fe_raviart_thomas_nodal.cc

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// ---------------------------------------------------------------------
//
// Copyright (C) 2003 - 2021 by the deal.II authors
//
// This file is part of the deal.II library.
//
// The deal.II library is free software; you can use it, redistribute
// it, and/or modify it under the terms of the GNU Lesser General
// Public License as published by the Free Software Foundation; either
// version 2.1 of the License, or (at your option) any later version.
// The full text of the license can be found in the file LICENSE.md at
// the top level directory of deal.II.
//
// ---------------------------------------------------------------------
 
 
#include <deal.II/base/qprojector.h>
#include <deal.II/base/quadrature_lib.h>
 
#include <deal.II/dofs/dof_accessor.h>
 
#include <deal.II/fe/fe.h>
#include <deal.II/fe/fe_nothing.h>
#include <deal.II/fe/fe_raviart_thomas.h>
#include <deal.II/fe/fe_tools.h>
#include <deal.II/fe/fe_values.h>
#include <deal.II/fe/mapping.h>
 
#include <deal.II/grid/tria.h>
#include <deal.II/grid/tria_iterator.h>
 
#include <memory>
#include <sstream>
 
 
DEAL_II_NAMESPACE_OPEN
 
// TODO: implement the adjust_quad_dof_index_for_face_orientation_table and
// adjust_line_dof_index_for_line_orientation_table fields, and write tests
// similar to bits/face_orientation_and_fe_q_*
 
template <int dim>
FE_RaviartThomasNodal<dim>::FE_RaviartThomasNodal(const unsigned int deg)
  : FE_PolyTensor<dim>(PolynomialsRaviartThomas<dim>(deg),
                       FiniteElementData<dim>(get_dpo_vector(deg),
                                              dim,
                                              deg + 1,
                                              FiniteElementData<dim>::Hdiv),
                       get_ria_vector(deg),
                       std::vector<ComponentMask>(
                         PolynomialsRaviartThomas<dim>::n_polynomials(deg),
                         std::vector<bool>(dim, true)))
{
  Assert(dim >= 2, ExcImpossibleInDim(dim));
  const unsigned int n_dofs = this->n_dofs_per_cell();
 
  this->mapping_kind = {mapping_raviart_thomas};
  // First, initialize the
  // generalized support points and
  // quadrature weights, since they
  // are required for interpolation.
  initialize_support_points(deg);
 
  // Now compute the inverse node matrix, generating the correct
  // basis functions from the raw ones. For a discussion of what
  // exactly happens here, see FETools::compute_node_matrix.
  const FullMatrix<double> M = FETools::compute_node_matrix(*this);
  this->inverse_node_matrix.reinit(n_dofs, n_dofs);
  this->inverse_node_matrix.invert(M);
  // From now on, the shape functions provided by FiniteElement::shape_value
  // and similar functions will be the correct ones, not
  // the raw shape functions from the polynomial space anymore.
 
  // Reinit the vectors of
  // prolongation matrices to the
  // right sizes. There are no
  // restriction matrices implemented
  for (unsigned int ref_case = RefinementCase<dim>::cut_x;
       ref_case < RefinementCase<dim>::isotropic_refinement + 1;
       ++ref_case)
    {
      const unsigned int nc =
        GeometryInfo<dim>::n_children(RefinementCase<dim>(ref_case));
 
      for (unsigned int i = 0; i < nc; ++i)
        this->prolongation[ref_case - 1][i].reinit(n_dofs, n_dofs);
    }
 
  // TODO: the implementation makes the assumption that all faces have the
  // same number of dofs
  AssertDimension(this->n_unique_faces(), 1);
  const unsigned int face_no = 0;
 
