Part: improve TopoShape::findPlane()

Make the returned plane normal consistent with the underlying face
geometry.

src/Mod/Part/App/TopoShape.cpp

@@ -171,6 +171,8 @@
 # include <APIHeaderSection_MakeHeader.hxx>
 # include <ShapeAnalysis_FreeBoundsProperties.hxx>
 # include <ShapeAnalysis_FreeBoundData.hxx>
+# include <BRepLProp_SLProps.hxx>
+# include <BRepGProp_Face.hxx>
 
 #if OCC_VERSION_HEX < 0x070300
 # include <BRepAlgo_Fuse.hxx>
@@ -4161,33 +4163,38 @@ bool TopoShape::findPlane(gp_Pln &pln, double tol) const {
     if(_Shape.IsNull())
         return false;
     TopoDS_Shape shape = _Shape;
-    TopExp_Explorer exp(_Shape,TopAbs_FACE);
+    TopExp_Explorer exp(_Shape,TopAbs_EDGE);
     if(exp.More()) {
-        auto face = exp.Current();
+        TopoDS_Shape edge = exp.Current();
         exp.Next();
-        if(!exp.More()) {
-            BRepAdaptor_Surface adapt(TopoDS::Face(face));
-            if(adapt.GetType() != GeomAbs_Plane)
-                return false;
-            pln = adapt.Plane();
-            return true;
-        }
-    }else{
-        TopExp_Explorer exp(_Shape,TopAbs_EDGE);
-        if(exp.More()) {
-            TopoDS_Shape edge = exp.Current();
-            exp.Next();
-            if(!exp.More()) {
-                // To deal with OCCT bug of wrong edge transformation
-                shape = BRepBuilderAPI_Copy(edge).Shape();
-            }
+        if (!exp.More()) {
+            // To deal with OCCT bug of wrong edge transformation
+            shape = BRepBuilderAPI_Copy(_Shape).Shape();
         }
     }
     try {
         BRepLib_FindSurface finder(shape,tol,Standard_True);
         if (!finder.Found())
             return false;
         pln = GeomAdaptor_Surface(finder.Surface()).Plane();
+
+        // To make the returned plane normal more stable, if the shape has any
+        // face, use the normal of the first face.
+        TopExp_Explorer exp(shape, TopAbs_FACE);
+        if(exp.More()) {
+            BRepAdaptor_Surface adapt(TopoDS::Face(exp.Current()));
+            double u = adapt.FirstUParameter()
+                + (adapt.LastUParameter() - adapt.FirstUParameter())/2.;
+            double v = adapt.FirstVParameter()
+                + (adapt.LastVParameter() - adapt.FirstVParameter())/2.;
+            BRepLProp_SLProps prop(adapt,u,v,2,Precision::Confusion());
+            if(prop.IsNormalDefined()) {
+                gp_Pnt pnt; gp_Vec vec;
+                // handles the orientation state of the shape
+                BRepGProp_Face(TopoDS::Face(exp.Current())).Normal(u,v,pnt,vec);
+                pln = gp_Pln(pnt, gp_Dir(vec));
+            }
+        }
         return true;
     }catch (Standard_Failure &e) {
         // For some reason the above BRepBuilderAPI_Copy failed to copy