Determination of local material properties of OSB sample by coupling advanced imaging techniques and morphology-based FEM simulation

This is the publisher’s final pdf. The published article is copyrighted by Walter de Gruyter & Co. and can be found at: http://www.degruyter.com/.The goal was to determine local mechanical properties inside of oriented strand board (OSB) based on a realistic morphology-based finite element (FE) model and data acquired from a physical test performed on the same material. The spatial information and local grayscale intensity from CT-scans obtained from small OSB sample was transformed into a 2D regular morphology-based FE mesh with corresponding material properties. The model was then used to simulate the actual compression test performed on the specimen using simplified boundary conditions. The simulated strain fields from the model were compared with the actual strain field measured on the specimen surface during the compression test by means of a full-field optical method, named digital image correlation (DIC). Finally, the original set of material properties was adjusted by an iterative procedure to minimize the difference between the simulated and the measured strain data. The results show that the developed procedure is useful to find local material properties as well as for morphological modeling without the need of segmentation of the image data. The achieved results serve as a prerequisite for full 3D analyses of the complex materials

New approaches to probing Minkowski functionals

We generalize the concept of the ordinary skew-spectrum to probe the effect of non-Gaussianity on the morphology of cosmic microwave background (CMB) maps in several domains: in real space (where they are commonly known as cumulant-correlators), and in harmonic and needlet bases. The essential aim is to retain more information than normally contained in these statistics, in order to assist in determining the source of any measured non-Gaussianity, in the same spirit as Munshi & Heavens skew-spectra were used to identify foreground contaminants to the CMB bispectrum in Planck data. Using a perturbative series to construct the Minkowski functionals (MFs), we provide a pseudo-C based approach in both harmonic and needlet representations to estimate these spectra in the presence of a mask and inhomogeneous noise. Assuming homogeneous noise, we present approximate expressions for error covariance for the purpose of joint estimation of these spectra. We present specific results for four different models of primordial non-Gaussianity local, equilateral, orthogonal and enfolded models, as well as non-Gaussianity caused by unsubtracted point sources. Closed form results of nextorder corrections to MFs too are obtained in terms of a quadruplet of kurt-spectra. We also use the method of modal decomposition of the bispectrum and trispectrum to reconstruct the MFs as an alternative method of reconstruction of morphological properties of CMB maps. Finally, we introduce the odd-parity skew-spectra to probe the odd-parity bispectrum and its impact on the morphology of the CMB sky. Although developed for the CMB, the generic results obtained here can be useful in other areas of cosmology

Virtual sculpting and 3D printing for young people with disabilities

In this paper, we present the SHIVA project which was designed to provide virtual sculpting tools for young people with complex disabilities, to allow them to engage with artistic and creative activities that they might otherwise never be able to access. Modern 3D printing then allows us to physically build their creations. To achieve this, we combined our expertise in education, accessible technology, user interfaces and geometric modelling. We built a generic accessible graphical user interface (GUI) and a suitable geometric modelling system and used these to produce two prototype modelling exercises. These tools were deployed in a school for students with complex disabilities and are now being used for a variety of educational and developmental purposes. In this paper, we present the project's motivations, approach and implementation details together with initial results, including 3D printed objects designed by young people who have disabilties

Link Prediction for Flow-Driven Spatial Networks

Link prediction algorithms aim to infer the existence of connections (or links) between nodes in network-structured data and are typically applied to refine the connectivity among nodes. In this work, we focus on link prediction for flow-driven spatial networks, which are embedded in a Euclidean space and relate to physical exchange and transportation processes (e.g., blood flow in vessels or traffic flow in road networks). To this end, we propose the Graph Attentive Vectors (GAV) link prediction framework. GAV models simplified dynamics of physical flow in spatial networks via an attentive, neighborhood-aware message-passing paradigm, updating vector embeddings in a constrained manner. We evaluate GAV on eight flow-driven spatial networks given by whole-brain vessel graphs and road networks. GAV demonstrates superior performances across all datasets and metrics and outperformed the state-of-the-art on the ogbl-vessel benchmark at the time of submission by 12% (98.38 vs. 87.98 AUC). All code is publicly available on GitHub

