Calculate average normal for poly mesh faces. Otherwise an arbitrary triangle is taken

git-svn-id: http://www.openmesh.org/svnrepo/OpenMesh/trunk@309 fdac6126-5c0c-442c-9429-916003d36597

src/OpenMesh/Core/Mesh/PolyMeshT.cc

@@ -95,15 +95,37 @@ calc_face_normal(FaceHandle _fh) const
 {
   assert(halfedge_handle(_fh).is_valid());
   ConstFaceVertexIter fv_it(cfv_iter(_fh));
+  
+  Point p0 = point(fv_it);
+  Point p0i = p0; //save point of vertex 0
+  ++fv_it;
+  Point p1 = point(fv_it);
+  Point p1i = p1; //save point of vertex 1
+  ++fv_it;
+  Point p2;
+  
+  //calculate area-weighted average normal of polygon's ears
+  Normal n(0,0,0);
+  for(; fv_it; ++fv_it)
+  {
+    p2 = point(fv_it);
+    n += vector_cast<Normal>((p2-p1)%(p0-p1)); 
+    p0 = p1;
+    p1 = p2;
+  }
+  
+  //two additional steps since we started at vertex 2, not 0
+  n += vector_cast<Normal>((p0i-p1)%(p0-p1)); 
+  n += vector_cast<Normal>((p1i-p0i)%(p1-p0i));
 
-  const Point& p0(point(fv_it));  ++fv_it;
-  const Point& p1(point(fv_it));  ++fv_it;
-  const Point& p2(point(fv_it));
-
-  return calc_face_normal(p0, p1, p2);
+  typename Normal::value_type norm = n.length();
+  
+  // The expression ((n *= (1.0/norm)),n) is used because the OpenSG
+  // vector class does not return self after component-wise
+  // self-multiplication with a scalar!!!
+  return (norm != typename Normal::value_type(0)) ? ((n *= (typename Normal::value_type(1)/norm)),n) : Normal(0,0,0);
 }
 
-
 //-----------------------------------------------------------------------------
 
 

src/OpenMesh/Core/Mesh/PolyMeshT.hh

@@ -218,7 +218,7 @@ public:
   void update_face_normals();
 
   /** Calculate normal vector for face _fh. */
-  Normal calc_face_normal(FaceHandle _fh) const;
+  virtual Normal calc_face_normal(FaceHandle _fh) const;
 
   /** Calculate normal vector for face (_p0, _p1, _p2). */
   Normal calc_face_normal(const Point& _p0, const Point& _p1,

src/OpenMesh/Core/Mesh/TriMeshT.cc

@@ -66,6 +66,21 @@ namespace OpenMesh {
 
 //== IMPLEMENTATION ==========================================================
 
+template <class Kernel>
+typename TriMeshT<Kernel>::Normal
+TriMeshT<Kernel>::
+calc_face_normal(FaceHandle _fh) const
+{
+  assert(halfedge_handle(_fh).is_valid());
+  ConstFaceVertexIter fv_it(cfv_iter(_fh));
+
+  const Point& p0(point(fv_it));  ++fv_it;
+  const Point& p1(point(fv_it));  ++fv_it;
+  const Point& p2(point(fv_it));
+
+  return PolyMesh::calc_face_normal(p0, p1, p2);
+}
+
 //=============================================================================
 } // namespace OpenMesh
 //=============================================================================

src/OpenMesh/Core/Mesh/TriMeshT.hh

@@ -183,14 +183,21 @@ public:
   inline void split(EdgeHandle _eh, VertexHandle _vh)
   { PolyMesh::split(_eh, _vh); }
 
-  /// Face split (= 1-to-3 split, calls corresponding PolyMeshT function).
-
   /// Face split (= 1-to-3 split, calls corresponding PolyMeshT function).
   inline void split(FaceHandle _fh, const Point& _p)
   { PolyMesh::split(_fh, _p); }
 
   inline void split(FaceHandle _fh, VertexHandle _vh)
   { PolyMesh::split(_fh, _vh); }
+  
+  /** \name Normal vector computation
+  */
+  //@{
+
+  /** Calculate normal vector for face _fh (specialized for TriMesh). */
+  Normal calc_face_normal(FaceHandle _fh) const;
+
+  //@}
 };