com.vividsolutions.jts.precision

Class MinimumClearance

java.lang.Object

com.vividsolutions.jts.precision.MinimumClearance

public class MinimumClearance
extends Object

Computes the Minimum Clearance of a Geometry.

The Minimum Clearance is a measure of what magnitude of perturbation of the vertices of a geometry can be tolerated before the geometry becomes topologically invalid. The smaller the Minimum Clearance distance, the less vertex pertubation the geometry can tolerate before becoming invalid.

The concept was introduced by Thompson and Van Oosterom [TV06], based on earlier work by Milenkovic [Mi88].

The Minimum Clearance of a geometry G is defined to be the value r such that "the movement of all points by a distance of r in any direction will guarantee to leave the geometry valid" [TV06]. An equivalent constructive definition [Mi88] is that r is the largest value such:

1. No two distinct vertices of G are closer than r
2. No vertex of G is closer than r to an edge of G of which the vertex is not an endpoint

The following image shows an example of the Minimum Clearance of a simple polygon.

If G has only a single vertex (i.e. is a Point), the value of the minimum clearance is Double.MAX_VALUE.

If G is a Puntal or Lineal geometry, then in fact no amount of perturbation will render the geometry invalid. In this case a Minimum Clearance is still computed based on the vertex and segment distances according to the constructive definition.

It is possible for no Minimum Clearance to exist. For instance, a MultiPoint with all members identical has no Minimum Clearance (i.e. no amount of perturbation will cause the member points to become non-identical). Empty geometries also have no such distance. The lack of a meaningful MinimumClearance distance is detected and suitable values are returned by getDistance() and getLine().

The computation of Minimum Clearance utilizes the STRtree.nearestNeighbour(ItemDistance) method to provide good performance even for large inputs.

An interesting note is that for the case of MultiPoints, the computed Minimum Clearance line effectively determines the Nearest Neighbours in the collection.

References

• [Mi88] Milenkovic, V. J., Verifiable implementations of geometric algorithms using finite precision arithmetic. in Artificial Intelligence, 377-401. 1988
• [TV06] Thompson, Rod and van Oosterom, Peter, Interchange of Spatial Data-Inhibiting Factors, Agile 2006, Visegrad, Hungary. 2006

Author:

Martin Davis

Constructor Summary

Constructors

Constructor and Description
MinimumClearance(Geometry geom) Creates an object to compute the Minimum Clearance for the given Geometry

Method Summary

Methods

Modifier and Type	Method and Description
double	getDistance() Gets the Minimum Clearance distance.
static double	getDistance(Geometry g) Computes the Minimum Clearance distance for the given Geometry.
LineString	getLine() Gets a LineString containing two points which are at the Minimum Clearance distance.
static Geometry	getLine(Geometry g) Gets a LineString containing two points which are at the Minimum Clearance distance for the given Geometry.

Methods inherited from class java.lang.Object

clone, equals, finalize, getClass, hashCode, notify, notifyAll, toString, wait, wait, wait

Constructor Detail

MinimumClearance

public MinimumClearance(Geometry geom)

Creates an object to compute the Minimum Clearance for the given Geometry

Parameters:

geom - the input geometry

Method Detail

getDistance

public static double getDistance(Geometry g)

Computes the Minimum Clearance distance for the given Geometry.

Parameters:

g - the input geometry

Returns:

the Minimum Clearance distance

getLine

public static Geometry getLine(Geometry g)

Gets a LineString containing two points which are at the Minimum Clearance distance for the given Geometry.

Parameters:

g - the input geometry

Returns:

the value of the minimum clearance distance or LINESTRING EMPTY if no Minimum Clearance distance exists

getDistance

public double getDistance()

Gets the Minimum Clearance distance.

If no distance exists (e.g. in the case of two identical points) Double.MAX_VALUE is returned.

Returns:

the value of the minimum clearance distance or Double.MAX_VALUE if no Minimum Clearance distance exists

getLine

public LineString getLine()

Gets a LineString containing two points which are at the Minimum Clearance distance.

If no distance could be found (e.g. in the case of two identical points) LINESTRING EMPTY is returned.

Returns:

the value of the minimum clearance distance or LINESTRING EMPTY if no Minimum Clearance distance exists