fix(core): arc bounding box inflated for near-zero sweep arcs

Arcs with sweep angles smaller than Tolerance.Epsilon were treated as
full circles by IsBetweenRad's shortcut check, causing UpdateBounds to
expand the bounding box to Center ± Radius. This made zoom-to-fit zoom
out far beyond the actual part extents.

Skip cardinal angle expansion when sweep is near-zero so the bounding
box uses only the arc's start/end points.

Co-Authored-By: Claude Opus 4.6 <noreply@anthropic.com>

OpenNest.Core/Drawing.cs

@@ -54,9 +54,9 @@ namespace OpenNest
             Id = Interlocked.Increment(ref nextId);
             Name = name;
             Material = new Material();
-            Program = pgm;
             Constraints = new NestConstraints();
             Source = new SourceInfo();
+            Program = pgm;
         }
 
         public int Id { get; }
@@ -78,9 +78,29 @@ namespace OpenNest
             {
                 program = value;
                 UpdateArea();
+                RecomputeCanonicalAngle();
             }
         }
 
+        /// <summary>
+        /// Recomputes and stores the canonical angle from the current Program.
+        /// Callers that mutate Program in place (rather than reassigning it) must invoke this explicitly.
+        /// Cut-off drawings are left with Angle=0.
+        /// </summary>
+        public void RecomputeCanonicalAngle()
+        {
+            if (Source == null)
+                Source = new SourceInfo();
+
+            if (program == null || IsCutOff)
+            {
+                Source.Angle = 0.0;
+                return;
+            }
+
+            Source.Angle = CanonicalAngle.Compute(this);
+        }
+
         public Color Color { get; set; }
 
         public bool IsCutOff { get; set; }
@@ -163,5 +183,15 @@ namespace OpenNest
         /// Offset distances to the original location.
         /// </summary>
         public Vector Offset { get; set; }
+
+        /// <summary>
+        /// Rotation (radians) that maps the source program geometry to its canonical
+        /// (MBR-axis-aligned) frame. Populated automatically by the <see cref="Drawing.Program"/>
+        /// setter via <see cref="CanonicalAngle.Compute"/>. A value of 0 means the drawing is
+        /// already canonical or <see cref="Drawing.IsCutOff"/> is true. Callers that mutate
+        /// <see cref="Drawing.Program"/> in place must invoke
+        /// <see cref="Drawing.RecomputeCanonicalAngle"/> to refresh.
+        /// </summary>
+        public double Angle { get; set; }
     }
 }

OpenNest.Core/Geometry/Arc.cs

@@ -404,10 +404,12 @@ namespace OpenNest.Geometry
                 maxY = startpt.Y;
             }
 
+            var sweep = SweepAngle();
+            if (sweep > Tolerance.Epsilon)
+            {
                 var angle1 = StartAngle;
                 var angle2 = EndAngle;
 
-            // switch the angle to counter clockwise.
                 if (IsReversed)
                     Generic.Swap(ref angle1, ref angle2);
 
@@ -424,6 +426,7 @@ namespace OpenNest.Geometry
 
                 if (Angle.IsBetweenRad(Angle.TwoPI, angle1, angle2))
                     maxX = Center.X + Radius;
+            }
 
             boundingBox.X = minX;
             boundingBox.Y = minY;

OpenNest.Engine/BestFit/BestFitCache.cs

@@ -24,6 +24,9 @@ namespace OpenNest.Engine.BestFit
             if (_cache.TryGetValue(key, out var cached))
                 return cached;
 
+            // Operate on the canonical frame so cached pair positions are orientation-invariant.
+            var canonical = CanonicalFrame.AsCanonicalCopy(drawing);
+
             IPairEvaluator evaluator = null;
             ISlideComputer slideComputer = null;
 
@@ -31,7 +34,7 @@ namespace OpenNest.Engine.BestFit
             {
                 if (CreateEvaluator != null)
                 {
-                    try { evaluator = CreateEvaluator(drawing, spacing); }
+                    try { evaluator = CreateEvaluator(canonical, spacing); }
                     catch { /* fall back to default evaluator */ }
                 }
 
@@ -42,7 +45,7 @@ namespace OpenNest.Engine.BestFit
                 }
 
                 var finder = new BestFitFinder(plateWidth, plateHeight, evaluator, slideComputer);
-                var results = finder.FindBestFits(drawing, spacing, StepSize);
+                var results = finder.FindBestFits(canonical, spacing, StepSize);
 
