/// <summary>
/// https://en.wikipedia.org/wiki/Sutherland%E2%80%93Hodgman_algorithm#Pseudo_code
/// Fails if result would be empty
/// </summary>
public static bool TryConvexClip(Polygon2 subject, Polygon2 clip, out Polygon2 result)
{
bool Inside(IntVector2 p, IntLineSegment2 edge) => GeometryOperations.Clockness(edge.First, edge.Second, p) != Clockness.CounterClockwise;
List <IntVector2> outputList = subject.Points;
for (var i = 0; i < clip.Points.Count - 1; i++)
{
var clipEdge = new IntLineSegment2(clip.Points[i], clip.Points[i + 1]);
List <IntVector2> inputList = outputList;
outputList = new List <IntVector2>();
var S = inputList[inputList.Count - 2];
for (var j = 0; j < inputList.Count - 1; j++)
{
var E = inputList[j];
if (Inside(E, clipEdge))
{
if (!Inside(S, clipEdge))
{
var SE = new IntLineSegment2(S, E);
if (!GeometryOperations.TryFindLineLineIntersection(SE, clipEdge, out var intersection))
{
throw new NotImplementedException();
}
outputList.Add(intersection.LossyToIntVector2());
}
outputList.Add(E);
}
else if (Inside(S, clipEdge))
{
var SE = new IntLineSegment2(S, E);
if (!GeometryOperations.TryFindLineLineIntersection(SE, clipEdge, out var intersection))
{
throw new NotImplementedException();
}
outputList.Add(intersection.LossyToIntVector2());
}
S = E;
}
if (outputList.Count == 0)
{
result = null;
return(false);
}
outputList.Add(outputList[0]);
}
result = new Polygon2(outputList);
return(true);
}
public static PolyTree ExtrudePolygon(IReadOnlyList <IntVector2> points, int offset)
{
if (points.Count <= 1)
{
throw new NotImplementedException("Not implemented: extrude 0 or 1 points");
}
if (offset <= 0)
{
throw new ArgumentOutOfRangeException();
}
List <IntVector2> polygonPoints = new List <IntVector2>(points);
// hack: fidget the last point slightly to enforce having area
var last = points[points.Count - 1];
var secondToLast = points[points.Count - 2];
var secondToLastToLast = secondToLast.To(last);
if (secondToLastToLast == IntVector2.Zero)
{
polygonPoints.Add(last + new IntVector2(1, 0));
}
else if (secondToLastToLast.X == 0)
{
polygonPoints.Add(last + new IntVector2(1, 0));
}
else
{
polygonPoints.Add(last + new IntVector2(0, 1));
}
for (int i = points.Count - 2; i >= 0; i--)
{
polygonPoints.Add(points[i]);
}
var inputHairlinePolygon = new Polygon2(polygonPoints);
var outputPolygons = PolygonOperations.Offset()
.Include(inputHairlinePolygon)
.Dilate(offset)
.Execute();
return(outputPolygons);
}
