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#### Query point visibility computation in polygons with holes

, Article Computational Geometry: Theory and Applications ; Volume 39, Issue 2 , 2008 , Pages 78-90 ; 09257721 (ISSN) ; Ghodsi, M ; Sharif University of Technology
2008

Abstract

In this paper, we consider the problem of computing the visibility of a query point inside polygons with holes. The goal is to perform this computation efficiently per query considering the cost of the preprocessing phase. Our algorithm is based on solutions in [A.L.P. Bose, J.I. Munro, Efficient visibility queries in simple polygons, Computational Geometry: Theory and Applications 23 (3) (2002) 313-335] and [M.T.B. Aronov, L. Guibas, L. Zhang, Visibility queries and maintenance in simple polygons, Discrete and Computational Geometry 27 (4) (2002) 461-483] proposed for simple polygons. In our solution, the preprocessing is done in time O( n3logn) to construct a data structure of size O( n3)....

#### Visibility of a moving segment observer

, Article 2009 14th International CSI Computer Conference, CSICC 2009, 20 October 2009 through 21 October 2009 ; 2009 , Pages 152-157 ; 9781424442621 (ISBN) ; Ghodsi, M ; Sharif University of Technology
Abstract

In this paper we study combinatorial segment visibility, and show how to compute and maintain it as the observer moves in the plane. There are n non-overlapping convex objects in the plane, and we have a segment observer among them. We first consider static case of the problem, in which the observer and objects are static, and then we study dynamic case of the problem, in which the observer can move among obstacles. ©2009 IEEE

#### α-Visibility

, Article Computational Geometry: Theory and Applications ; Vol. 47, issue. 3 PART A , April , 2014 , pp. 435-446 ; ISSN: 09257721 ; ISBN: 9783642311543 ; Maheshwari, A ; Nouri-Baygim, M ; Sack, J. R ; Zarrabi-Zadeh, H ; Sharif University of Technology
Abstract

We study a new class of visibility problems based on the notion of α-visibility. Given an angle α and a collection of line segments S in the plane, a segment t is said to be α-visible from a point p, if there exists an empty triangle with one vertex at p and the side opposite to p on t such that the angle at p is α. In this model of visibility, we study the classical variants of point visibility, weak and complete segment visibility, and the construction of the visibility graph. We also investigate the natural query versions of these problems, when α is either fixed or specified at query time

#### α-visibility

, Article Lecture Notes in Computer Science (including subseries Lecture Notes in Artificial Intelligence and Lecture Notes in Bioinformatics) ; Volume 7357 LNCS , 2012 , Pages 1-12 ; 03029743 (ISSN) ; 9783642311543 (ISBN) ; Maheshwari, A ; Nouri, M ; Sack, J. R ; Zarrabi Zadeh, H ; Sharif University of Technology
2012

Abstract

We study a new class of visibility problems based on the notion of α-visibility. Given an angle α and a collection of line segments in the plane, a segment t is said to be α-visible from a point p, if there exists an empty triangle with one vertex at p and the side opposite to p on t such that the angle at p is α. In this model of visibility, we study the classical variants of point visibility, weak and complete segment visibility, and the construction of the visibility graph. We also investigate the natural query versions of these problems, when α is either fixed or specified at query time

#### Efficient visibility maintenance of a moving segment observer inside a simple polygon

, Article 19th Annual Canadian Conference on Computational Geometry, CCCG 2007, Ottawa, ON, 20 August 2007 through 22 August 2007 ; 2007 , Pages 249-252 ; Zarei, A ; Ghodsi, M ; Sharif University of Technology
2007

Abstract

In this paper we consider maintaining the visibility of a segment observer moving inside a simple polygon. A practical instance of this problem is to identify the regions of a planar scene illuminated by a fluorescent lamp while the lamp moves around. We consider both strong and weak visibility in this paper. Our method is based on the shortest path tree which builds a linear-sized data structure in O(n) time, where n is the number of the vertices of the underlying simple polygon P. We first compute VP(st̄), the initial view of the segment observer st̄. Then, as st̄ moves, each change of VP(st̄) can be computed in O(log2(|V P(st̄)|)) time when the observer is allowed to change its direction,...

