projection_impl.hh

// SPDX-FileCopyrightInfo: Copyright © DUNE Project contributors, see file LICENSE.md in module root
// SPDX-License-Identifier: LGPL-3.0-or-later OR LicenseRef-GPL-2.0-only-with-dune-grid-glue-exception
#include <dune/common/fmatrix.hh>
 
#include <cmath>
 
namespace Dune {
namespace GridGlue {
 
namespace ProjectionImplementation {
 
template<typename Coordinate, typename Field>
inline Coordinate
corner(unsigned c)
{
  Coordinate x(Field(0));
  if (c == 0)
    return x;
  x[c-1] = Field(1);
  return x;
}
 
inline std::pair<unsigned, unsigned>
edgeToCorners(unsigned edge)
{
  switch(edge) {
    case 0: return {0, 1};
    case 1: return {0, 2};
    case 2: return {1, 2};
  }
  DUNE_THROW(Dune::Exception, "Unexpected edge number.");
}
 
template<typename Coordinate, typename Corners>
inline typename Corners::value_type
interpolate(const Coordinate& x, const Corners& corners)
{
  auto y = corners[0];
  for (unsigned i = 0; i < corners.size() - 1; ++i)
    y.axpy(x[i], corners[i+1] - corners[0]);
  return y;
}
 
template<typename Coordinate, typename Normals>
inline typename Normals::value_type
interpolate_unit_normals(const Coordinate& x, const Normals& normals)
{
  auto n = interpolate(x, normals);
  n /= n.two_norm();
  return n;
}
 
template<typename Coordinate, typename Field>
inline bool
inside(const Coordinate& x, const Field& epsilon)
{
  const unsigned dim = Coordinate::dimension;
  Field sum(0);
  for (unsigned i = 0; i < dim-1; ++i) {
    if (x[i] < -epsilon)
      return false;
    sum += x[i];
  }
  /* If any xᵢ is NaN, sum will be NaN and this comparison false! */
  if (sum <= Field(1) + epsilon)
    return true;
  return false;
}
 
} /* namespace ProjectionImplementation */
 
template<typename Coordinate>
Projection<Coordinate>
::Projection(const Field overlap, const Field max_normal_product)
  : m_overlap(overlap)
  , m_max_normal_product(max_normal_product)
{
  /* Nothing. */
}
 
template<typename Coordinate>
void
Projection<Coordinate>
::epsilon(const Field epsilon)
{
  m_epsilon = epsilon;
}
 
template<typename Coordinate>
template<typename Corners, typename Normals>
void
Projection<Coordinate>
::doProjection(const std::tuple<Corners&, Corners&>& corners, const std::tuple<Normals&, Normals&>& normals)
{
  /* Try to obtain Φ(xᵢ) for each corner xᵢ of the preimage triangle.
   * This means solving a linear system of equations
   *    Φ(xᵢ) = (1-α-β) y₀ + α y₁ + β y₂ = xᵢ + δ nᵢ
   * or α (y₁ - y₀) + β (y₂ - y₀) - δ nᵢ = xᵢ - y₀
   * to obtain the barycentric coordinates (α, β) of Φ(xᵢ) in the image
   * triangle and the distance δ.
   *
   * In the matrix m corresponding to the system, only the third column and the
   * right-hand side depend on i. The first two columns can be assembled before
   * and reused.
   */
  using namespace ProjectionImplementation;
  using std::get;
  typedef Dune::FieldMatrix<Field, dim, dim> Matrix;
  Matrix m;
 
  const auto& origin = get<0>(corners);
  const auto& origin_normals = get<0>(normals);
  const auto& target = get<1>(corners);
  const auto& target_normals = get<1>(normals);
  auto& images = get<0>(m_images);
  auto& success = get<0>(m_success);
 
  /* directionsᵢ = (yᵢ - y₀) / ||yᵢ - y₀||
   * These are the first to columns of the system matrix; the rescaling is done
   * to ensure all columns have a comparable norm (the last has the normal with norm 1.
   */
  std::array<Coordinate, dim-1> directions;
  std::array<Field, dim-1> scales;
  /* estimator for the diameter of the target face */
  Field scaleSum(0);
  for (unsigned i = 0; i < dim-1; ++i) {
    directions[i] = target[i+1] - target[0];
    scales[i] = directions[i].infinity_norm();
    directions[i] /= scales[i];
    scaleSum += scales[i];
  }
 
  for (unsigned i = 0; i < dim-1; ++i) {
    for (unsigned j = 0; j < dim; ++j) {
      m[j][i] = directions[i][j];
    }
  }
 
  m_projection_valid = true;
  success.reset();
 
  /* Now project xᵢ for each i */
  for (unsigned i = 0; i < origin.size(); ++i) {
    for (unsigned j = 0; j < dim; ++j)
      m[j][dim-1] = origin_normals[i][j];
 
    const Coordinate rhs = origin[i] - target[0];
 
    try {
      /* y = (α, β, δ) */
      auto& y = images[i];
      m.solve(y, rhs);
      for (unsigned j = 0; j < dim-1; ++j)
        y[j] /= scales[j];
      /* Solving gave us -δ as the term is "-δ nᵢ". */
      y[dim-1] *= Field(-1);
 
