PrusaSlicer/src/libslic3r/SLA/Hollowing.cpp

#include <functional>
#include <optional>
#include <numeric>
#include <unordered_set>

#include <libslic3r/OpenVDBUtils.hpp>
#include <libslic3r/TriangleMesh.hpp>
#include <libslic3r/TriangleMeshSlicer.hpp>
#include <libslic3r/SLA/Hollowing.hpp>
#include <libslic3r/AABBMesh.hpp>
#include <libslic3r/ClipperUtils.hpp>
#include <libslic3r/QuadricEdgeCollapse.hpp>
#include <libslic3r/SLA/SupportTreeMesher.hpp>
#include <libslic3r/Execution/ExecutionSeq.hpp>
#include <libslic3r/Model.hpp>

#include <boost/log/trivial.hpp>

#include <libslic3r/MTUtils.hpp>
#include <libslic3r/I18N.hpp>

//! macro used to mark string used at localization,
//! return same string
#define L(s) Slic3r::I18N::translate(s)

namespace Slic3r {
namespace sla {

struct Interior {
    indexed_triangle_set mesh;
    VoxelGridPtr gridptr;

    double iso_surface = 0.;
    double thickness = 0.;
    double full_narrowb = 2.;

    void reset_accessor() const  // This resets the accessor and its cache
    // Not a thread safe call!
    {
        if (gridptr)
            Slic3r::reset_accessor(*gridptr);
    }
};

void InteriorDeleter::operator()(Interior *p)
{
    delete p;
}

indexed_triangle_set &get_mesh(Interior &interior)
{
    return interior.mesh;
}

const indexed_triangle_set &get_mesh(const Interior &interior)
{
    return interior.mesh;
}

const VoxelGrid &get_grid(const Interior &interior)
{
    return *interior.gridptr;
}

VoxelGrid &get_grid(Interior &interior)
{
    return *interior.gridptr;
}

InteriorPtr generate_interior(const VoxelGrid       &vgrid,
                              const HollowingConfig &hc,
                              const JobController   &ctl)
{
    double offset   = hc.min_thickness;
    double D        = hc.closing_distance;
    float  in_range = 1.1f * float(offset + D);
    auto   narrowb  = 3.f / get_voxel_scale(vgrid);
    float  out_range = narrowb;

    if (ctl.stopcondition()) return {};
    else ctl.statuscb(0, L("Hollowing"));

    auto gridptr = dilate_grid(vgrid, out_range, in_range);

    if (ctl.stopcondition()) return {};
    else ctl.statuscb(30, L("Hollowing"));

    double iso_surface = D;
    if (D > EPSILON) {
        in_range = narrowb;
        gridptr = redistance_grid(*gridptr, -(offset + D), narrowb, in_range);

        gridptr = dilate_grid(*gridptr, std::ceil(iso_surface), 0.f);

        out_range = iso_surface;
    } else {
        iso_surface = -offset;
    }

    if (ctl.stopcondition()) return {};
    else ctl.statuscb(70, L("Hollowing"));

    double adaptivity = 0.;
    InteriorPtr interior = InteriorPtr{new Interior{}};

    interior->mesh = grid_to_mesh(*gridptr, iso_surface, adaptivity);
    interior->gridptr = std::move(gridptr);

    if (ctl.stopcondition()) return {};
    else ctl.statuscb(100, L("Hollowing"));

    interior->iso_surface = iso_surface;
    interior->thickness   = offset;
    interior->full_narrowb = (out_range + in_range) / 2.;

    return interior;
}

indexed_triangle_set DrainHole::to_mesh() const
{
    auto r = double(radius);
    auto h = double(height);
    indexed_triangle_set hole = its_make_cylinder(r, h); //sla::cylinder(r, h, steps);
    Eigen::Quaternionf q;
    q.setFromTwoVectors(Vec3f::UnitZ(), normal);
    for(auto& p : hole.vertices) p = q * p + pos;
    
    return hole;
}

bool DrainHole::operator==(const DrainHole &sp) const
{
    return (pos == sp.pos) && (normal == sp.normal) &&
            is_approx(radius, sp.radius) &&
            is_approx(height, sp.height);
}

bool DrainHole::is_inside(const Vec3f& pt) const
{
    Eigen::Hyperplane<float, 3> plane(normal, pos);
    float dist = plane.signedDistance(pt);
    if (dist < float(EPSILON) || dist > height)
        return false;