  // Fill prolongation matrices with embedding operators
  FETools::compute_embedding_matrices(*this, this->prolongation);
  // TODO[TL]: for anisotropic refinement we will probably need a table of
  // submatrices with an array for each refine case
  FullMatrix<double> face_embeddings[GeometryInfo<dim>::max_children_per_face];
  for (unsigned int i = 0; i < GeometryInfo<dim>::max_children_per_face; ++i)
    face_embeddings[i].reinit(this->n_dofs_per_face(face_no),
                              this->n_dofs_per_face(face_no));
  FETools::compute_face_embedding_matrices<dim, double>(*this,
                                                        face_embeddings,
                                                        0,
                                                        0);
  this->interface_constraints.reinit((1 << (dim - 1)) *
                                       this->n_dofs_per_face(face_no),
                                     this->n_dofs_per_face(face_no));
  unsigned int target_row = 0;
  for (unsigned int d = 0; d < GeometryInfo<dim>::max_children_per_face; ++d)
    for (unsigned int i = 0; i < face_embeddings[d].m(); ++i)
      {
        for (unsigned int j = 0; j < face_embeddings[d].n(); ++j)
          this->interface_constraints(target_row, j) = face_embeddings[d](i, j);
        ++target_row;
      }
 
  // We need to initialize the dof permutation table and the one for the sign
  // change.
  initialize_quad_dof_index_permutation_and_sign_change();
}
 
 
 
template <int dim>
std::string
FE_RaviartThomasNodal<dim>::get_name() const
{
  // note that the
  // FETools::get_fe_by_name
  // function depends on the
  // particular format of the string
  // this function returns, so they
  // have to be kept in synch
 
  // note that this->degree is the maximal
  // polynomial degree and is thus one higher
  // than the argument given to the
  // constructor
  std::ostringstream namebuf;
  namebuf << "FE_RaviartThomasNodal<" << dim << ">(" << this->degree - 1 << ")";
 
  return namebuf.str();
}
 
 
template <int dim>
std::unique_ptr<FiniteElement<dim, dim>>
FE_RaviartThomasNodal<dim>::clone() const
{
  return std::make_unique<FE_RaviartThomasNodal<dim>>(*this);
}
 
 
//---------------------------------------------------------------------------
// Auxiliary and internal functions
//---------------------------------------------------------------------------
 
 
 
template <int dim>
void
FE_RaviartThomasNodal<dim>::initialize_support_points(const unsigned int deg)
{
  // TODO: the implementation makes the assumption that all faces have the
  // same number of dofs
  AssertDimension(this->n_unique_faces(), 1);
  const unsigned int face_no = 0;
 
  this->generalized_support_points.resize(this->n_dofs_per_cell());
  this->generalized_face_support_points[face_no].resize(
    this->n_dofs_per_face(face_no));
 
  // Number of the point being entered
  unsigned int current = 0;
 
  // On the faces, we choose as many
  // Gauss points as necessary to
  // determine the normal component
  // uniquely. This is the deg of
  // the Raviart-Thomas element plus
  // one.
  if (dim > 1)
    {
      QGauss<dim - 1> face_points(deg + 1);
      Assert(face_points.size() == this->n_dofs_per_face(face_no),
             ExcInternalError());
      for (unsigned int k = 0; k < this->n_dofs_per_face(face_no); ++k)
        this->generalized_face_support_points[face_no][k] =
          face_points.point(k);
      Quadrature<dim> faces =
        QProjector<dim>::project_to_all_faces(this->reference_cell(),
                                              face_points);
      for (unsigned int k = 0; k < this->n_dofs_per_face(face_no) *
                                     GeometryInfo<dim>::faces_per_cell;
           ++k)
        this->generalized_support_points[k] = faces.point(
          k + QProjector<dim>::DataSetDescriptor::face(this->reference_cell(),
                                                       0,
                                                       true,
                                                       false,
                                                       false,
                                                       this->n_dofs_per_face(
                                                         face_no)));
 
      current =
        this->n_dofs_per_face(face_no) * GeometryInfo<dim>::faces_per_cell;
    }
 
  if (deg == 0)
    return;
  // In the interior, we need
  // anisotropic Gauss quadratures,
  // different for each direction.
  QGauss<1> high(deg + 1);
  QGauss<1> low(deg);
 
  for (unsigned int d = 0; d < dim; ++d)
    {
      std::unique_ptr<QAnisotropic<dim>> quadrature;
      switch (dim)
        {
          case 1:
            quadrature = std::make_unique<QAnisotropic<dim>>(high);
            break;
          case 2:
            quadrature =
              std::make_unique<QAnisotropic<dim>>(((d == 0) ? low : high),
                                                  ((d == 1) ? low : high));
            break;
          case 3:
            quadrature =
              std::make_unique<QAnisotropic<dim>>(((d == 0) ? low : high),
                                                  ((d == 1) ? low : high),
                                                  ((d == 2) ? low : high));
            break;
          default:
            Assert(false, ExcNotImplemented());
        }
 
      for (unsigned int k = 0; k < quadrature->size(); ++k)
        this->generalized_support_points[current++] = quadrature->point(k);
    }
  Assert(current == this->n_dofs_per_cell(), ExcInternalError());
}
 
