BambuStudio/libslic3r/PrintObject.cpp

#include "Exception.hpp"
#include "Print.hpp"
#include "BoundingBox.hpp"
#include "ClipperUtils.hpp"
#include "ElephantFootCompensation.hpp"
#include "Geometry.hpp"
#include "I18N.hpp"
#include "Layer.hpp"
#include "MutablePolygon.hpp"
#include "Support/SupportMaterial.hpp"
#include "Support/TreeSupport.hpp"
#include "Surface.hpp"
#include "Slicing.hpp"
#include "Tesselate.hpp"
#include "TriangleMeshSlicer.hpp"
#include "Utils.hpp"
#include "Fill/FillAdaptive.hpp"
#include "Fill/FillLightning.hpp"
#include "Format/STL.hpp"
#include "InternalBridgeDetector.hpp"
#include "AABBTreeLines.hpp"

#include <float.h>
#include <string_view>
#include <utility>

#include <boost/log/trivial.hpp>

#include <tbb/parallel_for.h>
#include <tbb/concurrent_vector.h>

#include <Shiny/Shiny.h>

#include "format.hpp"

using namespace std::literals;

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

#ifdef SLIC3R_DEBUG_SLICE_PROCESSING
#define SLIC3R_DEBUG
#endif

// #define SLIC3R_DEBUG
// Make assert active if SLIC3R_DEBUG
#ifdef SLIC3R_DEBUG
    #undef NDEBUG
    #define DEBUG
    #define _DEBUG
    #include "SVG.hpp"
    #undef assert
    #include <cassert>
#endif

namespace Slic3r {

// Constructor is called from the main thread, therefore all Model / ModelObject / ModelIntance data are valid.
PrintObject::PrintObject(Print* print, ModelObject* model_object, const Transform3d& trafo, PrintInstances&& instances) :
    PrintObjectBaseWithState(print, model_object),
    m_trafo(trafo),
    // BBS
    m_tree_support_preview_cache(nullptr)
{
    // Compute centering offet to be applied to our meshes so that we work with smaller coordinates
    // requiring less bits to represent Clipper coordinates.

	// Snug bounding box of a rotated and scaled object by the 1st instantion, without the instance translation applied.
	// All the instances share the transformation matrix with the exception of translation in XY and rotation by Z,
	// therefore a bounding box from 1st instance of a ModelObject is good enough for calculating the object center,
	// snug height and an approximate bounding box in XY.
    BoundingBoxf3  bbox        = model_object->raw_bounding_box();
    Vec3d 		   bbox_center = bbox.center();
    
	// We may need to rotate the bbox / bbox_center from the original instance to the current instance.
	double z_diff = Geometry::rotation_diff_z(model_object->instances.front()->get_rotation(), instances.front().model_instance->get_rotation());
	if (std::abs(z_diff) > EPSILON) {
		auto z_rot  = Eigen::AngleAxisd(z_diff, Vec3d::UnitZ());
		bbox 		= bbox.transformed(Transform3d(z_rot));
		bbox_center = (z_rot * bbox_center).eval();
	}

    // Center of the transformed mesh (without translation).
    m_center_offset = Point::new_scale(bbox_center.x(), bbox_center.y());
    // Size of the transformed mesh. This bounding may not be snug in XY plane, but it is snug in Z.
    m_size = (bbox.size() * (1. / SCALING_FACTOR)).cast<coord_t>();
    m_max_z = scaled(model_object->instance_bounding_box(0).max(2));

    this->set_instances(std::move(instances));
}

PrintObject::~PrintObject()
{
    BOOST_LOG_TRIVIAL(info) << __FUNCTION__ << boost::format(": this=%1%, m_shared_object %2%")%this%m_shared_object;
    if (m_shared_regions && -- m_shared_regions->m_ref_cnt == 0) delete m_shared_regions;
    clear_layers();
    clear_support_layers();
}