                 _cache.TryAdd(key, results);
                 return results;
@@ -86,9 +89,12 @@ namespace OpenNest.Engine.BestFit
 
             try
             {
+                // Operate on the canonical frame so cached pair positions are orientation-invariant.
+                var canonical = CanonicalFrame.AsCanonicalCopy(drawing);
+
                 if (CreateEvaluator != null)
                 {
-                    try { evaluator = CreateEvaluator(drawing, spacing); }
+                    try { evaluator = CreateEvaluator(canonical, spacing); }
                     catch { /* fall back to default evaluator */ }
                 }
 
@@ -100,7 +106,7 @@ namespace OpenNest.Engine.BestFit
 
                 // Compute candidates and evaluate once with the largest plate.
                 var finder = new BestFitFinder(maxWidth, maxHeight, evaluator, slideComputer);
-                var baseResults = finder.FindBestFits(drawing, spacing, StepSize);
+                var baseResults = finder.FindBestFits(canonical, spacing, StepSize);
 
                 // Cache a filtered copy for each plate size.
                 foreach (var size in needed)

OpenNest.Engine/CanonicalFrame.cs

@@ -0,0 +1,76 @@
+using OpenNest.CNC;
+using OpenNest.Geometry;
+using OpenNest.Math;
+using System.Collections.Generic;
+
+namespace OpenNest.Engine
+{
+    /// <summary>
+    /// Produces transient canonical (MBR-axis-aligned) copies of drawings for engine consumption
+    /// and un-rotates placed parts back to the drawing's original frame.
+    /// </summary>
+    public static class CanonicalFrame
+    {
+        /// <summary>
+        /// Returns a new Drawing whose Program geometry is rotated to the canonical frame.
+        /// The source drawing is not mutated.
+        /// </summary>
+        public static Drawing AsCanonicalCopy(Drawing drawing)
+        {
+            if (drawing == null)
+                return null;
+
+            var angle = drawing.Source?.Angle ?? 0.0;
+
+            // Clone program (never mutate the source).
+            var pgm = (drawing.Program.Clone() as OpenNest.CNC.Program)
+                      ?? new OpenNest.CNC.Program();
+
+            if (!Tolerance.IsEqualTo(angle, 0))
+                pgm.Rotate(angle, pgm.BoundingBox().Center);
+
+            var copy = new Drawing(drawing.Name ?? string.Empty, pgm)
+            {
+                Color = drawing.Color,
+                Constraints = drawing.Constraints,
+                Material = drawing.Material,
+                Priority = drawing.Priority,
+                Customer = drawing.Customer,
+                IsCutOff = drawing.IsCutOff,
+                Source = new SourceInfo
+                {
+                    Path = drawing.Source?.Path,
+                    Offset = drawing.Source?.Offset ?? new Vector(0, 0),
+                    Angle = 0.0,
+                },
+            };
+            return copy;
+        }
+
+        /// <summary>
+        /// Composes the source drawing's canonical angle onto each placed part so the
+        /// returned list is in the drawing's original (visible) frame.
+        ///
+        /// Derivation: let sourceAngle = S (rotation mapping source -> canonical).
+        /// Canonical part at rotation R shows visible orientation R.
+        /// Source part at rotation R' shows visible orientation R' + (-S), because the
+        /// source geometry is already rotated by -S relative to canonical.
+        /// Setting equal gives R' = R + S, so we ADD sourceAngle to each placed part.
+        ///
+        /// Rotation is performed around the part's Location so its placement position is preserved;
+        /// only the orientation composes.
+        /// </summary>
+        public static List<Part> FromCanonical(List<Part> placed, double sourceAngle)
+        {
+            if (placed == null || placed.Count == 0)
+                return placed;
+            if (Tolerance.IsEqualTo(sourceAngle, 0))
+                return placed;
+
+            foreach (var p in placed)
+                p.Rotate(sourceAngle, p.Location);
+
+            return placed;
+        }
+    }
+}

OpenNest.Engine/DefaultNestEngine.cs

@@ -47,14 +47,29 @@ namespace OpenNest
             PhaseResults.Clear();
             AngleResults.Clear();
 
