geos::edgegraph::HalfEdge Class Reference

#include <HalfEdge.h>

Inheritance diagram for geos::edgegraph::HalfEdge:

Public Member Functions
	HalfEdge (const geom::Coordinate &p_orig)
void	link (HalfEdge *p_sym)
const geom::Coordinate &	orig () const
const geom::Coordinate &	dest () const
double	directionX () const
double	directionY () const
HalfEdge *	sym () const
HalfEdge *	next () const
HalfEdge *	prev () const
HalfEdge *	oNext () const
void	setNext (HalfEdge *e)
HalfEdge *	find (const geom::Coordinate &dest)
bool	equals (const geom::Coordinate &p0, const geom::Coordinate &p1) const
void	insert (HalfEdge *eAdd)
bool	isEdgesSorted () const
int	compareAngularDirection (const HalfEdge *e) const
int	compareTo (const HalfEdge *e) const
int	degree ()
HalfEdge *	prevNode ()

Static Public Member Functions
static HalfEdge *	create (const geom::Coordinate &p0, const geom::Coordinate &p1)
static void	toStringNode (const HalfEdge *he, std::ostream &os)

Protected Member Functions
virtual const geom::Coordinate &	directionPt () const

Friends
std::ostream &	operator<< (std::ostream &os, const HalfEdge &el)

Detailed Description

Represents a directed component of an edge in an EdgeGraph. HalfEdges link vertices whose locations are defined by geom::Coordinates. HalfEdges start at an origin vertex, and terminate at a destination vertex. HalfEdges always occur in symmetric pairs, with the sym() method giving access to the oppositely-oriented component. HalfEdges and the methods on them form an edge algebra, which can be used to traverse and query the topology of the graph formed by the edges.

To support graphs where the edges are sequences of coordinates each edge may also have a direction point supplied. This is used to determine the ordering of the edges around the origin. HalfEdges with the same origin are ordered so that the ring of edges formed by them is oriented CCW.

By design HalfEdges carry minimal information about the actual usage of the graph they represent. They can be subclassed to carry more information if required.

HalfEdges form a complete and consistent data structure by themselves, but an EdgeGraph is useful to allow retrieving edges by vertex and edge location, as well as ensuring edges are created and linked appropriately.

Author

Martin Davis

Constructor & Destructor Documentation

HalfEdge()

geos::edgegraph::HalfEdge::HalfEdge ( const geom::Coordinate &  p_orig )

inline

Creates a half-edge originating from a given coordinate.

Parameters

p_orig

the origin coordinate

Member Function Documentation

compareAngularDirection()

int geos::edgegraph::HalfEdge::compareAngularDirection

(

const HalfEdge *

e

)

const

Implements the total order relation:

The angle of edge a is greater than the angle of edge b, where the angle of an edge is the angle made by the first segment of the edge with the positive x-axis

When applied to a list of edges originating at the same point, this produces a CCW ordering of the edges around the point.

Using the obvious algorithm of computing the angle is not robust, since the angle calculation is susceptible to roundoff error. A robust algorithm is:

• First, compare the quadrants the edge vectors lie in. If the quadrants are different, it is trivial to determine which edge has a greater angle.
• if the vectors lie in the same quadrant, the geom::Orientation::index() function can be used to determine the relative orientation of the vectors.

create()

static HalfEdge * geos::edgegraph::HalfEdge::create ( const geom::Coordinate &  p0, const geom::Coordinate &  p1  )

static

Creates a HalfEge pair, using the HalfEdge type of the graph subclass.

Parameters

p0	a vertex coordinate
p1	a vertex coordinate

Returns

the HalfEdge with origin at p0

degree()

int geos::edgegraph::HalfEdge::degree

(

)

Computes the degree of the origin vertex. The degree is the number of edges originating from the vertex.

Returns

the degree of the origin vertex

dest()

const geom::Coordinate & geos::edgegraph::HalfEdge::dest ( ) const

inline

Gets the destination coordinate of this edge.

