185 research outputs found
Searching Polyhedra by Rotating Half-Planes
The Searchlight Scheduling Problem was first studied in 2D polygons, where
the goal is for point guards in fixed positions to rotate searchlights to catch
an evasive intruder. Here the problem is extended to 3D polyhedra, with the
guards now boundary segments who rotate half-planes of illumination. After
carefully detailing the 3D model, several results are established. The first is
a nearly direct extension of the planar one-way sweep strategy using what we
call exhaustive guards, a generalization that succeeds despite there being no
well-defined notion in 3D of planar "clockwise rotation". Next follow two
results: every polyhedron with r>0 reflex edges can be searched by at most r^2
suitably placed guards, whereas just r guards suffice if the polyhedron is
orthogonal. (Minimizing the number of guards to search a given polyhedron is
easily seen to be NP-hard.) Finally we show that deciding whether a given set
of guards has a successful search schedule is strongly NP-hard, and that
deciding if a given target area is searchable at all is strongly PSPACE-hard,
even for orthogonal polyhedra. A number of peripheral results are proved en
route to these central theorems, and several open problems remain for future
work.Comment: 45 pages, 26 figure
Guarding and Searching Polyhedra
Guarding and searching problems have been of fundamental interest since the early years of Computational Geometry. Both are well-developed areas of research and have been thoroughly studied in planar polygonal settings.
In this thesis we tackle the Art Gallery Problem and the Searchlight Scheduling Problem in 3-dimensional polyhedral environments, putting special emphasis on edge guards and orthogonal polyhedra.
We solve the Art Gallery Problem with reflex edge guards in orthogonal polyhedra having reflex edges in just two directions: generalizing a classic theorem by O'Rourke, we prove that r/2 + 1 reflex edge guards are sufficient and occasionally necessary, where r is the number of reflex edges. We also show how to compute guard locations in O(n log n) time.
Then we investigate the Art Gallery Problem with mutually parallel edge guards in orthogonal polyhedra with e edges, showing that 11e/72 edge guards are always sufficient and can be found in linear time, improving upon the previous state of the art, which was e/6. We also give tight inequalities relating e with the number of reflex edges r, obtaining an upper bound on the guard number of 7r/12 + 1.
We further study the Art Gallery Problem with edge guards in polyhedra having faces oriented in just four directions, obtaining a lower bound of e/6 - 1 edge guards and an upper bound of (e+r)/6 edge guards.
All the previously mentioned results hold for polyhedra of any genus. Additionally, several guard types and guarding modes are discussed, namely open and closed edge guards, and orthogonal and non-orthogonal guarding.
Next, we model the Searchlight Scheduling Problem, the problem of searching a given polyhedron by suitably turning some half-planes
around their axes, in order to catch an evasive intruder. After discussing several generalizations of classic theorems, we study the problem of efficiently placing guards in a given polyhedron, in order to make it searchable. For general polyhedra, we give an upper bound of r^2 on the number of guards, which reduces to r for orthogonal polyhedra.
Then we prove that it is strongly NP-hard to decide if a given polyhedron is entirely searchable by a given set of guards. We further prove that, even under the assumption that an orthogonal polyhedron is searchable, approximating the minimum search time within a small-enough constant factor to the optimum is still strongly NP-hard.
Finally, we show that deciding if a specific region of an orthogonal polyhedron is searchable is strongly PSPACE-hard. By further improving our construction, we show that the same problem is strongly PSPACE-complete even for planar orthogonal polygons. Our last results are especially meaningful because no similar hardness theorems for 2-dimensional scenarios were previously known
Algorithms for Optimizing Search Schedules in a Polygon
In the area of motion planning, considerable work has been done on guarding
problems, where "guards", modelled as points, must guard a polygonal
space from "intruders". Different variants
of this problem involve varying a number of factors. The guards performing
the search may vary in terms of their number, their mobility, and their
range of vision. The model of intruders may or may not allow them to
move. The polygon being searched may have a specified starting point,
a specified ending point, or neither of these. The typical question asked
about one of these problems is whether or not certain polygons can be
searched under a particular guarding paradigm defined by the types
of guards and intruders.
In this thesis, we focus on two cases of a chain of guards searching
a room (polygon with a specific starting point) for mobile intruders.
The intruders must never be allowed to escape through the door undetected.
In the case of the two guard problem, the guards must start at the door
point and move in opposite directions along the boundary of the
polygon, never crossing the door point. At all times, the
guards must be able to see each other. The search is complete once both
guards occupy the same spot elsewhere on the polygon. In the case of
a chain of three guards, consecutive guards in the chain must always
be visible. Again, the search starts at the door point, and the outer
guards of the chain must move from the door in opposite directions.
These outer guards must always remain on the boundary of the polygon.
The search is complete once the chain lies entirely on a portion of
the polygon boundary not containing the door point.
Determining whether a polygon can be searched is a problem in the area
of visibility in polygons; further to that, our work is related
to the area of planning algorithms. We look for ways to find optimal schedules that minimize
the distance or time required to complete the search. This is done
by finding shortest paths in visibility diagrams that indicate valid
positions for the guards. In the case of
the two-guard room search, we are able to find the shortest distance
schedule and the quickest schedule. The shortest distance schedule
is found in O(n^2) time by solving an L_1 shortest path problem
among curved obstacles in two dimensions. The quickest search schedule is
found in O(n^4) time by solving an L_infinity shortest path
problem among curved obstacles in two dimensions.
