LocalRelaxation.cpp

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#include <polyfem/mesh/remesh/PhysicsRemesher.hpp>
#include <polyfem/mesh/remesh/wild_remesh/LocalRelaxationData.hpp>
#include <polyfem/solver/forms/BodyForm.hpp>
#include <polyfem/solver/NLProblem.hpp>
#include <polyfem/time_integrator/ImplicitTimeIntegrator.hpp>
 
#include <polysolve/nonlinear/Solver.hpp>
 
namespace polyfem::mesh
{
    void add_solver_timings(
        decltype(Remesher::timings) &timings,
        const polyfem::json &solver_info)
    {
        // Copy over timing data
        const int solver_iters = solver_info["iterations"];
        for (const auto &[key, value] : solver_info.items())
        {
            if (utils::StringUtils::startswith(key, "time_")
                && key != "total_time"
                && key != "time_line_search")
            {
                // The solver reports the time per iteration, so we need to
                // multiply by the number of iterations.
                const std::string new_key = "NonlinearSolver::" + key.substr(5, key.size() - 5);
                timings[new_key] += solver_iters * value.get<double>();
            }
        }
    }
 
    template <class WMTKMesh>
    bool PhysicsRemesher<WMTKMesh>::local_relaxation(
        const std::vector<Tuple> &local_mesh_tuples,
        const double acceptance_tolerance)
    {
        // --------------------------------------------------------------------
        // 1. Get the n-ring of elements around the vertex.
 
        const bool include_global_boundary =
            state.is_contact_enabled() && std::any_of(local_mesh_tuples.begin(), local_mesh_tuples.end(), [&](const Tuple &t) {
                const size_t tid = this->element_id(t);
                for (int i = 0; i < Super::FACETS_PER_ELEMENT; ++i)
                    if (this->is_boundary_facet(this->tuple_from_facet(tid, i)))
                        return true;
                return false;
            });
 
        LocalMesh<Super> local_mesh(*this, local_mesh_tuples, include_global_boundary);
 
        // --------------------------------------------------------------------
        // 2. Perform "relaxation" by minimizing the elastic energy of the
        // n-ring with the internal boundary edges fixed.
 
        LocalRelaxationData data(this->state, local_mesh, this->current_time, include_global_boundary);
        solver::SolveData &solve_data = data.solve_data;
 
        const int n_free_dof = data.n_free_dof();
        if (n_free_dof <= 0)
        {
            assert(n_free_dof == 0);
            return false;
        }
 
        this->total_ndofs += n_free_dof;
        this->num_solves++;
 
        // Nonlinear solver
        auto nl_solver = state.make_nl_solver(/*for_al=*/false); // TODO: Use Eigen::LLT
        nl_solver->stop_criteria().iterations = args["local_relaxation"]["max_nl_iterations"];
        if (this->is_boundary_op())
            nl_solver->stop_criteria().iterations = std::max(nl_solver->stop_criteria().iterations, size_t(5));
        nl_solver->allow_out_of_iterations = true;
 
        Eigen::VectorXd reduced_sol = solve_data.nl_problem->full_to_reduced(data.sol());
 
        const auto level_before = logger().level();
        logger().set_level(spdlog::level::warn);
        try
        {
            POLYFEM_REMESHER_SCOPED_TIMER("Local relaxation solve");
            nl_solver->minimize(*(solve_data.nl_problem), reduced_sol);
        }
        catch (const std::runtime_error &e)
        {
            logger().set_level(level_before);
            assert(false);
            return false;
        }
        logger().set_level(level_before);
 
        // Copy over timing data
        add_solver_timings(this->timings, nl_solver->info());
 
        Eigen::VectorXd sol = solve_data.nl_problem->reduced_to_full(reduced_sol);
 
        // --------------------------------------------------------------------
        // 3. Determine if we should accept the operation based on a decrease in
        // energy.
 
        const double local_energy_after = solve_data.nl_problem->value(sol);
        assert(std::isfinite(local_energy_before()));
        assert(std::isfinite(local_energy_after));
        const double abs_diff = local_energy_before() - local_energy_after; // > 0 if energy decreased
        // TODO: compute global_energy_before
        // Right now using: starting_energy = state.solve_data.nl_problem->value(sol)
        // const double global_energy_before = abs(starting_energy);
        // const double rel_diff = abs_diff / global_energy_before;
 
        // NOTE: only use abs_diff
        // accept = rel_diff >= energy_relative_tolerance && abs_diff >= energy_absolute_tolerance;
        // NOTE: account for Δt² in energy by multiplying acceptance tol by Δt²
        const double dt_sqr = solve_data.time_integrator ? solve_data.time_integrator->acceleration_scaling() : 1.0;
        const bool accept = abs_diff >= dt_sqr * acceptance_tolerance;
 
        // Update positions only on acceptance
        if (accept)
        {
            static int save_i = 0;
            // local_mesh.write_mesh(state.resolve_output_path(fmt::format("local_mesh_{:04d}.vtu", save_i)), target_x);
            // write_mesh(state.resolve_output_path(fmt::format("relaxation_{:04d}.vtu", save_i++)));
 
            // Re-solve with more iterations
            if (!is_converged_status(nl_solver->status()))
            {
                nl_solver->stop_criteria().iterations = 100;
 
                const auto level_before = logger().level();
                logger().set_level(spdlog::level::warn);
                try
                {
                    POLYFEM_REMESHER_SCOPED_TIMER("Local relaxation resolve");
                    nl_solver->minimize(*(solve_data.nl_problem), reduced_sol);
                }
                catch (const std::runtime_error &e)
                {
                    logger().set_level(level_before);
                    assert(false);
                    return false;
                }
                logger().set_level(level_before);
 
                // Copy over timing data
                add_solver_timings(this->timings, nl_solver->info());
 
                sol = solve_data.nl_problem->reduced_to_full(reduced_sol);
            }
 
            for (const auto &[glob_vi, loc_vi] : local_mesh.global_to_local())
            {
                const auto u = sol.middleRows(this->dim() * loc_vi, this->dim());
                const auto u_old = vertex_attrs[glob_vi].displacement();
                vertex_attrs[glob_vi].position = vertex_attrs[glob_vi].rest_position + u;
            }
 
            // local_mesh.write_mesh(state.resolve_output_path(fmt::format("local_mesh_{:04d}.vtu", save_i)), sol);
            // write_mesh(state.resolve_output_path(fmt::format("relaxation_{:04d}.vtu", save_i++)));
        }
 
        static const std::string accept_str =
            fmt::format(fmt::fg(fmt::terminal_color::green), "accept");
        static const std::string reject_str =
            fmt::format(fmt::fg(fmt::terminal_color::yellow), "reject");
        logger().debug(
            "[{:s}] E0={:<10g} E1={:<10g} (E0-E1)={:<11g} tol={:g} local_ndof={:d} n_iters={:d}",
            accept ? accept_str : reject_str, local_energy_before(),
            local_energy_after, abs_diff, dt_sqr * acceptance_tolerance,
            n_free_dof, nl_solver->current_criteria().iterations);
 
        return accept;
    }
 
    // ----------------------------------------------------------------------------------------------
    // Template specializations
    template class PhysicsRemesher<wmtk::TriMesh>;
    template class PhysicsRemesher<wmtk::TetMesh>;
} // namespace polyfem::mesh