 Computational geometry

Computational geometry is a branch of computer science devoted to the study of algorithms which can be stated in terms of geometry. Some purely geometrical problems arise out of the study of computational geometric algorithms, and such problems are also considered to be part of computational geometry.
The main impetus for the development of computational geometry as a discipline was progress in computer graphics and computeraided design and manufacturing (CAD/CAM), but many problems in computational geometry are classical in nature, and may come from mathematical visualization.
Other important applications of computational geometry include robotics (motion planning and visibility problems), geographic information systems (GIS) (geometrical location and search, route planning), integrated circuit design (IC geometry design and verification), computeraided engineering (CAE) (mesh generation).
The main branches of computational geometry are:
 Combinatorial computational geometry, also called algorithmic geometry, which deals with geometric objects as discrete entities. A groundlaying book in the subject by Preparata and Shamos dates the first use of the term "computational geometry" in this sense by 1975.^{[1]}
 Numerical computational geometry, also called machine geometry, computeraided geometric design (CAGD), or geometric modeling, which deals primarily with representing realworld objects in forms suitable for computer computations in CAD/CAM systems. This branch may be seen as a further development of descriptive geometry and is often considered a branch of computer graphics or CAD. The term "computational geometry" in this meaning has been in use since 1971.^{[2]}
Contents
Combinatorial computational geometry
The primary goal of research in combinatorial computational geometry is to develop efficient algorithms and data structures for solving problems stated in terms of basic geometrical objects: points, line segments, polygons, polyhedra, etc.
Some of these problems seem so simple that they were not regarded as problems at all until the advent of computers. Consider, for example, the Closest pair problem:
 Given n points in the plane, find the two with the smallest distance from each other.
One could compute the distances between all the pairs of points, of which there are n(n1)/2, then pick the pair with the smallest distance. This bruteforce algorithm takes O(n^{2}) time; i.e. its execution time is proportional to the square of the number of points. A classic result in computational geometry was the formulation of an algorithm that takes O(n log n). Randomized algorithms that take O(n) expected time, as well as a deterministic algorithm that takes O(n log log n) time, have also been discovered.^{[citation needed]}
Computational geometry focuses heavily on computational complexity since the algorithms are meant to be used on very large datasets containing tens or hundreds of millions of points. For large data sets, the difference between O(n^{2}) and O(n log n) can be the difference between days and seconds of computation.
Problem classes
The core problems in computational geometry may be classified in different ways, according to various criteria. The following general classes may be distinguished.
Static problems
In the problems of this category, some input is given and the corresponding output needs to be constructed or found. Some fundamental problems of this type are:
 Convex hull: Given a set of points, find the smallest convex polyhedron/polygon containing all the points.
 Line segment intersection: Find the intersections between a given set of line segments.
 Delaunay triangulation
 Voronoi diagram: Given a set of points, partition the space according to which points is closest to the given points.
 Linear programming
 Closest pair of points: Given a set of points, find the two with the smallest distance from each other.
 Euclidean shortest path: Connect two points in a Euclidean space (with polyhedral obstacles) by a shortest path.
 Polygon triangulation: Given a polygon, partition its interior into triangles
 Mesh generation
The computational complexity for this class of problems is estimated by the time and space (computer memory) required to solve a given problem instance.
Geometric query problems
In geometric query problems, commonly known as geometric search problems, the input consists of two parts: the search space part and the query part, which varies over the problem instances. The search space typically needs to be preprocessed, in a way that multiple queries can be answered efficiently.
Some fundamental geometric query problems are:
 Range searching: Preprocess a set of points, in order to efficiently count the number of points inside a query region.
 Point location: Given a partitioning of the space into cells, produce a data structure that efficiently tells in which cell a query point is located.
 Nearest neighbor: Preprocess a set of points, in order to efficiently find which point is closest to a query point.
 Ray tracing: Given a set of objects in space, produce a data structure that efficiently tells which object a query ray intersects first.
If the search space is fixed, the computational complexity for this class of problems is usually estimated by:
 the time and space required to construct the data structure to be searched in
 the time (and sometimes an extra space) to answer queries.
For the case when the search space is allowed to vary, see "Dynamic problems".
