483,186 research outputs found
Parallel algorithm for determining motion vectors in ice floe images by matching edge features
A parallel algorithm is described to determine motion vectors of ice floes using time sequences of images of the Arctic ocean obtained from the Synthetic Aperture Radar (SAR) instrument flown on-board the SEASAT spacecraft. Researchers describe a parallel algorithm which is implemented on the MPP for locating corresponding objects based on their translationally and rotationally invariant features. The algorithm first approximates the edges in the images by polygons or sets of connected straight-line segments. Each such edge structure is then reduced to a seed point. Associated with each seed point are the descriptions (lengths, orientations and sequence numbers) of the lines constituting the corresponding edge structure. A parallel matching algorithm is used to match packed arrays of such descriptions to identify corresponding seed points in the two images. The matching algorithm is designed such that fragmentation and merging of ice floes are taken into account by accepting partial matches. The technique has been demonstrated to work on synthetic test patterns and real image pairs from SEASAT in times ranging from .5 to 0.7 seconds for 128 x 128 images
Rearrangement of the vortex lattice due to instabilities of vortex flow
With increasing applied current we show that the moving vortex lattice
changes its structure from a triangular one to a set of parallel vortex rows in
a pinning free superconductor. This effect originates from the change of the
shape of the vortex core due to non-equilibrium effects (similar to the
mechanism of vortex motion instability in the Larkin-Ovchinnikov theory). The
moving vortex creates a deficit of quasiparticles in front of its motion and an
excess of quasiparticles behind the core of the moving vortex. This results in
the appearance of a wake (region with suppressed order parameter) behind the
vortex which attracts other vortices resulting in an effective
direction-dependent interaction between vortices. When the vortex velocity
reaches the critical value quasi-phase slip lines (lines with fast vortex
motion) appear which may coexist with slowly moving vortices between such
lines. Our results are found within the framework of the time-dependent
Ginzburg-Landau equations and are strictly valid when the coherence length
is larger or comparable with the decay length of the
non-equilibrium quasiparticle distribution function. We qualitatively explain
experiments on the instability of vortex flow at low magnetic fields when the
distance between vortices . We speculate that a
similar instability of the vortex lattice should exist for even when
.Comment: 10 pages, 11 figure
Dynamics of 2D pancake vortices in layered superconductors
The dynamics of 2D pancake vortices in Josephson-coupled
superconducting/normal - metal multilayers is considered within the
time-dependent Ginzburg-Landau theory. For temperatures close to a
viscous drag force acting on a moving 2D vortex is shown to depend strongly on
the conductivity of normal metal layers. For a tilted vortex line consisting of
2D vortices the equation of viscous motion in the presence of a transport
current parallel to the layers is obtained. The specific structure of the
vortex line core leads to a new dynamic behavior and to substantial deviations
from the Bardeen-Stephen theory. The viscosity coefficient is found to depend
essentially on the angle between the magnetic field and the
axis normal to the layers. For field orientations close to the layers
the nonlinear effects in the vortex motion appear even for slowly moving vortex
lines (when the in-plane transport current is much smaller than the
Ginzburg-Landau critical current). In this nonlinear regime the viscosity
coefficient depends logarithmically on the vortex velocity .Comment: 15 pages, revtex, no figure
Stable Camera Motion Estimation Using Convex Programming
We study the inverse problem of estimating n locations (up to
global scale, translation and negation) in from noisy measurements of a
subset of the (unsigned) pairwise lines that connect them, that is, from noisy
measurements of for some pairs (i,j) (where the
signs are unknown). This problem is at the core of the structure from motion
(SfM) problem in computer vision, where the 's represent camera locations
in . The noiseless version of the problem, with exact line measurements,
has been considered previously under the general title of parallel rigidity
theory, mainly in order to characterize the conditions for unique realization
of locations. For noisy pairwise line measurements, current methods tend to
produce spurious solutions that are clustered around a few locations. This
sensitivity of the location estimates is a well-known problem in SfM,
especially for large, irregular collections of images.