#### Approximation algorithms for visibility computation and testing over a terrain

, Article Applied Geomatics ; Volume 9, Issue 1 , 2017 , Pages 53-59 ; 18669298 (ISSN) ; Ghodsi, M ; Güdükbay, U ; Golkari, M ; Sharif University of Technology
Springer Verlag
2017

Abstract

Given a 2.5D terrain and a query point p on or above it, we want to find the triangles of terrain that are visible from p. We present an approximation algorithm to solve this problem. We implement the algorithm and test it on real data sets. The experimental results show that our approximate solution is very close to the exact solution and compared to the other similar works, the computational cost of our algorithm is lower. We analyze the computational complexity of the algorithm. We consider the visibility testing problem where the goal is to test whether a given triangle of the terrain is visible or not with respect to p. We present an algorithm for this problem and show that the average...

#### New Results on Different Types of Visibility Problems

, Ph.D. Dissertation Sharif University of Technology ; Ghodsi, Mohammad (Supervisor)
Abstract

The study of visibility is at least 100 years old, when in 1913 Brunn proved a theorem about the kernel of a set. By now, visibility has become one of the most studied notions in computational geometry. The reasons are two-fold: 1) such problems arise naturally in areas where computational geometry tools and algorithms _nd applications including: computer graphics, robotics, motion planning, geographic information systems, computer games, computer-aided architecture, and pattern recognition; and where 2) their solutions are required, or serve as building blocks in the development of solutions to other problems, such as shortest paths or motion planning problems. Many natural problem...

#### Weak Visibility of Line Segments in Different Environments

, Ph.D. Dissertation Sharif University of Technology ; Ghodsi, Mohammad (Supervisor)
Abstract

Visibility is an important topic in computational geometry, computer graphics, and motion planning. Two points inside a polygon are visible to each other if their connecting segment remains completely inside the polygon. Visibility polygon of a point in a simple polygon P is the set of points inside P that are visible from the point. The visibility problem has also been considered for line segments. A point v is said to be weakly visible to a line segment pq if there exists a point w 2 pq, such that w and v are visible to each other. The problem of computing the weak visibility polygon (or WVP) of pq inside a polygon P is to compute all points of P that are weakly visible from pq. In this...

#### Efficient Observer-Dependent Simplification in Polygonal Domains

, Article Algorithmica (New York) ; April , 2011 , Pages 1-21 ; 01784617 (ISSN) ; Ghodsi, M ; Sharif University of Technology
Abstract

In this paper, we consider a special version of the well-known line-simplification problem for simplifying the boundary of a region illuminated by a point light source q, or its visibility polygon VP(q). In this simplification approach, we should take the position of q as an essential factor into account to determine the quality of the resulting simplification. For this purpose, we redefine the known distance- and area-distortion error criteria as the main simplification criteria to take into account the distance between the observer q and the boundary of VP(q). Based on this, we propose algorithms for simplifying VP(q). More precisely, we propose simplification algorithms of O(n2) and...

#### The social work services in screening process of compulsory treatment interventions in addicts in Iran: a content analysis study

, Article Asian Social Work and Policy Review ; Volume 12, Issue 2 , 2018 , Pages 86-93 ; 17531403 (ISSN) ; Fathi, M ; Barati Sedeh, F ; Deilamizade, A ; Puyan, D ; Sharif University of Technology
Blackwell Publishing
2018

Abstract

Compulsory treatment has had a detrimental impact on drug abuse treatment policies and programs in Iran. Physicians are currently required to conduct initial treatment screening. Social workers are a part of the treatment team but have no authority to initiate a compulsory drug treatment plan. For this reason, the present article investigates social work service participation in the process of compulsory drug treatment. The study methodology is content analysis. Nine social workers participated in seven focus group discussions. Overall, 110 codes were extracted from the sessions and were categorized into five main themes: interview and initial assessment; referral; investigation of...

