56,388 research outputs found
Segmentation of Three-dimensional Images with Parametric Active Surfaces and Topology Changes
In this paper, we introduce a novel parametric method for segmentation of
three-dimensional images. We consider a piecewise constant version of the
Mumford-Shah and the Chan-Vese functionals and perform a region-based
segmentation of 3D image data. An evolution law is derived from energy
minimization problems which push the surfaces to the boundaries of 3D objects
in the image. We propose a parametric scheme which describes the evolution of
parametric surfaces. An efficient finite element scheme is proposed for a
numerical approximation of the evolution equations. Since standard parametric
methods cannot handle topology changes automatically, an efficient method is
presented to detect, identify and perform changes in the topology of the
surfaces. One main focus of this paper are the algorithmic details to handle
topology changes like splitting and merging of surfaces and change of the genus
of a surface. Different artificial images are studied to demonstrate the
ability to detect the different types of topology changes. Finally, the
parametric method is applied to segmentation of medical 3D images
Variational methods for solving nonlinear boundary problems of statics of hyper-elastic membranes
A number of important results of studying large deformations of hyper-elastic
shells are obtained using discrete methods of mathematical physics. In the
present paper, using the variational method for solving nonlinear boundary
problems of statics of hyper-elastic membranes under the regular hydrostatic
load, we investigate peculiarities of deformation of a circular membrane whose
mechanical characteristics are described by the Bidermann-type elastic
potential. We develop an algorithm for solving a singular perturbation of
nonlinear problem for the case of membrane loaded by heavy liquid. This
algorithm enables us to obtain approximate solutions both in the presence of
boundary layer and without it. The class of admissible functions, on which the
variational method is realized, is chosen with account of the structure of
formal asymptotic expansion of solutions of the corresponding linearized
equations that have singularities in a small parameter at higher derivatives
and in the independent variable. We give examples of calculations that
illustrate possibilities of the method suggested for solving the problem under
consideration
Procedural function-based modelling of volumetric microstructures
We propose a new approach to modelling heterogeneous objects containing internal volumetric structures with size of details orders of magnitude smaller than the overall size of the object. The proposed function-based procedural representation provides compact, precise, and arbitrarily parameterised models of coherent microstructures, which can undergo blending, deformations, and other geometric operations, and can be directly rendered and fabricated without generating any auxiliary representations (such as polygonal meshes and voxel arrays). In particular, modelling of regular lattices and cellular microstructures as well as irregular porous media is discussed and illustrated. We also present a method to estimate parameters of the given model by fitting it to microstructure data obtained with magnetic resonance imaging and other measurements of natural and artificial objects. Examples of rendering and digital fabrication of microstructure models are presented
Modified Linear Projection for Large Spatial Data Sets
Recent developments in engineering techniques for spatial data collection
such as geographic information systems have resulted in an increasing need for
methods to analyze large spatial data sets. These sorts of data sets can be
found in various fields of the natural and social sciences. However, model
fitting and spatial prediction using these large spatial data sets are
impractically time-consuming, because of the necessary matrix inversions.
Various methods have been developed to deal with this problem, including a
reduced rank approach and a sparse matrix approximation. In this paper, we
propose a modification to an existing reduced rank approach to capture both the
large- and small-scale spatial variations effectively. We have used simulated
examples and an empirical data analysis to demonstrate that our proposed
approach consistently performs well when compared with other methods. In
particular, the performance of our new method does not depend on the dependence
properties of the spatial covariance functions.Comment: 29 pages, 5 figures, 4 table
Geometric Properties of Isostables and Basins of Attraction of Monotone Systems
In this paper, we study geometric properties of basins of attraction of
monotone systems. Our results are based on a combination of monotone systems
theory and spectral operator theory. We exploit the framework of the Koopman
operator, which provides a linear infinite-dimensional description of nonlinear
dynamical systems and spectral operator-theoretic notions such as eigenvalues
and eigenfunctions. The sublevel sets of the dominant eigenfunction form a
family of nested forward-invariant sets and the basin of attraction is the
largest of these sets. The boundaries of these sets, called isostables, allow
studying temporal properties of the system. Our first observation is that the
dominant eigenfunction is increasing in every variable in the case of monotone
systems. This is a strong geometric property which simplifies the computation
of isostables. We also show how variations in basins of attraction can be
bounded under parametric uncertainty in the vector field of monotone systems.
Finally, we study the properties of the parameter set for which a monotone
system is multistable. Our results are illustrated on several systems of two to
four dimensions.Comment: 12 pages, to appear in IEEE Transaction on Automatic Contro
Structures of the magnetoionic media around the FR I radio galaxies 3C 31 and Hydra A
We use high-quality VLA images of the Fanaroff & Riley Class I radio galaxy
3C 31 at six frequencies in the range 1365 to 8440MHz to explore the spatial
scale and origin of the rotation measure (RM) fluctuations on the line of sight
to the radio source. We analyse the distribution of the degree of polarization
to show that the large depolarization asymmetry between the North and South
sides of the source seen in earlier work largely disappears as the resolution
is increased. We show that the depolarization seen at low resolution results
primarily from unresolved gradients in a Faraday screen in front of the
synchrotron-emitting plasma. We establish that the residual degree of
polarization in the short-wavelength limit should follow a Burn law and we fit
such a law to our data to estimate the residual depolarization at high
resolution. We show that the observed RM variations over selected areas of 3C
31 are consistent with a power spectrum of magnetic fluctuations in front of 3C
31 whose power-law slope changes significantly on the scales sampled by our
data. The power spectrum can only have the form expected for Kolmogorov
turbulence on scales <5 kpc. On larger scales we find a flatter slope. We also
compare the global variations of RM across 3C 31 with the results of
three-dimensional simulations of the magnetic-field fluctuations in the
surrounding magnetoionic medium. We show that our data are consistent with a
field distribution that favours the plane perpendicular to the jet axis -
probably because the radio source has evacuated a large cavity in the
surrounding medium. We also apply our analysis techniques to the case of Hydra
A, where the shape and the size of the cavities produced by the source in the
surrounding medium are known from X-ray data. (Abridged)Comment: 33 pages, 25 figures, accepted for publication in MNRA
Viscous regularization and r-adaptive remeshing for finite element analysis of lipid membrane mechanics
As two-dimensional fluid shells, lipid bilayer membranes resist bending and
stretching but are unable to sustain shear stresses. This property gives
membranes the ability to adopt dramatic shape changes. In this paper, a finite
element model is developed to study static equilibrium mechanics of membranes.
In particular, a viscous regularization method is proposed to stabilize
tangential mesh deformations and improve the convergence rate of nonlinear
solvers. The Augmented Lagrangian method is used to enforce global constraints
on area and volume during membrane deformations. As a validation of the method,
equilibrium shapes for a shape-phase diagram of lipid bilayer vesicle are
calculated. These numerical techniques are also shown to be useful for
simulations of three-dimensional large-deformation problems: the formation of
tethers (long tube-like exetensions); and Ginzburg-Landau phase separation of a
two-lipid-component vesicle. To deal with the large mesh distortions of the
two-phase model, modification of vicous regularization is explored to achieve
r-adaptive mesh optimization
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