In this paper we introduce a semidefinite programming (SDP) formulation,
specially tailored to overcome the clustering phenomenon. We further identify
the implications of parallel rigidity theory for the location estimation
problem to be well-posed, and prove exact (in the noiseless case) and stable
location recovery results. We also formulate an alternating direction method to
solve the resulting semidefinite program, and provide a distributed version of
our formulation for large numbers of locations. Specifically for the camera
location estimation problem, we formulate a pairwise line estimation method
based on robust camera orientation and subspace estimation. Lastly, we
demonstrate the utility of our algorithm through experiments on real images.Comment: 40 pages, 12 figures, 6 tables; notation and some unclear parts
updated, some typos correcte
Pose Invariant Gait Analysis And Reconstruction
One of the unique advantages of human gait is that it can be perceived from a distance. A varied range of research has been undertaken within the field of gait recognition. However, in almost all circumstances subjects have been constrained to walk fronto-parallel to the camera with a single walking speed. In this thesis we show that gait has sufficient properties that allows us to exploit the structure of articulated leg motion within single view sequences, in order to remove the unknown subject pose and reconstruct the underlying gait signature, with no prior knowledge of the camera calibration. Articulated leg motion is approximately planar, since almost all of the perceived motion is contained within a single limb swing plane. The variation of motion out of this plane is subtle and negligible in comparison to this major plane of motion. Subsequently, we can model human motion by employing a cardboard person assumption. A subject's body and leg segments may be represented by repeating spatio-temporal motion patterns within a set of bilaterally symmetric limb planes. The static features of gait are defined as quantities that remain invariant over the full range of walking motions. In total, we have identified nine static features of articulated leg motion, corresponding to the fronto-parallel view of gait, that remain invariant to the differences in the mode of subject motion. These features are hypothetically unique to each individual, thus can be used as suitable parameters for biometric identification. We develop a stratified approach to linear trajectory gait reconstruction that uses the rigid bone lengths of planar articulated leg motion in order to reconstruct the fronto-parallel view of gait. Furthermore, subject motion commonly occurs within a fixed ground plane and is imaged by a static camera. In general, people tend to walk in straight lines with constant velocity. Imaged gait can then be split piecewise into natural segments of linear motion. If two or more sufficiently different imaged trajectories are available then the calibration of the camera can be determined. Subsequently, the total pattern of gait motion can be globally parameterised for all subjects within an image sequence. We present the details of a sparse method that computes the maximum likelihood estimate of this set of parameters, then conclude with a reconstruction error analysis corresponding to an example image sequence of subject motion
Integral Field Unit Observations of NGC 891: Kinematics of the Diffuse Ionized Gas Halo
We present high and moderate spectral resolution spectroscopy of diffuse
ionized gas (DIG) emission in the halo of NGC 891. The data were obtained with
the SparsePak integral field unit at the WIYN Observatory. The wavelength
coverage includes the [NII]6548,6583, Halpha, and [SII]6716,6731 emission
lines. Position-velocity (PV) diagrams, constructed using spectra extracted
from four SparsePak pointings in the halo, are used to examine the kinematics
of the DIG. Using two independent methods, a vertical gradient in azimuthal
velocity is found to be present in the northeast quadrant of the halo, with
magnitude approximately 15-18 km/s/kpc, in agreement with results from HI
observations. The kinematics of the DIG suggest that this gradient begins at
approximately 1 kpc above the midplane. In another part of the halo, the
southeast quadrant, the kinematics are markedly different, and suggest rotation
at about 175 km/s, much slower than the disk but with no vertical gradient. We
utilize an entirely ballistic model of disk-halo flow in an attempt to
reproduce the kinematics observed in the northeast quadrant. Analysis shows
that the velocity gradient predicted by the ballistic model is far too shallow.
Based on intensity cuts made parallel to the major axis in the ballistic model
and an Halpha image of NGC 891 from the literature, we conclude that the DIG
halo is much more centrally concentrated than the model, suggesting that
hydrodynamics dominate over ballistic motion in shaping the density structure
of the halo. Velocity dispersion measurements along the minor axis of NGC 891
seem to indicate a lack of radial motions in the halo, but the uncertainties do
not allow us to set firm limits.Comment: 31 pages, 10 figures. Accepted for publication in the Astrophysical
Journa
MHD Modeling for Formation Process of Coronal Mass Ejections: Interaction between Ejecting Flux Rope and Ambient Field
We performed magnetohydrodynamic simulation of a formation process of coronal
mass ejections (CMEs), focusing on interaction (reconnection) between an
ejecting flux rope and its ambient field. We examined three cases with
different ambient fields: no ambient field, and cases with dipole field of two
opposite directions which are parallel and anti-parallel to that of the flux
rope surface. As a result, while the flux rope disappears in the anti-parallel
case, in other cases the flux ropes can evolve to CMEs and show different
amounts of rotation of the flux rope. The results imply that the interaction
between an ejecting flux rope and its ambient field is an important process for
determining CME formation and CME orientation, and also show that the amount
and direction of magnetic flux within the flux rope and the ambient field are
key parameters for CME formation. Especially, the interaction (reconnection)
plays a significant role to the rotation of the flux rope, with a process
similar to "tilting instability" in a spheromak-type experiment of laboratory
plasma.Comment: 24 pages, 5 figures. Accepted for publication in Ap
The structure of standing Alfvén waves in a dipole magnetosphere with moving plasma
The structure and spectrum of standing Alfv&#233;n waves were theoretically investigated in a dipole magnetosphere with moving plasma. Plasma motion was simulated with its azimuthal rotation. The model's scope allowed for describing a transition from the inner plasmasphere at rest to the outer magnetosphere with convecting plasma and, through the magnetopause, to the moving plasma of the solar wind. Solutions were found to equations describing longitudinal and transverse (those formed, respectively, along field lines and across magnetic shells) structures of standing Alfv&#233;n waves with high azimuthal wave numbers <i>m</i>>>1. Spectra were constructed for a number of first harmonics of poloidal and toroidal standing Alfv&#233;n waves inside the magnetosphere. For charged particles with velocities greatly exceeding the velocity of the background plasma, an effective parallel wave component of the electric field appears in the region occupied by such waves. This results in structured high-energy-particle flows and in the appearance of multiband aurorae. The transverse structure of the standing Alfv&#233;n waves' basic harmonic was shown to be analogous to the structure of a discrete auroral arc
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