#### Recognition and Reconstruction of Visibility Gragh in Special Cases

, M.Sc. Thesis Sharif University of Technology ; Zarei, Alireza (Supervisor)
Abstract

For a simple polygon in the plane, the visibility graph is a graph whose vertices are the vertices of the polygon and there is an edge between two vertices if and only if their corresponding vertices see each other in the polygon. Two points of the polygon see each other if the line segment connecting them lies completely inside (or on the boundary of) the polygon. Although computing the visibility graph of a polygon has been solved efficiently, its reverse problem after three decades is still an open problem. The reverse problem is known as recognition and reconstruction visibility graphs. Recognizing visibility graphs is to determine the necessary and sufficient conditions on a graph to be...

#### Weak visibility queries in simple polygons

, Article Proceedings of the 23rd Annual Canadian Conference on Computational Geometry, CCCG 2011 ; 2011 ; Ghodsi, M ; Sharif University of Technology
Abstract

In this paper, we consider the problem of computing the weak visibility (WV ) of a query line segment in- side a simple polygon. Our algorithm first preprocesses the polygon and creates data structures from which any WV query is answered efficiently in an output sensitive manner. In our solution, the preprocessing is performed in time O(n3 log n) and the size of the constructed data structure is O(n3). It is then possible to report the WV polygon of any query line segment in time O(log n+k), where k is the size of the output. Our algorithm im- proves the current results for this problem

#### A practical approach for planar visibility maintenance

, Article Journal for Geometry and Graphics ; Volume 13, Issue 1 , 2009 , Pages 75-90 ; 14338157 (ISSN) ; Ghodsi, M ; Sharif University of Technology
2009

Abstract

In this paper, we propose a method for maintaining the region visible from a moving point observer inside a planar scene. In this method, we check the observer position in discrete time-stamps to detect and apply changes to the visible or illuminated region of a moving point observer q, or VP(q). We efficiently maintain a list C(q) of edges in VP(q) which are subject to change during the motion. In each time-stamp that VP(q) is to be updated, we only refine and redraw the view against the edges of C(q) that indicate the positions of the visibility changes. We build an enriched representation of the visibility graph in a preprocessing step to apply the required updates on C(q) efficiently and...

#### Recognizing visibility graphs of triangulated irregular networks

, Article Fundamenta Informaticae ; Volume 179, Issue 4 , 2021 , Pages 345-360 ; 01692968 (ISSN) ; Ostovari, M ; Zarei, A ; Sharif University of Technology
IOS Press BV
2021

Abstract

A Triangulated Irregular Network (TIN) is a data structure that is usually used for representing and storing monotone geographic surfaces, approximately. In this representation, the surface is approximated by a set of triangular faces whose projection on the XY-plane is a triangulation. The visibility graph of a TIN is a graph whose vertices correspond to the vertices of the TIN and there is an edge between two vertices if their corresponding vertices on TIN see each other, i.e. the segment that connects these vertices completely lies above the TIN. Computing the visibility graph of a TIN and its properties have been considered thoroughly in the literature. In this paper, we consider this...

####
A Parallel Algorithm for Visibility Problem on GPU

,
M.Sc. Thesis
Sharif University of Technology
;
Ghodsi, Mohammad
(Supervisor)
;
Abam, Mohammad Ali
(Co-Advisor)
Abstract

Finding visibility region of an observer is one of the old and well-known problems in computational geometry and computer science.In this Problem, considering an observer (point, line segment or a geometrical object)inside an environment, the goal is to find the visible portion of that environment from the observer. Today visibility is used in many fields of computer science including robotics, computer vision and graphics. Nowadays tending to parallelism due to increases in data size and the need for accurate and complicated computations is inevitable. The GPU is composed of many weak processing units, which makes it a cheap and reachable resource for high performance computing. Due to it’s...

#### Pseudo-triangulating a simple polygon from its visibility graph

, Article 2015 5th International Conference on Computer and Knowledge Engineering, ICCKE 2015, 29 October 2015 ; October , 2015 , Pages 106-111 ; 9781467392808 (ISBN) ; Sharif University of Technology
Institute of Electrical and Electronics Engineers Inc
2015

Abstract

Visibility graph of a simple polygon in a plain is a graph in which the number of its vertices corresponds with the number of vertices in the polygon and each of its edges corresponds with a pair of visible vertices in the polygon. Visibility graph reconstruction of a polygon is one of the old and important problems in computational geometry for which no algorithm has been offered yet. Considering that the problem of visibility graph reconstruction of a pseudo-triangle has been solved, we present an O(n2)-time algorithm for pseudo-triangulation of a simple polygon, using the visibility graph corresponding with the polygon (n is number of vertices of the polygon). To do so, first we present a...