-            // Fast path: for very small quantities, skip the full strategy pipeline.
+            // Replace the item's Drawing with a canonical copy for the duration of this fill.
-            if (item.Quantity > 0 && item.Quantity <= 2)
+            // All internal methods see canonical geometry; this wrapper un-canonicalizes the final result.
+            var sourceAngle = item.Drawing?.Source?.Angle ?? 0.0;
+            var originalDrawing = item.Drawing;
+            var canonicalItem = new NestItem
             {
-                var fast = TryFillSmallQuantity(item, workArea);
+                Drawing = CanonicalFrame.AsCanonicalCopy(item.Drawing),
-                if (fast != null && fast.Count >= item.Quantity)
+                Quantity = item.Quantity,
+                Priority = item.Priority,
+                RotationStart = item.RotationStart,
+                RotationEnd = item.RotationEnd,
+                StepAngle = item.StepAngle,
+            };
+
+            // Fast path for qty 1-2.
+            if (canonicalItem.Quantity > 0 && canonicalItem.Quantity <= 2)
             {
-                    Debug.WriteLine($"[Fill] Fast path: placed {fast.Count} parts for qty={item.Quantity}");
+                var fast = TryFillSmallQuantity(canonicalItem, workArea);
+                if (fast != null && fast.Count >= canonicalItem.Quantity)
+                {
+                    Debug.WriteLine($"[Fill] Fast path: placed {fast.Count} parts for qty={canonicalItem.Quantity}");
                     WinnerPhase = NestPhase.Pairs;
+                    fast = RebindAndUnCanonicalize(fast, originalDrawing, sourceAngle);
                     ReportProgress(progress, new ProgressReport
                     {
                         Phase = WinnerPhase,
@@ -68,32 +83,30 @@ namespace OpenNest
                 }
             }
 
-            // For low quantities, shrink the work area in both dimensions to avoid
-            // running expensive strategies against the full plate.
             var effectiveWorkArea = workArea;
-            if (item.Quantity > 0)
+            if (canonicalItem.Quantity > 0)
             {
-                effectiveWorkArea = ShrinkWorkArea(item, workArea, Plate.PartSpacing);
+                effectiveWorkArea = ShrinkWorkArea(canonicalItem, workArea, Plate.PartSpacing);
-
                 if (effectiveWorkArea != workArea)
-                    Debug.WriteLine($"[Fill] Low-qty shrink: {item.Quantity} requested, " +
+                    Debug.WriteLine($"[Fill] Low-qty shrink: {canonicalItem.Quantity} requested, " +
                         $"from {workArea.Width:F1}x{workArea.Length:F1} " +
                         $"to {effectiveWorkArea.Width:F1}x{effectiveWorkArea.Length:F1}");
             }
 
-            var best = RunFillPipeline(item, effectiveWorkArea, progress, token);
+            var best = RunFillPipeline(canonicalItem, effectiveWorkArea, progress, token);
 
-            // Fallback: if the reduced area didn't yield enough, retry with full area.
+            if (canonicalItem.Quantity > 0 && best.Count < canonicalItem.Quantity && effectiveWorkArea != workArea)
-            if (item.Quantity > 0 && best.Count < item.Quantity && effectiveWorkArea != workArea)
             {
-                Debug.WriteLine($"[Fill] Low-qty fallback: got {best.Count}, need {item.Quantity}, retrying full area");
+                Debug.WriteLine($"[Fill] Low-qty fallback: got {best.Count}, need {canonicalItem.Quantity}, retrying full area");
                 PhaseResults.Clear();
                 AngleResults.Clear();
-                best = RunFillPipeline(item, workArea, progress, token);
+                best = RunFillPipeline(canonicalItem, workArea, progress, token);
             }
 
-            if (item.Quantity > 0 && best.Count > item.Quantity)
+            if (canonicalItem.Quantity > 0 && best.Count > canonicalItem.Quantity)
-                best = ShrinkFiller.TrimToCount(best, item.Quantity, TrimAxis);
+                best = ShrinkFiller.TrimToCount(best, canonicalItem.Quantity, TrimAxis);
+
+            best = RebindAndUnCanonicalize(best, originalDrawing, sourceAngle);
 
             ReportProgress(progress, new ProgressReport
             {
@@ -108,6 +121,31 @@ namespace OpenNest
             return best;
         }
 