PrintBase::ApplyStatus PrintObject::set_instances(PrintInstances &&instances)
{
    for (PrintInstance &i : instances)
    	// Add the center offset, which will be subtracted from the mesh when slicing.
    	i.shift += m_center_offset;
    // Invalidate and set copies.
    PrintBase::ApplyStatus status = PrintBase::APPLY_STATUS_UNCHANGED;
    bool equal_length = instances.size() == m_instances.size();
    bool equal = equal_length && std::equal(instances.begin(), instances.end(), m_instances.begin(),
    	[](const PrintInstance& lhs, const PrintInstance& rhs) { return lhs.model_instance == rhs.model_instance && lhs.shift == rhs.shift; });
    if (! equal) {
        status = PrintBase::APPLY_STATUS_CHANGED;
        if (m_print->invalidate_steps({ psSkirtBrim, psGCodeExport }) ||
            (! equal_length && m_print->invalidate_step(psWipeTower)))
            status = PrintBase::APPLY_STATUS_INVALIDATED;
        m_instances = std::move(instances);
	    for (PrintInstance &i : m_instances)
	    	i.print_object = this;
    }
    return status;
}

std::vector<std::reference_wrapper<const PrintRegion>> PrintObject::all_regions() const
{
    std::vector<std::reference_wrapper<const PrintRegion>> out;
    out.reserve(m_shared_regions->all_regions.size());
    for (const std::unique_ptr<Slic3r::PrintRegion> &region : m_shared_regions->all_regions)
        out.emplace_back(*region.get());
    return out;
}

// 1) Merges typed region slices into stInternal type.
// 2) Increases an "extra perimeters" counter at region slices where needed.
// 3) Generates perimeters, gap fills and fill regions (fill regions of type stInternal).
void PrintObject::make_perimeters()
{
    // prerequisites
    this->slice();

    if (! this->set_started(posPerimeters))
        return;

    m_print->set_status(15, L("Generating walls"));
    BOOST_LOG_TRIVIAL(info) << "Generating walls..." << log_memory_info();

    // Revert the typed slices into untyped slices.
    if (m_typed_slices) {
        for (Layer *layer : m_layers) {
            layer->restore_untyped_slices();
            m_print->throw_if_canceled();
        }
        m_typed_slices = false;
    }

    // compare each layer to the one below, and mark those slices needing
    // one additional inner perimeter, like the top of domed objects-

    // this algorithm makes sure that at least one perimeter is overlapping
    // but we don't generate any extra perimeter if fill density is zero, as they would be floating
    // inside the object - infill_only_where_needed should be the method of choice for printing
    // hollow objects
    for (size_t region_id = 0; region_id < this->num_printing_regions(); ++ region_id) {
        const PrintRegion &region = this->printing_region(region_id);
        //BBS: remove extra_perimeters, always false
        //if (! region.config().extra_perimeters || region.config().wall_loops == 0 || region.config().sparse_infill_density == 0 || this->layer_count() < 2)
            continue;

        BOOST_LOG_TRIVIAL(debug) << "Generating extra perimeters for region " << region_id << " in parallel - start";
        tbb::parallel_for(
            tbb::blocked_range<size_t>(0, m_layers.size() - 1),
            [this, &region, region_id](const tbb::blocked_range<size_t>& range) {
                for (size_t layer_idx = range.begin(); layer_idx < range.end(); ++ layer_idx) {
                    m_print->throw_if_canceled();
                    LayerRegion &layerm                     = *m_layers[layer_idx]->get_region(region_id);
                    const LayerRegion &upper_layerm         = *m_layers[layer_idx+1]->get_region(region_id);
                    const Polygons upper_layerm_polygons    = to_polygons(upper_layerm.slices.surfaces);
                    // Filter upper layer polygons in intersection_ppl by their bounding boxes?
                    // my $upper_layerm_poly_bboxes= [ map $_->bounding_box, @{$upper_layerm_polygons} ];
                    const double total_loop_length      = total_length(upper_layerm_polygons);
                    const coord_t perimeter_spacing     = layerm.flow(frPerimeter).scaled_spacing();
                    const Flow ext_perimeter_flow       = layerm.flow(frExternalPerimeter);
                    const coord_t ext_perimeter_width   = ext_perimeter_flow.scaled_width();
                    const coord_t ext_perimeter_spacing = ext_perimeter_flow.scaled_spacing();

