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#### Visibility testing and counting

, Article Information Processing Letters ; Volume 115, Issue 9 , September , 2015 , Pages 649-654 ; 00200190 (ISSN) ; Ghodsi, M ; Zarei, A ; Pourreza, M ; Sharif University of Technology
Elsevier
2015

Abstract

For a set of n disjoint line segments S in R2, the visibility testing problem (VTP) is to test whether the query point p sees a query segment s∈S. For this configuration, the visibility counting problem (VCP) is to preprocess S such that the number of visible segments in S from any query point p can be computed quickly. In this paper, we solve VTP in expected logarithmic query time using quadratic preprocessing time and space. Moreover, we propose a (1+δ)-approximation algorithm for VCP using at most quadratic preprocessing time and space. The query time of this method is Oε (1/δ 2√n) where Oε (f(n))=O(f(n)nε) and ε>0 is an arbitrary constant number

#### Visibility testing and counting

, Article Lecture Notes in Computer Science (including subseries Lecture Notes in Artificial Intelligence and Lecture Notes in Bioinformatics), 28 May 2011 through 31 May 2011, Jinhua ; Volume 6681 LNCS , 2011 , Pages 343-351 ; 03029743 (ISSN) ; 9783642212031 (ISBN) ; Zarei, A ; Sharif University of Technology
2011

Abstract

For a set of n disjoint line segments S in R2, the visibility counting problem (VCP) is to preprocess S such that the number of visible segments in S from a query point p can be computed quickly. For this configuration, the visibility testing problem (VTP) is to test whether p sees a fixed segment s. These problems can be solved in logarithmic query time by using O(n4) preprocessing time and space. In this paper, we approximately solve this problem using quadratic preprocessing time and space. Our methods are superior to current approximation algorithms in terms of both approximation factor and preprocessing cost. In this paper, we propose a 2-approximation algorithm for the VCP using at...

#### 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

#### Randomized approximation algorithms for planar visibility counting problem

, Article Theoretical Computer Science ; Volume 707 , 2018 , Pages 46-55 ; 03043975 (ISSN) ; Ghodsi, M ; Jafari, A ; Sharif University of Technology
Elsevier B.V
2018

Abstract

Given a set S of n disjoint line segments in R2, the visibility counting problem (VCP) is to preprocess S such that the number of segments in S visible from any query point p can be computed quickly. This problem can be solved trivially in O(logn) query time using O(n4logn) preprocessing time and O(n4) space. Gudmundsson and Morin (2010) [10] proposed a 2-approximation algorithm for this problem with a tradeoff between the space and the query time. For any constant 0≤α≤1, their algorithm answers any query in Oϵ(m(1−α)/2) time with Oϵ(m1+α) of preprocessing time and space, where ϵ>0 is a constant that can be made arbitrarily small and Oϵ(f(n))=O(f(n)nϵ) and m=O(n2) is a number that depends...

#### Space-query-time tradeoff for computing the visibility polygon

, Article 3rd International Frontiers of Algorithmics Workshop, FAW 2009, Hefei, 20 June 2009 through 23 June 2009 ; Volume 5598 LNCS , 2009 , Pages 120-131 ; 03029743 (ISSN); 3642022693 (ISBN); 9783642022692 (ISBN) ; Ghodsi, M ; Sharif University of Technology
2009

Abstract

Computing the visibility polygon, VP, of a point in a polygonal scene, is a classical problem that has been studied extensively. In this paper, we consider the problem of computing VP for any query point efficiently, with some additional preprocessing phase. The scene consists of a set of obstacles, of total complexity O(n). We show for a query point q, VP(q) can be computed in logarithmic time using O(n 4) space and O(n 4 logn) preprocessing time. Furthermore to decrease space usage and preprocessing time, we make a tradeoff between space usage and query time; so by spending O(m) space, we can achieve O(n 2 log √ m/n)/√ m)query time, where n 2≤m≤n 4. These results are also applied to...