Returns

the destination coordinate

directionPt()

virtual const geom::Coordinate & geos::edgegraph::HalfEdge::directionPt ( ) const

inlineprotectedvirtual

Gets the direction point of this edge. In the base case this is the dest coordinate of the edge. Subclasses may override to allow a HalfEdge to represent an edge with more than two coordinates.

Returns

the direction point for the edge

Reimplemented in geos::operation::overlayng::OverlayEdge.

directionX()

double geos::edgegraph::HalfEdge::directionX ( ) const

inline

The X component of the direction vector.

Returns

the X component of the direction vector

References geos::geom::Coordinate::x.

directionY()

double geos::edgegraph::HalfEdge::directionY ( ) const

inline

The Y component of the direction vector.

Returns

the Y component of the direction vector

References geos::geom::Coordinate::y.

equals()

bool geos::edgegraph::HalfEdge::equals	(	const geom::Coordinate &	p0,
		const geom::Coordinate &	p1
	)		const

Tests whether this edge has the given orig and dest vertices.

Parameters

p0	the origin vertex to test
p1	the destination vertex to test

Returns

true if the vertices are equal to the ones of this edge

find()

HalfEdge * geos::edgegraph::HalfEdge::find

(

const geom::Coordinate &

dest

)

Finds the edge starting at the origin of this edge with the given dest vertex, if any.

Parameters

dest	the dest vertex to search for

Returns

the edge with the required dest vertex, if it exists, or null

insert()

void geos::edgegraph::HalfEdge::insert

(

HalfEdge *

eAdd

)

Inserts an edge into the ring of edges around the origin vertex of this edge, ensuring that the edges remain ordered CCW. The inserted edge must have the same origin as this edge.

Parameters

eAdd	the edge to insert

isEdgesSorted()

bool geos::edgegraph::HalfEdge::isEdgesSorted

(

)

const

Tests whether the edges around the origin are sorted correctly. Note that edges must be strictly increasing, which implies no two edges can have the same direction point.

Returns

true if the origin edges are sorted correctly

link()

void geos::edgegraph::HalfEdge::link

(

HalfEdge *

p_sym

)

Links this edge with its sym (opposite) edge. This must be done for each pair of edges created.

Parameters

p_sym

the sym edge to link.

next()

HalfEdge * geos::edgegraph::HalfEdge::next ( ) const

inline

Gets the next edge CCW around the destination vertex of this edge, with the dest vertex as its origin. If the vertex has degree 1 then this is the sym edge.

Returns

the next edge

oNext()

HalfEdge * geos::edgegraph::HalfEdge::oNext ( ) const

inline

Gets the next edge CCW around the origin of this edge, with the same origin.

e.oNext() is equal to e.sym().next()

Returns

the next edge around the origin

orig()

const geom::Coordinate & geos::edgegraph::HalfEdge::orig ( ) const

inline

Gets the origin coordinate of this edge.

Returns

the origin coordinate

prev()

HalfEdge * geos::edgegraph::HalfEdge::prev

(

)

const

Gets the edge previous to this one (with dest being the same as this orig).

It is always true that e.next().prev() == e

Note that this requires a scan of the origin edges, so may not be efficient for some uses.

Returns

the previous edge to this one

prevNode()

HalfEdge * geos::edgegraph::HalfEdge::prevNode

(

)

Finds the first node previous to this edge, if any. A node has degree <> 2. If no such node exists (i.e. the edge is part of a ring) then null is returned.

Returns

an edge originating at the node prior to this edge, if any, or null if no node exists

setNext()

void geos::edgegraph::HalfEdge::setNext ( HalfEdge *  e )

inline

Sets the next edge CCW around the destination vertex of this edge.

Parameters

e	the next edge

sym()

HalfEdge * geos::edgegraph::HalfEdge::sym ( ) const

inline

Gets the symmetric pair edge of this edge.

Returns

the symmetric pair edge

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The documentation for this class was generated from the following file:

• HalfEdge.h