+        /// <summary>
+        /// Single exit point for canonical -> source frame conversion. Rebinds every Part to the
+        /// original Drawing (so consumers see the user's drawing identity, not the transient canonical copy)
+        /// and composes sourceAngle onto each Part's rotation via CanonicalFrame.FromCanonical.
+        /// </summary>
+        private static List<Part> RebindAndUnCanonicalize(List<Part> parts, Drawing original, double sourceAngle)
+        {
+            if (parts == null || parts.Count == 0)
+                return parts;
+
+            for (var i = 0; i < parts.Count; i++)
+            {
+                var p = parts[i];
+                // Rebind to `original` while preserving world pose. CreateAtOrigin rotates
+                // at the origin (keeping bbox at world (0,0)) then we offset to match p's bbox.
+                var rebound = Part.CreateAtOrigin(original, p.Rotation);
+                var delta = p.BoundingBox.Location - rebound.BoundingBox.Location;
+                rebound.Offset(delta);
+                rebound.UpdateBounds();
+                parts[i] = rebound;
+            }
+
+            return CanonicalFrame.FromCanonical(parts, sourceAngle);
+        }
+
         /// <summary>
         /// Fast path for qty 1-2: place a single part or a best-fit pair
         /// without running the full strategy pipeline.
@@ -139,6 +177,10 @@ namespace OpenNest
             var bestFits = BestFitCache.GetOrCompute(
                 drawing, Plate.Size.Length, Plate.Size.Width, Plate.PartSpacing);
 
+            // Build pair candidates with a canonical drawing so their geometry matches
+            // the coordinate frame of the cached fit results.
+            var canonicalDrawing = CanonicalFrame.AsCanonicalCopy(drawing);
+
             List<Part> bestPlacement = null;
 
             foreach (var fit in bestFits)
@@ -152,7 +194,7 @@ namespace OpenNest
                 if (fit.LongestSide > System.Math.Max(workArea.Width, workArea.Length) + Tolerance.Epsilon)
                     continue;
 
-                var landscape = fit.BuildParts(drawing);
+                var landscape = fit.BuildParts(canonicalDrawing);
                 var portrait = RotatePair90(landscape);
 
                 var lFits = TryOffsetToWorkArea(landscape, workArea);
@@ -174,6 +216,8 @@ namespace OpenNest
                     bestPlacement = candidate;
             }
 
+            // Parts are returned in canonical frame, bound to the canonical drawing.
+            // The outer Fill wrapper (Task 7) rebinds to `drawing` and composes sourceAngle onto rotation.
             return bestPlacement;
         }
 

OpenNest.Engine/ML/FeatureExtractor.cs

@@ -27,7 +27,10 @@ namespace OpenNest.Engine.ML
     {
         public static PartFeatures Extract(Drawing drawing)
         {
-            var entities = OpenNest.Converters.ConvertProgram.ToGeometry(drawing.Program)
+            // Normalize to canonical frame so features are invariant to import orientation.
+            var canonical = CanonicalFrame.AsCanonicalCopy(drawing);
+
+            var entities = OpenNest.Converters.ConvertProgram.ToGeometry(canonical.Program)
                 .Where(e => e.Layer != SpecialLayers.Rapid)
                 .ToList();
 
@@ -45,18 +48,18 @@ namespace OpenNest.Engine.ML
 
             var features = new PartFeatures
             {
-                Area = drawing.Area,
+                Area = canonical.Area,
-                Convexity = drawing.Area / (hullArea > 0 ? hullArea : 1.0),
+                Convexity = canonical.Area / (hullArea > 0 ? hullArea : 1.0),
                 AspectRatio = bb.Length / (bb.Width > 0 ? bb.Width : 1.0),
-                BoundingBoxFill = drawing.Area / (bb.Area() > 0 ? bb.Area() : 1.0),
+                BoundingBoxFill = canonical.Area / (bb.Area() > 0 ? bb.Area() : 1.0),
                 VertexCount = polygon.Vertices.Count,
                 Bitmask = GenerateBitmask(polygon, 32)
             };
 
             // Circularity = 4 * PI * Area / Perimeter^2
             var perimeterLen = polygon.Perimeter();
-            features.Circularity = (4 * System.Math.PI * drawing.Area) / (perimeterLen * perimeterLen);
+            features.Circularity = (4 * System.Math.PI * canonical.Area) / (perimeterLen * perimeterLen);
-            features.PerimeterToAreaRatio = drawing.Area > 0 ? perimeterLen / drawing.Area : 0;
+            features.PerimeterToAreaRatio = canonical.Area > 0 ? perimeterLen / canonical.Area : 0;
 
             return features;
         }

OpenNest.Engine/NestEngineBase.cs

@@ -334,6 +334,12 @@ namespace OpenNest
                 var bestFits = BestFitCache.GetOrCompute(
                     item.Drawing, Plate.Size.Length, Plate.Size.Width, Plate.PartSpacing);
 
