Class PolygonHullSimplifier

Computes topology-preserving simplified hulls of polygonal geometry. Both outer and inner hulls can be computed. Outer hulls contain the input geometry and are larger in area. Inner hulls are contained by the input geometry and are smaller in area. In both the hull vertices are a subset of the input vertices. The hull construction attempts to minimize the area difference with the input geometry.

Hulls are generally concave if the input is. Computed hulls are topology-preserving: they do not contain any self-intersections or overlaps, so the result polygonal geometry is valid.

Polygons with holes and MultiPolygons are supported. The result has the same geometric type and structure as the input.

The number of vertices in the computed hull is determined by a target parameter. Two parameters are supported:

• Vertex Number fractionthe fraction of the input vertices retained in the result. Value 1 produces the original geometry. Smaller values produce less concave results. For outer hulls, value 0 produces the convex hull (with triangles for any holes). For inner hulls, value 0 produces a triangle (if no holes are present).
• Area Delta ratio the ratio of the change in area to the input area.Value 0 produces the original geometry. Larger values produce less concave results.

The algorithm ensures that the result does not cause the target parameter to be exceeded. This allows computing outer or inner hulls with a small area delta ratio as an effective way of removing narrow gores and spikes.

Inheritance

Object

PolygonHullSimplifier

Inherited Members

Object.Equals(Object)

Object.Equals(Object, Object)

Object.GetHashCode()

Object.GetType()

Object.MemberwiseClone()

Object.ReferenceEquals(Object, Object)

Object.ToString()

Namespace: NetTopologySuite.Simplify

Assembly: NetTopologySuite.dll

Syntax

public class PolygonHullSimplifier

Constructors

PolygonHullSimplifier(Geometry, Boolean)

Creates a new instance to compute a simplified hull of a polygonal geometry. An outer or inner hull is computed depending on the value of isOuter.

Declaration

public PolygonHullSimplifier(Geometry inputGeom, bool isOuter)

Parameters

Type	Name	Description
Geometry	inputGeom	The polygonal geometry to process
Boolean	isOuter	Indicates whether to compute an outer or inner hull

Properties

AreaDeltaRatio

Gets or sets a value indicating the target maximum ratio of the change in area of the result to the input area. The value must be 0 or greater.

Declaration

public double AreaDeltaRatio { get; set; }

Property Value

Type	Description
Double

VertexNumFraction

Gets or sets a value indicating the target fraction of input vertices which are retained in the result. The value should be in the range [0,1].

Declaration

public double VertexNumFraction { get; set; }

Property Value

Type	Description
Double

Methods

GetResult()

Gets the result polygonal hull geometry.

Declaration

public Geometry GetResult()

Returns

Type	Description
Geometry	The polygonal geometry for the hull

Hull(Geometry, Boolean, Double)

Computes topology-preserving simplified hull of a polygonal geometry, with hull shape determined by a target parameter specifying the fraction of the input vertices retained in the result. Larger values compute less concave results. A value of 1 produces the convex hull; a value of 0 produces the original geometry. Either outer or inner hulls can be computed.

Declaration

public static Geometry Hull(Geometry geom, bool isOuter, double vertexNumFraction)

Parameters

Type	Name	Description
Geometry	geom	The polygonal geometry to process
Boolean	isOuter	A flag indicating whether to compute an outer or inner hull
Double	vertexNumFraction	The target fraction of number of input vertices in result

Returns

Type	Description
Geometry	The hull geometry

HullByAreaDelta(Geometry, Boolean, Double)

Computes a boundary-respecting hull of a polygonal geometry, with hull shape determined by a target parameter specifying the ratio of maximum difference in area to original area. Larger values compute less concave results. A value of 0 produces the original geometry. Either outer or inner hulls can be computed.

Declaration

public static Geometry HullByAreaDelta(Geometry geom, bool isOuter, double areaDeltaRatio)

Parameters

Type	Name	Description
Geometry	geom	The polygonal geometry to process
Boolean	isOuter	A flag indicating whether to compute an outer or inner hull
Double	areaDeltaRatio	The target ratio of area difference to original area

Returns

Type	Description
Geometry	The hull geometry