#### Weak visibility counting in simple polygons

, Article Journal of Computational and Applied Mathematics ; Volume 288 , November , 2015 , Pages 215-222 ; 03770427 (ISSN) ; Daneshpajouh, S ; Alipour, S ; Ghodsi, M ; Sharif University of Technology
Elsevier
2015

Abstract

For a simple polygon P of size n, we define weak visibility counting problem (WVCP) as finding the number of visible segments of P from a query line segment pq. We present different algorithms to compute WVCP in sub-linear time. In our first algorithm, we spend O(n7) time to preprocess the polygon and build a data structure of size O(n6), so that we can optimally answer WVCP in O(logn) time. Then, we reduce the preprocessing costs to O(n4+ε) time and space at the expense of more query time of O(log5n). We also obtain a trade-off between preprocessing and query time costs. Finally, we propose an approximation method to reduce the preprocessing costs to O(n2) time and space and O(n1/2+ε) query...

#### Geometric spanners for points inside a polygonal domain

, Article 31st International Symposium on Computational Geometry, SoCG 2015, 22 June 2015 through 25 June 2015 ; Volume 34 , 2015 , Pages 186-197 ; 18688969 (ISSN) ; 9783939897835 (ISBN) ; Adeli, M ; Homapour, H ; Zafar-Asadollahpoor, P ; Sharif University of Technology
Schloss Dagstuhl- Leibniz-Zentrum fur Informatik GmbH, Dagstuhl Publishing
2015

Abstract

Let P be a set of n points inside a polygonal domain D. A polygonal domain with h holes (or obstacles) consists of h disjoint polygonal obstacles surrounded by a simple polygon which itself acts as an obstacle. We first study t-spanners for the set P with respect to the geodesic distance function π where for any two points p and q, (p, q) is equal to the Euclidean length of the shortest path from p to q that avoids the obstacles interiors. For a case where the polygonal domain is a simple polygon (i.e., h = 0), we construct a (10 + ε)-spanner that has O(n log2 n) edges. For a case where there are h holes, our construction gives a (5 + ε)-spanner with the size of O(nh log2 n). Moreover, we...

#### GPU-based parallel algorithm for computing point visibility inside simple polygons

, Article Computers and Graphics (Pergamon) ; Volume 49 , 2015 , Pages 1-9 ; 00978493 (ISSN) ; Ghodsi, M ; Sharif University of Technology
Elsevier Ltd
2015

Abstract

Given a simple polygon P in the plane, we present a parallel algorithm for computing the visibility polygon of an observer point q inside P. We use chain visibility concept and a bottom-up merge method for constructing the visibility polygon of point q. The algorithm is simple and mainly designed for GPU architectures, where it runs in O(logn) time using O(n) processors. This is the first work on designing a GPU-based parallel algorithm for the visibility problem. To the best of our knowledge, the presented algorithm is also the first suboptimal parallel algorithm for the visibility problem that can be implemented on existing parallel architectures. We evaluated a sample implementation of...

#### Near optimal line segment queries in simple polygons

, Article Journal of Discrete Algorithms ; Volume 35 , November , 2015 , Pages 51-61 ; 15708667 (ISSN) ; Ghodsi, M ; Sharif University of Technology
Elsevier
2015

Abstract

This paper considers the problem of computing the weak visibility polygon (WVP) of any query line segment pq (or WVP(pq)) inside a given simple polygon P. We present an algorithm that preprocesses P and creates a data structure from which WVP(pq) is efficiently reported in an output sensitive manner. Our algorithm needs O(n2log n) time and O(n2) space in the preprocessing phase to report WVP(pq) of any query line segment pq in time O(|WVP(pq)|+log2 n+κlog2 (nκ)), where κ is an input and output sensitive parameter of at most |WVP(pq)|. We improve the preprocessing time and space of current results for this problem [11,6] at the expense of more query time