    Eigen::ParametrizedLine<float, 3> axis(pos, normal);
    if ( axis.squaredDistance(pt) < pow(radius, 2.f))
        return true;

    return false;
}

// Given a line s+dir*t, find parameter t of intersections with the hole
// and the normal (points inside the hole). Outputs through out reference,
// returns true if two intersections were found.
bool DrainHole::get_intersections(const Vec3f& s, const Vec3f& dir,
                                  std::array<std::pair<float, Vec3d>, 2>& out)
                                  const
{
    assert(is_approx(normal.norm(), 1.f));
    const Eigen::ParametrizedLine<float, 3> ray(s, dir.normalized());

    for (size_t i=0; i<2; ++i)
        out[i] = std::make_pair(AABBMesh::hit_result::infty(), Vec3d::Zero());

    const float sqr_radius = pow(radius, 2.f);

    // first check a bounding sphere of the hole:
    Vec3f center = pos+normal*height/2.f;
    float sqr_dist_limit = pow(height/2.f, 2.f) + sqr_radius ;
    if (ray.squaredDistance(center) > sqr_dist_limit)
        return false;

    // The line intersects the bounding sphere, look for intersections with
    // bases of the cylinder.

    size_t found = 0; // counts how many intersections were found
    Eigen::Hyperplane<float, 3> base;
    if (! is_approx(ray.direction().dot(normal), 0.f)) {
        for (size_t i=1; i<=1; --i) {
            Vec3f cylinder_center = pos+i*height*normal;
            if (i == 0) {
                // The hole base can be identical to mesh surface if it is flat
                // let's better move the base outward a bit
                cylinder_center -= EPSILON*normal;
            }
            base = Eigen::Hyperplane<float, 3>(normal, cylinder_center);
            Vec3f intersection = ray.intersectionPoint(base);
            // Only accept the point if it is inside the cylinder base.
            if ((cylinder_center-intersection).squaredNorm() < sqr_radius) {
                out[found].first = ray.intersectionParameter(base);
                out[found].second = (i==0 ? 1. : -1.) * normal.cast<double>();
                ++found;
            }
        }
    }
    else
    {
        // In case the line was perpendicular to the cylinder axis, previous
        // block was skipped, but base will later be assumed to be valid.
        base = Eigen::Hyperplane<float, 3>(normal, pos-EPSILON*normal);
    }

    // In case there is still an intersection to be found, check the wall
    if (found != 2 && ! is_approx(std::abs(ray.direction().dot(normal)), 1.f)) {
        // Project the ray onto the base plane
        Vec3f proj_origin = base.projection(ray.origin());
        Vec3f proj_dir = base.projection(ray.origin()+ray.direction())-proj_origin;
        // save how the parameter scales and normalize the projected direction
        float par_scale = proj_dir.norm();
        proj_dir = proj_dir/par_scale;
        Eigen::ParametrizedLine<float, 3> projected_ray(proj_origin, proj_dir);
        // Calculate point on the secant that's closest to the center
        // and its distance to the circle along the projected line
        Vec3f closest = projected_ray.projection(pos);
        float dist = sqrt((sqr_radius - (closest-pos).squaredNorm()));
        // Unproject both intersections on the original line and check
        // they are on the cylinder and not past it:
        for (int i=-1; i<=1 && found !=2; i+=2) {
            Vec3f isect = closest + i*dist * projected_ray.direction();
            Vec3f to_isect = isect-proj_origin;
            float par = to_isect.norm() / par_scale;
            if (to_isect.normalized().dot(proj_dir.normalized()) < 0.f)
                par *= -1.f;
            Vec3d hit_normal = (pos-isect).normalized().cast<double>();
            isect = ray.pointAt(par);
            // check that the intersection is between the base planes:
            float vert_dist = base.signedDistance(isect);
            if (vert_dist > 0.f && vert_dist < height) {
                out[found].first = par;
                out[found].second = hit_normal;
                ++found;
            }
        }
    }

    // If only one intersection was found, it is some corner case,
    // no intersection will be returned:
    if (found != 2)
        return false;

