14,029 research outputs found
Estimation of vector fields in unconstrained and inequality constrained variational problems for segmentation and registration
Vector fields arise in many problems of computer vision, particularly in non-rigid registration. In this paper, we develop coupled partial differential equations (PDEs) to estimate vector fields that define the deformation between
objects, and the contour or surface that defines the segmentation of the objects as well.We also explore the utility of inequality constraints applied to variational problems in vision such as estimation of deformation fields in non-rigid registration and tracking. To solve inequality constrained vector
field estimation problems, we apply tools from the Kuhn-Tucker theorem in optimization theory. Our technique differs from recently popular joint segmentation and registration algorithms, particularly in its coupled set of PDEs derived from the same set of energy terms for registration and
segmentation. We present both the theory and results that demonstrate our approach
Sparsity in Variational Autoencoders
Working in high-dimensional latent spaces, the internal encoding of data in
Variational Autoencoders becomes naturally sparse. We discuss this known but
controversial phenomenon sometimes refereed to as overpruning, to emphasize the
under-use of the model capacity. In fact, it is an important form of
self-regularization, with all the typical benefits associated with sparsity: it
forces the model to focus on the really important features, highly reducing the
risk of overfitting. Especially, it is a major methodological guide for the
correct tuning of the model capacity, progressively augmenting it to attain
sparsity, or conversely reducing the dimension of the network removing links to
zeroed out neurons. The degree of sparsity crucially depends on the network
architecture: for instance, convolutional networks typically show less
sparsity, likely due to the tighter relation of features to different spatial
regions of the input.Comment: An Extended Abstract of this survey will be presented at the 1st
International Conference on Advances in Signal Processing and Artificial
Intelligence (ASPAI' 2019), 20-22 March 2019, Barcelona, Spai
Multi-object segmentation using coupled nonparametric shape and relative pose priors
We present a new method for multi-object segmentation in a maximum a posteriori estimation framework. Our method is motivated by the observation that neighboring or coupling objects in images generate configurations and co-dependencies which could potentially aid in segmentation if properly exploited. Our approach employs coupled shape and inter-shape pose priors that are computed using training images in a nonparametric multi-variate kernel density estimation framework. The coupled shape prior is obtained by estimating the joint shape distribution of multiple objects and the inter-shape pose priors are modeled via standard moments. Based on such statistical models, we formulate an optimization problem for segmentation, which we solve by an algorithm based on active contours. Our technique provides significant improvements in the segmentation of weakly contrasted objects in a number of applications. In particular for medical image analysis, we use our method to extract brain Basal Ganglia structures, which are members of a complex multi-object system posing a challenging segmentation problem. We also apply our technique to the problem of handwritten character segmentation. Finally, we use our method to segment cars in urban scenes
Buckling without bending: a new paradigm in morphogenesis
A curious feature of organ and organoid morphogenesis is that in certain
cases, spatial oscillations in the thickness of the growing "film" are
out-of-phase with the deformation of the slower-growing "substrate," while in
other cases, the oscillations are in-phase. The former cannot be explained by
elastic bilayer instability, and contradict the notion that there is a
universal mechanism by which brains, intestines, teeth, and other organs
develop surface wrinkles and folds. Inspired by the microstructure of the
embryonic cerebellum, we develop a new model of 2d morphogenesis in which
system-spanning elastic fibers endow the organ with a preferred radius, while a
separate fiber network resides in the otherwise fluid-like film at the outer
edge of the organ and resists thickness gradients thereof. The tendency of the
film to uniformly thicken or thin is described via a "growth potential".
Several features of cerebellum, +blebbistatin organoid, and retinal fovea
morphogenesis, including out-of-phase behavior and a film thickness amplitude
that is comparable to the radius amplitude, are readily explained by our simple
analytical model, as may be an observed scale-invariance in the number of folds
in the cerebellum. We also study a nonlinear variant of the model, propose
further biological and bio-inspired applications, and address how our model is
and is not unique to the developing nervous system.Comment: version accepted by Physical Review
A trust-region method for stochastic variational inference with applications to streaming data
Stochastic variational inference allows for fast posterior inference in
complex Bayesian models. However, the algorithm is prone to local optima which
can make the quality of the posterior approximation sensitive to the choice of
hyperparameters and initialization. We address this problem by replacing the
natural gradient step of stochastic varitional inference with a trust-region
update. We show that this leads to generally better results and reduced
sensitivity to hyperparameters. We also describe a new strategy for variational
inference on streaming data and show that here our trust-region method is
crucial for getting good performance.Comment: in Proceedings of the 32nd International Conference on Machine
Learning, 201
Excitons in T-shaped quantum wires
We calculate energies, oscillator strengths for radiative recombination, and
two-particle wave functions for the ground state exciton and around 100 excited
states in a T-shaped quantum wire. We include the single-particle potential and
the Coulomb interaction between the electron and hole on an equal footing, and
perform exact diagonalisation of the two-particle problem within a finite basis
set. We calculate spectra for all of the experimentally studied cases of
T-shaped wires including symmetric and asymmetric GaAs/AlGaAs and
InGaAs/AlGaAs structures. We study in detail the
shape of the wave functions to gain insight into the nature of the various
states for selected symmetric and asymmetric wires in which laser emission has
been experimentally observed. We also calculate the binding energy of the
ground state exciton and the confinement energy of the 1D quantum-wire-exciton
state with respect to the 2D quantum-well exciton for a wide range of
structures, varying the well width and the Al molar fraction . We find that
the largest binding energy of any wire constructed to date is 16.5 meV. We also
notice that in asymmetric structures, the confinement energy is enhanced with
respect to the symmetric forms with comparable parameters but the binding
energy of the exciton is then lower than in the symmetric structures. For
GaAs/AlGaAs wires we obtain an upper limit for the binding energy
of around 25 meV in a 10 {\AA} wide GaAs/AlAs structure which suggests that
other materials must be explored in order to achieve room temperature
applications. There are some indications that
InGaAs/AlGaAs might be a good candidate.Comment: 20 pages, 10 figures, uses RevTeX and psfig, submitted to Physical
Review
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