Fix OCS→WCS coordinate transformation for ARC and CIRCLE entities

DXF stores ARC/CIRCLE center points in Object Coordinate System (OCS)
when the entity Normal differs from (0,0,1). Without applying the
AutoCAD Arbitrary Axis Algorithm to convert OCS→WCS, circles and arcs
with reversed or tilted normals appeared at incorrect positions (e.g.
mirrored along the X axis for Normal=(0,0,-1)).

Apply OcsToWcs() in CreateArc (including the degenerate arc→circle
fallback) and CreateCircle. Add regression test verifying that an ARC
with Normal=(0,0,-1) at OCS (-1000,-500) lands at WCS (1000,-500).

Co-Authored-By: Claude Sonnet 4.6 <noreply@anthropic.com>
Claude-Session: https://claude.ai/code/session_01Ka4ocDhmumPuQDD2fDs7ad

Author: claude

CADability/ImportDxf.cs

@@ -229,6 +229,22 @@ private IGeoObject GeoObjectFromEntity(Entity item)
         private static GeoPoint GeoPoint(XYZ p) => new GeoPoint(p.X, p.Y, p.Z);
         private static GeoVector GeoVector(XYZ p) => new GeoVector(p.X, p.Y, p.Z);
 
+        // Convert a 2D entity's position from OCS (Object Coordinate System) to WCS.
+        // DXF stores ARC/CIRCLE centers, TEXT insert points, etc. in OCS when Normal ≠ (0,0,1).
+        // The OCS axes are derived via the AutoCAD Arbitrary Axis Algorithm.
+        private static GeoPoint OcsToWcs(XYZ ocsPoint, XYZ normal)
+        {
+            GeoVector n = GeoVector(normal);
+            GeoVector ax = (Math.Abs(normal.X) < 1.0 / 64 && Math.Abs(normal.Y) < 1.0 / 64)
+                ? CADability.GeoVector.YAxis ^ n
+                : CADability.GeoVector.ZAxis ^ n;
+            GeoVector ay = n ^ ax;
+            return new GeoPoint(
+                ocsPoint.X * ax.x + ocsPoint.Y * ay.x + ocsPoint.Z * n.x,
+                ocsPoint.X * ax.y + ocsPoint.Y * ay.y + ocsPoint.Z * n.y,
+                ocsPoint.X * ax.z + ocsPoint.Y * ay.z + ocsPoint.Z * n.z);
+        }
+
         internal static Plane Plane(XYZ center, XYZ normal)
         {
             // AutoCAD Arbitrary Axis Algorithm — must use this for correct plane orientation
@@ -448,17 +464,18 @@ private IGeoObject CreateArc(ACadSharp.Entities.Arc arc)
             GeoObject.Ellipse e = GeoObject.Ellipse.Construct();
             GeoVector nor = GeoVector(arc.Normal);
             Plane plane = Plane(arc.Center, arc.Normal);
+            GeoPoint wcsCenter = OcsToWcs(arc.Center, arc.Normal);
             double start = arc.StartAngle;
             double end = arc.EndAngle;
             double sweep = end - start;
             if (sweep < 0.0) sweep += Math.PI * 2.0;
             if (start == end) sweep = 0.0;
             if (start == Math.PI * 2.0 && end == 0.0) sweep = 0.0;
-            e.SetArcPlaneCenterRadiusAngles(plane, GeoPoint(arc.Center), arc.Radius, start, sweep);
+            e.SetArcPlaneCenterRadiusAngles(plane, wcsCenter, arc.Radius, start, sweep);
             if (e.IsCircle && sweep == 0.0 && Precision.IsEqual(e.StartPoint, e.EndPoint))
             {
                 GeoObject.Ellipse circle = GeoObject.Ellipse.Construct();
-                circle.SetCirclePlaneCenterRadius(plane, GeoPoint(arc.Center), arc.Radius);
+                circle.SetCirclePlaneCenterRadius(plane, wcsCenter, arc.Radius);
                 e = circle;
             }
             double th = arc.Thickness;
@@ -471,7 +488,8 @@ private IGeoObject CreateCircle(ACadSharp.Entities.Circle circle)
         {
             GeoObject.Ellipse e = GeoObject.Ellipse.Construct();
             Plane plane = Plane(circle.Center, circle.Normal);
-            e.SetCirclePlaneCenterRadius(plane, GeoPoint(circle.Center), circle.Radius);
+            GeoPoint wcsCenter = OcsToWcs(circle.Center, circle.Normal);
+            e.SetCirclePlaneCenterRadius(plane, wcsCenter, circle.Radius);
             double th = circle.Thickness;
             GeoVector no = GeoVector(circle.Normal);
             if (th != 0.0 && !no.IsNullVector())

