Class ConcaveHullOfPolygons

Constructs a concave hull of a set of polygons, respecting the polygons as constraints. A concave hull is a possibly non-convex polygon containing all the input polygons. A given set of polygons has a sequence of hulls of increasing concaveness, determined by a numeric target parameter. The computed hull "fills the gap" between the polygons, and does not intersect their interior.

The concave hull is constructed by removing the longest outer edges of the Delaunay Triangulation of the space between the polygons, until the target criterion parameter is reached.

The target criteria are:

Maximum Edge Lengththe length of the longest edge between the polygons is no larger than this value.
Maximum Edge Length Ratiodetermine the Maximum Edge Length as a fraction of the difference between the longest and shortest edge lengths between the polygons. This normalizes the Maximum Edge Length to be scale-free. A value of 1 produces the convex hull; a value of 0 produces the original polygons.

The preferred criterion is the Maximum Edge Length Ratio, since it is scale-free and local (so that no assumption needs to be made about the total amount of concaveness present).

Optionally the concave hull can be allowed to contain holes, via HolesAllowed.

The hull can be specified as being "tight", which means it follows the outer boundaries of the input polygons.

The input polygons must form a valid MultiPolygon (i.e.they must be non - overlapping).

Inheritance

object

ConcaveHullOfPolygons

Inherited Members

object.Equals(object)

object.Equals(object, object)

object.GetHashCode()

object.GetType()

object.MemberwiseClone()

object.ReferenceEquals(object, object)

object.ToString()

Namespace: NetTopologySuite.Algorithm.Hull

Assembly: NetTopologySuite.dll

Syntax

public class ConcaveHullOfPolygons

Constructors

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ConcaveHullOfPolygons(Geometry)

Creates a new instance for a given geometry.

Declaration

public ConcaveHullOfPolygons(Geometry polygons)

Parameters

Type	Name	Description
Geometry	polygons	The input geometry

Properties

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HolesAllowed

Gets or sets a flag indicating whether holes are allowed in the concave hull polygon.

Declaration

public bool HolesAllowed { get; set; }

Property Value

Type	Description
bool

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MaximumEdgeLength

Gets or sets the target maximum edge length for the concave hull. The length value must be zero or greater.

The value 0.0 produces the input polygons.
Larger values produce less concave results.
Above a certain large value the result is the convex hull of the input.

The edge length ratio provides a scale-free parameter which is intended to produce similar concave results for a variety of inputs.

Declaration

public double MaximumEdgeLength { get; set; }

Property Value

Type	Description
double

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MaximumEdgeLengthRatio

Gets or sets the target maximum edge length ratio for the concave hull. The edge length ratio is a fraction of the difference between the longest and shortest edge lengths in the Delaunay Triangulation of the area between the input polygons. (Roughly speaking, it is a fraction of the difference between the shortest and longest distances between the input polygons.) It is a value in the range 0 to 1.

The value 0.0 produces the original input polygons.
The value 1.0 produces the convex hull.

Declaration

public double MaximumEdgeLengthRatio { get; set; }

Property Value

Type	Description
double

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Tight

Gets or sets a flag indicating whether the boundary of the hull polygon is kept tight to the outer edges of the input polygons.

Declaration

public bool Tight { get; set; }

Property Value

Type	Description
bool

Methods

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ConcaveFillByLength(Geometry, double)

Computes a concave fill area between a set of polygons, using the target criterion of maximum edge length.

Declaration

public static Geometry ConcaveFillByLength(Geometry polygons, double maxLength)

Parameters

Type	Name	Description
Geometry	polygons	The input polygons
double	maxLength	The target maximum edge length

Returns

Type	Description
Geometry	The concave fill

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ConcaveFillByLengthRatio(Geometry, double)

Computes a concave fill area between a set of polygons, using the target criterion of maximum edge length ratio.

Declaration

public static Geometry ConcaveFillByLengthRatio(Geometry polygons, double lengthRatio)

Parameters

Type	Name	Description
Geometry	polygons	The input polygons
double	lengthRatio	The target maximum edge length ratio

Returns

Type	Description
Geometry	The concave fill

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ConcaveHullByLength(Geometry, double)

Computes a concave hull of set of polygons using the target criterion of maximum edge length.

Declaration

public static Geometry ConcaveHullByLength(Geometry polygons, double maxLength)

Parameters

Type	Name	Description
Geometry	polygons	The input polygons
double	maxLength	The target maximum edge length

Returns

Type	Description
Geometry	The concave hull

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ConcaveHullByLength(Geometry, double, bool, bool)

Computes a concave hull of set of polygons using the target criterion of maximum edge length, and allowing control over whether the hull boundary is tight and can contain holes.

Declaration

public static Geometry ConcaveHullByLength(Geometry polygons, double maxLength, bool isTight, bool isHolesAllowed)

Parameters

Type	Name	Description
Geometry	polygons	The input polygons
double	maxLength	The target maximum edge length
bool	isTight	A flag indicating if the hull should be tight to the outside of the polygons
bool	isHolesAllowed	A flag indicating if holes are allowed in the hull polygon

Returns

Type	Description
Geometry	The concave hull

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ConcaveHullByLengthRatio(Geometry, double)

Computes a concave hull of set of polygons using the target criterion of maximum edge length ratio.

Declaration

public static Geometry ConcaveHullByLengthRatio(Geometry polygons, double lengthRatio)

Parameters

Type	Name	Description
Geometry	polygons	The input polygons
double	lengthRatio	The target maximum edge length ratio

Returns

Type	Description
Geometry

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ConcaveHullByLengthRatio(Geometry, double, bool, bool)

Computes a concave hull of set of polygons using the target criterion of maximum edge length ratio, and allowing control over whether the hull boundary is tight and can contain holes.

Declaration

public static Geometry ConcaveHullByLengthRatio(Geometry polygons, double lengthRatio, bool isTight, bool isHolesAllowed)

Parameters

Type	Name	Description
Geometry	polygons	The input polygons
double	lengthRatio	The target maximum edge length ratio
bool	isTight	A flag indicating if the hull should be tight to the outside of the polygons
bool	isHolesAllowed	A flag indicating if holes are allowed in the hull polygon

Returns

Type	Description
Geometry	The concave hull

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GetFill()

Gets the concave fill, which is the area between the input polygons, subject to the concaveness control parameter.

Declaration

public Geometry GetFill()

Returns

Type	Description
Geometry	The concave fill

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GetHull()

Gets the computed concave hull.

Declaration

public Geometry GetHull()

Returns

Type	Description
Geometry	The concave hull