Table of Contents

Class MonotoneChain

Namespace
NetTopologySuite.Index.Chain
Assembly
NetTopologySuite.dll

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

public class MonotoneChain
Inheritance
MonotoneChain
Inherited Members

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[], int, int, object)

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

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

Parameters

pts Coordinate[]

The points containing the chain

start int

The index of the first coordinate in the chain

end int

The index of the last coordinate in the chain

context object

A user-defined data object

Properties

Context

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

public object Context { get; }

Property Value

object

Coordinates

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

public Coordinate[] Coordinates { get; }

Property Value

Coordinate[]

EndIndex

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

public int EndIndex { get; }

Property Value

int

Envelope

Gets the envelope of this chain

public Envelope Envelope { get; }

Property Value

Envelope

Id

Gets or sets the Id of this chain

public int Id { get; set; }

Property Value

int

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.

public double OverlapDistance { get; set; }

Property Value

double

StartIndex

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

public int StartIndex { get; }

Property Value

int

Methods

ComputeOverlaps(MonotoneChain, MonotoneChainOverlapAction)

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

public void ComputeOverlaps(MonotoneChain mc, MonotoneChainOverlapAction mco)

Parameters

mc MonotoneChain

The chain to compare to

mco MonotoneChainOverlapAction

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.

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

Parameters

mc MonotoneChain

The chain to compare to

overlapTolerance double

The overlap tolerance distance (may be 0)

mco MonotoneChainOverlapAction

The overlap action to execute on selected segments

GetEnvelope(double)

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

public Envelope GetEnvelope(double expansionDistance)

Parameters

expansionDistance double

Distance to expand the envelope by

Returns

Envelope

The expanded envelope of the chain

GetLineSegment(int, ref LineSegment)

Gets the line segment starting at index

public void GetLineSegment(int index, ref LineSegment ls)

Parameters

index int

The index of the segment

ls LineSegment

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.

public void Select(Envelope searchEnv, MonotoneChainSelectAction mcs)

Parameters

searchEnv Envelope

The search envelope

mcs MonotoneChainSelectAction

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.