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On the capture and representation of fonts
This thesis was submitted for the degree of Doctor of Philosophy and awarded by Brunel University.The commercial need to capture, process and represent the shape and form of an outline has lead to the development of a number of spline routines. These use a mathematical curve format that approximates the contours of a given shape. The modelled outline lends itself to be used on, and for, a variety of purposes. These include graphic screens, laser printers and numerically controlled machines. The latter can be employed for cutting foil, metal. plastic and stone. One of the most widely used software design packages has been the lKARUS system. This, developed by URW of Hamburg (Gennany), employs a number of mathematical descriptions that facilitate the process of both modelling and representation of font characters. It uses a variety of curve formats, including Bezier cubics, general conics and parabolics. The work reported in this dissertation focuses on developing improved techniques, primarily. for the lKARUS system. This includes two algorithms
which allow a Bezier cubic description, two for a general conic representation and, yet another, two for the parabolic case. In addition, a number of algorithms are presented which promote conversions between these mathematical forms; for example, Bezier cubics to a general conic form. Furthennore, algorithms are developed to assist the process of rasterising both cubic and quadratic arcs.This study was partly funded by the Science and Education Research Council (SERC)
Subdivision schemes for curve design and image analysis
Subdivision schemes are able to produce functions, which are smooth up to pixel accuracy, in a few steps through an iterative process. They take as input a coarse control polygon and iteratively generate new points using some algebraic or geometric rules. Therefore, they are a powerful tool for creating and displaying functions, in particular in computer graphics, computer-aided design, and signal analysis. A lot of research on univariate subdivision schemes is concerned with the convergence and the smoothness of the limit curve, especially for schemes where the new points are a linear combination of points from the previous iteration. Much less is known for non-linear schemes: in many cases there are only ad hoc proofs or numerical evidence about the regularity of these schemes. For schemes that use a geometric construction, it could be interesting to study the continuity of geometric entities. Dyn and Hormann propose sufficient conditions such that the subdivision process converges and the limit curve is tangent continuous. These conditions can be satisfied by any interpolatory scheme and they depend only on edge lengths and angles. The goal of my work is to generalize these conditions and to find a sufficient constraint, which guarantees that a generic interpolatory subdivision scheme gives limit curves with continuous curvature. To require the continuity of the curvature it seems natural to come up with a condition that depends on the difference of curvatures of neighbouring circles. The proof of the proposed condition is not completed, but we give a numerical evidence of it. A key feature of subdivision schemes is that they can be used in different fields of approximation theory. Due to their well-known relation with multiresolution analysis they can be exploited also in image analysis. In fact, subdivision schemes allow for an efficient computation of the wavelet transform using the filterbank. One current issue in signal processing is the analysis of anisotropic signals. Shearlet transforms allow to do it using the concept of multiple subdivision schemes. One drawback, however, is the big number of filters needed for analysing the signal given. The number of filters is related to the determinant of the expanding matrix considered. Therefore, a part of my work is devoted to find expanding matrices that give a smaller number of filters compared to the shearlet case. We present a family of anisotropic matrices for any dimension d with smaller determinant than shearlets. At the same time, these matrices allow for the definition of a valid directional transform and associated multiple subdivision schemes
Adaptive multiscale detection of filamentary structures in a background of uniform random points
We are given a set of points that might be uniformly distributed in the
unit square . We wish to test whether the set, although mostly
consisting of uniformly scattered points, also contains a small fraction of
points sampled from some (a priori unknown) curve with -norm
bounded by . An asymptotic detection threshold exists in this problem;
for a constant , if the number of points sampled from the
curve is smaller than , reliable detection
is not possible for large . We describe a multiscale significant-runs
algorithm that can reliably detect concentration of data near a smooth curve,
without knowing the smoothness information or in advance,
provided that the number of points on the curve exceeds
. This algorithm therefore has an optimal
detection threshold, up to a factor . At the heart of our approach is
an analysis of the data by counting membership in multiscale multianisotropic
strips. The strips will have area and exhibit a variety of lengths,
orientations and anisotropies. The strips are partitioned into anisotropy
classes; each class is organized as a directed graph whose vertices all are
