/// <summary>
/// Determines if a non complex polygon is oriented clockwise (CW) or counter-clockwise (CCW).
/// Works for convex as well as concave polygons.
/// </summary>
/// <param name="vertices">Ordered vertices defining th polygon</param>
/// <returns>Polygon oriented CW (true) or CCW (false)</returns>
/// /// <exception cref="ArgumentException">If fewer than 3 vertices are provided.</exception>
public static bool PolygonOrientedClockwise(List <Vertex> vertices)
{
if (vertices.Count < 3)
{
throw new ArgumentException($"A polygon needs at least 3 vertices. Vertices provided: {vertices.Count}");
}
//https://stackoverflow.com/questions/1165647/how-to-determine-if-a-list-of-polygon-points-are-in-clockwise-order
var sum = 0f;
var vertexCount = vertices.Count; //only get count once
for (var i = 0; i < vertexCount; i++)
{
var indexOfNextVertex = CollectionsHelper.WrapIndex(i + 1, vertexCount);
var x1 = vertices[i].Position.x;
var y1 = vertices[i].Position.y;
var x2 = vertices[indexOfNextVertex].Position.x;
var y2 = vertices[indexOfNextVertex].Position.y;
sum += (x2 - x1) * (y2 + y1);
}
// sum > 0 -> clockwise
// sum = 0 -> Positive and negative areas cancel out, as in a figure-eight, probably not intended if that ever happens. No idea if this would still work; log warning
// sum < 0 -> counter-clockwise
if (Math.Abs(sum) < 0.000010f)
{
Debug.LogWarning("Sum of all positive and negative areas of polygon cancel out, something's probably wrong with your polygon.");
}
return(sum > 0);
}
/// <summary>
/// Triangulates a convex or concave polygon.
/// The points on the polygon should be ordered counter-clockwise.
/// This algorithm is called ear clipping and it's O(n*n) Another common algorithm is dividing it into trapezoids and it's O(n log n).
/// </summary>
/// <param name="vertices">Ordered list of vertices making up the polygon. Can be oriented CW as well as CCW.</param>
/// <returns></returns>
/// <exception cref="ArgumentException">If fewer than 3 vertices are provided.</exception>
public static List <Triangle> TriangulateConcaveOrConvexPolygon(List <Vertex> vertices)
{
if (vertices.Count < 3)
{
throw new ArgumentException($"A polygon needs at least 3 vertices. Vertices provided: {vertices.Count}");
}
//Rest of functions needs orientation to be ccw. Change orientation if it's cw
if (GeometryHelper.PolygonOrientedClockwise(vertices))
{
vertices.Reverse();
}
var triangles = new List <Triangle>();
//If we just have three points, we can just return a single triangle
if (vertices.Count == 3)
{
triangles.Add(new Triangle(vertices[0], vertices[2], vertices[1])); //vertices are expected to be in ccw order but Unity draw order for triangles in meshes is cw, hence 0,2,1 and not 0,1,2
return(triangles);
}
//Step 1. Set the next and prev vertex for every vertex
//Find the next and previous vertex
for (var i = 0; i < vertices.Count; i++)
{
var nextPos = CollectionsHelper.WrapIndex(i + 1, vertices.Count);
var prevPos = CollectionsHelper.WrapIndex(i - 1, vertices.Count);
vertices[i].PreviousVertex = vertices[prevPos];
vertices[i].NextVertex = vertices[nextPos];
}
//Step 2. Find the reflex (concave) and convex vertices, and ear vertices
foreach (var v in vertices)
{
SetIfVertexIsConcaveOrConvex(v);
}
//Have to find the ears after we have found if the vertex is concave or convex
var earVertices = new List <Vertex>();
foreach (var v in vertices)
{
SetIfVertexIsEar(v, vertices);
if (v.IsEar)
{
earVertices.Add(v);
}
}
//Step 3. Triangulate!
while (true)
{
//This means we have just one triangle left
if (vertices.Count == 3)
{
//The final triangle
triangles.Add(new Triangle(vertices[0], vertices[0].PreviousVertex, vertices[0].NextVertex));
break;
}
//Make a triangle of the first ear
var earVertex = earVertices[0];
var earVertexPrev = earVertex.PreviousVertex;
var earVertexNext = earVertex.NextVertex;
var newTriangle = new Triangle(earVertex, earVertexPrev, earVertexNext);
triangles.Add(newTriangle);
//Remove the vertex from the lists
earVertices.Remove(earVertex);
vertices.Remove(earVertex);
//Update the previous vertex and next vertex so that they are now directly linked together (take current ear vertex out of doubly linked list)
earVertexPrev.NextVertex = earVertexNext;
earVertexNext.PreviousVertex = earVertexPrev;
//...see if we have found a new ear by investigating the two vertices that was part of the ear and add them to the list of ears, if that's the case.
SetIfVertexIsConcaveOrConvex(earVertexPrev);
SetIfVertexIsConcaveOrConvex(earVertexNext);
earVertices.Remove(earVertexPrev);
earVertices.Remove(earVertexNext);
SetIfVertexIsEar(earVertexPrev, vertices);
if (earVertexPrev.IsEar)
{
earVertices.Add(earVertexPrev);
}
SetIfVertexIsEar(earVertexNext, vertices);
if (earVertexNext.IsEar)
{
earVertices.Add(earVertexNext);
}
}
return(triangles);
}