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{{short description|Sweep line algorithm}}
In [[computational geometry]], the '''Bentley–Ottmann algorithm''' is a [[sweep line algorithm]] for listing all [[line segment intersection|''crossings'' in a set of line segments]], i.e. it finds the ''intersection points'' (or, simply, ''intersections'') of line segments. It extends the [[Shamos–Hoey algorithm]],{{sfnp|Shamos|Hoey|1976}} a similar previous algorithm for testing whether or not a set of line segments has any crossings. For an input consisting of <math>n</math> line segments with <math>k</math> crossings (or intersections), the Bentley–Ottmann algorithm takes time <math>\mathcal{O}((n + k) \log n)</math>. In cases where <math>k = \mathcal{o}\left(\frac{n^2}{\log n} \right)</math>, this is an improvement on a naïve algorithm that tests every pair of segments, which takes <math>\Theta(n^2)</math>.
The algorithm was initially developed by {{harvs|first1=Jon|last1=Bentley|author1-link=Jon Bentley (computer scientist)|first2=Thomas|last2=Ottmann|year=1979|txt}}; it is described in more detail in the textbooks {{harvtxt|Preparata|Shamos|1985}}, {{harvtxt|O'Rourke|1998}}, and {{harvtxt|de Berg|van Kreveld|Overmars|Schwarzkopf|2000}}. Although [[asymptotic analysis|asymptotically]] faster algorithms are now known by {{
==Overall strategy==
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#No three line segments intersect at a single point.
In such a case, ''L'' will always intersect the input line segments in a set of points whose vertical ordering changes only at a finite set of discrete ''events''. Specifically, a discrete event can either be associated with an endpoint (left or right) of a line-segment or intersection point of two line-segments. Thus, the continuous motion of ''L'' can be broken down into a finite sequence of steps, and simulated by an algorithm that runs in a finite amount of time.
There are two types of events that may happen during the course of this simulation. When ''L'' sweeps across an endpoint of a line segment ''s'', the intersection of ''L'' with ''s'' is added to or removed from the vertically ordered set of intersection points. These events are easy to predict, as the endpoints are known already from the input to the algorithm. The remaining events occur when ''L'' sweeps across a crossing between (or intersection of) two line segments ''s'' and ''t''. These events may also be predicted from the fact that, just prior to the event, the points of intersection of ''L'' with ''s'' and ''t'' are adjacent in the vertical ordering of the intersection points{{clarify|reason=It may not be easy to visualize this concept without a simple animation, unless you already know the algorithm.|date=March 2018}}.
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The algorithm does not need to maintain explicitly a representation of the sweep line ''L'' or its position in the plane. Rather, the position of ''L'' is represented indirectly: it is the vertical line through the point associated with the most recently processed event.
The binary search tree may be any [[Self-balancing binary search tree|balanced binary search tree]] data structure, such as a [[
==Detailed algorithm==
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#Initialize a [[self-balancing binary search tree]] ''T'' of the line segments that cross the sweep line ''L'', ordered by the ''y''-coordinates of the crossing points. Initially, ''T'' is empty. (Even though the line sweep ''L'' is not explicitly represented, it may be helpful to imagine it as a vertical line which, initially, is at the left of all input segments.)
#While ''Q'' is nonempty, find and remove the event from ''Q'' associated with a point ''p'' with minimum ''x''-coordinate. Determine what type of event this is and process it according to the following case analysis:
#*If ''p'' is the left endpoint of a line segment ''s'', insert ''s'' into ''T''. Find the line-segments ''r'' and ''t'' that are respectively immediately
#*If ''p'' is the right endpoint of a line segment ''s'', remove ''s'' from ''T''. Find the segments ''r'' and ''t'' that (prior to the removal of ''s'') were respectively immediately above and below it in ''T'' (if they exist). If ''r'' and ''t'' cross, add that crossing point as a potential future event in the event queue.
#*If ''p'' is the crossing point of two segments ''s'' and ''t'' (with ''s'' below ''t'' to the left of the crossing), swap the positions of ''s'' and ''t'' in ''T''.
==Analysis==
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==References==
*{{citation|last=Balaban|first=I. J.|contribution=An optimal algorithm for finding segments intersections|title=Proc. 11th ACM Symp. Computational Geometry|year=1995|pages=211–219|doi=10.1145/220279.220302|s2cid=6342118|doi-access=free|isbn=0-89791-724-3 }}.
*{{citation|last1=Bartuschka|first1=U.|last2=Mehlhorn|first2=K.|author2-link=Kurt Mehlhorn|last3=Näher|first3=S.|contribution=A robust and efficient implementation of a sweep line algorithm for the straight line segment intersection problem|url=http://www.dsi.unive.it/~wae97/proceedings/|title=Proc. Worksh. Algorithm Engineering|year=1997|contribution-url=http://www.dsi.unive.it/~wae97/proceedings/ONLY_PAPERS/pap13.ps.gz|editor1-first=G. F.|editor1-last=Italiano|editor1-link=Giuseppe F. Italiano|editor2-first=S.|editor2-last=Orlando|access-date=2009-05-27|archive-date=2017-06-06|archive-url=https://web.archive.org/web/20170606120507/http://www.dsi.unive.it/~wae97/proceedings/|url-status=dead}}.
