8,964 research outputs found
A Fast and Scalable Graph Coloring Algorithm for Multi-core and Many-core Architectures
Irregular computations on unstructured data are an important class of
problems for parallel programming. Graph coloring is often an important
preprocessing step, e.g. as a way to perform dependency analysis for safe
parallel execution. The total run time of a coloring algorithm adds to the
overall parallel overhead of the application whereas the number of colors used
determines the amount of exposed parallelism. A fast and scalable coloring
algorithm using as few colors as possible is vital for the overall parallel
performance and scalability of many irregular applications that depend upon
runtime dependency analysis.
Catalyurek et al. have proposed a graph coloring algorithm which relies on
speculative, local assignment of colors. In this paper we present an improved
version which runs even more optimistically with less thread synchronization
and reduced number of conflicts compared to Catalyurek et al.'s algorithm. We
show that the new technique scales better on multi-core and many-core systems
and performs up to 1.5x faster than its predecessor on graphs with high-degree
vertices, while keeping the number of colors at the same near-optimal levels.Comment: To appear in the proceedings of Euro Par 201
Effectiveness of 3D Geoelectrical Resistivity Imaging using Parallel 2D Profiles
Acquisition geometry for 3D geoelectrical resistivity
imaging in which apparent resistivity data of a set of
parallel 2D profiles are collated to 3D dataset was
evaluated. A set of parallel 2D apparent resistivity
data was generated over two model structures. The
models, horst and trough, simulate the geological
environment of a weathered profile and refuse dump
site in a crystalline basement complex respectively.
The apparent resistivity data were generated for
Wenner–alpha, Wenner–beta, Wenner–Schlumberger,
dipole–dipole, pole–dipole and pole–pole arrays with
minimum electrode separation, a (a = 2, 4, 5 and 10 m)
and inter-line spacing, L (L = a, 2a, 2.5a, 4a, 5a and
10a). The 2D apparent resistivity data for each of the
arrays were collated to 3D dataset and inverted using
a full 3D inversion code. The 3D imaging capability
and resolution of the arrays for the set of parallel 2D
profiles are presented. Grid orientation effects are
observed in the inversion images produced. Inter-line
spacing of not greater than four times the minimum
electrode separation gives reasonable inverse models.
The resolution of the inverse models can be greatly
improved if the 3D dataset is built by collating sets of
orthogonal 2D profile
PyFrac: A planar 3D hydraulic fracture simulator
Fluid driven fractures propagate in the upper earth crust either naturally or
in response to engineered fluid injections. The quantitative prediction of
their evolution is critical in order to better understand their dynamics as
well as to optimize their creation. We present a Python implementation of an
open-source hydraulic fracture propagation simulator based on the implicit
level set algorithm originally developed by Peirce & Detournay (2008) -- "An
implicit level set method for modeling hydraulically driven fractures". Comp.
Meth. Appl. Mech. Engng, (33-40):2858--2885. This algorithm couples a finite
discretization of the fracture with the use of the near tip asymptotic
solutions of a steadily propagating semi-infinite hydraulic fracture. This
allows to resolve the multi-scale processes governing hydraulic fracture growth
accurately, even with relatively coarse meshes. We present an overview of the
mathematical formulation, the numerical scheme and the details of our
implementation. A series of problems including a radial hydraulic fracture
verification benchmark, the propagation of a height contained hydraulic
fracture, the lateral spreading of a magmatic dyke and the handling of fracture
closure are presented to demonstrate the capabilities, accuracy and robustness
of the implemented algorithm
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
Log-Euclidean Bag of Words for Human Action Recognition
Representing videos by densely extracted local space-time features has
recently become a popular approach for analysing actions. In this paper, we
tackle the problem of categorising human actions by devising Bag of Words (BoW)
models based on covariance matrices of spatio-temporal features, with the
features formed from histograms of optical flow. Since covariance matrices form
a special type of Riemannian manifold, the space of Symmetric Positive Definite
(SPD) matrices, non-Euclidean geometry should be taken into account while
discriminating between covariance matrices. To this end, we propose to embed
SPD manifolds to Euclidean spaces via a diffeomorphism and extend the BoW
approach to its Riemannian version. The proposed BoW approach takes into
account the manifold geometry of SPD matrices during the generation of the
codebook and histograms. Experiments on challenging human action datasets show
that the proposed method obtains notable improvements in discrimination
accuracy, in comparison to several state-of-the-art methods
Joint Total Variation ESTATICS for Robust Multi-Parameter Mapping
Quantitative magnetic resonance imaging (qMRI) derives tissue-specific
parameters -- such as the apparent transverse relaxation rate R2*, the
longitudinal relaxation rate R1 and the magnetisation transfer saturation --
that can be compared across sites and scanners and carry important information
about the underlying microstructure. The multi-parameter mapping (MPM) protocol
takes advantage of multi-echo acquisitions with variable flip angles to extract
these parameters in a clinically acceptable scan time. In this context,
ESTATICS performs a joint loglinear fit of multiple echo series to extract R2*
and multiple extrapolated intercepts, thereby improving robustness to motion
and decreasing the variance of the estimators. In this paper, we extend this
model in two ways: (1) by introducing a joint total variation (JTV) prior on
the intercepts and decay, and (2) by deriving a nonlinear maximum \emph{a
posteriori} estimate. We evaluated the proposed algorithm by predicting
left-out echoes in a rich single-subject dataset. In this validation, we
outperformed other state-of-the-art methods and additionally showed that the
proposed approach greatly reduces the variance of the estimated maps, without
introducing bias.Comment: 11 pages, 2 figures, 1 table, conference paper, accepted at MICCAI
202
Towards gigantic RVE sizes for 3D stochastic fibrous networks
The size of representative volume element (RVE) for 3D stochastic fibrous media is investigated. A statistical RVE size determination method is applied to a specific model of random microstructure: Poisson fibers. The definition of RVE size is related to the concept of integral range. What happens in microstructures exhibiting an infinite integral range? Computational homogenization for thermal and elastic properties is performed through finite elements, over hundreds of realizations of the stochastic microstructural model, using uniform and mixed boundary conditions. The generated data undergoes statistical treatment, from which gigantic RVE sizes emerge. The method used for determining RVE sizes was found to be operational, even for pathological media, i.e., with infinite integral range, interconnected percolating porous phase and infinite contrast of propertie
Fisheye Photogrammetry to Survey Narrow Spaces in Architecture and a Hypogea Environment
Nowadays, the increasing computation power of commercial grade processors has actively led to a vast spreading of image-based reconstruction software as well as its application in different disciplines. As a result, new frontiers regarding the use of photogrammetry in a vast range of investigation activities are being explored. This paper investigates the implementation of
fisheye lenses in non-classical survey activities along with the related problematics. Fisheye lenses are outstanding because of their large field of view.
This characteristic alone can be a game changer in reducing the amount of data required, thus speeding up the photogrammetric process when needed. Although they come at a cost, field of view (FOV), speed and manoeuvrability are key to the success of those optics as shown by two of the presented case studies: the survey of a very narrow spiral staircase located in the Duomo di Milano and the survey of a very narrow hypogea structure in Rome. A third case study, which deals with low-cost sensors, shows the metric evaluation of a commercial spherical camera equipped with fisheye lenses
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