org.h2gis.utilities.jts_utils

Class VisibilityAlgorithm

java.lang.Object

org.h2gis.utilities.jts_utils.VisibilityAlgorithm

public class VisibilityAlgorithm
extends Object

This class compute an IsoVist from a coordinate and a set of originalSegments This code is adapted from Byron Knoll byron-at-byronknoll.com javascript library: https://github.com/byronknoll/visibility-polygon-js First, sort all vertices according to their angle to the observer. Now iterate (angle sweep) through the sorted vertices. For each vertex imagine a ray projecting outwards from the observer towards that vertex compute the closest "active" line segment in order to construct the visibility polygon. The closest active line segment must be computed in O(log n) time (for each vertex). A special type of heap accomplish this: The heap keeps track of all active line segments (arranged by distance to the observer). The closest line segment is at the root of the heap. Since line segments don't intersect (a constraint in the problem definition), the heap remains consistent. This property is essential - if the distance ordering between two line segments could change depending on the angle, then a heap would no longer work. So, we can find the closest segment in O(1) time and insert new segments in O(log n) time. The reason the heap I used is "special" is because it also allows removing line segments (when they are no longer active) in O(log n) time. Inactive line segments can't just be ignored and left in the heap. they need to be removed to maintain a consistent distance ordering. In a standard heap, removing an arbitrary element takes O(n) time (since it takes linear time just to find the element). Heap contains an additional map structure from element value to heap index, so elements can be found in O(1) time. Once an element is found, we swap in the last element in the tree and propagate it either up or down (which takes O(log n) time) to maintain heap correctness.

Author:

Nicolas Fortin, Ifsttar UMRAE

Constructor Summary

Constructors

Constructor and Description
VisibilityAlgorithm(double maxDistance)

Method Summary

Modifier and Type	Method and Description
void	addGeometry(org.locationtech.jts.geom.Geometry geometry) Explode geometry and add occlusion segments in isovist
void	addLineString(org.locationtech.jts.geom.LineString lineString)
void	addSegment(org.locationtech.jts.geom.Coordinate p0, org.locationtech.jts.geom.Coordinate p1) Add occlusion segment in isovist
void	fixSegments()
double	getEpsilon()
org.locationtech.jts.geom.Polygon	getIsoVist(org.locationtech.jts.geom.Coordinate position, boolean addEnvelope) Compute isovist polygon
void	setEpsilon(double epsilon)
void	setNumPoints(int numPoints)

Methods inherited from class java.lang.Object

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

Constructor Detail

VisibilityAlgorithm

public VisibilityAlgorithm(double maxDistance)

Parameters:

maxDistance - maximum distance constraint for visibility polygon, from view point

Method Detail

fixSegments

public void fixSegments()

setNumPoints

public void setNumPoints(int numPoints)

Parameters:

numPoints - Number of points of the bounding circle polygon. Default 100

addSegment

public void addSegment(org.locationtech.jts.geom.Coordinate p0,
                       org.locationtech.jts.geom.Coordinate p1)

Add occlusion segment in isovist

Parameters:

p0 - segment origin

p1 - segment destination

getIsoVist

public org.locationtech.jts.geom.Polygon getIsoVist(org.locationtech.jts.geom.Coordinate position,
                                                    boolean addEnvelope)

Compute isovist polygon

Parameters:

position - View coordinate

addEnvelope - If true add circle bounding box. This function does not work properly if the view point is not enclosed by segments

Returns:

Visibility polygon

getEpsilon

public double getEpsilon()

setEpsilon

public void setEpsilon(double epsilon)

addLineString

public void addLineString(org.locationtech.jts.geom.LineString lineString)

addGeometry

public void addGeometry(org.locationtech.jts.geom.Geometry geometry)

Explode geometry and add occlusion segments in isovist

Parameters:

geometry - Geometry collection, LineString or Polygon instance