13,831 research outputs found
A massively parallel multi-level approach to a domain decomposition method for the optical flow estimation with varying illumination
We consider a variational method to solve the optical flow problem with
varying illumination. We apply an adaptive control of the regularization
parameter which allows us to preserve the edges and fine features of the
computed flow. To reduce the complexity of the estimation for high resolution
images and the time of computations, we implement a multi-level parallel
approach based on the domain decomposition with the Schwarz overlapping method.
The second level of parallelism uses the massively parallel solver MUMPS. We
perform some numerical simulations to show the efficiency of our approach and
to validate it on classical and real-world image sequences
Scattering line polarization in rotating, optically thick disks
To interpret observations of astrophysical disks it is essential to
understand the formation process of the emitted light. If the disk is optically
thick, scattering dominated and permeated by a Keplerian velocity field,
Non-Local Thermodynamic Equilibrium radiative transfer modeling must be done to
compute the emergent spectrum from a given disk model. We investigate Non-local
thermodynamic equilibrium polarized line formation in different simple disk
models and aim to demonstrate the importance of both radiative transfer effects
and scattering as well as the effects of velocity fields. We self-consistently
solve the coupled equations of radiative transfer and statistical equilibrium
for a two level atom model by means of Jacobi iteration. We compute scattering
polarization, that is Q/I and U/I line profiles. The degree of scattering
polarization is significantly influenced by the inclination of the disk with
respect to observer, but also by the optical thickness of the disk and the
presence of rotation. Stokes U shows double-lobed profiles with amplitude which
increases with the disk rotation. Our results suggest that the line profiles,
especially the polarized ones, emerging from gaseous disks differ significantly
from the profiles predicted by simple approximations. The profiles are diverse
in shape, but typically symmetric in Stokes Q and antisymmetric in Stokes U. A
clear indicator of disk rotation is the presence of Stokes U, which might prove
to be a useful diagnostic tool. We also demonstrate that, for moderate
rotational velocities, an approximate treatment can be used, where non-local
thermodynamic equilibrium radiative transfer is done in the velocity field-free
approximation and Doppler shift is applied in the process of spatial
integration over the whole emitting surface.Comment: 16 pages; 12 figures; Accepted with revision for A&A. This is the
version after first round of referee's suggestion
Photoacoustic generation by a gold nanosphere: From linear to nonlinear thermoelastics in the long-pulse illumination regime
We investigate theoretically the photoacoustic generation by a gold
nanosphere in water in the thermoelastic regime. Specifically, we consider the
long-pulse illumination regime, in which the time for electron-phonon
thermalisation can be neglected and photoacoustic wave generation arises solely
from the thermo-elastic stress caused by the temperature increase of the
nanosphere or its liquid environment. Photoacoustic signals are predicted
computed based on the successive resolution of a thermal diffusion problem and
a thermoelastic problem, taking into account the finite size of the gold
nanosphere and the temperature-dependence of the thermal expansion coefficient
of water. For sufficiently high illumination fluences, this temperature
dependence yields a nonlinear relationship between the photoacoustic amplitude
and the fluence. For nanosecond pulses in the linear regime, we show that more
than 90 % of the emitted photoacoustic energy is generated in water, and the
thickness of the generating layer around the particle scales close to the
square root of the pulse duration. Our results demonstrate that the
point-absorber model introduced by Calasso et al.[17] significantly
overestimates the amplitude of photoacoustic waves in the nonlinear regime. We
therefore provide quantitative estimates of a critical energy, defined as the
absorbed energy required such that the nonlinear contribution is equal to that
of the linear contribution. Our results suggest that the critical energy scales
as the volume of water over which heat diffuses during the illumination pulse.
Moreover, thermal nonlinearity is shown to be expected only for sufficiently
high ultrasound frequency. Finally, we show that the relationship between the
photoacoustic amplitude and the equilibrium temperature at sufficiently high
fluence reflects the thermal diffusion at the nanoscale around the gold
nanosphere.Comment: Published in Physical Review B, 16 pages, 14 figure
Procedural Modeling and Physically Based Rendering for Synthetic Data Generation in Automotive Applications
We present an overview and evaluation of a new, systematic approach for
generation of highly realistic, annotated synthetic data for training of deep
neural networks in computer vision tasks. The main contribution is a procedural
world modeling approach enabling high variability coupled with physically
accurate image synthesis, and is a departure from the hand-modeled virtual
worlds and approximate image synthesis methods used in real-time applications.