+                // BestFitCache stores pair coordinates in canonical frame. Build candidates
+                // from a canonical drawing copy so geometry and coords share a frame; rebind
+                // + un-rotate winning pair to the original drawing's frame before returning.
+                var canonicalDrawing = CanonicalFrame.AsCanonicalCopy(item.Drawing);
+                var sourceAngle = item.Drawing?.Source?.Angle ?? 0.0;
+
                 List<Part> bestPlacement = null;
                 Box bestTarget = null;
 
@@ -342,7 +348,7 @@ namespace OpenNest
                     if (!fit.Keep)
                         continue;
 
-                    var parts = fit.BuildParts(item.Drawing);
+                    var parts = fit.BuildParts(canonicalDrawing);
                     var pairBbox = ((IEnumerable<IBoundable>)parts).GetBoundingBox();
                     var pairW = pairBbox.Width;
                     var pairL = pairBbox.Length;
@@ -374,6 +380,10 @@ namespace OpenNest
 
                 if (bestPlacement == null) continue;
 
+                // Rebind to the original drawing and compose sourceAngle onto rotation so the
+                // final placed parts sit in the user's visible frame.
+                bestPlacement = RebindPairToOriginal(bestPlacement, item.Drawing, sourceAngle);
+
                 result.AddRange(bestPlacement);
                 item.Quantity = 0;
 
@@ -388,6 +398,30 @@ namespace OpenNest
             return result;
         }
 
+        /// <summary>
+        /// Rebinds each canonical-frame Part in the pair to the original Drawing at its current
+        /// world pose, then composes sourceAngle onto each via CanonicalFrame.FromCanonical so
+        /// the returned list is in the original drawing's visible frame. Mirrors
+        /// DefaultNestEngine.RebindAndUnCanonicalize.
+        /// </summary>
+        private static List<Part> RebindPairToOriginal(List<Part> parts, Drawing original, double sourceAngle)
+        {
+            if (parts == null || parts.Count == 0)
+                return parts;
+
+            for (var i = 0; i < parts.Count; i++)
+            {
+                var p = parts[i];
+                var rebound = Part.CreateAtOrigin(original, p.Rotation);
+                var delta = p.BoundingBox.Location - rebound.BoundingBox.Location;
+                rebound.Offset(delta);
+                rebound.UpdateBounds();
+                parts[i] = rebound;
+            }
+
+            return CanonicalFrame.FromCanonical(parts, sourceAngle);
+        }
+
         /// <summary>
         /// Determines whether a drawing should use grid-fill (true) or bin-pack (false).
         /// Low-quantity items whose total area is a small fraction of the plate are

OpenNest.Engine/PartClassifier.cs

@@ -64,8 +64,8 @@ namespace OpenNest.Engine
             var mbrArea = mbr.Area;
             var mbrPerimeter = 2 * (mbr.Width + mbr.Height);
 
-            // Store primary angle (negated to align MBR with axes, same as RotationAnalysis).
+            // Share the single angle formula with CanonicalAngle (no duplicate MBR compute).
-            result.PrimaryAngle = -mbr.Angle;
+            result.PrimaryAngle = CanonicalAngle.FromMbr(mbr);
 
             // Drawing perimeter for circularity and perimeter ratio.
             var drawingPerimeter = polygon.Perimeter();

OpenNest.IO/Extensions.cs

@@ -181,13 +181,22 @@ namespace OpenNest.IO
         {
             var center = new Vector(ellipse.Center.X, ellipse.Center.Y);
             var majorAxis = new Vector(ellipse.MajorAxisEndPoint.X, ellipse.MajorAxisEndPoint.Y);
-            var semiMajor = System.Math.Sqrt(majorAxis.X * majorAxis.X + majorAxis.Y * majorAxis.Y);
-            var semiMinor = semiMajor * ellipse.RadiusRatio;
-            var rotation = System.Math.Atan2(majorAxis.Y, majorAxis.X);
 
             var startParam = ellipse.StartParameter;
             var endParam = ellipse.EndParameter;
 
