431,773 research outputs found
Local Higher-Order Statistics (LHS) describing images with statistics of local non-binarized pixel patterns
Accepted for publication in International Journal of Computer Vision and Image Understanding (CVIU)International audienceWe propose a new image representation for texture categorization and facial analysis, relying on the use of higher-order local differential statistics as features. It has been recently shown that small local pixel pattern distributions can be highly discriminative while being extremely efficient to compute, which is in contrast to the models based on the global structure of images. Motivated by such works, we propose to use higher-order statistics of local non-binarized pixel patterns for the image description. The proposed model does not require either (i) user specified quantization of the space (of pixel patterns) or (ii) any heuristics for discarding low occupancy volumes of the space. We propose to use a data driven soft quantization of the space, with parametric mixture models, combined with higher-order statistics, based on Fisher scores. We demonstrate that this leads to a more expressive representation which, when combined with discriminatively learned classifiers and metrics, achieves state-of-the-art performance on challenging texture and facial analysis datasets, in low complexity setup. Further, it is complementary to higher complexity features and when combined with them improves performance
Bilevel Parameter Learning for Higher-Order Total Variation Regularisation Models.
We consider a bilevel optimisation approach for parameter learning in higher-order total variation image reconstruction models. Apart from the least squares cost functional, naturally used in bilevel learning, we propose and analyse an alternative cost based on a Huber-regularised TV seminorm. Differentiability properties of the solution operator are verified and a first-order optimality system is derived. Based on the adjoint information, a combined quasi-Newton/semismooth Newton algorithm is proposed for the numerical solution of the bilevel problems. Numerical experiments are carried out to show the suitability of our approach and the improved performance of the new cost functional. Thanks to the bilevel optimisation framework, also a detailed comparison between TGV 2 and ICTV is carried out, showing the advantages and shortcomings of both regularisers, depending on the structure of the processed images and their noise level.King Abdullah University of Science and Technology (KAUST) (Grant ID: KUKI1-007-43), Engineering and Physical Sciences Research Council (Grant IDs: Nr. EP/J009539/1 “Sparse & Higher-order Image Restoration” and Nr. EP/M00483X/1 “Efficient computational tools for inverse imaging problems”), Escuela Politécnica Nacional de Quito (Grant ID: PIS 12-14, MATHAmSud project SOCDE “Sparse Optimal Control of Differential Equations”), Leverhulme Trust (project on “Breaking the non-convexity barrier”), SENESCYT (Ecuadorian Ministry of Higher Education, Science, Technology and Innovation) (Prometeo Fellowship)This is the final version of the article. It first appeared from Springer via http://dx.doi.org/10.1007/s10851-016-0662-
Simulation of spin-polarized scanning tunneling microscopy on complex magnetic surfaces: Case of a Cr monolayer on Ag(111)
We propose an atom-superposition-based method for simulating spin-polarized
scanning tunneling microscopy (SP-STM) from first principles. Our approach
provides bias dependent STM images in high spatial resolution, with the
capability of using either constant current or constant height modes of STM. In
addition, topographic and magnetic contributions can clearly be distinguished,
which are directly comparable to results of SP-STM experiments in the
differential magnetic mode. Advantages of the proposed method are that it is
computationally cheap, it is easy to parallelize, and it can employ the results
of any ab initio electronic structure code. Its capabilities are illustrated
for the prototype frustrated hexagonal antiferromagnetic system, Cr monolayer
on Ag(111) in a noncollinear magnetic N\'eel state. We show
evidence that the magnetic contrast is sensitive to the tip electronic
structure, and this contrast can be reversed depending on the bias voltage.Comment: 28 pages manuscript, 1 table, 5 figure
Gas dynamics in the inner few AU around the Herbig B[e] star MWC297: Indications of a disk wind from kinematic modeling and velocity-resolved interferometric imaging
We present near-infrared AMBER (R = 12, 000) and CRIRES (R = 100, 000)
observations of the Herbig B[e] star MWC297 in the hydrogen Br-gamma-line.
Using the VLTI unit telescopes, we obtained a uv-coverage suitable for aperture
synthesis imaging. We interpret our velocity-resolved images as well as the
derived two-dimensional photocenter displacement vectors, and fit kinematic
models to our visibility and phase data in order to constrain the gas velocity
field on sub-AU scales. The measured continuum visibilities constrain the
orientation of the near-infrared-emitting dust disk, where we determine that
the disk major axis is oriented along a position angle of 99.6 +/- 4.8 degrees.