*{{citation|last1=Bentley|first1=J. L.|author1-link=Jon Bentley (computer scientist)|last2=Ottmann|first2=T. A.|title=Algorithms for reporting and counting geometric intersections|journal=IEEE Transactions on Computers|volume=C-28|issue=9|pages=643–647|year=1979|doi=10.1109/TC.1979.1675432|s2cid=1618521}}.
*{{citation|last1=de Berg|first1=Mark|last2=van Kreveld|first2=Marc|last3=Overmars|first3=Mark|author3-link=Mark Overmars|last4=Schwarzkopf|first4=Otfried|title=Computational Geometry|publisher=Springer-Verlag|year=2000|isbn=978-3-540-65620-3|edition=2nd|chapter=Chapter 2: Line segment intersection|pages=[https://archive.org/details/computationalgeo00berg/page/19 19–44]|chapter-url-access=registration|chapter-url=https://archive.org/details/computationalgeo00berg/page/19}}.
*{{citation|last1=Boissonat|first1=J.-D.|last2=Preparata|first2=F. P.|author2-link=Franco P. Preparata|title=Robust plane sweep for intersecting segments|journal=SIAM Journal on Computing|year=2000|url=
*{{citation|last=Brown|first=K. Q.|title=Comments on "Algorithms for Reporting and Counting Geometric Intersections"|journal=IEEE Transactions on Computers|year=1981|volume=C-30|issue=2|page=147|doi=10.1109/tc.1981.6312179|s2cid=206622367}}.
*{{citation|last1=Chazelle|first1=Bernard|author1-link=Bernard Chazelle|last2=Edelsbrunner|first2=Herbert|author2-link=Herbert Edelsbrunner|title=An optimal algorithm for intersecting line segments in the plane|journal=[[Journal of the ACM]]|volume=39|issue=1|pages=1–54|year=1992|doi=10.1145/147508.147511|s2cid=785741|doi-access=free}}.
*{{citation|last1=Chen|first1=E. Y.|last2=Chan|first2=T. M.|author2-link=Timothy M. Chan|contribution=A space-efficient algorithm for segment intersection|title=Proc. 15th Canadian Conference on Computational Geometry|year=2003|url=http://www.cccg.ca/proceedings/2003/31.pdf}}.
*{{citation|last=Clarkson|first=K. L.|authorlink=Kenneth L. Clarkson|contribution=Applications of random sampling in computational geometry, II|title=Proc. 4th ACM Symp. Computational Geometry|pages=1–11|year=1988|doi=10.1145/73393.73394|s2cid=15134654|doi-access=free|isbn=0-89791-270-5 }}.
*{{citation|last1=Clarkson|first1=K. L.|author1-link=Kenneth L. Clarkson|last2=Cole|first2=R.|last3=Tarjan|first3=R. E.|author3-link=Robert Tarjan|title=Randomized parallel algorithms for trapezoidal diagrams|journal=[[International Journal of Computational Geometry and Applications]]|volume=2|issue=2|year=1992|pages=117–133|doi=10.1142/S0218195992000081}}. Corrigendum, '''2''' (3): 341–343.
*{{citation|last1=Eppstein|first1=D.|author1-link=David Eppstein|last2=Goodrich|first2=M. |author2-link=Michael T. Goodrich|last3=Strash|first3=D.|contribution=Linear-time algorithms for geometric graphs with sublinearly many crossings|title=Proc. 20th ACM-SIAM Symp. Discrete Algorithms (SODA 2009)|year=2009|pages=150–159|doi=10.1137/090759112|arxiv=0812.0893|bibcode=2008arXiv0812.0893E|s2cid=13044724}}.
*{{citation|last=Mulmuley|first=K.|authorlink=Ketan Mulmuley|contribution=A fast planar partition algorithm, I|title=[[Symposium on Foundations of Computer Science|Proc. 29th IEEE Symp. Foundations of Computer Science (FOCS 1988)]]|year=1988|pages=580–589|doi=10.1109/SFCS.1988.21974|isbn=0-8186-0877-3 |s2cid=34582594}}.
*{{citation|last=O'Rourke|first=J.|authorlink= Joseph O'Rourke (professor)|title=Computational Geometry in C|edition=2nd|publisher=Cambridge University Press|year=1998|isbn=978-0-521-64976-6|chapter=Section 7.7: Intersection of segments|pages=263–265}}.
*{{citation|last1=Preparata|first1=F. P.|author1-link=Franco P. Preparata|last2=Shamos|first2=M. I.|author2-link=Michael Ian Shamos|title=Computational Geometry: An Introduction|publisher=Springer-Verlag|year=1985|chapter=Section 7.2.3: Intersection of line segments|pages=278–287|bibcode=1985cgai.book.....P}}.
*{{citation|last1=Pach|first1=J.|author1-link=János Pach|last2=Sharir|first2=M.|author2-link=Micha Sharir|title=On vertical visibility in arrangements of segments and the queue size in the Bentley–Ottmann line sweeping algorithm|journal=SIAM Journal on Computing|volume=20|year=1991|pages=460–470|issue=3|doi=10.1137/0220029|mr=1094525}}.
*{{citation|last1=Shamos|first1=M. I.|author1-link=Michael Ian Shamos|contribution=Geometric intersection problems|title=[[Symposium on Foundations of Computer Science|17th IEEE Conf. Foundations of Computer Science (FOCS 1976)]]|pages=208–215|year=1976|doi=10.1109/SFCS.1976.16|last2=Hoey|first2=Dan|s2cid=124804}}.
==External links==
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