+            if (ellipse.Normal.Z < 0)
+            {
+                var newStart = OpenNest.Math.Angle.TwoPI - endParam;
+                var newEnd = OpenNest.Math.Angle.TwoPI - startParam;
+                startParam = newStart;
+                endParam = newEnd;
+            }
+
+            var semiMajor = System.Math.Sqrt(majorAxis.X * majorAxis.X + majorAxis.Y * majorAxis.Y);
+            var semiMinor = semiMajor * ellipse.RadiusRatio;
+            var rotation = System.Math.Atan2(majorAxis.Y, majorAxis.X);
+
             var layer = ellipse.Layer.ToOpenNest();
             var color = ellipse.ResolveColor();
             var lineTypeName = ellipse.ResolveLineTypeName();

OpenNest.IO/OpenNest.IO.csproj

@@ -4,6 +4,9 @@
     <RootNamespace>OpenNest.IO</RootNamespace>
     <AssemblyName>OpenNest.IO</AssemblyName>
   </PropertyGroup>
+  <ItemGroup>
+    <InternalsVisibleTo Include="OpenNest.Tests" />
+  </ItemGroup>
   <ItemGroup>
     <ProjectReference Include="..\OpenNest.Core\OpenNest.Core.csproj" />
     <ProjectReference Include="..\OpenNest.Engine\OpenNest.Engine.csproj" />

OpenNest.Tests/Engine/CanonicalAngleTests.cs

@@ -97,3 +97,60 @@ public class CanonicalAngleTests
         Assert.Equal(0.0, CanonicalAngle.Compute(d), precision: 6);
     }
 }
+
+public class DrawingCanonicalAngleWiringTests
+{
+    private static OpenNest.CNC.Program RotatedRectProgram(double w, double h, double theta)
+    {
+        var pgm = new OpenNest.CNC.Program();
+        pgm.Codes.Add(new RapidMove(new Vector(0, 0)));
+        pgm.Codes.Add(new LinearMove(new Vector(w, 0)));
+        pgm.Codes.Add(new LinearMove(new Vector(w, h)));
+        pgm.Codes.Add(new LinearMove(new Vector(0, h)));
+        pgm.Codes.Add(new LinearMove(new Vector(0, 0)));
+        if (!OpenNest.Math.Tolerance.IsEqualTo(theta, 0))
+            pgm.Rotate(theta, pgm.BoundingBox().Center);
+        return pgm;
+    }
+
+    [Fact]
+    public void Constructor_ComputesAngleOnProgramAssignment()
+    {
+        var pgm = RotatedRectProgram(100, 50, 0.5);
+        var d = new Drawing("r", pgm);
+        Assert.InRange(d.Source.Angle, -0.52, -0.48);
+    }
+
+    [Fact]
+    public void SetProgram_RecomputesAngle()
+    {
+        var d = new Drawing("r", RotatedRectProgram(100, 50, 0.0));
+        Assert.Equal(0.0, d.Source.Angle, precision: 6);
+
+        d.Program = RotatedRectProgram(100, 50, 0.5);
+        Assert.InRange(d.Source.Angle, -0.52, -0.48);
+    }
+
+    [Fact]
+    public void IsCutOff_SkipsAngleComputation()
+    {
+        var d = new Drawing("cut", RotatedRectProgram(100, 50, 0.5)) { IsCutOff = true };
+        // Re-assign after flag is set so the setter observes IsCutOff.
+        d.Program = RotatedRectProgram(100, 50, 0.5);
+        Assert.Equal(0.0, d.Source.Angle, precision: 6);
+    }
+
+    [Fact]
+    public void RecomputeCanonicalAngle_UpdatesAfterMutation()
+    {
+        var d = new Drawing("r", RotatedRectProgram(100, 50, 0.0));
+        Assert.Equal(0.0, d.Source.Angle, precision: 6);
+
+        // Mutate in-place (doesn't trigger setter).
+        d.Program.Rotate(0.5, d.Program.BoundingBox().Center);
+        Assert.Equal(0.0, d.Source.Angle, precision: 6); // still stale
+
+        d.RecomputeCanonicalAngle();
+        Assert.InRange(d.Source.Angle, -0.52, -0.48);
+    }
+}