                    for (Surface &slice : layerm.slices.surfaces) {
                        for (;;) {
                            // compute the total thickness of perimeters
                            const coord_t perimeters_thickness = ext_perimeter_width/2 + ext_perimeter_spacing/2
                                + (region.config().wall_loops-1 + slice.extra_perimeters) * perimeter_spacing;
                            // define a critical area where we don't want the upper slice to fall into
                            // (it should either lay over our perimeters or outside this area)
                            const coord_t critical_area_depth = coord_t(perimeter_spacing * 1.5);
                            const Polygons critical_area = diff(
                                offset(slice.expolygon, float(- perimeters_thickness)),
                                offset(slice.expolygon, float(- perimeters_thickness - critical_area_depth))
                            );
                            // check whether a portion of the upper slices falls inside the critical area
                            const Polylines intersection = intersection_pl(to_polylines(upper_layerm_polygons), critical_area);
                            // only add an additional loop if at least 30% of the slice loop would benefit from it
                            if (total_length(intersection) <=  total_loop_length*0.3)
                                break;
                            /*
                            if (0) {
                                require "Slic3r/SVG.pm";
                                Slic3r::SVG::output(
                                    "extra.svg",
                                    no_arrows   => 1,
                                    expolygons  => union_ex($critical_area),
                                    polylines   => [ map $_->split_at_first_point, map $_->p, @{$upper_layerm->slices} ],
                                );
                            }
                            */
                            ++ slice.extra_perimeters;
                        }
                        #ifdef DEBUG
                            if (slice.extra_perimeters > 0)
                                printf("  adding %d more perimeter(s) at layer %zu\n", slice.extra_perimeters, layer_idx);
                        #endif
                    }
                }
            });
        m_print->throw_if_canceled();
        BOOST_LOG_TRIVIAL(debug) << "Generating extra perimeters for region " << region_id << " in parallel - end";
    }

    BOOST_LOG_TRIVIAL(debug) << "Generating perimeters in parallel - start";
    tbb::parallel_for(
        tbb::blocked_range<size_t>(0, m_layers.size()),
        [this](const tbb::blocked_range<size_t>& range) {
            for (size_t layer_idx = range.begin(); layer_idx < range.end(); ++ layer_idx) {
                m_print->throw_if_canceled();
                m_layers[layer_idx]->make_perimeters();
            }
        }
    );
    m_print->throw_if_canceled();
    BOOST_LOG_TRIVIAL(debug) << "Generating perimeters in parallel - end";

    this->set_done(posPerimeters);
}

void PrintObject::prepare_infill()
{
    if (! this->set_started(posPrepareInfill))
        return;
    m_print->set_status(25, L("Generating infill regions"));
    if (m_typed_slices) {
        // To improve robustness of detect_surfaces_type() when reslicing (working with typed slices), see GH issue #7442.
        // The preceding step (perimeter generator) only modifies extra_perimeters and the extra perimeters are only used by discover_vertical_shells()
        // with more than a single region. If this step does not use Surface::extra_perimeters or Surface::extra_perimeters is always zero, it is safe
        // to reset to the untyped slices before re-runnning detect_surfaces_type().
        //为了在重新切片（使用类型化切片）时提高detect_surfaces_type（）的鲁棒性，请参阅GH问题#7442。
        //前面的步骤（周长生成器）仅修改extra_perimeters，并且额外的周长仅由具有多个区域的discover_vertical_shell（）使用。
        //如果此步骤不使用Surface：：extra_perimeters或Surface：：extra_perimeter始终为零，则可以在重新运行detect_surfaces_type（）之前重置为非类型切片。
        for (Layer* layer : m_layers) {
            layer->restore_untyped_slices_no_extra_perimeters();
            m_print->throw_if_canceled();
        }
    }

    // This will assign a type (top/bottom/internal) to $layerm->slices.
    // Then the classifcation of $layerm->slices is transfered onto
    // the $layerm->fill_surfaces by clipping $layerm->fill_surfaces
    // by the cummulative area of the previous $layerm->fill_surfaces.
    this->detect_surfaces_type();
    m_print->throw_if_canceled();

    // Also tiny stInternal surfaces are turned to stInternalSolid.
    BOOST_LOG_TRIVIAL(info) << "Preparing fill surfaces..." << log_memory_info();
    for (auto *layer : m_layers)
        for (auto *region : layer->m_regions) {
            region->prepare_fill_surfaces();
            m_print->throw_if_canceled();
        }

    // Add solid fills to ensure the shell vertical thickness.
    this->discover_vertical_shells();
    m_print->throw_if_canceled();

    // this will detect bridges and reverse bridges
    // and rearrange top/bottom/internal surfaces
    // It produces enlarged overlapping bridging areas.
    //
    // 1) stBottomBridge / stBottom infill is grown by 3mm and clipped by the total infill area. Bridges are detected. The areas may overlap.
    // 2) stTop is grown by 3mm and clipped by the grown bottom areas. The areas may overlap.
    // 3) Clip the internal surfaces by the grown top/bottom surfaces.
    // 4) Merge surfaces with the same style. This will mostly get rid of the overlaps.
    //FIXME This does not likely merge surfaces, which are supported by a material with different colors, but same properties.
    this->process_external_surfaces();
    m_print->throw_if_canceled();