      /* If the forward projection is too far in the wrong direction
       * then this might result in artificial inverse projections or
       * edge intersections. To prevent these wrong cases but not
       * dismiss feasible intersections, the projection is dismissed
       * if the forward projection is further than two times the
       * approximate diameter of the image triangle.
       */
      if(y[dim-1] < -2*scaleSum) {
          success.set(i,false);
          m_projection_valid = false;
          return;
      }
 
      const bool feasible = projectionFeasible(origin[i], origin_normals[i], y, target, target_normals);
      success.set(i, feasible);
    }
    catch (const Dune::FMatrixError&) {
      success.set(i, false);
      m_projection_valid = false;
    }
  }
}
 
template<typename Coordinate>
template<typename Corners, typename Normals>
void
Projection<Coordinate>
::doInverseProjection(const std::tuple<Corners&, Corners&>& corners, const std::tuple<Normals&, Normals&>& normals)
{
  /* Try to obtain Φ⁻¹(yᵢ) for each corner yᵢ of the image triangle.
   * Instead of solving the problem directly (which would lead to
   * non-linear equations), we make use of the forward projection Φ
   * which projects the preimage triangle on the plane spanned by the
   * image triangle. The inverse projection is then given by finding
   * the barycentric coordinates of yᵢ with respect to the triangle
   * with the corners Φ(xᵢ). This way we only have to solve linear
   * equations.
   */
 
  using namespace ProjectionImplementation;
  using std::get;
  typedef Dune::FieldMatrix<Field, dim-1, dim-1> Matrix;
  typedef Dune::FieldVector<Field, dim-1> Vector;
 
  /* The inverse projection can only be computed if the forward projection
   * managed to project all xᵢ on the plane spanned by the yᵢ
   */
  if (!m_projection_valid) {
    get<1>(m_success).reset();
    return;
  }
 
  const auto& images = get<0>(m_images);
  const auto& target_corners = get<1>(corners);
  auto& preimages = get<1>(m_images);
  auto& success = get<1>(m_success);
 
  std::array<Coordinate, dim> v;
  for (unsigned i = 0; i < dim-1; ++i) {
    v[i] = interpolate(images[i+1], target_corners);
    v[i] -= interpolate(images[0], target_corners);
  }
 
  Matrix m;
  for (unsigned i = 0; i < dim-1; ++i) {
    for (unsigned j = 0; j < dim-1; ++j) {
      m[i][j] = v[i]*v[j];
    }
  }
 
  for (unsigned i = 0; i < dim; ++i) {
    /* Convert yᵢ to barycentric coordinates with respect to Φ(xⱼ) */
    v[dim-1] = target_corners[i];
    v[dim-1] -= interpolate(images[0], target_corners);
 
    Vector rhs, z;
    for (unsigned j = 0; j < dim-1; ++j)
      rhs[j] = v[dim-1]*v[j];
    m.solve(z, rhs);
 
    for (unsigned j = 0; j < dim-1; ++j)
      preimages[i][j] = z[j];
 
    /* Calculate distance along normal direction */
    const auto x = interpolate(z, get<0>(corners));
    preimages[i][dim-1] = (x - target_corners[i]) * get<1>(normals)[i];
 
    /* Check y_i lies inside the Φ(xⱼ) */
    const bool feasible = projectionFeasible(target_corners[i], get<1>(normals)[i], preimages[i], get<0>(corners), get<0>(normals));
    success.set(i, feasible);
  }
}
 
template<typename Coordinate>
template<typename Corners, typename Normals>
void
Projection<Coordinate>
::doEdgeIntersection(const std::tuple<Corners&, Corners&>& corners, const std::tuple<Normals&, Normals&>& normals)
{
  using namespace ProjectionImplementation;
  using std::get;
 
  m_number_of_edge_intersections = 0;
 
  /* There are no edge intersections for 2d, only for 3d */
  if (dim != 3)
    return;
 
  /* There are no edge intersections
   * - when the projection is invalid,
   * - when the projected triangle lies fully in the target triangle,
   * - or when the target triangle lies fully in the projected triangle.
   */
  if (!m_projection_valid || get<0>(m_success).all() || get<1>(m_success).all()) {
    return;
  }
 
  const auto& images = get<0>(m_images);
  const auto& ys = get<1>(corners);
 
  /* Intersect line through Φ(xᵢ), Φ(xⱼ) with line through yₖ, yₗ:
     We want α, β ∈ ℝ such that
       Φ(xᵢ) + α (Φ(xⱼ) - Φ(xᵢ)) = yₖ + β (yₗ - yₖ)
     or
       α (Φ(xⱼ)-Φ(xᵢ)) + β (yₗ-yₖ) = yₖ-Φ(xᵢ)
     To get a 2×2 system of equations, multiply with yₘ-y₀ for
     m ∈ {1,̣̣2} which are linear indep. (and so the system is
     equivalent to the original 3×2 system)
  */
  for (unsigned edgex = 0; edgex < dim; ++edgex) {
    unsigned i, j;
    std::tie(i, j) = edgeToCorners(edgex);
 