OpenNest.Tests/Engine/CanonicalFrameTests.cs

@@ -0,0 +1,84 @@
+using OpenNest.CNC;
+using OpenNest.Engine;
+using OpenNest.Geometry;
+using OpenNest.Math;
+
+namespace OpenNest.Tests.Engine;
+
+public class CanonicalFrameTests
+{
+    private static Drawing MakeRect(double w, double h, double rotation)
+    {
+        var pgm = new OpenNest.CNC.Program();
+        pgm.Codes.Add(new RapidMove(new Vector(0, 0)));
+        pgm.Codes.Add(new LinearMove(new Vector(w, 0)));
+        pgm.Codes.Add(new LinearMove(new Vector(w, h)));
+        pgm.Codes.Add(new LinearMove(new Vector(0, h)));
+        pgm.Codes.Add(new LinearMove(new Vector(0, 0)));
+        if (!Tolerance.IsEqualTo(rotation, 0))
+            pgm.Rotate(rotation, pgm.BoundingBox().Center);
+        return new Drawing("rect", pgm) { Source = new SourceInfo { Angle = -rotation } };
+    }
+
+    [Fact]
+    public void AsCanonicalCopy_AxisAlignsMbr()
+    {
+        var d = MakeRect(100, 50, 0.6);
+        var canonical = CanonicalFrame.AsCanonicalCopy(d);
+
+        var bb = canonical.Program.BoundingBox();
+        var longer = System.Math.Max(bb.Length, bb.Width);
+        var shorter = System.Math.Min(bb.Length, bb.Width);
+        Assert.InRange(longer, 100 - 0.1, 100 + 0.1);
+        Assert.InRange(shorter, 50 - 0.1, 50 + 0.1);
+        Assert.Equal(0.0, canonical.Source.Angle, precision: 6);
+    }
+
+    [Fact]
+    public void AsCanonicalCopy_DoesNotMutateSource()
+    {
+        var d = MakeRect(100, 50, 0.6);
+        var originalBbox = d.Program.BoundingBox();
+        var originalAngle = d.Source.Angle;
+
+        CanonicalFrame.AsCanonicalCopy(d);
+
+        var afterBbox = d.Program.BoundingBox();
+        Assert.Equal(originalBbox.Width, afterBbox.Width, precision: 6);
+        Assert.Equal(originalBbox.Length, afterBbox.Length, precision: 6);
+        Assert.Equal(originalAngle, d.Source.Angle, precision: 6);
+    }
+
+    [Fact]
+    public void FromCanonical_ComposesSourceAngleOntoRotation()
+    {
+        var d = MakeRect(100, 50, 0.0);
+        var part = new Part(d);
+        part.Rotate(0.2); // engine returned a canonical-frame part at R = 0.2
+
+        var placed = CanonicalFrame.FromCanonical(new List<Part> { part }, sourceAngle: -0.5);
+
+        // R' = R + sourceAngle = 0.2 + (-0.5) = -0.3
+        // Part.Rotation comes from Program.Rotation which is normalized to [0, 2PI),
+        // so compare after normalizing the expected value as well.
+        Assert.Single(placed);
+        Assert.Equal(Angle.NormalizeRad(-0.3), placed[0].Rotation, precision: 4);
+    }
+
+    [Fact]
+    public void RoundTrip_RestoresGeometry()
+    {
+        var d = MakeRect(100, 50, 0.4);
+        var canonical = CanonicalFrame.AsCanonicalCopy(d);
+
+        // Place a part at origin in the canonical frame.
+        var part = Part.CreateAtOrigin(canonical);
+        var canonicalBbox = part.BoundingBox;
+
+        var placed = CanonicalFrame.FromCanonical(new List<Part> { part }, d.Source.Angle);
+
+        var originalBbox = d.Program.BoundingBox();
+        Assert.Equal(originalBbox.Width, placed[0].BoundingBox.Width, precision: 2);
+        Assert.Equal(originalBbox.Length, placed[0].BoundingBox.Length, precision: 2);
+    }
+}

OpenNest.Tests/Geometry/EllipseConverterTests.cs

@@ -1,14 +1,19 @@
 using OpenNest.Geometry;
+using OpenNest.IO;
 using OpenNest.Math;
 using Xunit;
+using Xunit.Abstractions;
 using System.Linq;
 
 namespace OpenNest.Tests.Geometry;
 
 public class EllipseConverterTests
 {
+    private readonly ITestOutputHelper _output;
     private const double Tol = 1e-10;
 
+    public EllipseConverterTests(ITestOutputHelper output) => _output = output;
+
     [Fact]
     public void EvaluatePoint_AtZero_ReturnsMajorAxisEnd()
     {
@@ -244,6 +249,101 @@ public class EllipseConverterTests
         }
     }
 