For the chain of three guards, a search schedule minimizing the total
distance travelled by the outer guards is found in O(n^6) time by
solving an L_1 shortest path problem among curved obstacles in two dimensions
Planet Four: Terrains - Discovery of Araneiforms Outside of the South Polar Layered Deposits
We present the results of a systematic mapping of seasonally sculpted
terrains on the South Polar region of Mars with the Planet Four: Terrains (P4T)
online citizen science project. P4T enlists members of the general public to
visually identify features in the publicly released Mars Reconnaissance Orbiter
CTX images. In particular, P4T volunteers are asked to identify: 1) araneiforms
(including features with a central pit and radiating channels known as
'spiders'); 2) erosional depressions, troughs, mesas, ridges, and
quasi-circular pits characteristic of the South Polar Residual Cap (SPRC) which
we collectively refer to as 'Swiss cheese terrain', and 3) craters. In this
work we present the distributions of our high confidence classic spider
araneiforms and Swiss cheese terrain identifications. We find no locations
within our high confidence spider sample that also have confident Swiss cheese
terrain identifications. Previously spiders were reported as being confined to
the South Polar Layered Deposits (SPLD). Our work has provided the first
identification of spiders at locations outside of the SPLD, confirmed with high
resolution HiRISE imaging. We find araneiforms on the Amazonian and Hesperian
polar units and the Early Noachian highland units, with 75% of the identified
araneiform locations in our high confidence sample residing on the SPLD. With
our current coverage, we cannot confirm whether these are the only geologic
units conducive to araneiform formation on the Martian South Polar region. Our
results are consistent with the current CO2 jet formation scenario with the
process exploiting weaknesses in the surface below the seasonal CO2 ice sheet
to carve araneiform channels into the regolith over many seasons. These new
regions serve as additional probes of the conditions required for channel
creation in the CO2 jet process. (Abridged)Comment: accepted to Icarus - Supplemental data files are available at
https://www.zooniverse.org/projects/mschwamb/planet-four-terrains/about/results
- Icarus print version available at
http://www.sciencedirect.com/science/article/pii/S001910351730055
Connectivity Constraints in Network Analysis
This dissertation establishes mathematical foundations of connectivity requirements arising in both abstract and geometric network analysis. Connectivity constraints are ubiquitous in network design and network analysis. Aside from the obvious applications in communication and transportation networks, they have also appeared in forest planning, political distracting, activity detection in video sequences and protein-protein interaction networks. Theoretically, connectivity constraints can be analyzed via polyhedral methods, in which we investigate the structure of (vertex)-connected subgraph polytope (CSP).
One focus of this dissertation is on performing an extensive study of facets of CSP. We present the first systematic study of non-trivial facets of CSP. One advantage to study facets is that a facet-defining inequality is always among the tightest valid inequalities, so applying facet-defining inequalities when imposing connectivity constraints can guarantee good performance of the algorithm. We adopt lifting techniques to provide a framework to generate a wide class of facet-defining inequalities of CSP. We also derive the necessary and sufficient conditions when a vertex separator inequality, which plays a critical role in connectivity constraints, induces a facet of CSP. Another advantage to study facets is that CSP is uniquely determined by its facets, so full understanding of CSP's facets indicates full understanding of CSP itself. We are able to derive a full description of CSP for a wide class of graphs, including forest and several types of dense graphs, such as graphs with small independence number, s-plex with small s and s-defective cliques with small s. Furthermore, we investigate the relationship between lifting techniques, maximum weight connected subgraph problem and node-weight Steiner tree problem and study the computational complexity of generation of facet-defining inequalities.
Another focus of this dissertation is to study connectivity in geometric network analysis. In geometric applications like wireless networks and communication networks, the concept of connectivity can be defined in various ways. In one case, connectivity is imposed by distance, which can be modeled by unit disk graphs (UDG). We create a polytime algorithm to identify large 2-clique in UDG; in another case when connectivity is based on visibility, we provide a generalization of the two-guard problem
Efficient Image Segmentation and Segment-Based Analysis in Computer Vision Applications
This dissertation focuses on efficient image segmentation and
segment-based object recognition in computer vision
applications. Special attention is devoted to analyzing shape, of
particular importance for our two applications: plant species
identification from leaf photos, and object classification in remote
sensing images. Additionally, both problems are bound by efficiency,
constraining the choice of applicable methods: leaf recognition
results are to be used within an interactive system, while remote
sensing image analysis must scale well over very large image sets.
Leafsnap was the first mobile app to provide automatic recognition of
tree species, currently counting with over 1.7 million downloads. We
present an overview of the mobile app and corresponding back end
recognition system, as well as a preliminary analysis of
user-submitted data. More than 1.7 million valid leaf photos have been
uploaded by users, 1.3 million of which are GPS-tagged. We then focus
on the problem of segmenting photos of leaves taken against plain
light-colored backgrounds. These types of photos are used in practice
within Leafsnap for tree species recognition. A good segmentation is
essential in order to make use of the distinctive shape of leaves for
recognition. We present a comparative experimental evaluation of
several segmentation methods, including quantitative and qualitative
results. We then introduce a custom-tailored leaf segmentation method
that shows superior performance while maintaining computational
efficiency.
The other contribution of this work is a set of attributes for
analysis of image segments. The set of attributes is designed for use
in knowledge-based systems, so they are selected to be intuitive and
easily describable. The attributes can also be computed efficiently,
to allow applicability across different problems. We experiment with
several descriptive measures from the literature and encounter certain
limitations, leading us to introduce new attribute formulations and
more efficient computational methods. Finally, we experiment with the
attribute set on our two applications: plant species identification
from leaf photos and object recognition in remote sensing images
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