29,184 research outputs found
From 3D Point Clouds to Pose-Normalised Depth Maps
We consider the problem of generating either pairwise-aligned or pose-normalised depth maps from noisy 3D point clouds in a relatively unrestricted poses. Our system is deployed in a 3D face alignment application and consists of the following four stages: (i) data filtering, (ii) nose tip identification and sub-vertex localisation, (iii) computation of the (relative) face orientation, (iv) generation of either a pose aligned or a pose normalised depth map. We generate an implicit radial basis function (RBF) model of the facial surface and this is employed within all four stages of the process. For example, in stage (ii), construction of novel invariant features is based on sampling this RBF over a set of concentric spheres to give a spherically-sampled RBF (SSR) shape histogram. In stage (iii), a second novel descriptor, called an isoradius contour curvature signal, is defined, which allows rotational alignment to be determined using a simple process of 1D correlation. We test our system on both the University of York (UoY) 3D face dataset and the Face Recognition Grand Challenge (FRGC) 3D data. For the more challenging UoY data, our SSR descriptors significantly outperform three variants of spin images, successfully identifying nose vertices at a rate of 99.6%. Nose localisation performance on the higher quality FRGC data, which has only small pose variations, is 99.9%. Our best system successfully normalises the pose of 3D faces at rates of 99.1% (UoY data) and 99.6% (FRGC data)
The Area Quantum and Snyder Space
We show that in the Snyder space the area of the disc and of the sphere can
be quantized. It is also shown that the area spectrum of the sphere can be
related to the Bekenstein conjecture for the area spectrum of a black hole
horizon.Comment: 7 pages, in Press, Physics Letters
Gamma-rays from massive protostars
Massive protostars have associated bipolar outflows with velocities of
hundreds of km/s. Such outflows produce strong shocks when interact with the
ambient medium leading to regions of non-thermal radio emission. Under certain
conditions, the population of relativistic particles accelerated at the
terminal shocks of the protostellar jets can produce significant gamma-ray
emission. We estimate the conditions necessary for high-energy emission in the
non-thermal hot spots of jets associated with massive protostars embedded in
dense molecular clouds. Our results show that particle-matter interactions can
lead to the detection of molecular clouds hosting massive young stellar objects
by the Fermi satellite at MeV-GeV energies and even by Cherenkov telescope
arrays in the GeV-TeV range. Astronomy at gamma-rays can be used to probe the
physical conditions in star forming regions and particle acceleration processes
in the complex environment of massive molecular clouds.Comment: Proceeding of the conference "High Energy Phenomena in Massive
Stars". Jaen (Spain), 2-5 February 200
Kinematic groups across the MW disc: insights from models and from the RAVE catalogue
With the advent of the Gaia data, the unprecedented kinematic census of great
part of the Milky Way disc will allow us to characterise the local kinematic
groups and new groups in different disc neighbourhoods. First, we show here
that the models predict a stellar kinematic response to the spiral arms and bar
strongly dependent on disc position. For example, we find that the kinematic
groups induced by the spiral arm models change significantly if one moves only
~ 0.6 kpc in galactocentric radius, but ~ 2 kpc in azimuth. There are more and
stronger groups as one approaches the spiral arms. Depending on the spiral
pattern speed, the kinematic imprints are more intense in nearby vicinities or
far from the Sun. Secondly, we present a preliminary study of the kinematic
groups observed by RAVE. This sample will allow us, for the first time, to
study the dependence on Galactic position of the (thin and thick) disc moving
groups. In the solar neighbourhood, we find the same kinematics groups as
detected in previous surveys, but now with better statistics and over a larger
spatial volume around the Sun. This indicates that these structures are indeed
large scale kinematic features.Comment: 4 pages, 3 figures, to appear in the proceedings of "Assembling the
Puzzle of the Milky Way", Le Grand Bornand (April 17-22, 2011), C. Reyle, A.
Robin, M. Schultheis (eds.