 
 
template <int dim>
void
FE_RaviartThomasNodal<
  dim>::initialize_quad_dof_index_permutation_and_sign_change()
{
  // for 1D and 2D, do nothing
  if (dim < 3)
    return;
 
  // TODO: Implement this for this class
  return;
}
 
 
 
template <int dim>
std::vector<unsigned int>
FE_RaviartThomasNodal<dim>::get_dpo_vector(const unsigned int deg)
{
  // the element is face-based and we have
  // (deg+1)^(dim-1) DoFs per face
  unsigned int dofs_per_face = 1;
  for (unsigned int d = 1; d < dim; ++d)
    dofs_per_face *= deg + 1;
 
  // and then there are interior dofs
  const unsigned int interior_dofs = dim * deg * dofs_per_face;
 
  std::vector<unsigned int> dpo(dim + 1);
  dpo[dim - 1] = dofs_per_face;
  dpo[dim]     = interior_dofs;
 
  return dpo;
}
 
 
 
template <>
std::vector<bool>
FE_RaviartThomasNodal<1>::get_ria_vector(const unsigned int)
{
  Assert(false, ExcImpossibleInDim(1));
  return std::vector<bool>();
}
 
 
 
template <int dim>
std::vector<bool>
FE_RaviartThomasNodal<dim>::get_ria_vector(const unsigned int deg)
{
  const unsigned int dofs_per_cell =
    PolynomialsRaviartThomas<dim>::n_polynomials(deg);
  unsigned int dofs_per_face = deg + 1;
  for (unsigned int d = 2; d < dim; ++d)
    dofs_per_face *= deg + 1;
  // all face dofs need to be
  // non-additive, since they have
  // continuity requirements.
  // however, the interior dofs are
  // made additive
  std::vector<bool> ret_val(dofs_per_cell, false);
  for (unsigned int i = GeometryInfo<dim>::faces_per_cell * dofs_per_face;
       i < dofs_per_cell;
       ++i)
    ret_val[i] = true;
 
  return ret_val;
}
 
 
template <int dim>
bool
FE_RaviartThomasNodal<dim>::has_support_on_face(
  const unsigned int shape_index,
  const unsigned int face_index) const
{
  AssertIndexRange(shape_index, this->n_dofs_per_cell());
  AssertIndexRange(face_index, GeometryInfo<dim>::faces_per_cell);
 
  // The first degrees of freedom are
  // on the faces and each face has
  // degree degrees.
  const unsigned int support_face = shape_index / this->degree;
 
  // The only thing we know for sure
  // is that shape functions with
  // support on one face are zero on
  // the opposite face.
  if (support_face < GeometryInfo<dim>::faces_per_cell)
    return (face_index != GeometryInfo<dim>::opposite_face[support_face]);
 
  // In all other cases, return true,
  // which is safe
  return true;
}
 
 
 
template <int dim>
void
FE_RaviartThomasNodal<dim>::
  convert_generalized_support_point_values_to_dof_values(
    const std::vector<Vector<double>> &support_point_values,
    std::vector<double> &              nodal_values) const
{
  Assert(support_point_values.size() == this->generalized_support_points.size(),
         ExcDimensionMismatch(support_point_values.size(),
                              this->generalized_support_points.size()));
  Assert(nodal_values.size() == this->n_dofs_per_cell(),
         ExcDimensionMismatch(nodal_values.size(), this->n_dofs_per_cell()));
  Assert(support_point_values[0].size() == this->n_components(),
         ExcDimensionMismatch(support_point_values[0].size(),
                              this->n_components()));
 