    // Debugging output.
#ifdef SLIC3R_DEBUG_SLICE_PROCESSING
    for (size_t region_id = 0; region_id < this->num_printing_regions(); ++ region_id) {
        for (const Layer *layer : m_layers) {
            LayerRegion *layerm = layer->m_regions[region_id];
            layerm->export_region_slices_to_svg_debug("6_discover_vertical_shells-final");
            layerm->export_region_fill_surfaces_to_svg_debug("6_discover_vertical_shells-final");
        } // for each layer
    } // for each region
#endif /* SLIC3R_DEBUG_SLICE_PROCESSING */

    // Detect, which fill surfaces are near external layers.
    // They will be split in internal and internal-solid surfaces.
    // The purpose is to add a configurable number of solid layers to support the TOP surfaces
    // and to add a configurable number of solid layers above the BOTTOM / BOTTOMBRIDGE surfaces
    // to close these surfaces reliably.
    //FIXME Vojtech: Is this a good place to add supporting infills below sloping perimeters?
    this->discover_horizontal_shells();
    m_print->throw_if_canceled();

#ifdef SLIC3R_DEBUG_SLICE_PROCESSING
    for (size_t region_id = 0; region_id < this->num_printing_regions(); ++ region_id) {
        for (const Layer *layer : m_layers) {
            LayerRegion *layerm = layer->m_regions[region_id];
            layerm->export_region_slices_to_svg_debug("7_discover_horizontal_shells-final");
            layerm->export_region_fill_surfaces_to_svg_debug("7_discover_horizontal_shells-final");
        } // for each layer
    } // for each region
#endif /* SLIC3R_DEBUG_SLICE_PROCESSING */


    // Only active if config->infill_only_where_needed. This step trims the sparse infill,
    // so it acts as an internal support. It maintains all other infill types intact.
    // Here the internal surfaces and perimeters have to be supported by the sparse infill.
    //FIXME The surfaces are supported by a sparse infill, but the sparse infill is only as large as the area to support.
    // Likely the sparse infill will not be anchored correctly, so it will not work as intended.
    // Also one wishes the perimeters to be supported by a full infill.
    this->clip_fill_surfaces();
    m_print->throw_if_canceled();

#ifdef SLIC3R_DEBUG_SLICE_PROCESSING
    for (size_t region_id = 0; region_id < this->num_printing_regions(); ++ region_id) {
        for (const Layer *layer : m_layers) {
            LayerRegion *layerm = layer->m_regions[region_id];
            layerm->export_region_slices_to_svg_debug("8_clip_surfaces-final");
            layerm->export_region_fill_surfaces_to_svg_debug("8_clip_surfaces-final");
        } // for each layer
    } // for each region
#endif /* SLIC3R_DEBUG_SLICE_PROCESSING */

    // the following step needs to be done before combination because it may need
    // to remove only half of the combined infill
    this->bridge_over_infill();
    m_print->throw_if_canceled();

    // combine fill surfaces to honor the "infill every N layers" option
    this->combine_infill();
    m_print->throw_if_canceled();

#ifdef SLIC3R_DEBUG_SLICE_PROCESSING
    for (size_t region_id = 0; region_id < this->num_printing_regions(); ++ region_id) {
        for (const Layer *layer : m_layers) {
            LayerRegion *layerm = layer->m_regions[region_id];
            layerm->export_region_slices_to_svg_debug("9_prepare_infill-final");
            layerm->export_region_fill_surfaces_to_svg_debug("9_prepare_infill-final");
        } // for each layer
    } // for each region
    for (const Layer *layer : m_layers) {
        layer->export_region_slices_to_svg_debug("9_prepare_infill-final");
        layer->export_region_fill_surfaces_to_svg_debug("9_prepare_infill-final");
    } // for each layer
#endif /* SLIC3R_DEBUG_SLICE_PROCESSING */

    this->set_done(posPrepareInfill);
}

void PrintObject::infill()
{
    // prerequisites
    this->prepare_infill();

    if (this->set_started(posInfill)) {
        m_print->set_status(35, L("Generating infill toolpath"));

        const auto& adaptive_fill_octree = this->m_adaptive_fill_octrees.first;
        const auto& support_fill_octree = this->m_adaptive_fill_octrees.second;