The benefits of our approach include flexible, physically accurate and scalable
image synthesis, implicit wide coverage of classes and features, and complete
data introspection for annotations, which all contribute to quality and cost
efficiency. To evaluate our approach and the efficacy of the resulting data, we
use semantic segmentation for autonomous vehicles and robotic navigation as the
main application, and we train multiple deep learning architectures using
synthetic data with and without fine tuning on organic (i.e. real-world) data.
The evaluation shows that our approach improves the neural network's
performance and that even modest implementation efforts produce
state-of-the-art results.Comment: The project web page at
http://vcl.itn.liu.se/publications/2017/TKWU17/ contains a version of the
paper with high-resolution images as well as additional materia
An efficient 1-D periodic boundary integral equation technique to analyze radiation onto straight and meandering microstrip lines
A modeling technique to analyze the radiation onto arbitrary 1-D periodic metallizations residing on a microstrip substrate is presented. In particular, straight and meandering lines are being studied. The method is based on a boundary integral equation, more specifically on a mixed potential integral equation (MPIE), that is solved by means of the method of moments. A plane wave excites the microstrip structure, and according to the Floquet-Bloch theorem, the analysis can be restricted to one single unit cell. Thereto, the MPIE must be constructed using the pertinent 1-D periodic layered medium Green's functions. Here, these Green's functions are obtained in closed form by invoking the perfectly matched layer paradigm. The proposed method is applied to assess the radiation onto 1) a semi-infinite plate, 2) a straight microstrip line, and 3) a serpentine delay line. These three types of examples clearly illustrate and validate the method. Also, its efficiency, compared to a previously developed fast microstrip analysis technique, is demonstrated
X-ray reflection spectra from ionized slabs
X-ray reflection spectra are an important component in the X-ray spectra of
many active galactic nuclei and Galactic black hole candidates. It is likely
that reflection takes place from highly ionized surfaces of the accretion disc
in some cases. This can lead to strong Comptonization of the emergent iron, and
other, absorption and emission features. We present such reflection spectra
here, computed in a self-consistent manner with the method described by Ross
and Fabian. In particular we emphasise the range where the ionization parameter
(the flux to density ratio) \xi is around and above 10^4. Such spectra may be
relevant to the observed spectral features found in black hole candidates such
as Cygnus X-1 in the low/hard state.Comment: 7 pages with 5 postscript figures. Accepted for publication in MNRA
Vision-Based Road Detection in Automotive Systems: A Real-Time Expectation-Driven Approach
The main aim of this work is the development of a vision-based road detection
system fast enough to cope with the difficult real-time constraints imposed by
moving vehicle applications. The hardware platform, a special-purpose massively
parallel system, has been chosen to minimize system production and operational
costs. This paper presents a novel approach to expectation-driven low-level
image segmentation, which can be mapped naturally onto mesh-connected massively
parallel SIMD architectures capable of handling hierarchical data structures.
The input image is assumed to contain a distorted version of a given template;
a multiresolution stretching process is used to reshape the original template
in accordance with the acquired image content, minimizing a potential function.
The distorted template is the process output.Comment: See http://www.jair.org/ for any accompanying file
Solid state photomultiplier for astronomy, phase 2
Epitaxial layers with varying donor concentration profiles were grown on silicon substrate wafers using chemical vapor deposition (CVD) techniques, and solid state photomultiplier (SSPM) devices were fabricated from the wafers. Representative detectors were tested in a low background photon flux, low temperature environment to determine the device characteristics for comparison to NASA goals for astronomical applications. The SSPM temperatures varied between 6 and 11 K with background fluxes in the range from less than 5 x 10 to the 6th power to 10 to the 13th power photons/square cm per second at wavelengths of 3.2 and 20 cm. Measured parameters included quantum efficiency, dark count rate and bias current. Temperature for optimal performance is 10 K, the highest ever obtained for SSPMs. The devices exhibit a combination of the lowest dark current and highest quantum efficiency yet achieved. Experimental data were reduced, analyzed and used to generate recommendations for future studies. The background and present status of the microscopic theory of SSPM operation were reviewed and summarized. Present emphasis is on modeling of the avalanche process which is the basis for SSPM operation. Approaches to the solution of the Boltzmann transport equation are described and the treatment of electron scattering mechanisms is presented. The microscopic single-electron transport theory is ready to be implemented for large-scale computations
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