39,829 research outputs found
Photometric Depth Super-Resolution
This study explores the use of photometric techniques (shape-from-shading and
uncalibrated photometric stereo) for upsampling the low-resolution depth map
from an RGB-D sensor to the higher resolution of the companion RGB image. A
single-shot variational approach is first put forward, which is effective as
long as the target's reflectance is piecewise-constant. It is then shown that
this dependency upon a specific reflectance model can be relaxed by focusing on
a specific class of objects (e.g., faces), and delegate reflectance estimation
to a deep neural network. A multi-shot strategy based on randomly varying
lighting conditions is eventually discussed. It requires no training or prior
on the reflectance, yet this comes at the price of a dedicated acquisition
setup. Both quantitative and qualitative evaluations illustrate the
effectiveness of the proposed methods on synthetic and real-world scenarios.Comment: IEEE Transactions on Pattern Analysis and Machine Intelligence
(T-PAMI), 2019. First three authors contribute equall
Variational Uncalibrated Photometric Stereo under General Lighting
Photometric stereo (PS) techniques nowadays remain constrained to an ideal
laboratory setup where modeling and calibration of lighting is amenable. To
eliminate such restrictions, we propose an efficient principled variational
approach to uncalibrated PS under general illumination. To this end, the
Lambertian reflectance model is approximated through a spherical harmonic
expansion, which preserves the spatial invariance of the lighting. The joint
recovery of shape, reflectance and illumination is then formulated as a single
variational problem. There the shape estimation is carried out directly in
terms of the underlying perspective depth map, thus implicitly ensuring
integrability and bypassing the need for a subsequent normal integration. To
tackle the resulting nonconvex problem numerically, we undertake a two-phase
procedure to initialize a balloon-like perspective depth map, followed by a
"lagged" block coordinate descent scheme. The experiments validate efficiency
and robustness of this approach. Across a variety of evaluations, we are able
to reduce the mean angular error consistently by a factor of 2-3 compared to
the state-of-the-art.Comment: Haefner and Ye contributed equall
Magneto-optical Kramers-Kronig analysis
We describe a simple magneto-optical experiment and introduce a
magneto-optical Kramers-Kronig analysis (MOKKA) that together allow extracting
the complex dielectric function for left- and right-handed circular
polarizations in a broad range of frequencies without actually generating
circularly polarized light. The experiment consists of measuring reflectivity
and Kerr rotation, or alternatively transmission and Faraday rotation, at
normal incidence using only standard broadband polarizers without retarders or
quarter-wave plates. In a common case, where the magneto-optical rotation is
small (below 0.2 rad), a fast measurement protocol can be realized,
where the polarizers are fixed at 45 with respect to each other. Apart
from the time-effectiveness, the advantage of this protocol is that it can be
implemented at ultra-high magnetic fields and in other situations, where an
\emph{in-situ} polarizer rotation is difficult. Overall, the proposed technique
can be regarded as a magneto-optical generalization of the conventional
Kramers-Kronig analysis of reflectivity on bulk samples and the Kramers-Kronig
constrained variational analysis of more complex types of spectral data. We
demonstrate the application of this method to the textbook semimetals bismuth
and graphite and also use it to obtain handedness-resolved magneto-absorption
spectra of graphene on SiC.Comment: 11 pages, 4 figur
Small Bipolarons in the 2-dimensional Holstein-Hubbard Model. II Quantum Bipolarons
We study the effective mass of the bipolarons and essentially the possibility
to get both light and strongly bound bipolarons in the Holstein-Hubbard model
and some variations in the vicinity of the adiabatic limit. Several approaches
to investigate the quantum mobility of polarons and bipolarons are proposed for
this model. It is found that the bipolaron mass generally remains very large
except in the vicinity of the triple point of the phase diagram, where the
bipolarons have several degenerate configurations at the adiabatic limit
(single site (S0), two sites (S1) and quadrisinglet (QS)), while the polarons
are much lighter. This degeneracy reduces the bipolaron mass significantly. The
triple point of the phase diagram is washed out by the lattice quantum
fluctuations which thus suppress the light bipolarons. We show that some model
variations, for example a phonon dispersion may increase the stability of the
(QS) bipolaron against the quantum lattice fluctuations. The triple point of
the phase diagram may be stable to quantum lattice fluctuations and a very
sharp mass reduction may occur, leading to bipolaron masses of the order of 100
bare electronic mass for realistic parameters. Thus such very light bipolarons
could condense as a superconducting state at relatively high temperature when
their interactions are not too large, that is, their density is small enough.
This effect might be relevant for understanding the origin of the high Tc
superconductivity of doped cuprates far enough from half filling.Comment: accepted Eur. Phys. J. B (january 2000) Ref. B960
Variational Disparity Estimation Framework for Plenoptic Image
This paper presents a computational framework for accurately estimating the
disparity map of plenoptic images. The proposed framework is based on the
variational principle and provides intrinsic sub-pixel precision. The
light-field motion tensor introduced in the framework allows us to combine
advanced robust data terms as well as provides explicit treatments for
different color channels. A warping strategy is embedded in our framework for
tackling the large displacement problem. We also show that by applying a simple
regularization term and a guided median filtering, the accuracy of displacement
field at occluded area could be greatly enhanced. We demonstrate the excellent
performance of the proposed framework by intensive comparisons with the Lytro
software and contemporary approaches on both synthetic and real-world datasets
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