    // Sort the intersections:
    if (out[0].first > out[1].first)
        std::swap(out[0], out[1]);

    return true;
}

void cut_drainholes(std::vector<ExPolygons> & obj_slices,
                    const std::vector<float> &slicegrid,
                    float                     closing_radius,
                    const sla::DrainHoles &   holes,
                    std::function<void(void)> thr)
{
    TriangleMesh mesh;
    for (const sla::DrainHole &holept : holes)
        mesh.merge(TriangleMesh{holept.to_mesh()});
    
    if (mesh.empty()) return;
    
    std::vector<ExPolygons> hole_slices = slice_mesh_ex(mesh.its, slicegrid, closing_radius, thr);
    
    if (obj_slices.size() != hole_slices.size())
        BOOST_LOG_TRIVIAL(warning)
            << "Sliced object and drain-holes layer count does not match!";

    size_t until = std::min(obj_slices.size(), hole_slices.size());
    
    for (size_t i = 0; i < until; ++i)
        obj_slices[i] = diff_ex(obj_slices[i], hole_slices[i]);
}

void hollow_mesh(TriangleMesh &mesh, const HollowingConfig &cfg, int flags)
{
    InteriorPtr interior = generate_interior(mesh.its, cfg, JobController{});
    if (!interior) return;

    hollow_mesh(mesh, *interior, flags);
}

void hollow_mesh(TriangleMesh &mesh, const Interior &interior, int flags)
{
    if (mesh.empty() || interior.mesh.empty()) return;

    if (flags & hfRemoveInsideTriangles && interior.gridptr)
        remove_inside_triangles(mesh, interior);

    indexed_triangle_set interi = interior.mesh;
    sla::swap_normals(interi);
    TriangleMesh inter{std::move(interi)};

    mesh.merge(inter);
}

// Get the distance of p to the interior's zero iso_surface. Interior should
// have its zero isosurface positioned at offset + closing_distance inwards form
// the model surface.
static double get_distance_raw(const Vec3f &p, const Interior &interior)
{
    assert(interior.gridptr);

    return Slic3r::get_distance_raw(p, *interior.gridptr);
}

struct TriangleBubble { Vec3f center; double R; };

// Return the distance of bubble center to the interior boundary or NaN if the
// triangle is too big to be measured.
static double get_distance(const TriangleBubble &b, const Interior &interior)
{
    double R = b.R;
    double D = 2. * R;
    double Dst = get_distance_raw(b.center, interior);

    return D > interior.full_narrowb ||
           ((Dst - R) < 0. && 2 * R > interior.thickness) ?
                std::nan("") :
                Dst - interior.iso_surface;
}

inline double get_distance(const Vec3f &p, const Interior &interior)
{
    double d = get_distance_raw(p, interior) - interior.iso_surface;
    return d;
}

template<class T>
FloatingOnly<T> get_distance(const Vec<3, T> &p, const Interior &interior)
{
    return get_distance(Vec3f(p.template cast<float>()), interior);
}

// A face that can be divided. Stores the indices into the original mesh if its
// part of that mesh and the vertices it consists of.
enum { NEW_FACE = -1};
struct DivFace {
    Vec3i indx;
    std::array<Vec3f, 3> verts;
    long faceid = NEW_FACE;
    long parent = NEW_FACE;
};

// Divide a face recursively and call visitor on all the sub-faces.
template<class Fn>
void divide_triangle(const DivFace &face, Fn &&visitor)
{
    std::array<Vec3f, 3> edges = {(face.verts[0] - face.verts[1]),
                                  (face.verts[1] - face.verts[2]),
                                  (face.verts[2] - face.verts[0])};

    std::array<size_t, 3> edgeidx = {0, 1, 2};

    std::sort(edgeidx.begin(), edgeidx.end(), [&edges](size_t e1, size_t e2) {
        return edges[e1].squaredNorm() > edges[e2].squaredNorm();
    });

    DivFace child1, child2;

    child1.parent   = face.faceid == NEW_FACE ? face.parent : face.faceid;
    child1.indx(0)  = -1;
    child1.indx(1)  = face.indx(edgeidx[1]);
    child1.indx(2)  = face.indx((edgeidx[1] + 1) % 3);
    child1.verts[0] = (face.verts[edgeidx[0]] + face.verts[(edgeidx[0] + 1) % 3]) / 2.;
    child1.verts[1] = face.verts[edgeidx[1]];
    child1.verts[2] = face.verts[(edgeidx[1] + 1) % 3];

    if (visitor(child1))
        divide_triangle(child1, std::forward<Fn>(visitor));

    child2.parent   = face.faceid == NEW_FACE ? face.parent : face.faceid;
    child2.indx(0)  = -1;
    child2.indx(1)  = face.indx(edgeidx[2]);
    child2.indx(2)  = face.indx((edgeidx[2] + 1) % 3);
    child2.verts[0] = child1.verts[0];
    child2.verts[1] = face.verts[edgeidx[2]];
    child2.verts[2] = face.verts[(edgeidx[2] + 1) % 3];

    if (visitor(child2))
        divide_triangle(child2, std::forward<Fn>(visitor));
}

void remove_inside_triangles(TriangleMesh &mesh, const Interior &interior,
                             const std::vector<bool> &exclude_mask)
{
    enum TrPos { posInside, posTouch, posOutside };

    auto &faces       = mesh.its.indices;
    auto &vertices    = mesh.its.vertices;
    auto bb           = mesh.bounding_box();

    bool use_exclude_mask = faces.size() == exclude_mask.size();
    auto is_excluded = [&exclude_mask, use_exclude_mask](size_t face_id) {
        return use_exclude_mask && exclude_mask[face_id];
    };