        BOOST_LOG_TRIVIAL(debug) << "Filling layers in parallel - start";
        tbb::parallel_for(
            tbb::blocked_range<size_t>(0, m_layers.size()),
            [this, &adaptive_fill_octree = adaptive_fill_octree, &support_fill_octree = support_fill_octree](const tbb::blocked_range<size_t>& range) {
                for (size_t layer_idx = range.begin(); layer_idx < range.end(); ++ layer_idx) {
                    m_print->throw_if_canceled();
                    m_layers[layer_idx]->make_fills(adaptive_fill_octree.get(), support_fill_octree.get(), this->m_lightning_generator.get());
                }
            }
        );
        m_print->throw_if_canceled();
        BOOST_LOG_TRIVIAL(debug) << "Filling layers in parallel - end";
        /*  we could free memory now, but this would make this step not idempotent
        ### $_->fill_surfaces->clear for map @{$_->regions}, @{$object->layers};
        */
        this->set_done(posInfill);
    }
}

void PrintObject::ironing()
{
    if (this->set_started(posIroning)) {
        BOOST_LOG_TRIVIAL(debug) << "Ironing in parallel - start";
        tbb::parallel_for(
            // Ironing starting with layer 0 to support ironing all surfaces.
            tbb::blocked_range<size_t>(0, m_layers.size()),
            [this](const tbb::blocked_range<size_t>& range) {
                for (size_t layer_idx = range.begin(); layer_idx < range.end(); ++ layer_idx) {
                    m_print->throw_if_canceled();
                    m_layers[layer_idx]->make_ironing();
                }
            }
        );
        m_print->throw_if_canceled();
        BOOST_LOG_TRIVIAL(debug) << "Ironing in parallel - end";
        this->set_done(posIroning);
    }
}

// BBS
void PrintObject::clear_overhangs_for_lift()
{
    if (!m_shared_object) {
        for (Layer* l : m_layers)
            l->loverhangs.clear();
    }
}

static const float g_min_overhang_percent_for_lift = 0.3f;

void PrintObject::detect_overhangs_for_lift()
{
    if (this->set_started(posDetectOverhangsForLift)) {
        const float min_overlap = m_config.line_width * g_min_overhang_percent_for_lift;
        size_t num_layers = this->layer_count();
        size_t num_raft_layers = m_slicing_params.raft_layers();

        m_print->set_status(71, L("Detect overhangs for auto-lift"));

        this->clear_overhangs_for_lift();

        tbb::spin_mutex layer_storage_mutex;
        tbb::parallel_for(tbb::blocked_range<size_t>(num_raft_layers + 1, num_layers),
            [this, min_overlap](const tbb::blocked_range<size_t>& range)
            {
                for (size_t layer_id = range.begin(); layer_id < range.end(); ++layer_id) {
                    Layer& layer = *m_layers[layer_id];
                    Layer& lower_layer = *layer.lower_layer;

                    ExPolygons overhangs = diff_ex(layer.lslices, offset_ex(lower_layer.lslices, scale_(min_overlap)));
                    layer.loverhangs = std::move(offset2_ex(overhangs, -0.1f * scale_(m_config.line_width), 0.1f * scale_(m_config.line_width)));
                    layer.loverhangs_bbox = get_extents(layer.loverhangs);
                }
            });
        this->set_done(posDetectOverhangsForLift);
    }
}

void PrintObject::generate_support_material()
{
    if (this->set_started(posSupportMaterial)) {
        this->clear_support_layers();

        if(!has_support() && !m_print->get_no_check_flag()) {
            // BBS: pop a warning if objects have significant amount of overhangs but support material is not enabled
            // Note: we also need to pop warning if support is disabled and only raft is enabled
            m_print->set_status(50, L("Checking support necessity"));
            typedef std::chrono::high_resolution_clock clock_;
            typedef std::chrono::duration<double, std::ratio<1> > second_;
            std::chrono::time_point<clock_> t0{ clock_::now() };

