9,111 research outputs found
Formal models, usability and related work in IR (editorial for special edition)
The Glasgow IR group has carried out both theoretical and empirical work, aimed at giving end users efficient and effective access to large collections of multimedia data
The Bayesian Formulation of EIT: Analysis and Algorithms
We provide a rigorous Bayesian formulation of the EIT problem in an infinite
dimensional setting, leading to well-posedness in the Hellinger metric with
respect to the data. We focus particularly on the reconstruction of binary
fields where the interface between different media is the primary unknown. We
consider three different prior models - log-Gaussian, star-shaped and level
set. Numerical simulations based on the implementation of MCMC are performed,
illustrating the advantages and disadvantages of each type of prior in the
reconstruction, in the case where the true conductivity is a binary field, and
exhibiting the properties of the resulting posterior distribution.Comment: 30 pages, 10 figure
MAP Estimators for Piecewise Continuous Inversion
We study the inverse problem of estimating a field from data comprising a
finite set of nonlinear functionals of , subject to additive noise; we
denote this observed data by . Our interest is in the reconstruction of
piecewise continuous fields in which the discontinuity set is described by a
finite number of geometric parameters. Natural applications include groundwater
flow and electrical impedance tomography. We take a Bayesian approach, placing
a prior distribution on and determining the conditional distribution on
given the data . It is then natural to study maximum a posterior (MAP)
estimators. Recently (Dashti et al 2013) it has been shown that MAP estimators
can be characterised as minimisers of a generalised Onsager-Machlup functional,
in the case where the prior measure is a Gaussian random field. We extend this
theory to a more general class of prior distributions which allows for
piecewise continuous fields. Specifically, the prior field is assumed to be
piecewise Gaussian with random interfaces between the different Gaussians
defined by a finite number of parameters. We also make connections with recent
work on MAP estimators for linear problems and possibly non-Gaussian priors
(Helin, Burger 2015) which employs the notion of Fomin derivative.
In showing applicability of our theory we focus on the groundwater flow and
EIT models, though the theory holds more generally. Numerical experiments are
implemented for the groundwater flow model, demonstrating the feasibility of
determining MAP estimators for these piecewise continuous models, but also that
the geometric formulation can lead to multiple nearby (local) MAP estimators.
We relate these MAP estimators to the behaviour of output from MCMC samples of
the posterior, obtained using a state-of-the-art function space
Metropolis-Hastings method.Comment: 53 pages, 21 figure
Pickup usability dominates: a brief history of mobile text entry research and adoption
Text entry on mobile devices (e.g. phones and PDAs) has been a research challenge since devices shrank below laptop size: mobile devices are simply too small to have a traditional full-size keyboard. There has been a profusion of research into text entry techniques for smaller keyboards and touch screens: some of which have become mainstream, while others have not lived up to early expectations. As the mobile phone industry moves to mainstream touch screen interaction we will review the range of input techniques for mobiles, together with evaluations that have taken place to assess their validity: from theoretical modelling through to formal usability experiments. We also report initial results on iPhone text entry speed
Augmenting entry: the possibilities for utilizing geo-referenced information to improve mobile calendar applications
Today's mobile communication devices often offer extensive calendar facilities. However the use of these is often very limited through cumbersome interfaces and inappropriate designs for small devices. Prompted by previous work in mobile calendar usability, this paper discusses how augmentation of calendar entries with mobile spatial information could provide potential advantages and improve the usability of an electronic calendar
Hyperparameter Estimation in Bayesian MAP Estimation: Parameterizations and Consistency
The Bayesian formulation of inverse problems is attractive for three primary
reasons: it provides a clear modelling framework; means for uncertainty
quantification; and it allows for principled learning of hyperparameters. The
posterior distribution may be explored by sampling methods, but for many
problems it is computationally infeasible to do so. In this situation maximum a
posteriori (MAP) estimators are often sought. Whilst these are relatively cheap
to compute, and have an attractive variational formulation, a key drawback is
their lack of invariance under change of parameterization. This is a
particularly significant issue when hierarchical priors are employed to learn
hyperparameters. In this paper we study the effect of the choice of
parameterization on MAP estimators when a conditionally Gaussian hierarchical
prior distribution is employed. Specifically we consider the centred
parameterization, the natural parameterization in which the unknown state is
solved for directly, and the noncentred parameterization, which works with a
whitened Gaussian as the unknown state variable, and arises when considering
dimension-robust MCMC algorithms; MAP estimation is well-defined in the
nonparametric setting only for the noncentred parameterization. However, we
show that MAP estimates based on the noncentred parameterization are not
consistent as estimators of hyperparameters; conversely, we show that limits of
finite-dimensional centred MAP estimators are consistent as the dimension tends
to infinity. We also consider empirical Bayesian hyperparameter estimation,
show consistency of these estimates, and demonstrate that they are more robust
with respect to noise than centred MAP estimates. An underpinning concept
throughout is that hyperparameters may only be recovered up to measure
equivalence, a well-known phenomenon in the context of the Ornstein-Uhlenbeck
process.Comment: 36 pages, 8 figure
Constituent quark scaling violation due to baryon number transport
In ultra-relativistic heavy ion collisions at \roots\approx200 GeV, the
azimuthal emission anisotropy of hadrons with low and intermediate transverse
momentum ( GeV/c) displays an intriguing scaling. In particular,
the baryon (meson) emission patterns are consistent with a scenario in which a
bulk medium of flowing quarks coalesces into three-quark (two-quark) "bags."