    // TODO: Parallel mode not working yet
    constexpr auto &exec_policy = ex_seq;

    // Info about the needed modifications on the input mesh.
    struct MeshMods {

        // Just a thread safe wrapper for a vector of triangles.
        struct {
            std::vector<std::array<Vec3f, 3>> data;
            execution::SpinningMutex<decltype(exec_policy)> mutex;

            void emplace_back(const std::array<Vec3f, 3> &pts)
            {
                std::lock_guard lk{mutex};
                data.emplace_back(pts);
            }

            size_t size() const { return data.size(); }
            const std::array<Vec3f, 3>& operator[](size_t idx) const
            {
                return data[idx];
            }

        } new_triangles;

        // A vector of bool for all faces signaling if it needs to be removed
        // or not.
        std::vector<bool> to_remove;

        MeshMods(const TriangleMesh &mesh):
            to_remove(mesh.its.indices.size(), false) {}

        // Number of triangles that need to be removed.
        size_t to_remove_cnt() const
        {
            return std::accumulate(to_remove.begin(), to_remove.end(), size_t(0));
        }

    } mesh_mods{mesh};

    // Must return true if further division of the face is needed.
    auto divfn = [&interior, bb, &mesh_mods](const DivFace &f) {
        BoundingBoxf3 facebb { f.verts.begin(), f.verts.end() };

        // Face is certainly outside the cavity
        if (! facebb.intersects(bb) && f.faceid != NEW_FACE) {
            return false;
        }

        TriangleBubble bubble{facebb.center().cast<float>(), facebb.radius()};

        double D = get_distance(bubble, interior);
        double R = bubble.R;

        if (std::isnan(D)) // The distance cannot be measured, triangle too big
            return true;

        // Distance of the bubble wall to the interior wall. Negative if the
        // bubble is overlapping with the interior
        double bubble_distance = D - R;

        // The face is crossing the interior or inside, it must be removed and
        // parts of it re-added, that are outside the interior
        if (bubble_distance < 0.) {
            if (f.faceid != NEW_FACE)
                mesh_mods.to_remove[f.faceid] = true;

            if (f.parent != NEW_FACE) // Top parent needs to be removed as well
                mesh_mods.to_remove[f.parent] = true;

            // If the outside part is between the interior and the exterior
            // (inside the wall being invisible), no further division is needed.
            if ((R + D) < interior.thickness)
                return false;

            return true;
        } else if (f.faceid == NEW_FACE) {
            // New face completely outside needs to be re-added.
            mesh_mods.new_triangles.emplace_back(f.verts);
        }

        return false;
    };

    interior.reset_accessor();

    execution::for_each(
        exec_policy, size_t(0), faces.size(),
        [&](size_t face_idx) {
            const Vec3i &face = faces[face_idx];

            // If the triangle is excluded, we need to keep it.
            if (is_excluded(face_idx)) return;

            std::array<Vec3f, 3> pts = {vertices[face(0)], vertices[face(1)],
                                        vertices[face(2)]};

            BoundingBoxf3 facebb{pts.begin(), pts.end()};

            // Face is certainly outside the cavity
            if (!facebb.intersects(bb)) return;

            DivFace df{face, pts, long(face_idx)};

            if (divfn(df)) divide_triangle(df, divfn);
        },
        execution::max_concurrency(exec_policy)
    );

    auto new_faces = reserve_vector<Vec3i>(faces.size() +
                                           mesh_mods.new_triangles.size());

    for (size_t face_idx = 0; face_idx < faces.size(); ++face_idx) {
        if (!mesh_mods.to_remove[face_idx])
            new_faces.emplace_back(faces[face_idx]);
    }

    for(size_t i = 0; i < mesh_mods.new_triangles.size(); ++i) {
        size_t o = vertices.size();
        vertices.emplace_back(mesh_mods.new_triangles[i][0]);
        vertices.emplace_back(mesh_mods.new_triangles[i][1]);
        vertices.emplace_back(mesh_mods.new_triangles[i][2]);
        new_faces.emplace_back(int(o), int(o + 1), int(o + 2));
    }

