From 64945220b9712ceae88c20f7237951cbce57c654 Mon Sep 17 00:00:00 2001
From: AJ Isaacs <ajisaacs27@gmail.com>
Date: Thu, 2 Apr 2026 12:06:08 -0400
Subject: [PATCH] fix: account for contour winding direction in lead-in normal
 computation

ComputeNormal assumed CW winding for all contours. For CCW-wound cutouts,
line normals pointed to the material side instead of scrap, placing lead-ins
on the wrong side. Now accepts a winding parameter: lines flip the normal
for CCW winding, and arcs flip when arc direction differs from contour
winding (concave feature detection).

Co-Authored-By: Claude Opus 4.6 (1M context) <noreply@anthropic.com>
---
 .../CuttingStrategy/ContourCuttingStrategy.cs | 22 +++++++++++--------
 OpenNest/Actions/ActionLeadIn.cs              | 11 ++++++----
 2 files changed, 20 insertions(+), 13 deletions(-)

diff --git a/OpenNest.Core/CNC/CuttingStrategy/ContourCuttingStrategy.cs b/OpenNest.Core/CNC/CuttingStrategy/ContourCuttingStrategy.cs
index eef0d86..580add9 100644
--- a/OpenNest.Core/CNC/CuttingStrategy/ContourCuttingStrategy.cs
+++ b/OpenNest.Core/CNC/CuttingStrategy/ContourCuttingStrategy.cs
@@ -70,8 +70,8 @@ namespace OpenNest.CNC.CuttingStrategy
         private void EmitContour(Program program, Shape shape, Vector point, Entity entity, ContourType? forceType = null)
         {
             var contourType = forceType ?? DetectContourType(shape);
-            var normal = ComputeNormal(point, entity, contourType);
             var winding = DetermineWinding(shape);
+            var normal = ComputeNormal(point, entity, contourType, winding);
 
             var leadIn = SelectLeadIn(contourType);
             var leadOut = SelectLeadOut(contourType);
@@ -143,29 +143,33 @@ namespace OpenNest.CNC.CuttingStrategy
             return ContourType.Internal;
         }
 
-        public static double ComputeNormal(Vector point, Entity entity, ContourType contourType)
+        public static double ComputeNormal(Vector point, Entity entity, ContourType contourType,
+            RotationType winding = RotationType.CW)
         {
             double normal;
 
             if (entity is Line line)
             {
-                // Perpendicular to line direction
+                // Perpendicular to line direction: tangent + π/2 = left side.
+                // Left side = outward for CW winding; for CCW winding, outward
+                // is on the right side, so flip.
                 var tangent = line.EndPoint.AngleFrom(line.StartPoint);
                 normal = tangent + Math.Angle.HalfPI;
+                if (winding == RotationType.CCW)
+                    normal += System.Math.PI;
             }
             else if (entity is Arc arc)
             {
-                // Radial direction from center to point
+                // Radial direction from center to point.
+                // Flip when the arc direction differs from the contour winding —
+                // that indicates a concave feature where radial points inward.
                 normal = point.AngleFrom(arc.Center);
-
-                // For CCW arcs the radial points the wrong way — flip it.
-                // CW arcs are convex features (corners) where radial = outward.
-                // CCW arcs are concave features (slots) where radial = inward.
-                if (arc.Rotation == RotationType.CCW)
+                if (arc.Rotation != winding)
                     normal += System.Math.PI;
             }
             else if (entity is Circle circle)
             {
+                // Radial outward — always correct regardless of winding
                 normal = point.AngleFrom(circle.Center);
             }
             else
diff --git a/OpenNest/Actions/ActionLeadIn.cs b/OpenNest/Actions/ActionLeadIn.cs
index dbb7cbf..1788559 100644
--- a/OpenNest/Actions/ActionLeadIn.cs
+++ b/OpenNest/Actions/ActionLeadIn.cs
@@ -102,7 +102,7 @@ namespace OpenNest.Actions
                     snapPoint = closest;
                     snapEntity = entity;
                     snapContourType = info.ContourType;
-                    snapNormal = ContourCuttingStrategy.ComputeNormal(closest, entity, info.ContourType);
+                    snapNormal = ContourCuttingStrategy.ComputeNormal(closest, entity, info.ContourType, info.Winding);
                     hasSnap = true;
                     hoveredContour = info;
                 }
@@ -282,7 +282,7 @@ namespace OpenNest.Actions
             {
                 snapPoint = bestPoint;
                 snapEntity = bestEntity;
-                snapNormal = ContourCuttingStrategy.ComputeNormal(bestPoint, bestEntity, snapContourType);
+                snapNormal = ContourCuttingStrategy.ComputeNormal(bestPoint, bestEntity, snapContourType, hoveredContour.Winding);
                 activeSnapType = bestType;
             }
 
@@ -356,7 +356,8 @@ namespace OpenNest.Actions
                 contours.Add(new ShapeInfo
                 {
                     Shape = profile.Perimeter,
-                    ContourType = ContourType.External
+                    ContourType = ContourType.External,
+                    Winding = ContourCuttingStrategy.DetermineWinding(profile.Perimeter)
                 });
             }
 
@@ -366,7 +367,8 @@ namespace OpenNest.Actions
                 contours.Add(new ShapeInfo
                 {
                     Shape = cutout,
-                    ContourType = ContourCuttingStrategy.DetectContourType(cutout)
+                    ContourType = ContourCuttingStrategy.DetectContourType(cutout),
+                    Winding = ContourCuttingStrategy.DetermineWinding(cutout)
                 });
             }
         }
@@ -483,6 +485,7 @@ namespace OpenNest.Actions
         {
             public Shape Shape { get; set; }
             public ContourType ContourType { get; set; }
+            public RotationType Winding { get; set; }
         }
     }
 }