  // First do interpolation on
  // faces. There, the component
  // evaluated depends on the face
  // direction and orientation.
  unsigned int fbase = 0;
  unsigned int f     = 0;
  for (; f < GeometryInfo<dim>::faces_per_cell;
       ++f, fbase += this->n_dofs_per_face(f))
    {
      for (unsigned int i = 0; i < this->n_dofs_per_face(f); ++i)
        {
          nodal_values[fbase + i] = support_point_values[fbase + i](
            GeometryInfo<dim>::unit_normal_direction[f]);
        }
    }
 
  // The remaining points form dim
  // chunks, one for each component.
  const unsigned int istep = (this->n_dofs_per_cell() - fbase) / dim;
  Assert((this->n_dofs_per_cell() - fbase) % dim == 0, ExcInternalError());
 
  f = 0;
  while (fbase < this->n_dofs_per_cell())
    {
      for (unsigned int i = 0; i < istep; ++i)
        {
          nodal_values[fbase + i] = support_point_values[fbase + i](f);
        }
      fbase += istep;
      ++f;
    }
  Assert(fbase == this->n_dofs_per_cell(), ExcInternalError());
}
 
 
 
// TODO: There are tests that check that the following few functions don't
// produce assertion failures, but none that actually check whether they do the
// right thing. one example for such a test would be to project a function onto
// an hp-space and make sure that the convergence order is correct with regard
// to the lowest used polynomial degree
 
template <int dim>
bool
FE_RaviartThomasNodal<dim>::hp_constraints_are_implemented() const
{
  return true;
}
 
 
template <int dim>
std::vector<std::pair<unsigned int, unsigned int>>
FE_RaviartThomasNodal<dim>::hp_vertex_dof_identities(
  const FiniteElement<dim> &fe_other) const
{
  // we can presently only compute these
  // identities if both FEs are
  // FE_RaviartThomasNodals or the other is FE_Nothing.
  // In either case, no dofs are assigned on the vertex,
  // so we shouldn't be getting here at all.
  if (dynamic_cast<const FE_RaviartThomasNodal<dim> *>(&fe_other) != nullptr)
    return std::vector<std::pair<unsigned int, unsigned int>>();
  else if (dynamic_cast<const FE_Nothing<dim> *>(&fe_other) != nullptr)
    return std::vector<std::pair<unsigned int, unsigned int>>();
  else
    {
      Assert(false, ExcNotImplemented());
      return std::vector<std::pair<unsigned int, unsigned int>>();
    }
}
 
 
 
template <int dim>
std::vector<std::pair<unsigned int, unsigned int>>
FE_RaviartThomasNodal<dim>::hp_line_dof_identities(
  const FiniteElement<dim> &fe_other) const
{
  // we can presently only compute
  // these identities if both FEs are
  // FE_RaviartThomasNodals or if the other
  // one is FE_Nothing
  if (const FE_RaviartThomasNodal<dim> *fe_q_other =
        dynamic_cast<const FE_RaviartThomasNodal<dim> *>(&fe_other))
    {
      // dofs are located on faces; these are
      // only lines in 2d
      if (dim != 2)
        return std::vector<std::pair<unsigned int, unsigned int>>();
 
      // dofs are located along lines, so two
      // dofs are identical only if in the
      // following two cases (remember that
      // the face support points are Gauss
      // points):
      // 1. this->degree = fe_q_other->degree,
      //   in the case, all the dofs on
      //   the line are identical
      // 2. this->degree-1 and fe_q_other->degree-1
      //   are both even, i.e. this->dof_per_line
      //   and fe_q_other->dof_per_line are both odd,
      //   there exists only one point (the middle one)
      //   such that dofs are identical on this point
      //
      // to understand this, note that
      // this->degree is the *maximal*
      // polynomial degree, and is thus one
      // higher than the argument given to
      // the constructor
      const unsigned int p = this->degree - 1;
      const unsigned int q = fe_q_other->degree - 1;
 
      std::vector<std::pair<unsigned int, unsigned int>> identities;
 
      if (p == q)
        for (unsigned int i = 0; i < p + 1; ++i)
          identities.emplace_back(i, i);
 
