/// <summary>
/// A implementation of the Weiler-Atherthon algorithm that cuts away the provided clippling area from the subject polygon.
/// Assumes the cutting polygon is not entirely inside subject polygon!
/// </summary>
/// <param name="a_subject"></param>
/// <param name="a_clip"></param>
/// <returns></returns>
public static MultiPolygon2D CutOut(Polygon2D a_subject, Polygon2D a_clip)
{
// copy and maybe reverse
if (a_subject.IsClockwise())
{
a_subject = new Polygon2D(a_subject.Vertices);
}
else
{
a_subject = new Polygon2D(a_subject.Vertices.Reverse());
}
if (a_clip.IsClockwise())
{
a_clip = new Polygon2D(a_clip.Vertices);
}
else
{
a_clip = new Polygon2D(a_clip.Vertices.Reverse());
}
var WA = new WeilerAtherton(a_subject, a_clip);
//return new MultiPolygon2D();
return(WeilerAthertonCutOut(WA));
}
/// <summary>
/// Creates a cutout given the relevant weiler atherton object containing subject and clipping lists
/// Can result in multiple polygons.
/// Assumes the cutting polygon is not entirely inside subject polygon!
/// </summary>
/// <param name="WA"></param>
/// <returns></returns>
private static MultiPolygon2D WeilerAthertonCutOut(WeilerAtherton WA)
{
var multiPoly = new MultiPolygon2D();
// check if polygons have intersections
if (WA.EntryIntersections.Count == 0)
{
if (!WA.Subject.Vertices.ToList().Exists(p => WA.Clip.ContainsInside(p)))
{
multiPoly.AddPolygon(new Polygon2D(WA.Subject.Vertices));
}
}
while (WA.EntryIntersections.Count > 0)
{
var vertices = new List <Vector2>();
var visited = new HashSet <LinkedListNode <WAPoint> >();
// remove an entry intersection point
var startPoint = WA.EntryIntersections.LastOrDefault();
WA.EntryIntersections.Remove(startPoint);
LinkedListNode <WAPoint> node;
try
{
node = WA.ClipNode[startPoint];
}
catch (KeyNotFoundException e)
{
Debug.Log("entry point not found: " + e);
continue;
}
WAPoint last = null;
while (!visited.Contains(node))
{
// remove entry intersection from starting list
WA.EntryIntersections.Remove(node.Value);
// traverse clip polygon in counter-clockwise order until exit vertex found
while (node.Value.Type != WAList.Exit && !visited.Contains(node))
{
// check for duplicates
if (last == null || !MathUtil.EqualsEps(node.Value.Pos, last.Pos))
{
vertices.Add(node.Value.Pos);
visited.Add(node);
}
last = node.Value;
node = node.Previous ?? WA.ClipList.Last;
}
// might contains entry but no exit vertex
// should not occur, return no polygon
if (visited.Contains(node))
{
vertices.Clear(); break;
}
try
{
node = WA.SubjectNode[node.Value];
}
catch (KeyNotFoundException e)
{
Debug.Log("exit point not found: " + e.StackTrace);
break;
}
// traverse subject polygon in clockwise order until new entr vertex found
while (node.Value.Type != WAList.Entry && !visited.Contains(node))
{
if (last == null || !MathUtil.EqualsEps(node.Value.Pos, last.Pos))
{
vertices.Add(node.Value.Pos);
visited.Add(node);
}
last = node.Value;
node = node.Next ?? WA.SubjectList.First;
}
if (visited.Contains(node))
{
break;
}
try
{
node = WA.ClipNode[node.Value];
}
catch (KeyNotFoundException e)
{
Debug.Log("entry point not found: " + e);
break;
}
}
// add new polygon from the vertices
if (vertices.Count > 2)
{
multiPoly.AddPolygon(new Polygon2D(vertices));
}
}
return(multiPoly);
}
