1,177 research outputs found
Finite Boolean Algebras for Solid Geometry using Julia's Sparse Arrays
The goal of this paper is to introduce a new method in computer-aided
geometry of solid modeling. We put forth a novel algebraic technique to
evaluate any variadic expression between polyhedral d-solids (d = 2, 3) with
regularized operators of union, intersection, and difference, i.e., any CSG
tree. The result is obtained in three steps: first, by computing an independent
set of generators for the d-space partition induced by the input; then, by
reducing the solid expression to an equivalent logical formula between Boolean
terms made by zeros and ones; and, finally, by evaluating this expression using
bitwise operators. This method is implemented in Julia using sparse arrays. The
computational evaluation of every possible solid expression, usually denoted as
CSG (Constructive Solid Geometry), is reduced to an equivalent logical
expression of a finite set algebra over the cells of a space partition, and
solved by native bitwise operators.Comment: revised version submitted to Computer-Aided Geometric Desig
Geometric deep learning: going beyond Euclidean data
Many scientific fields study data with an underlying structure that is a
non-Euclidean space. Some examples include social networks in computational
social sciences, sensor networks in communications, functional networks in
brain imaging, regulatory networks in genetics, and meshed surfaces in computer
graphics. In many applications, such geometric data are large and complex (in
the case of social networks, on the scale of billions), and are natural targets
for machine learning techniques. In particular, we would like to use deep
neural networks, which have recently proven to be powerful tools for a broad
range of problems from computer vision, natural language processing, and audio
analysis. However, these tools have been most successful on data with an
underlying Euclidean or grid-like structure, and in cases where the invariances
of these structures are built into networks used to model them. Geometric deep
learning is an umbrella term for emerging techniques attempting to generalize
(structured) deep neural models to non-Euclidean domains such as graphs and
manifolds. The purpose of this paper is to overview different examples of
geometric deep learning problems and present available solutions, key
difficulties, applications, and future research directions in this nascent
field
Cusps of arithmetic orbifolds
This thesis investigates cusp cross-sections of arithmetic real, complex, and
quaternionic hyperbolic --orbifolds. We give a smooth classification of
these submanifolds and analyze their induced geometry. One of the primary tools
is a new subgroup separability result for general arithmetic lattices.Comment: 76 pages; Ph.D. thesi
Mutations and short geodesics in hyperbolic 3-manifolds
In this paper, we explicitly construct large classes of incommensurable
hyperbolic knot complements with the same volume and the same initial (complex)
length spectrum. Furthermore, we show that these knot complements are the only
knot complements in their respective commensurabiltiy classes by analyzing
their cusp shapes.
The knot complements in each class differ by a topological cut-and-paste
operation known as mutation. Ruberman has shown that mutations of hyperelliptic
surfaces inside hyperbolic 3-manifolds preserve volume. Here, we provide
geometric and topological conditions under which such mutations also preserve
the initial (complex) length spectrum. This work requires us to analyze when
least area surfaces could intersect short geodesics in a hyperbolic 3-manifold.Comment: This is the final (accepted) version of this pape
Topological Classification of Multiaxial U(n)-Actions
A U(n)-manifold is multiaxial if the isotropy groups are always conjugate to
unitary subgroups. The classification and the concordance of such manifolds
have been studied by Davis, Hsiang and Morgan under much more strict
conditions. We show that in general, without much extra condition, the homotopy
classification of multiaxial manifolds can be split into a direct sum of the
classification of pairs of adjacent strata, which can be computed by the
classical surgery theory. Moreover, we also compute the homotopy classification
for the case of the standard representation sphere. We also present the result
for the similar multiaxial Sp(n)-manifolds.Comment: 30 page
GRASS: Generative Recursive Autoencoders for Shape Structures
We introduce a novel neural network architecture for encoding and synthesis
of 3D shapes, particularly their structures. Our key insight is that 3D shapes
are effectively characterized by their hierarchical organization of parts,
which reflects fundamental intra-shape relationships such as adjacency and
symmetry. We develop a recursive neural net (RvNN) based autoencoder to map a
flat, unlabeled, arbitrary part layout to a compact code. The code effectively
captures hierarchical structures of man-made 3D objects of varying structural
complexities despite being fixed-dimensional: an associated decoder maps a code
back to a full hierarchy. The learned bidirectional mapping is further tuned
using an adversarial setup to yield a generative model of plausible structures,
from which novel structures can be sampled. Finally, our structure synthesis
framework is augmented by a second trained module that produces fine-grained
part geometry, conditioned on global and local structural context, leading to a
full generative pipeline for 3D shapes. We demonstrate that without
supervision, our network learns meaningful structural hierarchies adhering to
perceptual grouping principles, produces compact codes which enable
applications such as shape classification and partial matching, and supports
shape synthesis and interpolation with significant variations in topology and
geometry.Comment: Corresponding author: Kai Xu ([email protected]
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