Path.Area: improved section tolerance

Previously, a small offset in the same direction of section is added to
the section height to avoid tagent plane cut. Now we'll retry by adding
offset in the oppsite direction if the first attemp fails.

src/Mod/Path/App/Area.cpp

@@ -565,6 +565,7 @@ std::vector<shared_ptr<Area> > Area::makeSections(
             yMin<<','<<yMax<<"), Z("<<zMin<<','<<zMax<<')');
 
     std::vector<double> heights;
+    double tolerance = 0.0;
     if(_heights.empty()) {
         double z;
         double d = fabs(myParams.Stepdown);
@@ -593,7 +594,7 @@ std::vector<shared_ptr<Area> > Area::makeSections(
             z = zMax;
         else if(z < zMin)
             z = zMin;
-        double dz,tolerance;
+        double dz;
         if(myParams.Stepdown>0.0) {
             dz = z - zMin;
             tolerance = myParams.SectionTolerance;
@@ -660,56 +661,77 @@ std::vector<shared_ptr<Area> > Area::makeSections(
 
     std::vector<shared_ptr<Area> > sections;
     sections.reserve(heights.size());
+    tolerance *= 2.0;
+    bool can_retry = fabs(tolerance)>Precision::Confusion();
     for(double z : heights) {
-        gp_Pln pln(gp_Pnt(0,0,z),gp_Dir(0,0,1));
-        Standard_Real a,b,c,d;
-        pln.Coefficients(a,b,c,d);
-        BRepLib_MakeFace mkFace(pln,xMin,xMax,yMin,yMax);
-        const TopoDS_Shape &face = mkFace.Face();
-
-        shared_ptr<Area> area(new Area(&myParams));
-        area->setPlane(face);
-        for(const Shape &s : myShapes) {
-            BRep_Builder builder;
-            TopoDS_Compound comp;
-            builder.MakeCompound(comp);
-            for(TopExp_Explorer it(s.shape.Moved(loc), TopAbs_SOLID); it.More(); it.Next()) {
-                Part::CrossSection section(a,b,c,it.Current());
-                std::list<TopoDS_Wire> wires = section.slice(-d);
-                if(wires.empty()) {
-                    AREA_LOG("Section returns no wires");
-                    continue;
+        bool retried = !can_retry;
+        while(true) {
+            gp_Pln pln(gp_Pnt(0,0,z),gp_Dir(0,0,1));
+            Standard_Real a,b,c,d;
+            pln.Coefficients(a,b,c,d);
+            BRepLib_MakeFace mkFace(pln,xMin,xMax,yMin,yMax);
+            const TopoDS_Shape &face = mkFace.Face();
+
+            shared_ptr<Area> area(new Area(&myParams));
+            area->setPlane(face);
+            for(auto it=myShapes.begin();it!=myShapes.end();++it) {
+                const auto &s = *it;
+                BRep_Builder builder;
+                TopoDS_Compound comp;
+                builder.MakeCompound(comp);
+                for(TopExp_Explorer xp(s.shape.Moved(loc), TopAbs_SOLID); xp.More(); xp.Next()) {
+                    Part::CrossSection section(a,b,c,xp.Current());
+                    std::list<TopoDS_Wire> wires = section.slice(-d);
+                    if(wires.empty()) {
+                        AREA_LOG("Section returns no wires");
+                        continue;
+                    }
+                    Part::FaceMakerBullseye mkFace;
+                    mkFace.setPlane(pln);
+                    for(const TopoDS_Wire &wire : wires)
+                        mkFace.addWire(wire);
+                    try {
+                        mkFace.Build();
+                        if (mkFace.Shape().IsNull())
+                            AREA_WARN("FaceMakerBullseye return null shape on section");
+                        else {
+                            builder.Add(comp,mkFace.Shape());
+                            continue;
+                        }
+                    }catch (Base::Exception &e){
+                        AREA_WARN("FaceMakerBullseye failed on section: " << e.what());
+                    }
+                    for(const TopoDS_Wire &wire : wires)
+                        builder.Add(comp,wire);
                 }
 
-                Part::FaceMakerBullseye mkFace;
-                mkFace.setPlane(pln);
-                for(const TopoDS_Wire &wire : wires)
-                    mkFace.addWire(wire);
-                try {
-                    mkFace.Build();
-                    if (mkFace.Shape().IsNull())
-                        AREA_WARN("FaceMakerBullseye return null shape on section");
-                    else {
-                        builder.Add(comp,mkFace.Shape());
-                        continue;
+                // Make sure the compound has at least one edge
+                TopExp_Explorer xp(comp,TopAbs_EDGE);
+                if(xp.More()) {
+                    area->add(comp,s.op);
+                }else if(area->myShapes.empty()){
+                    auto itNext = it;
+                    if(++itNext != myShapes.end() &&
+                        (itNext->op==OperationIntersection ||
+                         itNext->op==OperationDifference))
+                    {
+                        break;
                     }
-                }catch (Base::Exception &e){
-                    AREA_WARN("FaceMakerBullseye failed on section: " << e.what());
                 }
-                for(const TopoDS_Wire &wire : wires)
-                    builder.Add(comp,wire);
             }
-
-            // Make sure the compound has at least one edge
-            for(TopExp_Explorer it(comp,TopAbs_EDGE);it.More();) {
-                area->add(comp,s.op);
+            if(area->myShapes.size()){
+                sections.push_back(area);
                 break;
             }
+            if(retried) {
+                AREA_WARN("Discard empty section");
+                break;
+            }else{
+                AREA_TRACE("retry section " <<z<<"->"<<z+tolerance);
+                z += tolerance;
+                retried = true;
+            }
         }
-        if(area->myShapes.size())
-            sections.push_back(area);
-        else
-            AREA_WARN("Discard empty section");
         TIME_PRINT(t1,"makeSection " << z);
     }
     TIME_PRINT(t,"makeSection count: " << sections.size()<<", total");