    BOOST_LOG_TRIVIAL(info)
            << "Trimming: " << mesh_mods.to_remove_cnt() << " triangles removed";
    BOOST_LOG_TRIVIAL(info)
            << "Trimming: " << mesh_mods.new_triangles.size() << " triangles added";

    faces.swap(new_faces);
    new_faces = {};

    mesh = TriangleMesh{mesh.its};
    //FIXME do we want to repair the mesh? Are there duplicate vertices or flipped triangles?
}

struct FaceHash {

    // A 64 bit number's max hex digits
    static constexpr size_t MAX_NUM_CHARS = 16;

    // A hash is created for each triangle to be identifiable. The hash uses
    // only the triangle's geometric traits, not the index in a particular mesh.
    std::unordered_set<std::string> facehash;

    // Returns the string in reverse, but that is ok for hashing
    static std::array<char, MAX_NUM_CHARS + 1> to_chars(int64_t val)
    {
        std::array<char, MAX_NUM_CHARS + 1> ret;

        static const constexpr char * Conv = "0123456789abcdef";

        auto ptr = ret.begin();
        auto uval = static_cast<uint64_t>(std::abs(val));
        while (uval) {
            *ptr = Conv[uval & 0xf];
            ++ptr;
            uval = uval >> 4;
        }
        if (val < 0) { *ptr = '-'; ++ptr; }
        *ptr = '\0'; // C style string ending

        return ret;
    }

    static std::string hash(const Vec<3, int64_t> &v)
    {
        std::string ret;
        ret.reserve(3 * MAX_NUM_CHARS);

        for (auto val : v)
            ret += to_chars(val).data();

        return ret;
    }

    static std::string facekey(const Vec3i &face, const std::vector<Vec3f> &vertices)
    {
        // Scale to integer to avoid floating points
        std::array<Vec<3, int64_t>, 3> pts = {
            scaled<int64_t>(vertices[face(0)]),
            scaled<int64_t>(vertices[face(1)]),
            scaled<int64_t>(vertices[face(2)])
        };

        // Get the first two sides of the triangle, do a cross product and move
        // that vector to the center of the triangle. This encodes all
        // information to identify an identical triangle at the same position.
        Vec<3, int64_t> a = pts[0] - pts[2], b = pts[1] - pts[2];
        Vec<3, int64_t> c = a.cross(b) + (pts[0] + pts[1] + pts[2]) / 3;

        // Return a concatenated string representation of the coordinates
        return hash(c);
    }

    FaceHash (const indexed_triangle_set &its): facehash(its.indices.size())
    {
        for (const Vec3i &face : its.indices)
            facehash.insert(facekey(face, its.vertices));
    }

    bool find(const std::string &key)
    {
        auto it = facehash.find(key);
        return it != facehash.end();
    }
};


static void exclude_neighbors(const Vec3i                &face,
                              std::vector<bool>          &mask,
                              const indexed_triangle_set &its,
                              const VertexFaceIndex      &index,
                              size_t                      recursions)
{
    for (int i = 0; i < 3; ++i) {
        const auto &neighbors_range = index[face(i)];
        for (size_t fi_n : neighbors_range) {
            mask[fi_n] = true;
            if (recursions > 0)
                exclude_neighbors(its.indices[fi_n], mask, its, index, recursions - 1);
        }
    }
}

std::vector<bool> create_exclude_mask(const indexed_triangle_set   &its,
                                      const Interior               &interior,
                                      const std::vector<DrainHole> &holes)
{
    FaceHash interior_hash{sla::get_mesh(interior)};

    std::vector<bool> exclude_mask(its.indices.size(), false);

    VertexFaceIndex neighbor_index{its};

    for (size_t fi = 0; fi < its.indices.size(); ++fi) {
        auto &face = its.indices[fi];

        if (interior_hash.find(FaceHash::facekey(face, its.vertices))) {
            exclude_mask[fi] = true;
            continue;
        }

        if (exclude_mask[fi]) {
            exclude_neighbors(face, exclude_mask, its, neighbor_index, 1);
            continue;
        }