While a full understanding of this number of constituent quark (NCQ) scaling
remains elusive, it is suggestive of a thermalized bulk system characterized by
colored dynamical degrees of freedom-- a quark-gluon plasma (QGP). In this
scenario, one expects the scaling to break down as the central energy density
is reduced below the QGP formation threshold; for this reason, NCQ-scaling
violation searches are of interest in the energy scan program at the
Relativistic Heavy Ion Collider (RHIC). However, as \roots is reduced, it is
not only the initial energy density that changes; there is also an increase in
the net baryon number at midrapidity, as stopping transports entrance-channel
partons to midrapidity. This phenomenon can result in violations of simple NCQ
scaling. Still in the context of the quark coalescence model, we describe a
specific pattern for the break-down of the scaling that includes different flow
strengths for particles and their anti-partners. Related complications in the
search for recently suggested exotic phenomena are also discussed.Comment: 7 pages, 2 tables, 2 figures. Wording sharpened. Two tables added, to
quantify the estimate of stopped quark fraction
Image retrieval by hypertext links
This paper presents a model for retrieval of images from a large World Wide Web based collection. Rather than considering complex visual recognition algorithms, the model presented is based on combining evidence of the text content and hypertext structure of the Web. The paper shows that certain types of query are amply served by this form of representation. It also presents a novel means of gathering relevance judgements
Time and M-theory
We review our recent proposal for a background independent formulation of a
holographic theory of quantum gravity. The present review incorporates the
necessary background material on geometry of canonical quantum theory,
holography and spacetime thermodynamics, Matrix theory, as well as our specific
proposal for a dynamical theory of geometric quantum mechanics, as applied to
Matrix theory. At the heart of this review is a new analysis of the conceptual
problem of time and the closely related and phenomenologically relevant problem
of vacuum energy in quantum gravity. We also present a discussion of some
observational implications of this new viewpoint on the problem of vacuum
energy.Comment: 86 pages, 5 figures, LaTeX, typos fixed, references added, and Sec.
6.2 revised; invited review for Int. J. Mod. Phys.
Assessing the effectiveness of multi-touch interfaces for DP operation
Navigating a vessel using dynamic positioning (DP) systems close to offshore installations is a challenge. The operator's only possibility of manipulating the system is through its interface, which can be categorized as the physical appearance of the equipment and the visualization of the system. Are there possibilities of interaction between the operator and the system that can reduce strain and cognitive load during DP operations? Can parts of the system (e.g. displays) be physically brought closer to the user to enhance the feeling of control when operating the system? Can these changes make DP operations more efficient and safe? These questions inspired this research project, which investigates the use of multi-touch and hand gestures known from consumer products to directly manipulate the visualization of a vessel in the 3D scene of a DP system. Usability methodologies and evaluation techniques that are widely used in consumer market research were used to investigate how these interaction techniques, which are new to the maritime domain, could make interaction with the DP system more efficient and transparent both during standard and safety-critical operations. After investigating which gestures felt natural to use by running user tests with a paper prototype, the gestures were implemented into a Rolls-Royce DP system and tested in a static environment. The results showed that the test participants performed significantly faster using direct gesture manipulation compared to using traditional button/menu interaction. To support the results from these tests, further tests were carried out. The purpose is to investigate how gestures are performed in a moving environment, using a motion platform to simulate rough sea conditions. The key results and lessons learned from a collection of four user experiments, together with a discussion of the choice of evaluation techniques will be discussed in this paper
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