      else if (p % 2 == 0 && q % 2 == 0)
        identities.emplace_back(p / 2, q / 2);
 
      return identities;
    }
  else if (dynamic_cast<const FE_Nothing<dim> *>(&fe_other) != nullptr)
    {
      // the FE_Nothing has no degrees of freedom, so there are no
      // equivalencies to be recorded
      return std::vector<std::pair<unsigned int, unsigned int>>();
    }
  else
    {
      Assert(false, ExcNotImplemented());
      return std::vector<std::pair<unsigned int, unsigned int>>();
    }
}
 
 
template <int dim>
std::vector<std::pair<unsigned int, unsigned int>>
FE_RaviartThomasNodal<dim>::hp_quad_dof_identities(
  const FiniteElement<dim> &fe_other,
  const unsigned int        face_no) const
{
  // we can presently only compute
  // these identities if both FEs are
  // FE_RaviartThomasNodals or if the other
  // one is FE_Nothing
  if (const FE_RaviartThomasNodal<dim> *fe_q_other =
        dynamic_cast<const FE_RaviartThomasNodal<dim> *>(&fe_other))
    {
      // dofs are located on faces; these are
      // only quads in 3d
      if (dim != 3)
        return std::vector<std::pair<unsigned int, unsigned int>>();
 
      // this works exactly like the line
      // case above
      const unsigned int p = this->n_dofs_per_quad(face_no);
 
      AssertDimension(fe_q_other->n_unique_faces(), 1);
      const unsigned int q = fe_q_other->n_dofs_per_quad(0);
 
      std::vector<std::pair<unsigned int, unsigned int>> identities;
 
      if (p == q)
        for (unsigned int i = 0; i < p; ++i)
          identities.emplace_back(i, i);
 
      else if (p % 2 != 0 && q % 2 != 0)
        identities.emplace_back(p / 2, q / 2);
 
      return identities;
    }
  else if (dynamic_cast<const FE_Nothing<dim> *>(&fe_other) != nullptr)
    {
      // the FE_Nothing has no degrees of freedom, so there are no
      // equivalencies to be recorded
      return std::vector<std::pair<unsigned int, unsigned int>>();
    }
  else
    {
      Assert(false, ExcNotImplemented());
      return std::vector<std::pair<unsigned int, unsigned int>>();
    }
}
 
 
template <int dim>
FiniteElementDomination::Domination
FE_RaviartThomasNodal<dim>::compare_for_domination(
  const FiniteElement<dim> &fe_other,
  const unsigned int        codim) const
{
  Assert(codim <= dim, ExcImpossibleInDim(dim));
  (void)codim;
 
  // vertex/line/face/cell domination
  // --------------------------------
  if (const FE_RaviartThomasNodal<dim> *fe_rt_nodal_other =
        dynamic_cast<const FE_RaviartThomasNodal<dim> *>(&fe_other))
    {
      if (this->degree < fe_rt_nodal_other->degree)
        return FiniteElementDomination::this_element_dominates;
      else if (this->degree == fe_rt_nodal_other->degree)
        return FiniteElementDomination::either_element_can_dominate;
      else
        return FiniteElementDomination::other_element_dominates;
    }
  else if (const FE_Nothing<dim> *fe_nothing =
             dynamic_cast<const FE_Nothing<dim> *>(&fe_other))
    {
      if (fe_nothing->is_dominating())
        return FiniteElementDomination::other_element_dominates;
      else
        // the FE_Nothing has no degrees of freedom and it is typically used
        // in a context where we don't require any continuity along the
        // interface
        return FiniteElementDomination::no_requirements;
    }
 
  Assert(false, ExcNotImplemented());
  return FiniteElementDomination::neither_element_dominates;
}
 
 
 
template <>
void
FE_RaviartThomasNodal<1>::get_face_interpolation_matrix(
  const FiniteElement<1, 1> & /*x_source_fe*/,
  FullMatrix<double> & /*interpolation_matrix*/,
  const unsigned int) const
{
  Assert(false, ExcImpossibleInDim(1));
}
 
 
template <>
void
FE_RaviartThomasNodal<1>::get_subface_interpolation_matrix(
  const FiniteElement<1, 1> & /*x_source_fe*/,
  const unsigned int /*subface*/,
  FullMatrix<double> & /*interpolation_matrix*/,
  const unsigned int) const
{
  Assert(false, ExcImpossibleInDim(1));
}
 