The near-infrared continuum emission is 3.6 times more compact than the
expected dust-sublimation radius, possibly indicating the presence of highly
refractory dust grains or optically thick gas emission in the inner disk. Our
velocity-resolved channel maps and moment maps reveal the motion of the
Br-gamma-emitting gas in six velocity channels, marking the first time that
kinematic effects in the sub-AU inner regions of a protoplanetary disk could be
directly imaged. We find a rotation-dominated velocity field, where the blue-
and red-shifted emissions are displaced along a position angle of 24 +/- 3
degrees and the approaching part of the disk is offset west of the star. The
visibility drop in the line as well as the strong non-zero phase signals can be
modeled reasonably well assuming a Keplerian velocity field, although this
model is not able to explain the 3 sigma difference that we measure between the
position angle of the line photocenters and the position angle of the dust
disk. We find that the fit can be improved by adding an outflowing component to
the velocity field, as inspired by a magneto-centrifugal disk-wind scenario.Comment: 15 pages, 13 Figure
Quantification of sub-resolution porosity in carbonate rocks by applying high-salinity contrast brine using X-ray microtomography differential imaging
Characterisation of the pore space in carbonate reservoirs and aquifers is of utmost importance in a number of applications such as enhanced oil recovery, geological carbon storage and contaminant transport. We present a new experimental methodology that uses high-salinity contrast brine and differential imaging acquired by X-ray tomography to non-invasively obtain three-dimensional spatially resolved information on porosity and connectivity of two rock samples, Portland and Estaillades limestones, including sub-resolution micro-porosity. We demonstrate that by injecting 30 wt% KI brine solution, a sufficiently high phase contrast can be achieved allowing accurate three-phase segmentation based on differential imaging. This results in spatially resolved maps of the solid grain phase, sub-resolution micro-pores within the grains, and macro-pores. The total porosity values from the three-phase segmentation for two carbonate rock samples are shown to be in good agreement with Helium porosity measurements. Furthermore, our flow-based method allows for an accurate estimate of pore connectivity and a distribution of porosity within the sub-resolution pores
The Anisoplanatic Point Spread Function in Adaptive Optics
The effects of anisoplanatism on the adaptive optics point spread function
are investigated. A model is derived that combines observations of the guide
star with an analytic formulation of anisoplanatism to generate predictions for
the adaptive optics point spread function at arbitrary locations within the
field of view. The analytic formulation captures the dependencies of
anisoplanatism on aperture diameter, observing wavelength, angular offset,
zenith angle and turbulence profile. The predictions of this model are compared
to narrowband 2.12 um and 1.65 um images of a 21 arcsec binary (mV=7.3, 7.6)
acquired with the Palomar Adaptive Optics System on the Hale 5 meter telescope.
Contemporaneous measurements of the turbulence profile made with a DIMM/MASS
unit are used together with images of the primary to predict the point spread
function of the binary companion. Predicted companion Strehl ratios are shown
to match measurements to within a few percent, whereas predictions based on the
isoplanatic angle approximation are highly discrepant. The predicted companion
point spread functions are shown to agree with observations to 10%. These
predictions are used to measure the differential photometry between binary
members to an accuracy of 1 part in 10^{3}, and the differential astrometry to
an accuracy of 1 mas. Errors in the differential astrometry are shown to be
dominated by differential atmospheric tilt jitter. These results are compared
to other techniques that have been employed for photometry, astrometry, and
high contrast imaging.Comment: 26 pages, 7 figure
Evidence that widespread star formation may be underway in G0.253+016, "The Brick"
Image cubes of differential column density as a function of dust temperature
are constructed for Galactic Centre molecular cloud G0.253+0.016 ("The Brick")
using the recently described PPMAP procedure. The input data consist of
continuum images from the Herschel Space Telescope in the wavelength range
70-500 m, supplemented by previously published interferometric data at 1.3
mm wavelength. While the bulk of the dust in the molecular cloud is consistent
with being heated externally by the local interstellar radiation field, our
image cube shows the presence, near one edge of the cloud, of a filamentary
structure whose temperature profile suggests internal heating. The structure
appears as a cool ( K) tadpole-like feature, pc in length, in
which is embedded a thin spine of much hotter ( 40-50 K) material. We
interpret these findings in terms of a cool filament whose hot central region
is undergoing gravitational collapse and fragmentation to form a line of
protostars. If confirmed, this would represent the first evidence of widespread
star formation having started within this cloud.Comment: 5 pages, 4 figures; accepted for publication in MNRAS Letter
Three-dimensional simulations of near-surface convection in main-sequence stars - II. Properties of granulation and spectral lines
The atmospheres of cool main-sequence stars are structured by convective
flows from the convective envelope that penetrate the optically thin layers and
lead to structuring of the stellar atmospheres analogous to solar granulation.
The flows have considerable influence on the 3D structure of temperature and
pressure and affect the profiles of spectral lines formed in the photosphere.
For the set of six 3D radiative (M)HD simulations of cool main-sequence stars
described in the first paper of this series, we analyse the near-surface
layers. We aim at describing the properties of granulation of different stars
and at quantifying the effects on spectral lines of the thermodynamic structure
and flows of 3D convective atmospheres. We detected and tracked granules in
brightness images from the simulations to analyse their statistical properties,
as well as their evolution and lifetime. We calculated spatially resolved
spectral line profiles using the line synthesis code SPINOR. To enable a
comparison to stellar observations, we implemented a numerical
disc-integration, which includes (differential) rotation. Although the stellar
parameters change considerably along the model sequence, the properties of the
granules are very similar. The impact of the 3D structure of the atmospheres on
line profiles is measurable in disc-integrated spectra. Line asymmetries caused
by convection are modulated by stellar rotation. The 3D structure of cool
stellar atmospheres as shaped by convective flows has to be taken into account
when using photospheric lines to determine stellar parameters.Comment: 18 pages, 22 figures, 3 tables; accepted for publication in A&
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