#### Query-point visibility constrained shortest paths in simple polygons

, Article Theoretical Computer Science ; Volume 389, Issue 1-2 , 2007 , Pages 1-11 ; 03043975 (ISSN) ; Ghodsi, M ; Sharif University of Technology
2007

Abstract

In this paper, we study the problem of finding the shortest path between two points inside a simple polygon such that there is at least one point on the path from which a query point is visible. We provide an algorithm which preprocesses the input in O (n2 + n K) time and space and provides logarithmic query time. The input polygon has n vertices and K is a parameter dependent on the input polygon which is O (n2) in the worst case but is much smaller for most polygons. The preprocessing algorithm sweeps an angular interval around every reflex vertex of the polygon to store the optimal contact points between the shortest paths and the windows separating the visibility polygons of the query...

#### Liver Segmentation in CT Images

, M.Sc. Thesis Sharif University of Technology ; Manzouri, Mohammad Taghi (Supervisor)
Abstract

Image segmentation has a huge amount of applications in machine vision, target detection, medical image processing, etc. In many medical researches such as Organ and Gland Volume Specification, Analysis of Anatomical Structures and Multimodal Image Registration, Organ

Segmentation is the first step of preprocessing.Since detection of diseases out of medical images depends on organ segmentation results,the segmentation process is done by experts which has a lot of disadvantages such as high time computation, high cost, etc. Hence, designing algorithms that can segment images with high accuracy and need minimum user interaction are desirable. So, in this thesis, a new

knowledge based...

Segmentation is the first step of preprocessing.Since detection of diseases out of medical images depends on organ segmentation results,the segmentation process is done by experts which has a lot of disadvantages such as high time computation, high cost, etc. Hence, designing algorithms that can segment images with high accuracy and need minimum user interaction are desirable. So, in this thesis, a new

knowledge based...

#### Optimization in Hub Location Allocation Problem

, M.Sc. Thesis Sharif University of Technology ; Eshghi, Koorosh (Supervisor)
Abstract

Hub location problem arises where the flow has to be routed between a set of origins and destinations, but direct links between all nodes are impossible or too costly. In these situations, e.g. transportation and communication systems, hubs serve as transshipment points to collect, transfer, and distribute the flow. In hub location allocation problem, hubs are located and non-hub nodes are allocated to them. In this thesis, two different models of capacitated multiple allocation hub location problem are discussed. In the first model, flow collection of hubs from non-hub nodes and in the second model, sum of flow collection and distribution of hubs from/to non-hub nodes is addressed. The...

#### Space/query-time tradeoff for computing the visibility polygon

, Article Computational Geometry: Theory and Applications ; Volume 46, Issue 3 , April , 2013 , Pages 371-381 ; 09257721 (ISSN) ; Ghodsi, M ; Sharif University of Technology
2013

Abstract

In this paper, we consider the problem of computing the visibility polygon (VP) of a query point q (or VP(q)) in a scene consisting of some obstacles with total complexity of n. We show that the combinatorial representation of VP(q) can be computed in logarithmic time by preprocessing the scene in O( n4logn) time and using O( n4) space. We can also report the actual VP(q) in additional O(|VP(q)|) time. As a main result of this paper, we will prove that we can have a tradeoff between the query time and the preprocessing time/space. In other words, we will show that using O(m) space, we can obtain O( n2log(m/n)/m) query time, where m is a parameter satisfying n2≤m≤ n4. For example, when m= n3,...

#### Kinetic Euclidean minimum spanning tree in the plane

, Article Journal of Discrete Algorithms ; Volume 16 , October , 2012 , Pages 2-11 ; 15708667 (ISSN) ; Zarei, A ; Sharif University of Technology
Elsevier
2012

Abstract

This paper presents a kinetic data structure (KDS) for maintenance of the Euclidean minimum spanning tree (EMST) on a set of moving points in 2-dimensional space. For a set of n points moving in the plane we build a KDS of size O(n) in O(nlogn) preprocessing time by which the EMST is maintained efficiently during the motion. This is done by applying the required changes to the combinatorial structure of the EMST which is changed in discrete timestamps. We assume that the motion of the points, i.e. x and y coordinates of the points, are defined by algebraic functions of constant maximum degree. In terms of the KDS performance parameters, our KDS is responsive, local, and compact. The...

