113,168 research outputs found
Three-dimensional decomposition of galaxies with bulge and long bar
Some observations indicate that the Milky Way has two inner components, a
bulge and a long bar, which present a misalignment of about 20 degrees that is
against the predictions of some theoretical models that are based on numerical
simulations. In this paper, we wish to determine whether this misalignment
between the bar and the bulge can be observed in barred galaxies other than the
Milky Way. For that, each galaxy of our sample was decomposed based on its
Ks-band 2MASS image by fitting and modelling in a three-dimensional (3D) space
the following components: a disc, a bar, and a bulge. The chi-square
goodness-of-fit estimation allowed retrieving the best-fit angle values for the
bar and the bulge to detect any misalignment. From the 3D decomposition of six
barred galaxies, we have detected at least three galaxies (NGC 2217, NGC 3992,
and NGC 4593) that present a significant misalignment between the bar and the
bulge of more than 20 degrees.Comment: 8 pages, 1 figure. Accepted for publication in A&A. Corrected typo
Hierarchical structure-and-motion recovery from uncalibrated images
This paper addresses the structure-and-motion problem, that requires to find
camera motion and 3D struc- ture from point matches. A new pipeline, dubbed
Samantha, is presented, that departs from the prevailing sequential paradigm
and embraces instead a hierarchical approach. This method has several
advantages, like a provably lower computational complexity, which is necessary
to achieve true scalability, and better error containment, leading to more
stability and less drift. Moreover, a practical autocalibration procedure
allows to process images without ancillary information. Experiments with real
data assess the accuracy and the computational efficiency of the method.Comment: Accepted for publication in CVI
Registration of Standardized Histological Images in Feature Space
In this paper, we propose three novel and important methods for the
registration of histological images for 3D reconstruction. First, possible
intensity variations and nonstandardness in images are corrected by an
intensity standardization process which maps the image scale into a standard
scale where the similar intensities correspond to similar tissues meaning.
Second, 2D histological images are mapped into a feature space where continuous
variables are used as high confidence image features for accurate registration.
Third, we propose an automatic best reference slice selection algorithm that
improves reconstruction quality based on both image entropy and mean square
error of the registration process. We demonstrate that the choice of reference
slice has a significant impact on registration error, standardization, feature
space and entropy information. After 2D histological slices are registered
through an affine transformation with respect to an automatically chosen
reference, the 3D volume is reconstructed by co-registering 2D slices
elastically.Comment: SPIE Medical Imaging 2008 - submissio
What is the orientation of the tip in a scanning tunneling microscope?
We introduce a statistical correlation analysis method to obtain information
on the local geometry and orientation of the tip used in scanning tunneling
microscopy (STM) experiments based on large scale simulations. The key quantity
is the relative brightness correlation of constant-current topographs between
experimental and simulated data. This correlation can be analyzed statistically
for a large number of modeled tip orientations and geometries. Assuming a
stable tip during the STM scans and based on the correlation distribution, it
is possible to determine the tip orientations that are most likely present in
an STM experiment, and exclude other orientations. This is especially important
for substrates such as highly oriented pyrolytic graphite (HOPG) since its STM
contrast is strongly tip dependent, which makes interpretation and comparison
of STM images very challenging. We illustrate the applicability of our method
considering the HOPG surface in combination with tungsten tip models of two
different apex geometries and 18144 different orientations. We calculate
constant-current profiles along the direction of the HOPG(0001)
surface in the V bias voltage range, and compare them with
experimental data. We find that a blunt tip model provides better correlation
with the experiment for a wider range of tip orientations and bias voltages
than a sharp tip model. Such a combination of experiments and large scale
simulations opens up the way for obtaining more detailed information on the
structure of the tip apex and more reliable interpretation of STM data in the
view of local tip geometry effects.Comment: Progress in Surface Science, accepted for publication, 25 pages
manuscript, 9 figures, abstract shortene
Histogram of Oriented Principal Components for Cross-View Action Recognition
Existing techniques for 3D action recognition are sensitive to viewpoint
variations because they extract features from depth images which are viewpoint
dependent. In contrast, we directly process pointclouds for cross-view action
recognition from unknown and unseen views. We propose the Histogram of Oriented
Principal Components (HOPC) descriptor that is robust to noise, viewpoint,
scale and action speed variations. At a 3D point, HOPC is computed by
projecting the three scaled eigenvectors of the pointcloud within its local
spatio-temporal support volume onto the vertices of a regular dodecahedron.
HOPC is also used for the detection of Spatio-Temporal Keypoints (STK) in 3D
pointcloud sequences so that view-invariant STK descriptors (or Local HOPC
descriptors) at these key locations only are used for action recognition. We
also propose a global descriptor computed from the normalized spatio-temporal
distribution of STKs in 4-D, which we refer to as STK-D. We have evaluated the
performance of our proposed descriptors against nine existing techniques on two
cross-view and three single-view human action recognition datasets. The
Experimental results show that our techniques provide significant improvement
over state-of-the-art methods
Difference of Normals as a Multi-Scale Operator in Unorganized Point Clouds
A novel multi-scale operator for unorganized 3D point clouds is introduced.
The Difference of Normals (DoN) provides a computationally efficient,
multi-scale approach to processing large unorganized 3D point clouds. The
application of DoN in the multi-scale filtering of two different real-world
outdoor urban LIDAR scene datasets is quantitatively and qualitatively
demonstrated. In both datasets the DoN operator is shown to segment large 3D
point clouds into scale-salient clusters, such as cars, people, and lamp posts
towards applications in semi-automatic annotation, and as a pre-processing step
in automatic object recognition. The application of the operator to
segmentation is evaluated on a large public dataset of outdoor LIDAR scenes
with ground truth annotations.Comment: To be published in proceedings of 3DIMPVT 201
Quantitative Measurements of CME-driven Shocks from LASCO Observations
In this paper, we demonstrate that CME-driven shocks can be detected in white
light coronagraph images and in which properties such as the density
compression ratio and shock direction can be measured. Also, their propagation
direction can be deduced via simple modeling. We focused on CMEs during the
ascending phase of solar cycle 23 when the large-scale morphology of the corona
was simple. We selected events which were good candidates to drive a shock due
to their high speeds (V>1500 km/s). The final list includes 15 CMEs. For each
event, we calibrated the LASCO data, constructed excess mass images and
searched for indications of faint and relatively sharp fronts ahead of the
bright CME front. We found such signatures in 86% (13/15) of the events and
measured the upstream/downstream densities to estimate the shock strength. Our
values are in agreement with theoretical expectations and show good
correlations with the CME kinetic energy and momentum. Finally, we used a
simple forward modeling technique to estimate the 3D shape and orientation of
the white light shock features. We found excellent agreement with the observed
density profiles and the locations of the CME source regions. Our results
strongly suggest that the observed brightness enhancements result from density
enhancements due to a bow-shock structure driven by the CME.Comment: to be published in Astrophysical Journa
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