37,397 research outputs found
Passive Non-line-of-sight Imaging for Moving Targets with an Event Camera
Non-line-of-sight (NLOS) imaging is an emerging technique for detecting
objects behind obstacles or around corners. Recent studies on passive NLOS
mainly focus on steady-state measurement and reconstruction methods, which show
limitations in recognition of moving targets. To the best of our knowledge, we
propose a novel event-based passive NLOS imaging method. We acquire
asynchronous event-based data which contains detailed dynamic information of
the NLOS target, and efficiently ease the degradation of speckle caused by
movement. Besides, we create the first event-based NLOS imaging dataset,
NLOS-ES, and the event-based feature is extracted by time-surface
representation. We compare the reconstructions through event-based data with
frame-based data. The event-based method performs well on PSNR and LPIPS, which
is 20% and 10% better than frame-based method, while the data volume takes only
2% of traditional method
Non-line-of-sight transient rendering
The capture and analysis of light in flight, or light in transient state, has enabled applications such as range imaging, reflectance estimation and especially non-line-of-sight (NLOS) imaging. For this last case, hidden geometry can be reconstructed using time-resolved measurements of indirect diffuse light emitted by a laser. Transient rendering is a key tool for developing such new applications, significantly more challenging than its steady-state counterpart. In this work, we introduce a set of simple yet effective subpath sampling techniques targeting transient light transport simulation in occluded scenes. We analyze the usual capture setups of NLOS scenes, where both the camera and light sources are focused on particular points in the scene. Also, the hidden geometry can be difficult to sample using conventional techniques. We leverage that configuration to reduce the integration path space. We implement our techniques in a modified version of Mitsuba 2 adapted for transient light transport, allowing us to support parallelization, polarization, and differentiable rendering. © 2022 The Author(s
Differentiable Transient Rendering
Recent differentiable rendering techniques have become key tools to tackle many inverse problems in graphics and vision. Existing models, however, assume steady-state light transport, i.e., infinite speed of light. While this is a safe assumption for many applications, recent advances in ultrafast imaging leverage the wealth of information that can be extracted from the exact time of flight of light. In this context, physically-based transient rendering allows to efficiently simulate and analyze light transport considering that the speed of light is indeed finite. In this paper, we introduce a novel differentiable transient rendering framework, to help bring the potential of differentiable approaches into the transient regime. To differentiate the transient path integral we need to take into account that scattering events at path vertices are no longer independent; instead, tracking the time of flight of light requires treating such scattering events at path vertices jointly as a multidimensional, evolving manifold. We thus turn to the generalized transport theorem, and introduce a novel correlated importance term, which links the time-integrated contribution of a path to its light throughput, and allows us to handle discontinuities in the light and sensor functions. Last, we present results in several challenging scenarios where the time of flight of light plays an important role such as optimizing indices of refraction, non-line-of-sight tracking with nonplanar relay walls, and non-line-of-sight tracking around two corners
Gas flow and dark matter in the inner parts of early-type barred galaxies
This paper presents the dynamical simulations run in the potential derived
from the light distribution of 5 late-type barred spiral galaxies. The aim is
to determine whether the mass distribution together with the hydrodynamical
simulations can reproduce the observed line-of-sight velocity curves and the
gas morphology in the inner regions of the sample barred galaxies. The light
distribution is obtained from the -band and the -band combined together.
The M/L is determined using population synthesis models. The observations and
the methodology of the mass distribution modelling are presented in a companion
paper. The SPH models using the stellar mass models obtained directly from the
-band light distributions give a good representation of the gas distribution
and dynamics of the modelled galaxies, supporting the maximum disk assumption.
This result indicates that the gravitational field in the inner region is
mostly provided by the stellar luminous component. When 40% of the total mass
is transferred to an axisymmetric dark halo, the modelled kinematics clearly
depart from the observed kinematics, whereas the departures are negligible for
dark mass halos of 5% and 20% of the total mass. This result sets a lower limit
for the contribution of the luminous component of about 80%, which is in
agreement with the maximum disk definition of the stellar mass contribution to
the rotation curve (about 85%10).Comment: 28 pages, 30 figures. Accepted for publication in A&A on 17/05/2004.