#### 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...

#### Enhancing physionet electrocardiogram records for fetal heart rate detection algorithm

, Article Proceedings - 2015 2nd International Conference on Biomedical Engineering, ICoBE 2015 ; 2015 ; 9781479917495 (ISBN) ; Ali, M. A. M ; Zahedi, E ; Sharif University of Technology
Abstract

The noninvasive fetal electrocardiogram (ECG) data available from Physionet data bank are suitable for developing fetal heart rate (FHR) detection algorithms. The data have been collected from single subject with a broad range of gestation weeks, and have a total data length of more than 9 hours arranged in 55 data sets. However, there are three additional data features which are currently not directly available from Physionet to facilitate the easy usage of these data: (1) the fetal peak visibility evaluation, (2) the gestation week, and (3) the data length. This article presents an improvement to the data bank by providing the additional features. The required pre-processing of the data is...

#### Weak visibility queries of line segments in simple polygons and polygonal domains

, Article International Journal of Computer Mathematics ; 2017 , Pages 1-18 ; 00207160 (ISSN) ; Ghodsi, M ; Sharif University of Technology
Taylor and Francis Ltd
2017

Abstract

In this paper we consider the problem of computing the weak visibility polygon of a query line segment pq (or (Formula presented.)) inside a given polygon (Formula presented.). Our first algorithm runs in simple polygons and needs (Formula presented.) time and (Formula presented.) space in the preprocessing phase to report (Formula presented.) of any query line segment pq in time (Formula presented.). We also give an algorithm to compute the weak visibility polygon of a query line segment in a non-simple polygon with (Formula presented.) pairwise-disjoint polygonal obstacles with a total of n vertices. Our algorithm needs (Formula presented.) time and (Formula presented.) space in the...

#### Weak visibility queries of line segments in simple polygons and polygonal domains

, Article International Journal of Computer Mathematics ; Volume 95, Issue 4 , 2018 , Pages 721-738 ; 00207160 (ISSN) ; Ghodsi, M ; Sharif University of Technology
Taylor and Francis Ltd
2018

Abstract

In this paper we consider the problem of computing the weak visibility polygon of a query line segment pq (or WVP(pq)) inside a given polygon P. Our first algorithm runs in simple polygons and needs O(n3 log n) time and O(n3) space in the preprocessing phase to report WVP(pq) of any query line segment pq in time O(log n + |WVP(pq)|).. We also give an algorithm to compute the weak visibility polygon of a query line segment in a non-simple polygon with h ≥ 1 pairwise-disjoint polygonal obstacles with a total of n vertices. Our algorithm needs O(n2 log n) time and O(n2) space in the preprocessing phase and WVP(pq) in query time of O(nh’ log n + k), in which h’ is an output sensitive parameter...

#### Windowing queries using Minkowski sum and their extension to MapReduce

, Article Journal of Supercomputing ; Volume 77, Issue 1 , 2021 , Pages 936-972 ; 09208542 (ISSN) ; Keikha, V ; Ghodsi, M ; Mohades, A ; Sharif University of Technology
Springer
2021

Abstract

Given a set of n segments and a query shape Q, the windowing length query asks for finding the sum of the lengths of the parts of the segments that lie inside Q. The popular places problem of a set of curves asks for the subset of the plane where each query shape centered at a point of that region intersects with at least f distinct curves. For square queries, an optimal O(n2) time algorithm and a matching lower bound exist. We solve the length query problem for convex polygons and disks as query shapes, with O(log n+ k) query time and polynomial preprocessing time that depends on the complexity of the query shape. We define a new version of the problem of finding popular places in a set of...