strips of the same anisotropy and whose edges link such strips to their ``good
continuations.'' The point-cloud data are reduced to counts that measure
membership in strips. Each anisotropy graph is reduced to a subgraph that
consist of strips with significant counts. The algorithm rejects
whenever some such subgraph contains a path that connects many consecutive
significant counts.Comment: Published at http://dx.doi.org/10.1214/009053605000000787 in the
Annals of Statistics (http://www.imstat.org/aos/) by the Institute of
Mathematical Statistics (http://www.imstat.org
Adaptive quadratures for nonlinear approximation of low-dimensional PDEs using smooth neural networks
Physics-informed neural networks (PINNs) and their variants have recently
emerged as alternatives to traditional partial differential equation (PDE)
solvers, but little literature has focused on devising accurate numerical
integration methods for neural networks (NNs), which is essential for getting
accurate solutions. In this work, we propose adaptive quadratures for the
accurate integration of neural networks and apply them to loss functions
appearing in low-dimensional PDE discretisations. We show that at opposite ends
of the spectrum, continuous piecewise linear (CPWL) activation functions enable
one to bound the integration error, while smooth activations ease the
convergence of the optimisation problem. We strike a balance by considering a
CPWL approximation of a smooth activation function. The CPWL activation is used
to obtain an adaptive decomposition of the domain into regions where the
network is almost linear, and we derive an adaptive global quadrature from this
mesh. The loss function is then obtained by evaluating the smooth network
(together with other quantities, e.g., the forcing term) at the quadrature
points. We propose a method to approximate a class of smooth activations by
CPWL functions and show that it has a quadratic convergence rate. We then
derive an upper bound for the overall integration error of our proposed
adaptive quadrature. The benefits of our quadrature are evaluated on a strong
and weak formulation of the Poisson equation in dimensions one and two. Our
numerical experiments suggest that compared to Monte-Carlo integration, our
adaptive quadrature makes the convergence of NNs quicker and more robust to
parameter initialisation while needing significantly fewer integration points
and keeping similar training times.Comment: Fixed typos and, clarified the legend of fig. 3 and proofs of lemma 1
and proposition
A Compact Representation of Drawing Movements with Sequences of Parabolic Primitives
Some studies suggest that complex arm movements in humans and monkeys may optimize several objective functions, while others claim that arm movements satisfy geometric constraints and are composed of elementary components. However, the ability to unify different constraints has remained an open question. The criterion for a maximally smooth (minimizing jerk) motion is satisfied for parabolic trajectories having constant equi-affine speed, which thus comply with the geometric constraint known as the two-thirds power law. Here we empirically test the hypothesis that parabolic segments provide a compact representation of spontaneous drawing movements. Monkey scribblings performed during a period of practice were recorded. Practiced hand paths could be approximated well by relatively long parabolic segments. Following practice, the orientations and spatial locations of the fitted parabolic segments could be drawn from only 2β4 clusters, and there was less discrepancy between the fitted parabolic segments and the executed paths. This enabled us to show that well-practiced spontaneous scribbling movements can be represented as sequences (βwordsβ) of a small number of elementary parabolic primitives (βlettersβ). A movement primitive can be defined as a movement entity that cannot be intentionally stopped before its completion. We found that in a well-trained monkey a movement was usually decelerated after receiving a reward, but it stopped only after the completion of a sequence composed of several parabolic segments. Piece-wise parabolic segments can be generated by applying affine geometric transformations to a single parabolic template. Thus, complex movements might be constructed by applying sequences of suitable geometric transformations to a few templates. Our findings therefore suggest that the motor system aims at achieving more parsimonious internal representations through practice, that parabolas serve as geometric primitives and that non-Euclidean variables are employed in internal movement representations (due to the special role of parabolas in equi-affine geometry)
Three-manifolds, Foliations and Circles, I
This paper investigates certain foliations of three-manifolds that are
hybrids of fibrations over the circle with foliated circle bundles over
surfaces: a 3-manifold slithers around the circle when its universal cover
fibers over the circle so that deck transformations are bundle automorphisms.
Examples include hyperbolic 3-manifolds of every possible homological type. We
show that all such foliations admit transverse pseudo-Anosov flows, and that in
the universal cover of the hyperbolic cases, the leaves limit to sphere-filling
Peano curves. The skew R-covered Anosov foliations of Sergio Fenley are
examples. We hope later to use this structure for geometrization of slithered
3-manifolds.Comment: 60 pages, 10 figure
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