 
 
template <int dim>
void
FE_RaviartThomasNodal<dim>::get_face_interpolation_matrix(
  const FiniteElement<dim> &x_source_fe,
  FullMatrix<double> &      interpolation_matrix,
  const unsigned int        face_no) const
{
  // this is only implemented, if the
  // source FE is also a
  // RaviartThomasNodal element
  AssertThrow((x_source_fe.get_name().find("FE_RaviartThomasNodal<") == 0) ||
                (dynamic_cast<const FE_RaviartThomasNodal<dim> *>(
                   &x_source_fe) != nullptr),
              typename FiniteElement<dim>::ExcInterpolationNotImplemented());
 
  Assert(interpolation_matrix.n() == this->n_dofs_per_face(face_no),
         ExcDimensionMismatch(interpolation_matrix.n(),
                              this->n_dofs_per_face(face_no)));
  Assert(interpolation_matrix.m() == x_source_fe.n_dofs_per_face(face_no),
         ExcDimensionMismatch(interpolation_matrix.m(),
                              x_source_fe.n_dofs_per_face(face_no)));
 
  // ok, source is a RaviartThomasNodal element, so
  // we will be able to do the work
  const FE_RaviartThomasNodal<dim> &source_fe =
    dynamic_cast<const FE_RaviartThomasNodal<dim> &>(x_source_fe);
 
  // Make sure, that the element,
  // for which the DoFs should be
  // constrained is the one with
  // the higher polynomial degree.
  // Actually the procedure will work
  // also if this assertion is not
  // satisfied. But the matrices
  // produced in that case might
  // lead to problems in the
  // hp-procedures, which use this
  // method.
  Assert(this->n_dofs_per_face(face_no) <= source_fe.n_dofs_per_face(face_no),
         typename FiniteElement<dim>::ExcInterpolationNotImplemented());
 
  // generate a quadrature
  // with the generalized support points.
  // This is later based as a
  // basis for the QProjector,
  // which returns the support
  // points on the face.
  Quadrature<dim - 1> quad_face_support(
    source_fe.generalized_face_support_points[face_no]);
 
  // Rule of thumb for FP accuracy,
  // that can be expected for a
  // given polynomial degree.
  // This value is used to cut
  // off values close to zero.
  double eps = 2e-13 * this->degree * (dim - 1);
 
  // compute the interpolation
  // matrix by simply taking the
  // value at the support points.
  const Quadrature<dim> face_projection =
    QProjector<dim>::project_to_face(this->reference_cell(),
                                     quad_face_support,
                                     0);
 
  for (unsigned int i = 0; i < source_fe.n_dofs_per_face(face_no); ++i)
    {
      const Point<dim> &p = face_projection.point(i);
 
      for (unsigned int j = 0; j < this->n_dofs_per_face(face_no); ++j)
        {
          double matrix_entry =
            this->shape_value_component(this->face_to_cell_index(j, 0), p, 0);
 
          // Correct the interpolated
          // value. I.e. if it is close
          // to 1 or 0, make it exactly
          // 1 or 0. Unfortunately, this
          // is required to avoid problems
          // with higher order elements.
          if (std::fabs(matrix_entry - 1.0) < eps)
            matrix_entry = 1.0;
          if (std::fabs(matrix_entry) < eps)
            matrix_entry = 0.0;
 
          interpolation_matrix(i, j) = matrix_entry;
        }
    }
 
  // make sure that the row sum of
  // each of the matrices is 1 at
  // this point. this must be so
  // since the shape functions sum up
  // to 1
  for (unsigned int j = 0; j < source_fe.n_dofs_per_face(face_no); ++j)
    {
      double sum = 0.;
 
      for (unsigned int i = 0; i < this->n_dofs_per_face(face_no); ++i)
        sum += interpolation_matrix(j, i);
 
