14,408 research outputs found
Solving the Direction Field for Discrete Agent Motion
Models for pedestrian dynamics are often based on microscopic approaches
allowing for individual agent navigation. To reach a given destination, the
agent has to consider environmental obstacles. We propose a direction field
calculated on a regular grid with a Moore neighborhood, where obstacles are
represented by occupied cells. Our developed algorithm exactly reproduces the
shortest path with regard to the Euclidean metric.Comment: 8 pages, 4 figure
Correction of Optical Aberrations in Elliptic Neutron Guides
Modern, nonlinear ballistic neutron guides are an attractive concept in
neutron beam delivery and instrumentation, because they offer increased
performance over straight or linearly tapered guides. However, like other
ballistic geometries they have the potential to create significantly
non-trivial instrumental resolution functions. We address the source of the
most prominent optical aberration, namely coma, and we show that for extended
sources the off-axis rays have a different focal length from on-axis rays,
leading to multiple reflections in the guide system. We illustrate how the
interplay between coma, sources of finite size, and mirrors with non-perfect
reflectivity can therefore conspire to produce uneven distributions in the
neutron beam divergence, the source of complicated resolution functions. To
solve these problems, we propose a hybrid elliptic-parabolic guide geometry.
Using this new kind of neutron guide shape, it is possible to condition the
neutron beam and remove almost all of the aberrations, whilst providing the
same performance in beam current as a standard elliptic neutron guide. We
highlight the positive implications for a number of neutron scattering
instrument types that this new shape can bring.Comment: Presented at NOP2010 Conference in Alpe d'Huez, France, in March 201
Evidence for Orbital Motion of Material Close to the Central Black Hole of Mrk 766
Time-resolved X-ray spectroscopy has been obtained for the narrow line
Seyfert galaxy Mrk766 from XMM-Newton observations. We present analysis in the
energy-time plane of EPIC pn data in the 4-8 keV band with energy resolution
R~50. A component of Fe Ka emission detected in the maps shows a variation of
photon energy with time that appears both to be statistically significant and
to be consistent with sinusoidal variation. We investigate the interpretation
that there exists a component of line emission from matter in a Keplerian orbit
around a supermassive black hole. The orbit has a period ~165 ks and a
line-of-sight velocity ~13,500 km/s. This yields a lower limit for the central
mass of M > 4.9x10^5 solar masses within a radius of 3.6 x 10^13 cm (2.4 A.U.).
The orbit parameters are consistent with higher black hole masses, but the lack
of any substantial gravitational redshift of the orbit implies an upper limit
to the black hole mass of 4.5x10^7 solar masses.Comment: 20 pages, 6 figures (some colour). Accepted for publication in A&A.
Only minor changes since V1 (including reordering of Figs 1a & b
Intima-Media Thickness: Setting a Standard for a Completely Automated Method of Ultrasound Measurement
The intima - media thickness (IMT) of the common carotid artery is a widely used clinical marker of severe cardiovascular diseases. IMT is usually manually measured on longitudinal B-Mode ultrasound images. Many computer-based techniques for IMT measurement have been proposed to overcome the limits of manual segmentation. Most of these, however, require a certain degree of user interaction. In this paper we describe a new completely automated layers extraction (CALEXia) technique for the segmentation and IMT measurement of carotid wall in ultrasound images. CALEXia is based on an integrated approach consisting of feature extraction, line fitting, and classification that enables the automated tracing of the carotid adventitial walls. IMT is then measured by relying on a fuzzy K-means classifier. We tested CALEXia on a database of 200 images. We compared CALEXia performances to those of a previously developed methodology that was based on signal analysis (CULEXsa). Three trained operators manually segmented the images and the average profiles were considered as the ground truth. The average error from CALEXia for lumen - intima (LI) and media - adventitia (MA) interface tracings were 1.46 ± 1.51 pixel (0.091 ± 0.093 mm) and 0.40 ± 0.87 pixel (0.025 ± 0.055 mm), respectively. The corresponding errors for CULEXsa were 0.55 ± 0.51 pixels (0.035 ± 0.032 mm) and 0.59 ± 0.46 pixels (0.037 ± 0.029 mm). The IMT measurement error was equal to 0.87 ± 0.56 pixel (0.054 ± 0.035 mm) for CALEXia and 0.12 ± 0.14 pixel (0.01 ± 0.01 mm) for CULEXsa. Thus, CALEXia showed limited performance in segmenting the LI interface, but outperformed CULEXsa in the MA interface and in the number of images correctly processed (10 for CALEXia and 16 for CULEXsa). Based on two complementary strategies, we anticipate fusing them for further IMT improvement
Factors contributing to the ballistocardiographic wave form in healthy middle aged males
Factors contributing to ballistocardiographic waveform in healthy middle aged male
- …