Vector solve_laplacian_matrix(const SparseMatrix &h,
const Vector &g,
bool verbose = false)
{
size_t N = h.rows();
assert(N == h.cols());
auto v = h.diagonal();
if (v.minCoeff() == 0)
{
size_t i;
std::cerr << "Error: hessian of Kantorovich's functional "
<< "is not invertible:\n";
std::cerr << "diag = " << v.head(10)
<< " ... in [" << v.minCoeff() << ","
<< v.minCoeff(&i) << "]\n";;
std::cerr << "minCoeff => " << i << "\n";
}
// remove last row and column so that the linear system is
// invertible
Vector gs = g.head(N-1);
SparseMatrix hs = h.block(0,0,N-1,N-1); // top-left submatrix
Eigen::SimplicialLLT<SparseMatrix> solver(hs);
Vector ds = solver.solve(gs);
// if cannot solve with Cholesky, use QR decomposition
double err = (hs*ds - gs).norm();
if (err > 1e-7) // FIXME: threshold
{
std::cerr << "WARNING: in solve_laplacian_matrix: err=" << err << "\n";
#ifdef MA_USE_SUITESPARSE_QR
std::cerr << "Resorting to QR decomposition";
Eigen::SPQR<SparseMatrix> solver(hs);
ds = solver.solve(gs);
if (verbose)
std::cerr << "rank(h) = " << solver.rank() << "\n";
#endif
}
// assemble result
Vector d(N);
d.head(N-1) = ds;
d(N-1) = 0;
return d;
}
Scalar condition(Eigen::Matrix<Scalar, Rows, Cols>& matrix, Scalar maxWanted) {
// TODO Separate case for self-adjoint matrices, which would be the case for
// TODO covariance matrices.
// TODO matrix.selfadjointView<Lower>().eigenvalues();
auto values = matrix.eigenvalues().cwiseAbs();
Scalar max = values.maxCoeff();
Scalar min = values.minCoeff();
// TODO Should I be using the signed min and max eigenvalues?
// TODO I'm not sure how to deal generally with complex values if so.
Scalar condition = max / min;
if (condition > maxWanted) {
// TODO If maxWanted is (near?) 1, then just set to identity?
Scalar bonus = (max - min * maxWanted) / (maxWanted - 1);
matrix.diagonal() = matrix.diagonal().array() + bonus;
}
return condition;
}
void Mesher::check_feature()
{
auto contour = get_contour();
const auto normals = get_normals(contour);
// Find the largest cone and the normals that enclose
// the largest angle as n0, n1.
float theta = 1;
Vec3f n0, n1;
for (auto ni : normals)
{
for (auto nj : normals)
{
float dot = ni.dot(nj);
if (dot < theta)
{
theta = dot;
n0 = ni;
n1 = nj;
}
}
}
// If there isn't a feature in this fan, then return immediately.
if (theta > 0.9)
return;
// Decide whether this is a corner or edge feature.
const Vec3f nstar = n0.cross(n1);
float phi = 0;
for (auto n : normals)
phi = fmax(phi, fabs(nstar.dot(n)));
bool edge = phi < 0.7;
// Find the center of the contour.
Vec3f center(0, 0, 0);
for (auto c : contour)
center += c;
center /= contour.size();
// Construct the matrices for use in our least-square fit.
Eigen::MatrixX3d A(normals.size(), 3);
{
int i=0;
for (auto n : normals)
A.row(i++) << n.transpose();
}
// When building the second matrix, shift position values to be centered
// about the origin (because that's what the least-squares fit will
// minimize).
Eigen::VectorXd B(normals.size(), 1);
{
auto n = normals.begin();
auto c = contour.begin();
int i=0;
while (n != normals.end())
B.row(i++) << (n++)->dot(*(c++) - center);
}
// Use singular value decomposition to solve the least-squares fit.
Eigen::JacobiSVD<Eigen::MatrixX3d> svd(A, Eigen::ComputeFullU |
Eigen::ComputeFullV);
// Set the smallest singular value to zero to make fitting happier.
if (edge)
{
auto singular = svd.singularValues();
svd.setThreshold(singular.minCoeff() / singular.maxCoeff() * 1.01);
}
// Solve for the new point's position.
const Vec3f new_pt = svd.solve(B) + center;
// Erase this triangle fan, as we'll be inserting a vertex in the center.
triangles.erase(fan_start, voxel_start);
// Construct a new triangle fan.
contour.push_back(contour.front());
{
auto p0 = contour.begin();
auto p1 = contour.begin();
p1++;
while (p1 != contour.end())
push_swappable_triangle(Triangle(*(p0++), *(p1++), new_pt));
}
}
LanczosBounds<real_t> minmax_eigenvalues(SparseMatrixX<scalar_t> const& matrix,
double precision_percent) {
auto const precision = static_cast<real_t>(precision_percent / 100);
auto const matrix_size = static_cast<int>(matrix.rows());
auto left = VectorX<scalar_t>{VectorX<scalar_t>::Zero(matrix_size)};
auto right_previous = VectorX<scalar_t>{VectorX<scalar_t>::Zero(matrix_size)};
auto right = num::make_random<VectorX<scalar_t>>(matrix_size);
right.normalize();
// Alpha and beta are the diagonals of the tridiagonal matrix.
// The final size is not known ahead of time, but it will be small.
auto alpha = std::vector<real_t>();
alpha.reserve(100);
auto beta = std::vector<real_t>();
beta.reserve(100);
// Energy values from the previous iteration. Used to test convergence.
// Initial values as far away from expected as possible.
auto previous_min = std::numeric_limits<real_t>::max();
auto previous_max = std::numeric_limits<real_t>::lowest();
constexpr auto loop_limit = 1000;
// This may iterate up to matrix_size, but since only the extreme eigenvalues are required it
// will converge very quickly. Exceeding `loop_limit` would suggest something is wrong.
for (int i = 0; i < loop_limit; ++i) {
// PART 1: Calculate tridiagonal matrix elements a and b
// =====================================================
// left = h_matrix * right
// matrix-vector multiplication (the most compute intensive part of each iteration)
compute::matrix_vector_mul(matrix, right, left);
auto const a = std::real(compute::dot_product(left, right));
auto const b_prev = !beta.empty() ? beta.back() : real_t{0};
// left -= a*right + b_prev*right_previous;
compute::axpy(scalar_t{-a}, right, left);
compute::axpy(scalar_t{-b_prev}, right_previous, left);
auto const b = left.norm();
right_previous.swap(right);
right = (1/b) * left;
alpha.push_back(a);
beta.push_back(b);
// PART 2: Check if the largest magnitude eigenvalues have converged
// =================================================================
auto const eigenvalues = compute::tridiagonal_eigenvalues(eigen_cast<ArrayX>(alpha),
eigen_cast<ArrayX>(beta));
auto const min = eigenvalues.minCoeff();
auto const max = eigenvalues.maxCoeff();
auto const is_converged_min = abs((previous_min - min) / min) < precision;
auto const is_converged_max = abs((previous_max - max) / max) < precision;
if (is_converged_min && is_converged_max) {
return {min, max, i};
}
previous_min = min;
previous_max = max;
};
throw std::runtime_error{"Lanczos algorithm did not converge for the min/max eigenvalues."};
}
