17,570 research outputs found
Non-linearity and Non-Gaussianity through Phase Information
In the standard picture of structure formation, initially random-phase
fluctuations are amplified by non-linear gravitational instability to produce a
final distribution of mass which is highly non-Gaussian and has highly coupled
Fourier phases. Second-order statistics, such as the power spectrum, are blind
to this kind of phase association. We discuss the information contained in the
phases of cosmological density fluctuations and their possible use in
statistical analysis tools. In particular, we show how the bispectrum measures
a particular form of phase association called quadratic phase coupling, show
how to visualise phase association using colour models. These techniques offer
the prospect of more complete tests of initial non-Gaussianity than those
available at present.Comment: 7 pages, 1 figure (two parts). To appear in the proceedings of The
MPA/ESO/MPE Joint Astronomy Conference "Mining the Sky" held in Garching,
Germany, July 31 - August 4 2000. To be published in the Springer-Verlag
series "ESO Astrophysics Symposia
A fast Bayesian approach to discrete object detection in astronomical datasets - PowellSnakes I
A new fast Bayesian approach is introduced for the detection of discrete
objects immersed in a diffuse background. This new method, called PowellSnakes,
speeds up traditional Bayesian techniques by: i) replacing the standard form of
the likelihood for the parameters characterizing the discrete objects by an
alternative exact form that is much quicker to evaluate; ii) using a
simultaneous multiple minimization code based on Powell's direction set
algorithm to locate rapidly the local maxima in the posterior; and iii)
deciding whether each located posterior peak corresponds to a real object by
performing a Bayesian model selection using an approximate evidence value based
on a local Gaussian approximation to the peak. The construction of this
Gaussian approximation also provides the covariance matrix of the uncertainties
in the derived parameter values for the object in question. This new approach
provides a speed up in performance by a factor of `hundreds' as compared to
existing Bayesian source extraction methods that use MCMC to explore the
parameter space, such as that presented by Hobson & McLachlan. We illustrate
the capabilities of the method by applying to some simplified toy models.
Furthermore PowellSnakes has the advantage of consistently defining the
threshold for acceptance/rejection based on priors which cannot be said of the
frequentist methods. We present here the first implementation of this technique
(Version-I). Further improvements to this implementation are currently under
investigation and will be published shortly. The application of the method to
realistic simulated Planck observations will be presented in a forthcoming
publication.Comment: 30 pages, 15 figures, revised version with minor changes, accepted
for publication in MNRA
Image processing for plastic surgery planning
This thesis presents some image processing tools for plastic surgery planning. In particular,
it presents a novel method that combines local and global context in a probabilistic
relaxation framework to identify cephalometric landmarks used in Maxillofacial plastic
surgery. It also uses a method that utilises global and local symmetry to identify abnormalities
in CT frontal images of the human body. The proposed methodologies are
evaluated with the help of several clinical data supplied by collaborating plastic surgeons
Observation of surface wave patterns modified by sub-surface shear currents
We report experimental observations of two canonical surface wave patterns
--- ship waves and ring waves --- skewed by sub-surface shear, thus confirming
effects predicted by recent theory. Observed ring waves on a still surface with
sub-surface shear current are strikingly asymmetric, an effect of strongly
anisotropic wave dispersion. Ship waves for motion across a sub--surface
current on a still surface exhibit striking asymmetry about the ship's line of
motion, and large differences in wake angle and transverse wavelength for
upstream vs downstream motion are demonstrated, all of which in good agreement
with theoretical predictions. Neither of these phenomena can occur on a
depth-uniform current. A quantitative comparison of measured vs predicted
average phase shift for a ring wave is grossly mispredicted by no-shear theory,
but in good agreement with predictions for the measured shear current. A clear
difference in wave frequency within the ring wave packet is observed in the
upstream vs downstream direction for all shear flows, while it conforms with
theory for quiescent water for propagation normal to the shear current, as
expected. Peak values of the measured 2-dimensional Fourier spectrum for ship
waves are shown to agree well with the predicted criterion of stationary ship
waves, with the exception of some cases where results are imperfect due to the
limited wave-number resolution, transient effects and/or experimental noise.
Experiments were performed on controlled shear currents created in two
different ways, with a curved mesh, and beneath a blocked stagnant-surface
flow. Velocity profiles were measured with particle image velocimetry, and
surface waves with a synthetic schlieren method. Our observations lend strong
empirical support to recent predictions that wave forces on vessels and
structures can be greatly affected by shear in estuarine and tidal waters.Comment: 21 pages, 11 figure
A General Geometric Fourier Transform Convolution Theorem
The large variety of Fourier transforms in geometric algebras inspired the
straight forward definition of ``A General Geometric Fourier Transform`` in
Bujack et al., Proc. of ICCA9, covering most versions in the literature. We
showed which constraints are additionally necessary to obtain certain features
like linearity, a scaling, or a shift theorem. In this paper we extend the
former results by a convolution theorem
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