospatial data acquisition of terrains with contact-free sensors such as Terrestrial or Airbone Laser Scanners generates scattered and noisy point clouds. Performing a surface approximation is an efficient way to reduce and structure the recorded point clouds. To that end, LR B-splines are attractive as they allow a local refinement, on the contrary to the tensor product B-spline and raster surfaces. By comparing the approximation error with a given tolerance, a local refinement is performed. We apply this adaptive refinement strategy to landslides data sets from Alpine terrain in Austria. We show how different epochs of the point clouds can be analyzed with LR B-spline volumes for spatio-temporal visualisation of deformation. We highlight the potential of a time-differenced LR B-splines volume for analysing geomorphological changes. A further application of this method is the drawing of contour lines.
Locally refined B-spline (LRB) surfaces provide a representation that is well suited to scattered... more Locally refined B-spline (LRB) surfaces provide a representation that is well suited to scattered data approximation. When a data set has local details in some areas and is largely smooth elsewhere, LR B-splines allow the spatial distribution of degrees of freedom to follow the variations of the data set. An LRB surface approximating a data set is refined in areas where the accuracy does not meet a required tolerance. In this paper we address, in a systematic study, different LRB refinement strategies and polynomial degrees for surface approximation. We study their influence on the resulting data volume and accuracy when applied to geospatial data sets with different structural behaviour. The relative performance of the refinement strategies is reasonably coherent for the different data sets and this paper concludes with some recommendations. An overall evaluation indicates that bi-quadratic LRB are preferable for the use cases tested, and that the strategies we denote as “full span...
� Based on the accepted approach for the integration of CAD and FEA � FEM well established in 197... more � Based on the accepted approach for the integration of CAD and FEA � FEM well established in 1970 � Volumetric CAD research in 1970s, established in the 1990s � From the patch work of CADsurface establish a FEA type surface mesh � Approximate shape, watertight � Add volumetric mesh � Define boundary conditions � Simplify shape
New data acquisition techniques are emerging and are providing a fast and efficient means for mul... more New data acquisition techniques are emerging and are providing a fast and efficient means for multidimensional spatial data collection. Single and multi-beam echo-sounders, airborne LIDAR, SAR satellites and mobile mapping systems are increasingly used for the digital reconstruction of the environment. All these systems provide point clouds, often enriched with other sensor data providing extremely high volumes of raw data. With these acquisition approaches, a great deal of data is collected, but it often requires harmonisation and integration before reaching its maximum use potential. For use in modelling waves and flow in seas and oceans, collections of surveys of water depth are such an example of a potentially ‘big data’ source structured as a point cloud. Individual surveys vary both spatially and temporally and can overlap with many other similar surveys. Where measurements of water depth differ greatly between surveys a strategy needs to be employed to determine how to create...
In the industrial practice, additive manufacturing processes are often followed by post-processin... more In the industrial practice, additive manufacturing processes are often followed by post-processing operations such as subtractive machining, milling, etc. to achieve the desired surface quality and dimensional accuracy. Hence, a given part must be 3D printed with extra material to enable such finishing phase. This combined additive/subtractive technique can be optimized to reduce manufacturing costs by saving printing time and reducing material and energy usage. In this work, a numerical methodology based on parametric shape optimization is proposed for optimizing the thickness of the extra material, allowing for minimal machining operations while ensuring the finishing requirements. Moreover, the proposed approach is complemented by a novel algorithm for generating inner structures leading to reduced distortion and improved weight reduction. The computational effort induced by classical constrained optimization methods is alleviated by replacing both the objective and constraint functions by their sparse-grid surrogates. Numerical results showcase the effectiveness of the proposed approach.
Computers & Mathematics with Applications, 2018
Digital representations targeting design and simulation for Additive Manufacturing (AM) are addre... more Digital representations targeting design and simulation for Additive Manufacturing (AM) are addressed from the perspective of Computer Aided Geometric Design. We discuss the feasibility for multi-material AM for Brep based CAD, STL, sculptured triangles as well as trimmed and blockstructured trivariate locally refined spline representations. The trivariate spline representations support Isogeometric Analysis (IGA), and topology structures supporting these for CAD, IGA and AM are outlined. The ideas of (Truncated) Hierarchical B-splines, T-splines and LR B-splines are outlined and the approaches are compared. An example from the EC H2020 Factories of the Future Research and Innovation Actions CAxMan illustrates both trimmed and block-structured spline representations for IGA and AM.
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Papers by Vibeke Skytt