Class MonotoneChain

MonotoneChains are a way of partitioning the segments of a linestring to allow for fast searching of intersections.

Inheritance

Object

MonotoneChain

Inherited Members

Object.Equals(Object)

Object.Equals(Object, Object)

Object.GetHashCode()

Object.GetType()

Object.MemberwiseClone()

Object.ReferenceEquals(Object, Object)

Object.ToString()

Namespace: NetTopologySuite.Index.Chain

Assembly: NetTopologySuite.dll

Syntax

public class MonotoneChain

Remarks

They have the following properties:

• the segments within a monotone chain never intersect each other
• the envelope of any contiguous subset of the segments in a monotone chain is equal to the envelope of the endpoints of the subset.

Property 1 means that there is no need to test pairs of segments from within the same monotone chain for intersection.

Property 2 allows an efficient binary search to be used to find the intersection points of two monotone chains. For many types of real-world data, these properties eliminate a large number of segment comparisons, producing substantial speed gains.

One of the goals of this implementation of MonotoneChains is to be as space and time efficient as possible. One design choice that aids this is that a MonotoneChain is based on a subarray of a list of points. This means that new arrays of points (potentially very large) do not have to be allocated.

MonotoneChains support the following kinds of queries:

This implementation of MonotoneChains uses the concept of internal iterators (MonotoneChainSelectAction and MonotoneChainOverlapAction) to return the resultsets for the above queries. This has time and space advantages, since it is not necessary to build lists of instantiated objects to represent the segments returned by the query. Queries made in this manner are thread-safe.

MonotoneChains support being assigned an integer id value to provide a total ordering for a set of chains. This can be used during some kinds of processing to avoid redundant comparisons (i.e.by comparing only chains where the first id is less than the second).

MonotoneChains support using an tolerance distance for overlap tests. This allows reporting overlap in situations where intersection snapping is being used. If this is used the chain envelope must be computed providing an expansion distance using GetEnvelope(Double).

Constructors

MonotoneChain(Coordinate[], Int32, Int32, Object)

Creates a new MonotoneChain based on the given array of points.

Declaration

public MonotoneChain(Coordinate[] pts, int start, int end, object context)

Parameters

Type	Name	Description
Coordinate[]	pts	The points containing the chain
Int32	start	The index of the first coordinate in the chain
Int32	end	The index of the last coordinate in the chain
Object	context	A user-defined data object

Properties

Context

Gets the chain's user-defined context data value.

Declaration

public object Context { get; }

Property Value

Type	Description
Object

Coordinates

Return the subsequence of coordinates forming this chain. Allocates a new array to hold the Coordinates.

Declaration

public Coordinate[] Coordinates { get; }

Property Value

Type	Description
Coordinate[]

EndIndex

Gets the index of the end of the monotone chain in the underlying array of points.

Declaration

public int EndIndex { get; }

Property Value

Type	Description
Int32

Envelope

Gets the envelope of this chain

Declaration

public Envelope Envelope { get; }

Property Value

Type	Description
Envelope

Id

Gets or sets the Id of this chain

Declaration

public int Id { get; set; }

Property Value

Type	Description
Int32

Remarks

Useful for assigning an ordering to a set of chains, which can be used to avoid redundant processing.

OverlapDistance

Gets or sets the overlap distance used in overlap tests with other chains.

Declaration

public double OverlapDistance { get; set; }

Property Value

Type	Description
Double

StartIndex

Gets the index of the start of the monotone chain in the underlying array of points.

Declaration

public int StartIndex { get; }

Property Value

Type	Description
Int32

Methods

ComputeOverlaps(MonotoneChain, MonotoneChainOverlapAction)

Determines the line segments in two chains which may overlap, and passes them to an overlap action.

Declaration

public void ComputeOverlaps(MonotoneChain mc, MonotoneChainOverlapAction mco)

Parameters

Type	Name	Description
MonotoneChain	mc	The chain to compare to
MonotoneChainOverlapAction	mco	The overlap action to execute on selected segments

Remarks

The monotone chain search algorithm attempts to optimize performance by not calling the overlap action on chain segments which it can determine do not overlap. However, it* may* call the overlap action on segments which do not actually interact. This saves on the overhead of checking intersection each time, since clients may be able to do this more efficiently.

ComputeOverlaps(MonotoneChain, Double, MonotoneChainOverlapAction)

Determines the line segments in two chains which may overlap, using an overlap distance tolerance, and passes them to an overlap action.

Declaration

public void ComputeOverlaps(MonotoneChain mc, double overlapTolerance, MonotoneChainOverlapAction mco)

Parameters

Type	Name	Description
MonotoneChain	mc	The chain to compare to
Double	overlapTolerance	The overlap tolerance distance (may be 0)
MonotoneChainOverlapAction	mco	The overlap action to execute on selected segments

GetEnvelope(Double)

Gets the envelope for this chain, expanded by a given distance.

Declaration

public Envelope GetEnvelope(double expansionDistance)

Parameters

Type	Name	Description
Double	expansionDistance	Distance to expand the envelope by

Returns

Type	Description
Envelope	The expanded envelope of the chain

GetLineSegment(Int32, ref LineSegment)

Gets the line segment starting at index

Declaration

public void GetLineSegment(int index, ref LineSegment ls)

Parameters

Type	Name	Description
Int32	index	The index of the segment
LineSegment	ls	The line segment to extract to

Select(Envelope, MonotoneChainSelectAction)

Determine all the line segments in the chain whose envelopes overlap the searchEnvelope, and process them.

Declaration

public void Select(Envelope searchEnv, MonotoneChainSelectAction mcs)

Parameters

Type	Name	Description
Envelope	searchEnv	The search envelope
MonotoneChainSelectAction	mcs	The select action to execute on selected segments

Remarks

The monotone chain search algorithm attempts to optimize performance by not calling the select action on chain segments which it can determine are not in the search envelope. However, it may call the select action on segments which do not intersect the search envelope. This saves on the overhead of checking envelope intersection each time, since clients may be able to do this more efficiently.