Class HalfEdge

java.lang.Object
org.locationtech.jts.edgegraph.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.

  • Field Details

  • Constructor Details

    • HalfEdge

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

    • 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
    • setSym

      private void setSym(HalfEdge e)
      Sets the sym edge.
      Parameters:
      e - the sym edge to set
    • 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)
    • oNext

      public HalfEdge oNext()
    • 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
    • insertAfter

      private void insertAfter(HalfEdge e)
      Insert an edge with the same origin after this one. Assumes that the inserted edge is in the correct position around the ring.
      Parameters:
      e - the edge to insert (with same origin)
    • 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:

      • 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
        invalid reference
        Orientation#computeOrientation(Coordinate, Coordinate, Coordinate)
        function can be used to determine the relative orientation of the vectors.
    • 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