com.vividsolutions.jts.edgegraph

Class HalfEdge

  • Direct Known Subclasses:
    MarkHalfEdge 

    public class HalfEdge
extends Object
    Represents a directed component of an edge in an EdgeGraph. HalfEdges link vertices whose locations are defined by 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.

    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.

    • Constructor Summary

      Constructors

      Constructor and Description
      HalfEdge(Coordinate orig)
      Creates an edge originating from a given coordinate.

    • Method Summary

      Modifier and Type Method and Description
      int compareAngularDirection(HalfEdge e)
      Implements the total order relation:
      int compareTo(Object obj)
      Compares edges which originate at the same vertex based on the angle they make at their origin vertex with the positive X-axis.
      static HalfEdge create(Coordinate p0, Coordinate p1)
      Creates a HalfEdge pair representing an edge between two vertices located at coordinates p0 and p1.
      int degree()
      Computes the degree of the origin vertex.
      double deltaX()
      The X component of the distance between the orig and dest vertices.
      double deltaY()
      The Y component of the distance between the orig and dest vertices.
      Coordinate dest()
      Gets the destination coordinate of this edge.
      boolean equals(Coordinate p0, Coordinate p1)
      Tests whether this edge has the given orig and dest vertices.
      HalfEdge find(Coordinate dest)
      Finds the edge starting at the origin of this edge with the given dest vertex, if any.
      protected void init(HalfEdge e)
       
      static HalfEdge init(HalfEdge e0, HalfEdge e1)
      Initialize a symmetric pair of halfedges.
      void insert(HalfEdge e)
      Inserts an edge into the ring of edges around the origin vertex of this edge.
      HalfEdge next()
      Gets the next edge CCW around the destination vertex of this edge.
      HalfEdge oNext()
       
      Coordinate orig()
      Gets the origin coordinate of this edge.
      HalfEdge prev()
      Returns the edge previous to this one (with dest being the same as this orig).
      HalfEdge prevNode()
      Finds the first node previous to this edge, if any.
      void setNext(HalfEdge e)
       
      HalfEdge sym()
      Gets the symmetric pair edge of this edge.
      String toString()
      Computes a string representation of a HalfEdge.

    • Constructor Detail

      • HalfEdge

        public HalfEdge(Coordinate orig)
        Creates an edge originating from a given coordinate.
        Parameters:
        orig - the origin coordinate

    • Method Detail

      • create

        public static HalfEdge create(Coordinate p0,
                              Coordinate p1)
        Creates a HalfEdge pair representing an edge between two vertices located at coordinates p0 and p1.
        Parameters:
        p0 - a vertex coordinate
        p1 - a vertex coordinate
        Returns:
        the HalfEdge with origin at p0

      • init

        public static HalfEdge init(HalfEdge e0,
                            HalfEdge e1)
        Initialize a symmetric pair of halfedges. Intended for use by EdgeGraph subclasses. The edges are initialized to have each other as the sym edge, and to have next pointers which point to edge other. This effectively creates a graph containing a single edge.
        Parameters:
        e0 - a halfedge
        e1 - a symmetric halfedge
        Returns:
        the initialized edge e0

      • init

        protected void init(HalfEdge e)

      • orig

        public Coordinate orig()
        Gets the origin coordinate of this edge.
        Returns:
        the origin coordinate

      • dest

        public Coordinate dest()
        Gets the destination coordinate of this edge.
        Returns:
        the destination coordinate

      • sym

        public HalfEdge sym()
        Gets the symmetric pair edge of this edge.
        Returns:
        the symmetric pair edge

      • next

        public HalfEdge next()
        Gets the next edge CCW around the destination vertex of this edge. If the vertex has degree 1 then this is the sym edge.
        Returns:
        the next edge

      • prev

        public HalfEdge prev()
        Returns the edge previous to this one (with dest being the same as this orig).
        Returns:
        the previous edge to this one

      • setNext

        public void setNext(HalfEdge e)

      • find

        public HalfEdge find(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

      • equals

        public boolean equals(Coordinate p0,
                      Coordinate p1)
        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

      • insert

        public void insert(HalfEdge e)
        Inserts an edge into the ring of edges around the origin vertex of this edge. The inserted edge must have the same origin as this edge.
        Parameters:
        e - the edge to insert

      • compareTo

        public int compareTo(Object obj)
        Compares edges which originate at the same vertex based on the angle they make at their origin vertex with the positive X-axis. This allows sorting edges around their origin vertex in CCW order.

      • compareAngularDirection

        public int compareAngularDirection(HalfEdge e)
        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:

      • deltaX

        public double deltaX()
        The X component of the distance between the orig and dest vertices.
        Returns:
        the X component of the edge length

      • deltaY

        public double deltaY()
        The Y component of the distance between the orig and dest vertices.
        Returns:
        the Y component of the edge length

      • toString

        public String toString()
        Computes a string representation of a HalfEdge.
        Overrides:
        toString in class  Object
        Returns:
        a string representation

      • degree

        public int 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

      • prevNode

        public HalfEdge prevNode()
        Finds the first node previous to this edge, if any. 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