Doctor of Philosophy

dissertationThe medial axis of an object is a shape descriptor that intuitively presents the morphology or structure of the object as well as intrinsic geometric properties of the object’s shape. These properties have made the medial axis a vital ingredient for shape analysis applications, and therefore the computation of which is a fundamental problem in computational geometry. This dissertation presents new methods for accurately computing the 2D medial axis of planar objects bounded by B-spline curves, and the 3D medial axis of objects bounded by B-spline surfaces. The proposed methods for the 3D case are the first techniques that automatically compute the complete medial axis along with its topological structure directly from smooth boundary representations. Our approach is based on the eikonal (grassfire) flow where the boundary is offset along the inward normal direction. As the boundary deforms, different regions start intersecting with each other to create the medial axis. In the generic situation, the (self-) intersection set is born at certain creation-type transition points, then grows and undergoes intermediate transitions at special isolated points, and finally ends at annihilation-type transition points. The intersection set evolves smoothly in between transition points. Our approach first computes and classifies all types of transition points. The medial axis is then computed as a time trace of the evolving intersection set of the boundary using theoretically derived evolution vector fields. This dynamic approach enables accurate tracking of elements of the medial axis as they evolve and thus also enables computation of topological structure of the solution. Accurate computation of geometry and topology of 3D medial axes enables a new graph-theoretic method for shape analysis of objects represented with B-spline surfaces. Structural components are computed via the cycle basis of the graph representing the 1-complex of a 3D medial axis. This enables medial axis based surface segmentation, and structure based surface region selection and modification. We also present a new approach for structural analysis of 3D objects based on scalar functions defined on their surfaces. This approach is enabled by accurate computation of geometry and structure of 2D medial axes of level sets of the scalar functions. Edge curves of the 3D medial axis correspond to a subset of ridges on the bounding surfaces. Ridges are extremal curves of principal curvatures on a surface indicating salient intrinsic features of its shape, and hence are of particular interest as tools for shape analysis. This dissertation presents a new algorithm for accurately extracting all ridges directly from B-spline surfaces. The proposed technique is also extended to accurately extract ridges from isosurfaces of volumetric data using smooth implicit B-spline representations. Accurate ridge curves enable new higher-order methods for surface analysis. We present a new definition of salient regions in order to capture geometrically significant surface regions in the neighborhood of ridges as well as to identify salient segments of ridges

Operatori za multi-rezolucione komplekse Morza i ćelijske komplekse

The topic of the thesis is analysis of the topological structure of scalar fields and shapes represented through Morse and cell complexes, respectively. This is achieved by defining simplification and refinement operators on these complexes. It is shown that the defined operators form a basis for the set of operators that modify Morse and cell complexes. Based on the defined operators, a multi-resolution model for Morse and cell complexes is constructed, which contains a large number of representations at uniform and variable resolution.Тема дисертације је анализа тополошке структуре скаларних поља и облика представљених у облику комплекса Морза и ћелијских комплекса, редом. То се постиже дефинисањем оператора за симплификацију и рафинацију тих комплекса. Показано је да дефинисани оператори чине базу за скуп оператора на комплексима Морза и ћелијским комплексима. На основу дефинисаних оператора конструисан је мулти-резолуциони модел за комплексе Морза и ћелијске комплексе, који садржи велики број репрезентација униформне и варијабилне резолуције.Tema disertacije je analiza topološke strukture skalarnih polja i oblika predstavljenih u obliku kompleksa Morza i ćelijskih kompleksa, redom. To se postiže definisanjem operatora za simplifikaciju i rafinaciju tih kompleksa. Pokazano je da definisani operatori čine bazu za skup operatora na kompleksima Morza i ćelijskim kompleksima. Na osnovu definisanih operatora konstruisan je multi-rezolucioni model za komplekse Morza i ćelijske komplekse, koji sadrži veliki broj reprezentacija uniformne i varijabilne rezolucije

Spline-based medial axis transform representation of binary images

Medial axes are well-known descriptors used for representing, manipulating, and compressing binary images. In this paper, we present a full pipeline for computing a stable and accurate piece-wise B-spline representation of Medial Axis Transforms (MATs) of binary images. A comprehensive evaluation on a benchmark shows that our method, called Spline-based Medial Axis Transform (SMAT), achieves very high compression ratios while keeping quality high. Compared with the regular MAT representation, the SMAT yields a much higher compression ratio at the cost of a slightly lower image quality. We illustrate our approach on a multi-scale SMAT representation, generating super-resolution images, and free-form binary image deformation