            SupportNecessaryType sntype = this->is_support_necessary();

            double duration{ std::chrono::duration_cast<second_>(clock_::now() - t0).count() };
            BOOST_LOG_TRIVIAL(info) << std::fixed << std::setprecision(0) << "is_support_necessary takes " << duration << " secs.";

            if (sntype != NoNeedSupp) {
                std::map<SupportNecessaryType, std::string> reasons = {
                    {SharpTail,L("floating regions")},
                    {Cantilever,L("floating cantilever")},
                    {LargeOverhang,L("large overhangs")} };
                std::string warning_message = Slic3r::format(L("It seems object %s has %s. Please re-orient the object or enable support generation."),
                    this->model_object()->name, reasons[sntype]);
                this->active_step_add_warning(PrintStateBase::WarningLevel::NON_CRITICAL, warning_message, PrintStateBase::SlicingNeedSupportOn);
            }

#if 0
            // Printing without supports. Empty layer means some objects or object parts are levitating,
            // therefore they cannot be printed without supports.
            for (const Layer *layer : m_layers)
                if (layer->empty())
                    throw Slic3r::SlicingError("Levitating objects cannot be printed without supports.");
#endif
        }

        if ((this->has_support() && m_layers.size() > 1) || (this->has_raft() && !m_layers.empty())) {
            m_print->set_status(50, L("Generating support"));

            this->_generate_support_material();
            m_print->throw_if_canceled();
        }
        this->set_done(posSupportMaterial);
    }
}

void PrintObject::simplify_extrusion_path()
{
    if (this->set_started(posSimplifyWall)) {
        m_print->set_status(75, L("Optimizing toolpath"));
        BOOST_LOG_TRIVIAL(debug) << "Simplify wall extrusion path of object in parallel - start";
        //BBS: walls
        tbb::parallel_for(
            tbb::blocked_range<size_t>(0, m_layers.size()),
            [this](const tbb::blocked_range<size_t>& range) {
                for (size_t layer_idx = range.begin(); layer_idx < range.end(); ++ layer_idx) {
                    m_print->throw_if_canceled();
                    m_layers[layer_idx]->simplify_wall_extrusion_path();
                }
            }
        );
        m_print->throw_if_canceled();
        BOOST_LOG_TRIVIAL(debug) << "Simplify wall extrusion path of object in parallel - end";
        this->set_done(posSimplifyWall);
    }

    if (this->set_started(posSimplifyInfill)) {
        m_print->set_status(75, L("Optimizing toolpath"));
        BOOST_LOG_TRIVIAL(debug) << "Simplify infill extrusion path of object in parallel - start";
        //BBS: infills
        tbb::parallel_for(
            tbb::blocked_range<size_t>(0, m_layers.size()),
            [this](const tbb::blocked_range<size_t>& range) {
                for (size_t layer_idx = range.begin(); layer_idx < range.end(); ++layer_idx) {
                    m_print->throw_if_canceled();
                    m_layers[layer_idx]->simplify_infill_extrusion_path();
                }
            }
        );
        m_print->throw_if_canceled();
        BOOST_LOG_TRIVIAL(debug) << "Simplify infill extrusion path of object in parallel - end";
        this->set_done(posSimplifyInfill);
    }

    if (this->set_started(posSimplifySupportPath)) {
        //BBS: share same progress
        m_print->set_status(75, L("Optimizing toolpath"));
        BOOST_LOG_TRIVIAL(debug) << "Simplify extrusion path of support in parallel - start";
        tbb::parallel_for(
            tbb::blocked_range<size_t>(0, m_support_layers.size()),
            [this](const tbb::blocked_range<size_t>& range) {
                for (size_t layer_idx = range.begin(); layer_idx < range.end(); ++ layer_idx) {
                    m_print->throw_if_canceled();
                    m_support_layers[layer_idx]->simplify_support_extrusion_path();
                }
            }
        );
        m_print->throw_if_canceled();
        BOOST_LOG_TRIVIAL(debug) << "Simplify extrusion path of support in parallel - end";
        this->set_done(posSimplifySupportPath);
    }
}

std::pair<FillAdaptive::OctreePtr, FillAdaptive::OctreePtr> PrintObject::prepare_adaptive_infill_data(
    const std::vector<std::pair<const Surface *, float>> &surfaces_w_bottom_z) const
{
    using namespace FillAdaptive;

    auto [adaptive_line_spacing, support_line_spacing] = adaptive_fill_line_spacing(*this);
    if ((adaptive_line_spacing == 0. && support_line_spacing == 0.) || this->layers().empty())
        return std::make_pair(OctreePtr(), OctreePtr());

    indexed_triangle_set mesh = this->model_object()->raw_indexed_triangle_set();
    // Rotate mesh and build octree on it with axis-aligned (standart base) cubes.
    auto to_octree = transform_to_octree().toRotationMatrix();
    its_transform(mesh, to_octree * this->trafo_centered(), true);