#### Windowing queries using Minkowski sum and their extension to MapReduce

, Article Journal of Supercomputing ; Volume 77, Issue 1 , 2021 , Pages 936-972 ; 09208542 (ISSN) ; Keikha, V ; Ghodsi, M ; Mohades, A ; Sharif University of Technology
Springer
2021

Abstract

Given a set of n segments and a query shape Q, the windowing length query asks for finding the sum of the lengths of the parts of the segments that lie inside Q. The popular places problem of a set of curves asks for the subset of the plane where each query shape centered at a point of that region intersects with at least f distinct curves. For square queries, an optimal O(n2) time algorithm and a matching lower bound exist. We solve the length query problem for convex polygons and disks as query shapes, with O(log n+ k) query time and polynomial preprocessing time that depends on the complexity of the query shape. We define a new version of the problem of finding popular places in a set of...

#### Design and Hardware Implementation of Optical Character Recognition

, M.Sc. Thesis Sharif University of Technology ; Hashemi, Matin (Supervisor)
Abstract

The objective of OCR systems is to retrieve machine-encoded text from a raster image. Despite the abundance of powerful OCR algorithms for English, there are not many for Farsi. Our proposed algorithm is comprised of pre-processing, line detection, sub-word detection and segmentation, feature extraction and classification. Furthermore, hardware implementation and acceleration of this system on a GPGPU is presented. This algorithm was tested on 5 fonts including Titr, Lotus,Yekan, Koodak and Nazanin and an average accuracy above 90% was achieved

#### Persian Query Corrector Based on Deep Learning Networks (with Emphasis on Spatial Queries)

, M.Sc. Thesis Sharif University of Technology ; Izadi, Mohammad (Supervisor)
Abstract

The main spectrum of a location search engine is retrieving the locations that are most relevant to the user query. Geographic and spatial queries usually consist of a series of keywords that express the user's needs. The geographic search engine should retrieve the locations associated with the user request and then rank the retrieved results according to their relevance to the user request. In some cases, user requests may contain spelling errors that can greatly affect the results retrieved. Spell correction is automatic in the back spectrum recovery system. Spelling correction is the task of automatically recovering the intended text from a misspelled text. Therefore, spelling correction...

#### Investigation of brain default network's activation in autism spectrum disorders using group independent component analysis

, Article 2014 21st Iranian Conference on Biomedical Engineering, ICBME 2014 ; 2014 , p. 177-180 ; Fatemizadeh, E ; Deevband, M. R ; Sharif University of Technology
Abstract

Autism Spectrum Disorders (ADS), with unknown etiology, is one of the most understudy fields of research worldwide that requires complicated and delicate analytical study methods. The purpose of this study was to compare active regions of Brain Default Mode Network (DMN) using Group Independent Component Analysis (6ICA) among resting state patients with Autism Disorder and healthy subjects. Default Mode Network consists of posterior cingulate cortex (PCC), lateral parietal cortex/angular gyrus retrosplenial cortex, medial prefrontal cortex, superior frontal gyrus, parahippocampal gyrus and temporal lobe shows more prominent activity in passive resting conditions. The diagnosis of autism...

#### On the power of the semi-separated pair decomposition

, Article Computational Geometry: Theory and Applications ; Volume 46, Issue 6 , 2013 , Pages 631-639 ; 09257721 (ISSN) ; Carmi, P ; Farshi, M ; Smid, M ; Sharif University of Technology
2013

Abstract

A Semi-Separated Pair Decomposition (SSPD), with parameter s>1, of a set SâŠ"Rd is a set {(Ai,Bi)} of pairs of subsets of S such that for each i, there are balls DAi and DBi containing Ai and Bi respectively such that d(DAi,DBi)≥s×min(radius(DAi), radius(DBi)), and for any two points p,qâ̂̂S there is a unique index i such that pâ̂̂Ai and qâ̂̂Bi or vice versa. In this paper, we use the SSPD to obtain the following results: First, we consider the construction of geometric t-spanners in the context of imprecise points and we prove that any set SâŠ"Rd of n imprecise points, modeled as pairwise disjoint balls, admits a t-spanner with O(nlogn/(t-1)d) edges that can be computed in O(nlogn/(t-1)d)...