Polygon

Creating a polygon

The IPolygon interface provides a minimal interface to construct geometries consisting of points and segments, which may form a polygon (probably with holes), known as a planar straight-line graph (PSLG). The following examples will show different ways to create a simple box with a hole.

Using contours

A contour may constitute an inner or an outer boundary of a polygon. There are two methods for adding a Contour to a polygon:

• The Add(Contour contour, bool hole = false) method will add the contour and make it a hole, if the boolean hole parameter is set to true.
• The Add(Contour contour, Point hole) method will add the contour as a hole. The hole parameter must be a point inside the contour.

using TriangleNet.Geometry;

public static void Example()
{
    var p = new Polygon();

    // Add the outer box contour with boundary marker 1.
    p.Add(new Contour(new Vertex[4]
    {
        new Vertex(0.0, 0.0, 1),
        new Vertex(3.0, 0.0, 1),
        new Vertex(3.0, 3.0, 1),
        new Vertex(0.0, 3.0, 1)
    }, 1));
        
    // Add the inner box contour with boundary marker 2.
    p.Add(new Contour(new Vertex[4]
    {
        new Vertex(1.0, 1.0, 2),
        new Vertex(2.0, 1.0, 2),
        new Vertex(2.0, 2.0, 2),
        new Vertex(1.0, 2.0, 2)
    }, 2)
    , new Point(1.5, 1.5)); // Make it a hole.
}

Using segments

There are two methods for adding a Segment to a polygon:

• The Add(ISegment segment, bool insert = false) method will add the segment. Additionally, both endpoints will be added, if the boolean insert parameter is set to true.
• The Add(ISegment segment, int index) method will add the segment and one of its endpoints, determined by the index parameter. The following example code makes use of the second overload, using an index of 0, meaning that the first endpoint will automatically be added to the polygon Points collection:

using TriangleNet.Geometry;

public static void Example()
{
    var p = new Polygon();

    var v = new Vertex[4]
    {
        new Vertex(0.0, 0.0, 1),
        new Vertex(3.0, 0.0, 1),
        new Vertex(3.0, 3.0, 1),
        new Vertex(0.0, 3.0, 1)
    };

    // Add segments of the outer box.
    p.Add(new Segment(v[0], v[1], 1), 0);
    p.Add(new Segment(v[1], v[2], 1), 0);
    p.Add(new Segment(v[2], v[3], 1), 0);
    p.Add(new Segment(v[3], v[0], 1), 0);
        
    v = new Vertex[4]
    {
        new Vertex(1.0, 1.0, 2),
        new Vertex(2.0, 1.0, 2),
        new Vertex(2.0, 2.0, 2),
        new Vertex(1.0, 2.0, 2)
    };

    // Add segments of the inner box.
    p.Add(new Segment(v[0], v[1], 2), 0);
    p.Add(new Segment(v[1], v[2], 2), 0);
    p.Add(new Segment(v[2], v[3], 2), 0);
    p.Add(new Segment(v[3], v[0], 2), 0);

    // Add the hole.
    p.Holes.Add(new Point(1.5, 1.5));
}

Using a .poly file

The above geometry can be described by the following .poly file:

# A box with a hole: eight points, no attributes, one boundary marker.
8 2 0 1
# Outer box
  1   0.0 0.0   1
  2   3.0 0.0   1
  3   3.0 3.0   1
  4   0.0 3.0   1
# Inner square
  5   1.0 1.0   2
  6   2.0 1.0   2
  7   2.0 2.0   2
  8   1.0 2.0   2
# Eight segments with boundary markers.
8 1
# Outer box
  1   1 2   1
  2   2 3   1
  3   3 4   1
  4   4 1   1
# Inner square
  5   5 6   2
  6   6 7   2
  7   7 8   2
  8   8 5   2
# One hole in the middle of the inner square.
1
  1   1.5 1.5

The file can be loaded using the FileProcessor class:

using TriangleNet.IO;

public static void Example()
{
    // Load polygon from file.
    var polygon = FileProcessor.Read("box.poly");
}