Search for: visibility-counting
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    Approximation algorithms for visibility computation and testing over a terrain

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

    Weak Visibility of Line Segments in Different Environments

    , Ph.D. Dissertation Sharif University of Technology Nouri Bygi, Mojtaba (Author) ; Ghodsi, Mohammad (Supervisor)
    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... 

    Weak visibility counting in simple polygons

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

    Randomized approximation algorithms for planar visibility counting problem

    , Article Theoretical Computer Science ; Volume 707 , 2018 , Pages 46-55 ; 03043975 (ISSN) Alipour, S ; Ghodsi, M ; Jafari, A ; Sharif University of Technology
    Elsevier B.V  2018
    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(log⁡n) query time using O(n4log⁡n) 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...