+    [Fact]
+    public void DxfImport_ArcBoundingBoxes_Diagnostic()
+    {
+        var path = @"C:\Users\aisaacs\Desktop\11ga tab.dxf";
+        if (!System.IO.File.Exists(path)) return;
+
+        var result = Dxf.Import(path);
+        var all = (System.Collections.Generic.IEnumerable<IBoundable>)result.Entities;
+        var bbox = all.GetBoundingBox();
+        _output.WriteLine($"Overall: X={bbox.X:F4} Y={bbox.Y:F4} W={bbox.Length:F4} H={bbox.Width:F4}");
+
+        for (var i = 0; i < result.Entities.Count; i++)
+        {
+            var e = result.Entities[i];
+            var b = e.BoundingBox;
+            var flag = (b.Length > 1 || b.Width > 1) ? " ***" : "";
+            _output.WriteLine($"{i + 1,3}. {e.GetType().Name,-8} X={b.X:F4} Y={b.Y:F4} W={b.Length:F4} H={b.Width:F4}{flag}");
+        }
+    }
+
+    [Fact]
+    public void ToOpenNest_FlippedNormalZ_ProducesCorrectArcs()
+    {
+        var normal = new ACadSharp.Entities.Ellipse
+        {
+            Center = new CSMath.XYZ(-0.275, -0.245, 0),
+            MajorAxisEndPoint = new CSMath.XYZ(0.0001, 1.245, 0),
+            RadiusRatio = 0.28,
+            StartParameter = 0.017,
+            EndParameter = 1.571,
+            Normal = new CSMath.XYZ(0, 0, 1)
+        };
+
+        var flipped = new ACadSharp.Entities.Ellipse
+        {
+            Center = new CSMath.XYZ(0.275, -0.245, 0),
+            MajorAxisEndPoint = new CSMath.XYZ(-0.0001, 1.245, 0),
+            RadiusRatio = 0.28,
+            StartParameter = 0.017,
+            EndParameter = 1.571,
+            Normal = new CSMath.XYZ(0, 0, -1)
+        };
+
+        var normalArcs = normal.ToOpenNest();
+        var flippedArcs = flipped.ToOpenNest();
+
+        Assert.True(normalArcs.Count > 0);
+        Assert.True(flippedArcs.Count > 0);
+        Assert.True(normalArcs.All(e => e is Arc));
+        Assert.True(flippedArcs.All(e => e is Arc));
+
+        var normalFirst = (Arc)normalArcs.First();
+        var flippedFirst = (Arc)flippedArcs.First();
+        var normalStart = GetArcStart(normalFirst);
+        var flippedStart = GetArcStart(flippedFirst);
+
+        Assert.True(normalStart.X < 0, $"Normal ellipse start X should be negative, got {normalStart.X}");
+        Assert.True(flippedStart.X > 0, $"Flipped ellipse should bulge right, got {flippedStart.X}");
+
+        var normalBbox = GetBoundingBox(normalArcs.Cast<Arc>());
+        var flippedBbox = GetBoundingBox(flippedArcs.Cast<Arc>());
+        Assert.True(flippedBbox.minX > 0, $"Flipped ellipse should stay on positive X side, minX={flippedBbox.minX}");
+        Assert.True(normalBbox.maxX < 0, $"Normal ellipse should stay on negative X side, maxX={normalBbox.maxX}");
+    }
+
+    private static (double minX, double maxX) GetBoundingBox(IEnumerable<Arc> arcs)
+    {
+        var minX = double.MaxValue;
+        var maxX = double.MinValue;
+        foreach (var arc in arcs)
+        {
+            var s = GetArcStart(arc);
+            var e = GetArcEnd(arc);
+            minX = System.Math.Min(minX, System.Math.Min(s.X, e.X));
+            maxX = System.Math.Max(maxX, System.Math.Max(s.X, e.X));
+        }
+        return (minX, maxX);
+    }
+
+    private static Vector GetArcStart(Arc arc)
+    {
+        var angle = arc.IsReversed ? arc.EndAngle : arc.StartAngle;
+        return new Vector(
+            arc.Center.X + arc.Radius * System.Math.Cos(angle),
+            arc.Center.Y + arc.Radius * System.Math.Sin(angle));
+    }
+
+    private static Vector GetArcEnd(Arc arc)
+    {
+        var angle = arc.IsReversed ? arc.StartAngle : arc.EndAngle;
+        return new Vector(
+            arc.Center.X + arc.Radius * System.Math.Cos(angle),
+            arc.Center.Y + arc.Radius * System.Math.Sin(angle));
+    }
+
     private static double MaxDeviationFromEllipse(Arc arc, Vector ellipseCenter,
         double semiMajor, double semiMinor, double rotation, int samples)
     {