Class PolygonHullSimplifier

Namespace

NetTopologySuite.Simplify

Assembly

NetTopologySuite.dll

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.

public class PolygonHullSimplifier

Inheritance

object

PolygonHullSimplifier

Inherited Members

object.GetType()

object.MemberwiseClone()

object.ToString()

object.Equals(object)

object.Equals(object, object)

object.ReferenceEquals(object, object)

object.GetHashCode()

Constructors

PolygonHullSimplifier(Geometry, bool)

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.

public PolygonHullSimplifier(Geometry inputGeom, bool isOuter)

Parameters

inputGeom Geometry

The polygonal geometry to process

isOuter bool

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.

public double AreaDeltaRatio { get; set; }

Property Value

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].

public double VertexNumFraction { get; set; }

Property Value

double

Methods

GetResult()

Gets the result polygonal hull geometry.

public Geometry GetResult()

Returns

Geometry

The polygonal geometry for the hull

Hull(Geometry, bool, 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.

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

Parameters

geom Geometry

The polygonal geometry to process

isOuter bool

A flag indicating whether to compute an outer or inner hull

vertexNumFraction double

The target fraction of number of input vertices in result

Returns

Geometry

The hull geometry

HullByAreaDelta(Geometry, bool, 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.

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

Parameters

geom Geometry

The polygonal geometry to process

isOuter bool

A flag indicating whether to compute an outer or inner hull

areaDeltaRatio double

The target ratio of area difference to original area

Returns

Geometry

The hull geometry