Modeling - Fix thickness operation regression on circle-to-polygon lofts (#889)

- Fixed CheckMixedContinuity to detect actual mixed concavity
  (both convex and concave regions) instead of any G1/non-G1 transitions
- Added null edge and PCurve checks in RefEdgeInter to prevent crashes
- Added GTests for BRepOffset_MakeOffset covering various loft scenarios

src/ModelingAlgorithms/TKOffset/BRepOffset/BRepOffset_Analyse.cxx

@@ -152,7 +152,7 @@ Standard_Boolean CheckMixedContinuity(const TopoDS_Edge&  theEdge,
     // so, if aCurrOrder > C0 it means that faces are tangent along whole edge.
     return aMixedCont;
   }
-  // But we caqnnot trust result, if it is C0. because this value set by default.
+  // But we cannot trust result, if it is C0, because this value is set by default.
   Standard_Real TolC0 = std::max(0.001, 1.5 * BRep_Tool::Tolerance(theEdge));
 
   Standard_Real aFirst;
@@ -217,45 +217,15 @@ Standard_Boolean CheckMixedContinuity(const TopoDS_Edge&  theEdge,
 
   Standard_Integer aNbSamples = 23;
 
-  // Computation of the continuity.
-  Standard_Real    aPar;
-  Standard_Real    aDelta = (aLast - aFirst) / (aNbSamples - 1);
-  Standard_Integer i, istart = 1;
-  Standard_Boolean isG1 = Standard_False;
+  // Check for mixed concavity: convex in some regions, concave in others.
+  const Standard_Real aDelta      = (aLast - aFirst) / (aNbSamples - 1);
+  bool                aHasConvex  = false;
+  bool                aHasConcave = false;
+  Standard_Integer    aNbValid    = 0;
 
-  for (i = 1, aPar = aFirst; i <= aNbSamples; i++, aPar += aDelta)
+  for (Standard_Integer i = 1; i <= aNbSamples; i++)
   {
-    if (i == aNbSamples)
-      aPar = aLast;
-
-    LocalAnalysis_SurfaceContinuity aCont(aC2d1,
-                                          aC2d2,
-                                          aPar,
-                                          aSurf1,
-                                          aSurf2,
-                                          GeomAbs_G1,
-                                          0.001,
-                                          TolC0,
-                                          theAngTol,
-                                          theAngTol,
-                                          theAngTol);
-    if (aCont.IsDone())
-    {
-      istart = i + 1;
-      isG1   = aCont.IsG1();
-      break;
-    }
-  }
-
-  if (istart > aNbSamples / 2)
-  {
-    return aMixedCont;
-  }
-
-  for (i = istart, aPar = aFirst; i <= aNbSamples; i++, aPar += aDelta)
-  {
-    if (i == aNbSamples)
-      aPar = aLast;
+    const Standard_Real aPar = (i == aNbSamples) ? aLast : aFirst + (i - 1) * aDelta;
 
     LocalAnalysis_SurfaceContinuity aCont(aC2d1,
                                           aC2d2,
@@ -273,17 +243,24 @@ Standard_Boolean CheckMixedContinuity(const TopoDS_Edge&  theEdge,
       continue;
     }
 
-    if (aCont.IsG1() == isG1)
+    aNbValid++;
+
+    if (!aCont.IsG1() && (!aHasConvex || !aHasConcave))
     {
-      continue;
-    }
-    else
-    {
-      aMixedCont = Standard_True;
-      break;
+      const Standard_Real anAngle = aCont.C0Value();
+      aHasConvex                  = aHasConvex || (anAngle > M_PI_2 + theAngTol);
+      aHasConcave                 = aHasConcave || (anAngle < M_PI_2 - theAngTol);
     }
   }
 
+  if (aNbValid < aNbSamples / 2)
+  {
+    return aMixedCont;
+  }
+
+  // Mixed connectivity: both convex and concave regions exist.
+  aMixedCont = aHasConvex && aHasConcave;
+
   return aMixedCont;
 }
 

src/ModelingAlgorithms/TKOffset/BRepOffset/BRepOffset_Inter2d.cxx

@@ -702,13 +702,17 @@ static void RefEdgeInter(const TopoDS_Face&                         F,
   //
   if (E1.IsSame(E2))
     return;
+  if (E1.IsNull() || E2.IsNull())
+    return;
 
   Standard_Real    f[3], l[3];
   Standard_Real    TolDub = 1.e-7, TolLL = 0.0;
   Standard_Integer i;
 
-  // BRep_Tool::Range(E1, f[1], l[1]);
-  // BRep_Tool::Range(E2, f[2], l[2]);
+  Handle(Geom2d_Curve) pcurve1 = BRep_Tool::CurveOnSurface(E1, F, f[1], l[1]);
+  Handle(Geom2d_Curve) pcurve2 = BRep_Tool::CurveOnSurface(E2, F, f[2], l[2]);
+  if (pcurve1.IsNull() || pcurve2.IsNull())
+    return;
 
   BRepAdaptor_Curve CE1(E1, F);
   BRepAdaptor_Curve CE2(E2, F);
@@ -744,10 +748,8 @@ static void RefEdgeInter(const TopoDS_Face&                         F,
     }
   }
   //
-  Handle(Geom2d_Curve) pcurve1 = BRep_Tool::CurveOnSurface(E1, F, f[1], l[1]);
-  Handle(Geom2d_Curve) pcurve2 = BRep_Tool::CurveOnSurface(E2, F, f[2], l[2]);
-  Geom2dAdaptor_Curve  GAC1(pcurve1, f[1], l[1]);
-  Geom2dAdaptor_Curve  GAC2(pcurve2, f[2], l[2]);
+  Geom2dAdaptor_Curve GAC1(pcurve1, f[1], l[1]);
+  Geom2dAdaptor_Curve GAC2(pcurve2, f[2], l[2]);
   if ((GAC1.GetType() == GeomAbs_Line) && (GAC2.GetType() == GeomAbs_Line))
   {
     // Just quickly check if lines coincide

src/ModelingAlgorithms/TKOffset/GTests/FILES.cmake

@@ -2,4 +2,5 @@
 set(OCCT_TKOffset_GTests_FILES_LOCATION "${CMAKE_CURRENT_LIST_DIR}")
 
 set(OCCT_TKOffset_GTests_FILES
+  BRepOffset_MakeOffset_Test.cxx
 )