8,349 research outputs found
Low Complexity Regularization of Linear Inverse Problems
Inverse problems and regularization theory is a central theme in contemporary
signal processing, where the goal is to reconstruct an unknown signal from
partial indirect, and possibly noisy, measurements of it. A now standard method
for recovering the unknown signal is to solve a convex optimization problem
that enforces some prior knowledge about its structure. This has proved
efficient in many problems routinely encountered in imaging sciences,
statistics and machine learning. This chapter delivers a review of recent
advances in the field where the regularization prior promotes solutions
conforming to some notion of simplicity/low-complexity. These priors encompass
as popular examples sparsity and group sparsity (to capture the compressibility
of natural signals and images), total variation and analysis sparsity (to
promote piecewise regularity), and low-rank (as natural extension of sparsity
to matrix-valued data). Our aim is to provide a unified treatment of all these
regularizations under a single umbrella, namely the theory of partial
smoothness. This framework is very general and accommodates all low-complexity
regularizers just mentioned, as well as many others. Partial smoothness turns
out to be the canonical way to encode low-dimensional models that can be linear
spaces or more general smooth manifolds. This review is intended to serve as a
one stop shop toward the understanding of the theoretical properties of the
so-regularized solutions. It covers a large spectrum including: (i) recovery
guarantees and stability to noise, both in terms of -stability and
model (manifold) identification; (ii) sensitivity analysis to perturbations of
the parameters involved (in particular the observations), with applications to
unbiased risk estimation ; (iii) convergence properties of the forward-backward
proximal splitting scheme, that is particularly well suited to solve the
corresponding large-scale regularized optimization problem
Recent Progress in Image Deblurring
This paper comprehensively reviews the recent development of image
deblurring, including non-blind/blind, spatially invariant/variant deblurring
techniques. Indeed, these techniques share the same objective of inferring a
latent sharp image from one or several corresponding blurry images, while the
blind deblurring techniques are also required to derive an accurate blur
kernel. Considering the critical role of image restoration in modern imaging
systems to provide high-quality images under complex environments such as
motion, undesirable lighting conditions, and imperfect system components, image
deblurring has attracted growing attention in recent years. From the viewpoint
of how to handle the ill-posedness which is a crucial issue in deblurring
tasks, existing methods can be grouped into five categories: Bayesian inference
framework, variational methods, sparse representation-based methods,
homography-based modeling, and region-based methods. In spite of achieving a
certain level of development, image deblurring, especially the blind case, is
limited in its success by complex application conditions which make the blur
kernel hard to obtain and be spatially variant. We provide a holistic
understanding and deep insight into image deblurring in this review. An
analysis of the empirical evidence for representative methods, practical
issues, as well as a discussion of promising future directions are also
presented.Comment: 53 pages, 17 figure
Comparitive assessment of the vulnerability and resilience of 10 deltas, synthesis report
The proposed framework for delta assessment and especially the scorecards are intended to enhance awareness raising, discussion and prioritization on most relevant delta issues, in each delta but also in comparison with other deltas. This should lead to more efficient and effective (multi-sectoral) policy formulation, management design and implementation, in concrete Delta plans, pilot-projects and (research) programmes. The target groups are all stakeholders who are involved in delta management at different levels and with different interests (government, private companies, NGOs, public), and who wish to contribute to the resilience of their own delta and other deltas worldwide
Compressive Wave Computation
This paper considers large-scale simulations of wave propagation phenomena.
We argue that it is possible to accurately compute a wavefield by decomposing
it onto a largely incomplete set of eigenfunctions of the Helmholtz operator,
chosen at random, and that this provides a natural way of parallelizing wave
simulations for memory-intensive applications.
This paper shows that L1-Helmholtz recovery makes sense for wave computation,
and identifies a regime in which it is provably effective: the one-dimensional
wave equation with coefficients of small bounded variation. Under suitable
assumptions we show that the number of eigenfunctions needed to evolve a sparse
wavefield defined on N points, accurately with very high probability, is
bounded by C log(N) log(log(N)), where C is related to the desired accuracy and
can be made to grow at a much slower rate than N when the solution is sparse.
The PDE estimates that underlie this result are new to the authors' knowledge
and may be of independent mathematical interest; they include an L1 estimate
for the wave equation, an estimate of extension of eigenfunctions, and a bound
for eigenvalue gaps in Sturm-Liouville problems.
Numerical examples are presented in one spatial dimension and show that as
few as 10 percents of all eigenfunctions can suffice for accurate results.
Finally, we argue that the compressive viewpoint suggests a competitive
parallel algorithm for an adjoint-state inversion method in reflection
seismology.Comment: 45 pages, 4 figure
An Interdisciplinary Approach to Assessing, Planning and Managing Urban Rivers in the context of Greater London
PhDUrban rivers present complex management challenges due to the combined natural and
anthropocentric factors affecting developed catchments. Planning urban river
rehabilitation strategies and measures in parallel with green infrastructure initiatives
requires the combined expertise of multi-disciplinary partnerships, encompassing river
science and landscape engineering plus community engagement, to deliver integrated
and sustainable outcomes. This thesis takes an interdisciplinary approach to investigate
the assessment and management of urban rivers, focusing specifically upon the planning
of integrated restoration projects for River Thames tributaries within Greater London.
Comparisons of restored and unrestored sites on London tributary rivers at the reachand
catchment-scale explore the versatility of the Urban River Survey method for
assessing and communicating contrasts in the bio-physical condition and
engineering:habitat associations of heavily modified rivers. A trial of the Ecosystem
Services Assessment method for urban river restorations indicates the strengths and
limitations of this approach and areas of research need.
Urban river governance investigations and a review of changes in restoration practices
over time confirm a decreasing emphasis on channel control and progressively lighter
engineering, plus a greater social focus with urban river management becoming
increasingly driven by awareness of the symbiosis between rivers and local
communities. In some London boroughs partner organisations are developing new links
through sustainable development objectives, but connections are geographically
inconsistent and typically dependent upon key advocates.
Findings indicate that integrated planning can facilitate interdisciplinary processes
through the identification of cross-cutting themes (e.g. climate change) and open
knowledge exchange when delivered with appropriate levels of detail. While some
disciplinary boundaries are necessary (to define project scope and for task
management), socio-ecological benefits may be achieved when these are flexible,
permeable and managed responsively in relation to simple overarching goals; and by
allowing time for different kinds of knowledge to merge and stimulate new creative and
integrated interpretations
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