    // Triangulate internal bridging surfaces.
    std::vector<std::vector<Vec3d>> overhangs(std::max(surfaces_w_bottom_z.size(), size_t(1)));
    // ^ make sure vector is not empty, even with no briding surfaces we still want to build the adaptive trees later, some continue normally
    tbb::parallel_for(tbb::blocked_range<int>(0, surfaces_w_bottom_z.size()),
        [this, &to_octree, &overhangs, &surfaces_w_bottom_z](const tbb::blocked_range<int> &range) {
            for (int surface_idx = range.begin(); surface_idx < range.end(); ++surface_idx) {
                std::vector<Vec3d> &out = overhangs[surface_idx];
                m_print->throw_if_canceled();
                append(out, triangulate_expolygon_3d(surfaces_w_bottom_z[surface_idx].first->expolygon,
                    surfaces_w_bottom_z[surface_idx].second));
                for (Vec3d &p : out)
                    p = (to_octree * p).eval();
            }
        });
    // and gather them.
    for (size_t i = 1; i < overhangs.size(); ++ i)
        append(overhangs.front(), std::move(overhangs[i]));

    return std::make_pair(
        adaptive_line_spacing ? build_octree(mesh, overhangs.front(), adaptive_line_spacing, false) : OctreePtr(),
        support_line_spacing  ? build_octree(mesh, overhangs.front(), support_line_spacing, true) : OctreePtr());
}

FillLightning::GeneratorPtr PrintObject::prepare_lightning_infill_data()
{
    bool has_lightning_infill = false;
    for (size_t region_id = 0; region_id < this->num_printing_regions(); ++region_id)
        if (const PrintRegionConfig& config = this->printing_region(region_id).config(); config.sparse_infill_density > 0 && config.sparse_infill_pattern == ipLightning) {
            has_lightning_infill = true;
            break;
        }

    return has_lightning_infill ? FillLightning::build_generator(std::as_const(*this), [this]() -> void { this->throw_if_canceled(); }) : FillLightning::GeneratorPtr();
}

void PrintObject::clear_layers()
{
    if (!m_shared_object) {
        for (Layer *l : m_layers)
            delete l;
        m_layers.clear();
    }
}

Layer* PrintObject::add_layer(int id, coordf_t height, coordf_t print_z, coordf_t slice_z)
{
    m_layers.emplace_back(new Layer(id, this, height, print_z, slice_z));
    return m_layers.back();
}

const SupportLayer* PrintObject::get_support_layer_at_printz(coordf_t print_z, coordf_t epsilon) const
{
    coordf_t limit = print_z - epsilon;
    auto it = Slic3r::lower_bound_by_predicate(m_support_layers.begin(), m_support_layers.end(), [limit](const SupportLayer* layer) { return layer->print_z < limit; });
    return (it == m_support_layers.end() || (*it)->print_z > print_z + epsilon) ? nullptr : *it;
}

SupportLayer* PrintObject::get_support_layer_at_printz(coordf_t print_z, coordf_t epsilon)
{
    return const_cast<SupportLayer*>(std::as_const(*this).get_support_layer_at_printz(print_z, epsilon));
}

void PrintObject::clear_support_layers()
{
    if (!m_shared_object) {
        for (SupportLayer* l : m_support_layers)
            delete l;
        m_support_layers.clear();
        for (auto l : m_layers) {
            l->sharp_tails.clear();
            l->sharp_tails_height.clear();
            l->cantilevers.clear();
        }
    }
}

std::shared_ptr<TreeSupportData> PrintObject::alloc_tree_support_preview_cache()
{
    if (!m_tree_support_preview_cache) {
        const coordf_t layer_height = m_config.layer_height.value;
        const coordf_t xy_distance = m_config.support_object_xy_distance.value;
        const double angle = m_config.tree_support_branch_angle.value * M_PI / 180.;
        const coordf_t max_move_distance
            = (angle < M_PI / 2) ? (coordf_t)(tan(angle) * layer_height) : std::numeric_limits<coordf_t>::max();
        const coordf_t radius_sample_resolution = g_config_tree_support_collision_resolution;

        m_tree_support_preview_cache = std::make_shared<TreeSupportData>(*this, xy_distance, max_move_distance, radius_sample_resolution);
    }

    return m_tree_support_preview_cache;
}

SupportLayer* PrintObject::add_tree_support_layer(int id, coordf_t height, coordf_t print_z, coordf_t slice_z)
{
    m_support_layers.emplace_back(new SupportLayer(id, 0, this, height, print_z, slice_z));
    m_support_layers.back()->support_type = stInnerTree;
    return m_support_layers.back();
}