High resolution figures on publicatio
A closer look at a coronal loop rooted in a sunspot umbra
Extreme UV (EUV) and X-ray loops in the solar corona connect regions of
enhanced magnetic activity, but they are not usually rooted in the dark umbrae
of sunspots because the strong magnetic field found there suppresses
convection. This means that the Poynting flux of magnetic energy into the upper
atmosphere is not significant within the umbra as long as there are no light
bridges or umbral dots. Here we report a rare observation of a coronal loop
rooted in the dark umbra of a sunspot without any traces of light bridges or
umbral dots. We used the slit-jaw images and spectroscopic data from IRIS and
concentrate on the line profiles of O IV and Si IV that show persistent strong
redshifted components in the loop rooted in the umbra. Using the ratios of O
IV, we can estimate the density and thus investigate the mass flux. The coronal
context and temperature diagnostics of these observations is provided through
the EUV channels of AIA. The coronal loop, embedded within cooler downflows,
hosts supersonic downflows. The speed of more than 100 km s is on the
same order of magnitude in the transition region lines of O IV and Si IV, and
is even seen at comparable speed in the chromospheric Mg II lines. At a
projected distance of within of the footpoint, we see a shock transition
to smaller downflow speeds of about 15 km s being consistent with mass
conservation across a stationary isothermal shock. We see no direct evidence
for energy input into the loop because the loop is rooted in the dark uniform
part of the umbra with no light bridges or umbral dots near by. Thus one might
conclude that we are seeing a siphon flow driven from the footpoint at the
other end of the loop. However, for a final result data of similar quality at
the other footpoint are needed, but this is too far away to be covered by the
IRIS field of view.Comment: Accepted for publication in Astronomy and Astrophysics (abridged
abstract
Multiphase turbulent interstellar medium: some recent results from radio astronomy
The radio frequency 1.4 GHz transition of the atomic hydrogen is one of the
important tracers of the diffuse neutral interstellar medium. Radio
astronomical observations of this transition, using either a single dish
telescope or an array interferometer, reveal different properties of the
interstellar medium. Such observations are particularly useful to study the
multiphase nature and turbulence in the interstellar gas. Observations with
multiple radio telescopes have recently been used to study these two closely
related aspects in greater detail. Using various observational techniques, the
density and the velocity fluctuations in the Galactic interstellar medium was
found to have a Kolmogorov-like power law power spectra. The observed power law
scaling of the turbulent velocity dispersion with the length scale can be used
to derive the true temperature distribution of the medium. Observations from a
large ongoing atomic hydrogen absorption line survey have also been used to
study the distribution of gas at different temperature. The thermal steady
state model predicts that the multiphase neutral gas will exist in cold and
warm phase with temperature below 200 K and above 5000 K respectively. However,
these observations clearly show the presence of a large fraction of gas in the
intermediate unstable phase. These results raise serious doubt about the
validity of the standard model, and highlight the necessity of alternative
theoretical models. Interestingly, numerical simulations suggest that some of
the observational results can be explained consistently by including the
effects of turbulence in the models of the multiphase medium. This review
article presents a brief outline of some of the basic ideas of radio
astronomical observations and data analysis, summarizes the results from recent
observations, and discusses possible implications of the results.Comment: 20 pages, 10 figures. Invited review accepted for publication in the
Proceedings of the Indian National Science Academy. The definitive version
will be available at http://insaindia.org/journals/proceedings.ph
3D Structure of Microwave Sources from Solar Rotation Stereoscopy vs Magnetic Extrapolations
We use rotation stereoscopy to estimate the height of a steady-state solar
feature relative to the photosphere, based on its apparent motion in the image
plane recorded over several days of observation. The stereoscopy algorithm is
adapted to work with either one- or two-dimensional data (i.e. from images or
from observations that record the projected position of the source along an
arbitrary axis). The accuracy of the algorithm is tested on simulated data, and
then the algorithm is used to estimate the coronal radio source heights
associated with the active region NOAA 10956, based on multifrequency imaging
data over 7 days from the Siberian Solar Radio Telescope near 5.7 GHz, the
Nobeyama Radio Heliograph at 17 GHz, as well as one-dimensional scans at
multiple frequencies spanning the 5.98--15.95 GHz frequency range from the
RATAN-600 instrument. The gyroresonance emission mechanism, which is sensitive
to the coronal magnetic field strength, is applied to convert the estimated
radio source heights at various frequencies, h(f), to information about
magnetic field vs. height B(h), and the results are compared to a magnetic
field extrapolation derived from photospheric magnetic field observations
obtained by Hinode and MDI. We found that the gyroresonant emission comes from
the heights exceeding location of the third gyrolayer irrespectively on the
magnetic extrapolation method; implications of this finding for the coronal
magnetography and coronal plasma physics are discussed.Comment: 26 pages, 13 figures, ApJ accepte
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