Minor bugfix in polygon offsetting (#593)

Author: AngusJohnson

CPP/Clipper2Lib/src/clipper.offset.cpp

@@ -1,6 +1,6 @@
 /*******************************************************************************
 * Author    :  Angus Johnson                                                   *
-* Date      :  16 July 2023                                                    *
+* Date      :  7 August 2023                                                   *
 * Website   :  http://www.angusj.com                                           *
 * Copyright :  Angus Johnson 2010-2023                                         *
 * Purpose   :  Path Offset (Inflate/Shrink)                                    *
@@ -354,6 +354,17 @@ void ClipperOffset::OffsetPoint(Group& group, Path64& path, size_t j, size_t k)
 
 void ClipperOffset::OffsetPolygon(Group& group, Path64& path)
 {
+	// when the path is contracting, make sure 
+	// there is sufficient space to do so.                //#593
+	// nb: this will have a small impact on performance
+	double a = Area(path);
+	// contracting when orientation is opposite offset direction
+	if ((a < 0) != (group_delta_ < 0)) 
+	{
+		Rect64 rec = GetBounds(path);
+		if (std::fabs(group_delta_) * 2 > rec.Width()) return;
+	}
+
 	for (Path64::size_type j = 0, k = path.size() -1; j < path.size(); k = j, ++j)
 		OffsetPoint(group, path, j, k);
 	group.paths_out.push_back(group.path);

CSharp/Clipper2Lib/Clipper.Offset.cs

@@ -1,6 +1,6 @@
 /*******************************************************************************
 * Author    :  Angus Johnson                                                   *
-* Date      :  16 July 2023                                                    *
+* Date      :  7 August 2023                                                   *
 * Website   :  http://www.angusj.com                                           *
 * Copyright :  Angus Johnson 2010-2023                                         *
 * Purpose   :  Path Offset (Inflate/Shrink)                                    *
@@ -498,6 +498,16 @@ private void OffsetPoint(Group group, Path64 path, int j, ref int k)
     [MethodImpl(MethodImplOptions.AggressiveInlining)]
     private void OffsetPolygon(Group group, Path64 path)
     {
+      // when the path is contracting, make sure 
+      // there is sufficient space to do so.              //#593
+      //nb: this will have a small impact on performance
+      double a = Clipper.Area(path);
+      if ((a < 0) != (_groupDelta < 0))
+      {
+        Rect64 rec = Clipper.GetBounds(path);
+        if (Math.Abs(_groupDelta) * 2 > rec.Width) return;
+      }
+
       group.outPath = new Path64();
       int cnt = path.Count, prev = cnt - 1;
       for (int i = 0; i < cnt; i++)

Delphi/Clipper2Lib/Clipper.Offset.pas

@@ -2,7 +2,7 @@
 
 (*******************************************************************************
 * Author    :  Angus Johnson                                                   *
-* Date      :  16 July 2023                                                    *
+* Date      :  7 August 2023                                                   *
 * Website   :  http://www.angusj.com                                           *
 * Copyright :  Angus Johnson 2010-2023                                         *
 * Purpose   :  Path Offset (Inflate/Shrink)                                    *
@@ -429,7 +429,19 @@ procedure TClipperOffset.BuildNormals;
 procedure TClipperOffset.OffsetPolygon;
 var
   i,j: integer;
+  a: double;
+  rec: TRect64;
 begin
+  //when the path is contracting, make sure
+  //there is sufficient space to do so.                //#593
+  //nb: this will have a small impact on performance
+  a := Area(fInPath);
+  if (a < 0) <> (fGroupDelta < 0) then
+  begin
+    rec := GetBounds(fInPath);
+    if Abs(fGroupDelta) * 2 >= rec.Width then Exit;
+  end;
+
   j := high(fInPath);
   for i := 0 to high(fInPath) do
     OffsetPoint(i, j);