        // Lets deal with the holes. All the triangles of a hole and all the
        // neighbors of these triangles need to be kept. The neigbors were
        // created by CGAL mesh boolean operation that modified the original
        // interior inside the input mesh to contain the holes.
        Vec3d tr_center = (
                              its.vertices[face(0)] +
                              its.vertices[face(1)] +
                              its.vertices[face(2)]
                              ).cast<double>() / 3.;

        // If the center is more than half a mm inside the interior,
        // it cannot possibly be part of a hole wall.
        if (sla::get_distance(tr_center, interior) < -0.5)
            continue;

        Vec3f U = its.vertices[face(1)] - its.vertices[face(0)];
        Vec3f V = its.vertices[face(2)] - its.vertices[face(0)];
        Vec3f C = U.cross(V);
        Vec3f face_normal = C.normalized();

        for (const sla::DrainHole &dh : holes) {
            if (dh.failed) continue;

            Vec3d dhpos = dh.pos.cast<double>();
            Vec3d dhend = dhpos + dh.normal.cast<double>() * dh.height;

            Linef3 holeaxis{dhpos, dhend};

            double D_hole_center = line_alg::distance_to(holeaxis, tr_center);
            double D_hole        = std::abs(D_hole_center - dh.radius);
            float dot            = dh.normal.dot(face_normal);

            // Empiric tolerances for center distance and normals angle.
            // For triangles that are part of a hole wall the angle of
            // triangle normal and the hole axis is around 90 degrees,
            // so the dot product is around zero.
            double D_tol = dh.radius / sla::DrainHole::steps;
            float normal_angle_tol = 1.f / sla::DrainHole::steps;

            if (D_hole < D_tol && std::abs(dot) < normal_angle_tol) {
                exclude_mask[fi] = true;
                exclude_neighbors(face, exclude_mask, its, neighbor_index, 1);
            }
        }
    }

    return exclude_mask;
}

DrainHoles transformed_drainhole_points(const ModelObject &mo,
                                        const Transform3d &trafo)
{
    auto pts = mo.sla_drain_holes;
//    const Transform3d& vol_trafo = mo.volumes.front()->get_transformation().get_matrix();
    const Geometry::Transformation trans(trafo /** vol_trafo*/);
    const Transform3d& tr = trans.get_matrix();
    for (sla::DrainHole &hl : pts) {
        Vec3d pos = hl.pos.cast<double>();
        Vec3d nrm = hl.normal.cast<double>();

        pos = tr * pos;
        nrm = tr * nrm - tr.translation();

        // Now shift the hole a bit above the object and make it deeper to
        // compensate for it. This is to avoid problems when the hole is placed
        // on (nearly) flat surface.
        pos -= nrm.normalized() * sla::HoleStickOutLength;

        hl.pos = pos.cast<float>();
        hl.normal = nrm.cast<float>();
        hl.height += sla::HoleStickOutLength;
    }

    return pts;
}

double get_voxel_scale(double mesh_volume, const HollowingConfig &hc)
{
    static constexpr double MIN_SAMPLES_IN_WALL = 3.5;
    static constexpr double MAX_OVERSAMPL = 8.;
    static constexpr double UNIT_VOLUME   = 500000; // empiric

    // I can't figure out how to increase the grid resolution through openvdb
    // API so the model will be scaled up before conversion and the result
    // scaled down. Voxels have a unit size. If I set voxelSize smaller, it
    // scales the whole geometry down, and doesn't increase the number of
    // voxels.
    //
    // First an allowed range for voxel scale is determined from an initial
    // range of <MIN_SAMPLES_IN_WALL, MAX_OVERSAMPL>. The final voxel scale is
    // then chosen from this range using the 'quality:<0, 1>' parameter.
    // The minimum can be lowered if the wall thickness is great enough and
    // the maximum is lowered if the model volume very big.

    double sc_divider    = std::max(1.0, (mesh_volume / UNIT_VOLUME));
    double min_oversampl = std::max(MIN_SAMPLES_IN_WALL / hc.min_thickness, 1.);
    double max_oversampl_scaled = std::max(min_oversampl, MAX_OVERSAMPL / sc_divider);
    auto   voxel_scale          = min_oversampl + (max_oversampl_scaled - min_oversampl) * hc.quality;

    BOOST_LOG_TRIVIAL(debug) << "Hollowing: max oversampl will be: " << max_oversampl_scaled;
    BOOST_LOG_TRIVIAL(debug) << "Hollowing: voxel scale will be: " << voxel_scale;
    BOOST_LOG_TRIVIAL(debug) << "Hollowing: mesh volume is: " << mesh_volume;

    return voxel_scale;
}

}} // namespace Slic3r::sla