      Assert(std::fabs(sum - 1) < 2e-13 * this->degree * (dim - 1),
             ExcInternalError());
    }
}
 
 
template <int dim>
void
FE_RaviartThomasNodal<dim>::get_subface_interpolation_matrix(
  const FiniteElement<dim> &x_source_fe,
  const unsigned int        subface,
  FullMatrix<double> &      interpolation_matrix,
  const unsigned int        face_no) const
{
  // this is only implemented, if the
  // source FE is also a
  // RaviartThomasNodal element
  AssertThrow((x_source_fe.get_name().find("FE_RaviartThomasNodal<") == 0) ||
                (dynamic_cast<const FE_RaviartThomasNodal<dim> *>(
                   &x_source_fe) != nullptr),
              typename FiniteElement<dim>::ExcInterpolationNotImplemented());
 
  Assert(interpolation_matrix.n() == this->n_dofs_per_face(face_no),
         ExcDimensionMismatch(interpolation_matrix.n(),
                              this->n_dofs_per_face(face_no)));
  Assert(interpolation_matrix.m() == x_source_fe.n_dofs_per_face(face_no),
         ExcDimensionMismatch(interpolation_matrix.m(),
                              x_source_fe.n_dofs_per_face(face_no)));
 
  // ok, source is a RaviartThomasNodal element, so
  // we will be able to do the work
  const FE_RaviartThomasNodal<dim> &source_fe =
    dynamic_cast<const FE_RaviartThomasNodal<dim> &>(x_source_fe);
 
  // Make sure, that the element,
  // for which the DoFs should be
  // constrained is the one with
  // the higher polynomial degree.
  // Actually the procedure will work
  // also if this assertion is not
  // satisfied. But the matrices
  // produced in that case might
  // lead to problems in the
  // hp-procedures, which use this
  // method.
  Assert(this->n_dofs_per_face(face_no) <= source_fe.n_dofs_per_face(face_no),
         typename FiniteElement<dim>::ExcInterpolationNotImplemented());
 
  // generate a quadrature
  // with the generalized support points.
  // This is later based as a
  // basis for the QProjector,
  // which returns the support
  // points on the face.
  Quadrature<dim - 1> quad_face_support(
    source_fe.generalized_face_support_points[face_no]);
 
  // Rule of thumb for FP accuracy,
  // that can be expected for a
  // given polynomial degree.
  // This value is used to cut
  // off values close to zero.
  double eps = 2e-13 * this->degree * (dim - 1);
 
  // compute the interpolation
  // matrix by simply taking the
  // value at the support points.
 
  const Quadrature<dim> subface_projection =
    QProjector<dim>::project_to_subface(this->reference_cell(),
                                        quad_face_support,
                                        0,
                                        subface);
 
  for (unsigned int i = 0; i < source_fe.n_dofs_per_face(face_no); ++i)
    {
      const Point<dim> &p = subface_projection.point(i);
 
      for (unsigned int j = 0; j < this->n_dofs_per_face(face_no); ++j)
        {
          double matrix_entry =
            this->shape_value_component(this->face_to_cell_index(j, 0), p, 0);
 
          // Correct the interpolated
          // value. I.e. if it is close
          // to 1 or 0, make it exactly
          // 1 or 0. Unfortunately, this
          // is required to avoid problems
          // with higher order elements.
          if (std::fabs(matrix_entry - 1.0) < eps)
            matrix_entry = 1.0;
          if (std::fabs(matrix_entry) < eps)
            matrix_entry = 0.0;
 
          interpolation_matrix(i, j) = matrix_entry;
        }
    }
 
  // make sure that the row sum of
  // each of the matrices is 1 at
  // this point. this must be so
  // since the shape functions sum up
  // to 1
  for (unsigned int j = 0; j < source_fe.n_dofs_per_face(face_no); ++j)
    {
      double sum = 0.;
 
      for (unsigned int i = 0; i < this->n_dofs_per_face(face_no); ++i)
        sum += interpolation_matrix(j, i);
 
      Assert(std::fabs(sum - 1) < 2e-13 * this->degree * (dim - 1),
             ExcInternalError());
    }
}
 
 
 
// explicit instantiations
#include "fe_raviart_thomas_nodal.inst"
 
 
DEAL_II_NAMESPACE_CLOSE