tests/CADability.Tests/ProjectTest.cs

@@ -980,6 +980,30 @@ public void import_dxf_empty_block_insert_not_placed_at_origin()
                 $"Empty block insert must be skipped; expected 1 object, got {model.AllObjects.Count}");
         }
 
+        [TestMethod]
+        public void import_dxf_arc_with_reversed_normal_uses_wcs_center()
+        {
+            // Regression: ARC/CIRCLE entities with Normal=(0,0,-1) store their center in OCS.
+            // OCS→WCS via Arbitrary Axis Algorithm flips the X component for Normal=(0,0,-1).
+            // OCS (-1000,-500,0) with Normal=(0,0,-1) → WCS (1000,-500,0).
+            // Without the fix the center was imported at the raw OCS coordinate (-1000,-500).
+            const string dxf = "  0\r\nSECTION\r\n  2\r\nHEADER\r\n  9\r\n$ACADVER\r\n  1\r\nAC1015\r\n  0\r\nENDSEC\r\n  0\r\nSECTION\r\n  2\r\nBLOCKS\r\n  0\r\nBLOCK\r\n  8\r\n0\r\n  2\r\n*Model_Space\r\n 10\r\n0.0\r\n 20\r\n0.0\r\n 30\r\n0.0\r\n  3\r\n*Model_Space\r\n  4\r\n\r\n  0\r\nARC\r\n  8\r\n0\r\n 10\r\n-1000.0\r\n 20\r\n-500.0\r\n 30\r\n0.0\r\n 40\r\n100.0\r\n 50\r\n0.0\r\n 51\r\n90.0\r\n210\r\n0.0\r\n220\r\n0.0\r\n230\r\n-1.0\r\n  0\r\nENDBLK\r\n  8\r\n0\r\n  0\r\nENDSEC\r\n  0\r\nSECTION\r\n  2\r\nENTITIES\r\n  0\r\nENDSEC\r\n  0\r\nEOF\r\n";
+            var file = this.TestContext.TestName + ".dxf";
+            File.WriteAllText(file, dxf);
+            var project = Project.ReadFromFile(file, "dxf");
+            Assert.IsNotNull(project);
+            var model = project.GetActiveModel();
+            Assert.IsNotNull(model);
+            Assert.IsTrue(model.AllObjects.Count > 0, "ARC entity must be imported");
+            var arc = model.AllObjects[0] as GeoObject.Ellipse;
+            Assert.IsNotNull(arc, "Imported object must be an Ellipse/Arc");
+            // WCS center must be (1000,-500,0), not the raw OCS value (-1000,-500,0)
+            Assert.IsTrue(Math.Abs(arc.Center.x - 1000.0) < 1e-6,
+                $"Arc WCS X must be 1000, got {arc.Center.x}");
+            Assert.IsTrue(Math.Abs(arc.Center.y - (-500.0)) < 1e-6,
+                $"Arc WCS Y must be -500, got {arc.Center.y}");
+        }
+
         [TestMethod]
         [DeploymentItem(@"Files/Step/issue153.stp", nameof(import_step_issue153_succeeds))]
         public void import_step_issue153_succeeds()