Update documentation in GridOut.

diff --git a/include/deal.II/grid/grid_out.h b/include/deal.II/grid/grid_out.h
index 8547991e92a436caf793a6d4367a631a9e65e0c8..83ba798e3f380558dab51b412a30bfbddb952f79 100644 (file)
--- a/include/deal.II/grid/grid_out.h
+++ b/include/deal.II/grid/grid_out.h
@@ -1074,6 +1074,20 @@ public:
 
   /**
    * Write triangulation in VTK format.
+   *
+   * Due to the way this function writes data to the output stream,
+   * the resulting output files correspond to a faithful representation
+   * of the mesh in that all cells are visible for visualization. However,
+   * the data is not in a format that allows reading this file in again
+   * through the GridIn class. This is because every vertex of the mesh is
+   * duplicated as many times as there are adjacent cells. In other words,
+   * every cell has its own, separate set of vertices that are at the
+   * same location as the vertices of other cells, but are separately
+   * numbered. If such a file is read in through the GridIn class, then
+   * that will result in a mesh that has the correct cells and vertex
+   * locations, but because the vertices are logically separate (though at
+   * the same locations) all cells are unconnected and have no neighbors
+   * across faces.
    */
   template <int dim, int spacedim>
   void write_vtk (const Triangulation<dim,spacedim> &tria,
@@ -1081,6 +1095,20 @@ public:
 
   /**
    * Write triangulation in VTU format.
+   *
+   * Due to the way this function writes data to the output stream,
+   * the resulting output files correspond to a faithful representation
+   * of the mesh in that all cells are visible for visualization. However,
+   * the data is not in a format that allows reading this file in again
+   * through the GridIn class. This is because every vertex of the mesh is
+   * duplicated as many times as there are adjacent cells. In other words,
+   * every cell has its own, separate set of vertices that are at the
+   * same location as the vertices of other cells, but are separately
+   * numbered. If such a file is read in through the GridIn class, then
+   * that will result in a mesh that has the correct cells and vertex
+   * locations, but because the vertices are logically separate (though at
+   * the same locations) all cells are unconnected and have no neighbors
+   * across faces.
    */
   template <int dim, int spacedim>
   void write_vtu (const Triangulation<dim,spacedim> &tria,