    /* Both sides of edgex lie in the target triangle means no edge intersection */
    if (get<0>(m_success)[i] && get<0>(m_success)[j])
      continue;
 
    const auto pxi = interpolate(images[i], ys);
    const auto pxj = interpolate(images[j], ys);
    const auto pxjpxi = pxj - pxi;
 
    typedef Dune::FieldMatrix<Field, dim-1, dim-1> Matrix;
    typedef Dune::FieldVector<Field, dim-1> Vector;
 
    for (unsigned edgey = 0; edgey < dim; ++edgey) {
      unsigned k, l;
      std::tie(k, l) = edgeToCorners(edgey);
 
      /* Both sides of edgey lie in the projected triangle means no edge intersection */
      if (get<1>(m_success)[k] && get<1>(m_success)[l])
        continue;
 
      const auto ykyl = ys[k] - ys[l];
      const auto ykpxi = ys[k] - pxi;
 
      /* If edges are parallel then the intersection is already computed by vertex projections. */
      bool parallel = true;
      for (unsigned h=0; h<3; h++)
        parallel &= std::abs(ykyl[(h+1)%3]*pxjpxi[(h+2)%3] - ykyl[(h+2)%3]*pxjpxi[(h+1)%3])<1e-14;
      if (parallel)
        continue;
 
      Matrix mat;
      Vector rhs, z;
 
      for (unsigned m = 0; m < dim-1; ++m) {
        const auto ym1y0 = ys[m+1] - ys[0];
        mat[m][0] = pxjpxi * ym1y0;
        mat[m][1] = ykyl * ym1y0;
        rhs[m] = ykpxi * ym1y0;
      }
 
      try {
        using std::isfinite;
 
        mat.solve(z, rhs);
 
        /* If solving the system gives a NaN, the edges are probably parallel. */
        if (!isfinite(z[0]) || !isfinite(z[1]))
          continue;
 
        /* Filter out corner (pre)images. We only want "real" edge-edge intersections here. */
        if (z[0] < m_epsilon || z[0] > Field(1) - m_epsilon
            || z[1] < m_epsilon || z[1] > Field(1) - m_epsilon)
          continue;
 
        Coordinate local_x = corner<Coordinate, Field>(i);
        local_x.axpy(z[0], corner<Coordinate, Field>(j) - corner<Coordinate, Field>(i));
        Coordinate local_y = corner<Coordinate, Field>(k);
        local_y.axpy(z[1], corner<Coordinate, Field>(l) - corner<Coordinate, Field>(k));
 
        /* Make sure the intersection is in the triangle. */
        if (!inside(local_x, m_epsilon) || !inside(local_y, m_epsilon))
          continue;
 
        /* Make sure the intersection respects overlap. */
        auto xy = interpolate(local_x, get<0>(corners));
        xy -= interpolate(local_y, get<1>(corners));
        const auto nx = interpolate_unit_normals(local_x, get<0>(normals));
        const auto ny = interpolate_unit_normals(local_y, get<1>(normals));
        local_x[dim-1] = -(xy*nx);
        local_y[dim-1] = xy*ny;
 
        if (local_x[dim-1] < -m_overlap-m_epsilon || local_y[dim-1] < -m_overlap-m_epsilon)
          continue;
 
        /* Normals should be opposing. */
        if (nx*ny > m_max_normal_product + m_epsilon)
          continue;
 
        /* Intersection is feasible. Store it. */
        auto& intersection = m_edge_intersections[m_number_of_edge_intersections++];
        intersection = { {{edgex, edgey}}, {{local_x, local_y}} };
      }
      catch(const Dune::FMatrixError&) {
        /* Edges might be parallel, ignore and continue with next edge */
      }
    }
  }
}
 
template<typename Coordinate>
template<typename Corners, typename Normals>
bool Projection<Coordinate>
::projectionFeasible(const Coordinate& x, const Coordinate& nx, const Coordinate& px, const Corners& corners, const Normals& normals) const
{
  using namespace ProjectionImplementation;
 
  /* Image must be within simplex. */
  if (!inside(px, m_epsilon))
    return false;
 
  /* Distance along normal must not be smaller than -overlap. */
  if (px[dim-1] < -m_overlap-m_epsilon)
    return false;
 
  /* Distance along normal at image must not be smaller than -overlap. */
  auto xmy = x;
  xmy -= interpolate(px, corners);
  const auto n = interpolate_unit_normals(px, normals);
  const auto d = xmy * n;
  if (d < -m_overlap-m_epsilon)
    return false;
 
  /* Normals at x and Φ(x) are opposing. */
  if (nx * n > m_max_normal_product + m_epsilon)
    return false;
 
  /* Okay, projection is feasible. */
  return true;
}
 
template<typename Coordinate>
template<typename Corners, typename Normals>
void Projection<Coordinate>
::project(const std::tuple<Corners&, Corners&>& corners, const std::tuple<Normals&, Normals&>& normals)
{
  doProjection(corners, normals);
  doInverseProjection(corners, normals);
  doEdgeIntersection(corners, normals);
}
 
} /* namespace GridGlue */
} /* namespace Dune */