Part: issue #6488: Cone surface returns wrong dv derivative

This is a workaround to fix a bug in ElSLib::ConeDN

src/Mod/Part/App/Geometry.cpp

@@ -4246,6 +4246,12 @@ bool GeomSurface::normal(double u, double v, gp_Dir& dir) const
     return false;
 }
 
+gp_Vec GeomSurface::getDN(double u, double v, int Nu, int Nv) const
+{
+    Handle(Geom_Surface) s = Handle(Geom_Surface)::DownCast(handle());
+    return s->DN(u, v, Nu, Nv);
+}
+
 bool GeomSurface::isUmbillic(double u, double v) const
 {
     Handle(Geom_Surface) s = Handle(Geom_Surface)::DownCast(handle());
@@ -4499,6 +4505,50 @@ PyObject *GeomCone::getPyObject(void)
     return new ConePy(static_cast<GeomCone*>(this->clone()));
 }
 
+gp_Vec GeomCone::getDN(double u, double v, int Nu, int Nv) const
+{
+    // Copied from ElSLib::ConeDN() and applied the needed fix
+    auto ElSLib__ConeDN = [](const Standard_Real U,
+                             const Standard_Real V,
+                             const gp_Ax3& Pos,
+                             const Standard_Real Radius,
+                             const Standard_Real SAngle,
+                             const Standard_Integer Nu,
+                             const Standard_Integer Nv)
+    {
+       gp_XYZ Xdir = Pos.XDirection().XYZ();
+       gp_XYZ Ydir = Pos.YDirection().XYZ();
+       gp_XYZ ZDir = Pos.Direction ().XYZ();
+       Standard_Real Um = U + Nu * M_PI_2;  // M_PI * 0.5
+       Xdir.Multiply(cos(Um));
+       Ydir.Multiply(sin(Um));
+       Xdir.Add(Ydir);
+       if(Nv == 0) {
+         Xdir.Multiply(Radius + V * sin(SAngle));
+         if(Nu == 0) Xdir.Add(Pos.Location().XYZ());
+         return gp_Vec(Xdir);
+       }
+       else if(Nv == 1) {
+         Xdir.Multiply(sin(SAngle));
+         ZDir.Multiply(cos(SAngle));
+         Xdir.Add(ZDir);
+         return gp_Vec(Xdir);
+       }
+       return gp_Vec(0.0,0.0,0.0);
+    };
+
+    // Workaround for cones to get the correct derivatives
+    // https://forum.freecadweb.org/viewtopic.php?f=10&t=66677
+    Handle(Geom_ConicalSurface) s = Handle(Geom_ConicalSurface)::DownCast(handle());
+    Standard_RangeError_Raise_if (Nu + Nv < 1 || Nu < 0 || Nv < 0, " ");
+    if (Nv > 1) {
+        return gp_Vec (0.0, 0.0, 0.0);
+    }
+    else {
+      return ElSLib__ConeDN(u, v, s->Position(), s->RefRadius(), s->SemiAngle(), Nu, Nv);
+    }
+}
+
 // -------------------------------------------------
 
 TYPESYSTEM_SOURCE(Part::GeomToroid,Part::GeomSurface)

src/Mod/Part/App/Geometry.h

@@ -66,6 +66,7 @@
 #include <boost/uuid/uuid.hpp>
 #include <boost/uuid/uuid_generators.hpp>
 
+#include <Mod/Part/PartGlobal.h>
 #include "GeometryExtension.h"
 
 namespace Part {
@@ -795,6 +796,11 @@ public:
     bool tangentU(double u, double v, gp_Dir& dirU) const;
     bool tangentV(double u, double v, gp_Dir& dirV) const;
     bool normal(double u, double v, gp_Dir& dir) const;
+    /*!
+      Computes the derivative of order Nu in the direction U and Nv
+      in the direction V at the point P(U, V).
+     */
+    virtual gp_Vec getDN(double u, double v, int Nu, int Nv) const;
 
     /** @name Curvature information */
     //@{
@@ -892,6 +898,9 @@ public:
     void setHandle(const Handle(Geom_ConicalSurface)&);
     const Handle(Geom_Geometry)& handle() const;
 
+    // Overloaded for Geom_ConicalSurface because of an OCC bug
+    virtual gp_Vec getDN(double u, double v, int Nu, int Nv) const;
+
 private:
     Handle(Geom_ConicalSurface) mySurface;
 };

src/Mod/Part/App/GeometrySurfacePyImp.cpp

@@ -226,17 +226,13 @@ PyObject* GeometrySurfacePy::getD0(PyObject *args)
 
 PyObject* GeometrySurfacePy::getDN(PyObject *args)
 {
-    Handle(Geom_Geometry) g = getGeometryPtr()->handle();
-    Handle(Geom_Surface) s = Handle(Geom_Surface)::DownCast(g);
     try {
-        if (!s.IsNull()) {
-            int nu, nv;
-            double u,v;
-            if (!PyArg_ParseTuple(args, "ddii", &u, &v, &nu, &nv))
-                return nullptr;
-            gp_Vec v1 = s->DN(u, v, nu, nv);
-            return new Base::VectorPy(Base::Vector3d(v1.X(),v1.Y(),v1.Z()));
-        }
+        int nu, nv;
+        double u,v;
+        if (!PyArg_ParseTuple(args, "ddii", &u, &v, &nu, &nv))
+            return nullptr;
+        gp_Vec v1 = getGeomSurfacePtr()->getDN(u, v, nu, nv);
+        return new Base::VectorPy(Base::Vector3d(v1.X(),v1.Y(),v1.Z()));
     }
     catch (Standard_Failure& e) {
         PyErr_SetString(PartExceptionOCCError, e.GetMessageString());