Understanding the spiral structure of the Milky Way using the local kinematic groups
We study the spiral arm influence on the solar neighbourhood stellar
kinematics. As the nature of the Milky Way (MW) spiral arms is not completely
determined, we study two models: the Tight-Winding Approximation (TWA) model,
which represents a local approximation, and a model with self-consistent
material arms named PERLAS. This is a mass distribution with more abrupt
gravitational forces. We perform test particle simulations after tuning the two
models to the observational range for the MW spiral arm properties. We explore
the effects of the arm properties and find that a significant region of the
allowed parameter space favours the appearance of kinematic groups. The
velocity distribution is mostly sensitive to the relative spiral arm phase and
pattern speed. In all cases the arms induce strong kinematic imprints for
pattern speeds around 17 km/s/kpc (close to the 4:1 inner resonance) but no
substructure is induced close to corotation. The groups change significantly if
one moves only ~0.6 kpc in galactocentric radius, but ~2 kpc in azimuth. The
appearance time of each group is different, ranging from 0 to more than 1 Gyr.
Recent spiral arms can produce strong kinematic structures. The stellar
response to the two potential models is significantly different near the Sun,
both in density and kinematics. The PERLAS model triggers more substructure for
a larger range of pattern speed values. The kinematic groups can be used to
reduce the current uncertainty about the MW spiral structure and to test
whether this follows the TWA. However, groups such as the observed ones in the
solar vicinity can be reproduced by different parameter combinations. Data from
velocity distributions at larger distances are needed for a definitive
constraint.Comment: 18 pages, 21 figures, 4 tables; acccepted for publication in MNRA
Electron-Phonon Interactions in C-derived Molecular Solids
We present {\it ab initio} density-functional calculations of molecular
solids formed from C-derived closed-shell fullerenes. Solid
CH is found to bind weakly and exhibits many of the electronic
structure features of solid C with an enhanced electron-phonon
interaction potential. We show that chemical doping of this structure is
feasible, albeit more restrictive than its C counterpart, with an
estimated superconducting transition temperature exceeding those of the
alkali-doped C solids.Comment: Lower quality postscript file for Figure 1 is used in the manuscript
in order to meet submission quota for pre-print server. Higher quality
postscript file available from author: [email protected] This article has
been updated to reflect changes incorporated during the peer review process.
It is published in PRB 70, 140504(R) 200
Effects of dimensionality and anisotropy on the Holstein polaron
We apply weak-coupling perturbation theory and strong-coupling perturbation
theory to the Holstein molecular crystal model in order to elucidate the
effects of anisotropy on polaron properties in D dimensions. The ground state
energy is considered as a primary criterion through which to study the effects
of anisotropy on the self-trapping transition, the self-trapping line
associated with this transition, and the adiabatic critical point. The effects
of dimensionality and anisotropy on electron-phonon correlations and polaronic
mass enhancement are studied, with particular attention given to the polaron
radius and the characteristics of quasi-1D and quasi-2D structures.
Perturbative results are confirmed by selected comparisons with variational
calculations and quantum Monte Carlo data
Density functional theory study of the nematic-isotropic transition in an hybrid cell
We have employed the Density Functional Theory formalism to investigate the
nematic-isotropic capillary transitions of a nematogen confined by walls that
favor antagonist orientations to the liquid crystal molecules (hybrid cell). We
analyse the behavior of the capillary transition as a function of the
fluid-substrate interactions and the pore width. In addition to the usual
capillary transition between isotropic-like to nematic-like states, we find
that this transition can be suppressed when one substrate is wet by the
isotropic phase and the other by the nematic phase. Under this condition the
system presents interface-like states which allow to continuously transform the
nematic-like phase to the isotropic-like phase without undergoing a phase
transition. Two different mechanisms for the disappearance of the capillary
transition are identified. When the director of the nematic-like state is
homogeneously planar-anchored with respect to the substrates, the capillary
transition ends up in a critical point. This scenario is analogous to the
observed in Ising models when confined in slit pores with opposing surface
fields which have critical wetting transitions. When the nematic-like state has
a linearly distorted director field, the capillary transition continuously
transforms in a transition between two nematic-like states.Comment: 31 pages, 10 figures, submitted to J. Chem. Phy
Effect of parallel magnetic field on the Zero Differential Resistance State
The non-linear zero-differential resistance state (ZDRS) that occurs for
highly mobile two-dimensional electron systems in response to a dc bias in the
presence of a strong magnetic field applied perpendicular to the electron plane
is suppressed and disappears gradually as the magnetic field is tilted away
from the perpendicular at fixed filling factor . Good agreement is found
with a model that considers the effect of the Zeeman splitting of Landau levels
enhanced by the in-plane component of the magnetic field.Comment: 4 pages, 4 figure
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