Dynamic problems
Yet another major class is the dynamic problems, in which the goal is to find an efficient algorithm for finding a solution repeatedly after each incremental modification of the input data (addition or deletion input geometric elements). Algorithms for problems of this type typically involve dynamic data structures. Any of the computational geometric problems may be converted into a dynamic one, at the cost of increased processing time. For example, the range searching problem may be converted into the dynamic range searching problem by providing for addition and/or deletion of the points. The dynamic convex hull problem is to keep track of the convex hull, e.g., for the dynamically changing set of points, i.e., while the input points are inserted or deleted.
The computational complexity for this class of problems is estimated by:
 the time and space required to construct the data structure to be searched in
 the time and space to modify the searched data structure after an incremental change in the search space
 the time (and sometimes an extra space) to answer a query.
Variations
Some problems may be treated as belonging to either of the categories, depending on the context. For example, consider the following problem.
 Point in polygon: Decide whether a point is inside or outside a given polygon.
In many applications this problem is treated as a singleshot one, i.e., belonging to the first class. For example, in many applications of computer graphics a common problem is to find which area on the screen is clicked by a mouse cursor. However in some applications the polygon in question is invariant, while the point represents a query. For example, the input polygon may represent a border of a country and a point is a position of an aircraft, and the problem is to determine whether the aircraft violated the border. Finally, in the previously mentioned example of computer graphics, in CAD applications the changing input data are often stored in dynamic data structures, which may be exploited to speedup the pointinpolygon queries.
In some contexts of query problems there are reasonable expectations on the sequence of the queries, which may be exploited either for efficient data structures or for tighter computational complexity estimates. For example, in some cases it is important to know the worst case for the total time for the whole sequence of N queries, rather than for a single query. See also "amortized analysis".
Numerical computational geometry
Main article: computeraided geometric designThis branch is also known as geometric modelling and computeraided geometric design (CAGD).
Core problems are curve and surface modelling and representation.
The most important instruments here are parametric curves and parametric surfaces, such as Bezier curves, spline curves and surfaces. An important nonparametric approach is the level set method.
Application areas include shipbuilding, aircraft, and automotive industries. The modern ubiquity and power of computers means that even perfume bottles and shampoo dispensers are designed using techniques unheard of by shipbuilders of 1960s.
See also
 List of combinatorial computational geometry topics
 List of numerical computational geometry topics
 CAD/CAM/CAE
 Robotics
 Solid modeling
 Computational topology
 Digital geometry
 Discrete geometry (combinatorial geometry)
 Space partitioning
 Tricomplex number
 Wikiversity:Topic:Computational geometry
References
 ^ Franco P. Preparata and Michael Ian Shamos (1985). Computational Geometry  An Introduction. SpringerVerlag. 1st edition: ISBN 0387961313; 2nd printing, corrected and expanded, 1988: ISBN 3540961313.
 ^ A.R. Forrest, "Computational geometry", Proc. Royal Society London, 321, series 4, 187195 (1971)
Further reading
Journals
Combinatorial/algorithmic computational geometry
Below is the list of the major journals that have been publishing research in geometric algorithms. Please notice with the appearance of journals specifically dedicated to computational geometry, the share of geometric publications in generalpurpose computer science and computer graphics journals decreased.
 ACM Computing Surveys
 ACM Transactions on Graphics
 Acta Informatica
 Advances in Geometry
 Algorithmica
 Ars Combinatoria
 Computational Geometry: Theory and Applications
 Communications of the ACM
 Computer Aided Geometric Design
 Computer Graphics and Applications
 Computer Graphics World
 Discrete & Computational Geometry
 Geombinatorics
 Geometriae Dedicata
 IEEE Transactions on Graphics
 IEEE Transactions on Computers
 IEEE Transactions on Pattern Analysis and Machine Intelligence
 Information Processing Letters
 International Journal of Computational Geometry and Applications
 International Journal of Differential Geometry
 Journal of Combinatorial Theory, series B
 Journal of Computational Geometry
 Journal of the ACM
 Journal of Algorithms
 Journal of Computer and System Sciences
 Management Science
 Pattern Recognition
 Pattern Recognition Letters
 SIAM Journal on Computing
 SIGACT News; featured the "Computational Geometry Column" by Joseph O'Rourke
 Theoretical Computer Science
 The Visual Computer
External links
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