9,154 research outputs found
A study of the sensitivity of shape measurements to the input parameters of weak lensing image simulations
Improvements in the accuracy of shape measurements are essential to exploit
the statistical power of planned imaging surveys that aim to constrain
cosmological parameters using weak lensing by large-scale structure. Although a
range of tests can be performed using the measurements, the performance of the
algorithm can only be quantified using simulated images. This yields, however,
only meaningful results if the simulated images resemble the real observations
sufficiently well. In this paper we explore the sensitivity of the
multiplicative bias to the input parameters of Euclid-like image simulations.We
find that algorithms will need to account for the local density of sources. In
particular the impact of galaxies below the detection limit warrants further
study, because magnification changes their number density, resulting in
correlations between the lensing signal and multiplicative bias. Although
achieving sub-percent accuracy will require further study, we estimate that
sufficient archival Hubble Space Telescope data are available to create
realistic populations of galaxies.Comment: 18 pages, accepted for publications in MNRA
Dynamic flow distortion investigation in an S-duct using DDES and SPIV data
The dynamic flow distortion generated within convoluted aero-engine intakes can affect the performance and operability of the engine. There is a need for a better understanding of the main flow mechanisms which promote flow distortion at the exit of S-shaped intakes. This paper presents a detailed analysis of the main coherent structures in an S-duct flow field based on a Delayed Detached Eddy Simulation (DDES). The DDES capability to capture the characteristics of the highly unsteady flow field is demonstrated against high resolution, synchronous Stereoscopic Particle Image Velocimetry (SPIV) measurements at the Aerodynamic Interface Plane (AIP). The flow field mechanisms responsible for the main AIP perturbations are identified. Clockwise and counter-clockwise stream-wise vortices are alternately generated around the separation region at a frequency of St=0.53, which promotes the swirl switching at the AIP. Spanwise vortices are also shed from the separation region at a frequency of St=1.06, and convect downstream along the separated centreline shear layer. This results in a vertical modulation of the main loss region and a fluctuation of the velocity gradient between the high and low velocity flow at the AIP
Painterly rendering techniques: A state-of-the-art review of current approaches
In this publication we will look at the different methods presented over the past few decades which attempt to recreate digital paintings. While previous surveys concentrate on the broader subject of non-photorealistic rendering, the focus of this paper is firmly placed on painterly rendering techniques. We compare different methods used to produce different output painting styles such as abstract, colour pencil, watercolour, oriental, oil and pastel. Whereas some methods demand a high level of interaction using a skilled artist, others require simple parameters provided by a user with little or no artistic experience. Many methods attempt to provide more automation with the use of varying forms of reference data. This reference data can range from still photographs, video, 3D polygonal meshes or even 3D point clouds. The techniques presented here endeavour to provide tools and styles that are not traditionally available to an artist. Copyright © 2012 John Wiley & Sons, Ltd
Dictionary Learning-based Inpainting on Triangular Meshes
The problem of inpainting consists of filling missing or damaged regions in
images and videos in such a way that the filling pattern does not produce
artifacts that deviate from the original data. In addition to restoring the
missing data, the inpainting technique can also be used to remove undesired
objects. In this work, we address the problem of inpainting on surfaces through
a new method based on dictionary learning and sparse coding. Our method learns
the dictionary through the subdivision of the mesh into patches and rebuilds
the mesh via a method of reconstruction inspired by the Non-local Means method
on the computed sparse codes. One of the advantages of our method is that it is
capable of filling the missing regions and simultaneously removes noise and
enhances important features of the mesh. Moreover, the inpainting result is
globally coherent as the representation based on the dictionaries captures all
the geometric information in the transformed domain. We present two variations
of the method: a direct one, in which the model is reconstructed and restored
directly from the representation in the transformed domain and a second one,
adaptive, in which the missing regions are recreated iteratively through the
successive propagation of the sparse code computed in the hole boundaries,
which guides the local reconstructions. The second method produces better
results for large regions because the sparse codes of the patches are adapted
according to the sparse codes of the boundary patches. Finally, we present and
analyze experimental results that demonstrate the performance of our method
compared to the literature
Voigt-Profile Analysis of the Lyman-alpha Forest in a Cold Dark Matter Universe
We use an automated Voigt-profile fitting procedure to extract statistical
properties of the Ly forest in a numerical simulation of an ,
cold dark matter (CDM) universe. Our analysis method is similar to that used in
most observational studies of the forest, and we compare the simulations to
recently published results derived from Keck HIRES spectra. With the
Voigt-profile decomposition analysis, the simulation reproduces the large
number of weak lines (N_{\rm HI}\la 10^{13}\cdunits) found in the HIRES
spectra. The column density distribution evolves significantly between
and , with the number of lines at fixed column density dropping by a
factor in the range where line blending is not severe. At , the
-parameter distribution has a median of 35 \kms and a dispersion of 20
\kms, in reasonable agreement with the observed values. The comparison between
our new analysis and recent data strengthens earlier claims that the \lya
forest arises naturally in hierarchical structure formation as photoionized gas
falls into dark matter potential wells. However, there are two statistically
signficant discrepancies between the simulated forest and the HIRES results:
the model produces too many lines at by a factor , and it
produces more narrow lines (b<20 \kms) than are seen in the data. The first
result is sensitive to our adopted normalization of the mean \lya optical
depth, and the second is sensitive to our assumption that helium reionization
has not significantly raised gas temperatures at . It is therefore too
early to say whether these discrepancies indicate a fundamental problem with
the high-redshift structure of the CDM model or reflect errors of
detail in our modeling of the gas distribution or the observational procedure.Comment: 13 pages, 3 figures, AAS LaTex, accepted to Ap
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