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#### 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

#### 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...

#### 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...

#### Reducing motion estimation time with skin detection

, Article 2009 IEEE International Workshop on Imaging Systems and Techniques, IST 2009, Hong Kong, 11 May 2009 through 12 May 2009 ; 2009 , Pages 71-75 ; 9781424434831 (ISBN) ; Shirali Shahreza, S ; IEEE Instrumentation and Measurement Society ; Sharif University of Technology
2009

Abstract

Motion estimation (ME) is one of the key parts of video compression algorithms. But, motion estimation and computation of motion vectors (MVs) are very time con-suming. In this paper, we propose a method for reducing the cost of motion estimation process. During this process, a series of candidate blocks should be searched to find the best motion vectors. In our method, we compare the skin parts of two blocks before comparing all pixel pairs of the two blocks. Having a preprocessing phase, the skin part comparison is performed quickly. This method provides a parameter that can be used to create a balance between the processing time and the motion estimation accuracy. © 2009 IEEE

#### Enhanced cache attack on AES applicable on ARM-based devices with new operating systems

, Article Computer Networks ; Volume 198 , 2021 ; 13891286 (ISSN) ; Soleimany, H ; Aref, M. R ; Sharif University of Technology
Elsevier B.V
2021

Abstract

There are several key challenges in performing cache-based attacks on ARM-based devices. Lipp et al. introduced various techniques to tackle these challenges and applied successfully different cache-based attacks on ARM-based mobile devices. In the cache-based attacks proposed by Lipp et al. it is assumed that the attacker has access to the mapping of virtual addresses to physical addresses through/proc/self/pagemap which is an important limiting factor in Linux and newer versions of Android operating systems. To access this mapping, the attacker must know the root of the operating system. In this paper, we introduce an Evict+Reload attack on the T-table-based implementation of AES which...