SupportLayer* PrintObject::add_support_layer(int id, int interface_id, coordf_t height, coordf_t print_z)
{
    m_support_layers.emplace_back(new SupportLayer(id, interface_id, this, height, print_z, -1));
    return m_support_layers.back();
}

SupportLayerPtrs::iterator PrintObject::insert_support_layer(SupportLayerPtrs::iterator pos, size_t id, size_t interface_id, coordf_t height, coordf_t print_z, coordf_t slice_z)
{
    return m_support_layers.insert(pos, new SupportLayer(id, interface_id, this, height, print_z, slice_z));
}

// Called by Print::apply().
// This method only accepts PrintObjectConfig and PrintRegionConfig option keys.
bool PrintObject::invalidate_state_by_config_options(
    const ConfigOptionResolver &old_config, const ConfigOptionResolver &new_config, const std::vector<t_config_option_key> &opt_keys)
{
    if (opt_keys.empty())
        return false;

    std::vector<PrintObjectStep> steps;
    bool invalidated = false;
    for (const t_config_option_key &opt_key : opt_keys) {
        if (   opt_key == "brim_width"
            || opt_key == "brim_object_gap"
            || opt_key == "brim_type"
            // BBS: brim generation depends on printing speed
            || opt_key == "outer_wall_speed"
            || opt_key == "sparse_infill_speed"
            || opt_key == "inner_wall_speed"
            || opt_key == "support_speed"
            || opt_key == "internal_solid_infill_speed"
            || opt_key == "top_surface_speed") {
            // Brim is printed below supports, support invalidates brim and skirt.
            steps.emplace_back(posSupportMaterial);
            if (opt_key == "brim_type") {
                const auto* old_brim_type = old_config.option<ConfigOptionEnum<BrimType>>(opt_key);
                const auto* new_brim_type = new_config.option<ConfigOptionEnum<BrimType>>(opt_key);
                //BBS: When switch to manual brim, the object must have brim, then re-generate perimeter
                //to make the wall order of first layer to be outer-first
                if (old_brim_type->value == btOuterOnly || new_brim_type->value == btOuterOnly)
                    steps.emplace_back(posPerimeters);
            }
        } else if (
               opt_key == "wall_loops"
            || opt_key == "top_one_wall_type"
            || opt_key == "top_area_threshold"
            || opt_key == "only_one_wall_first_layer"
            || opt_key == "initial_layer_line_width"
            || opt_key == "inner_wall_line_width"
            || opt_key == "infill_wall_overlap") {
            steps.emplace_back(posPerimeters);
        } else if (opt_key == "gap_infill_speed" || opt_key == "filter_out_gap_fill") {
            // Return true if gap-fill speed has changed from zero value to non-zero or from non-zero value to zero.
            auto is_gap_fill_changed_state_due_to_speed = [&opt_key, &old_config, &new_config]() -> bool {
                if (opt_key == "gap_infill_speed") {
                    const auto *old_gap_fill_speed = old_config.option<ConfigOptionFloat>(opt_key);
                    const auto *new_gap_fill_speed = new_config.option<ConfigOptionFloat>(opt_key);
                    assert(old_gap_fill_speed && new_gap_fill_speed);
                    return (old_gap_fill_speed->value > 0.f && new_gap_fill_speed->value == 0.f) ||
                           (old_gap_fill_speed->value == 0.f && new_gap_fill_speed->value > 0.f);
                }
                return false;
            };

            // Filtering of unprintable regions in multi-material segmentation depends on if gap-fill is enabled or not.
            // So step posSlice is invalidated when gap-fill was enabled/disabled by option "gap_fill_enabled" or by
            // changing "gap_infill_speed" to force recomputation of the multi-material segmentation.
            if (this->is_mm_painted() && ((opt_key == "gap_infill_speed" || opt_key == "filter_out_gap_fill") && is_gap_fill_changed_state_due_to_speed()))
                steps.emplace_back(posSlice);
            steps.emplace_back(posPerimeters);
        } else if (
               opt_key == "layer_height"
            || opt_key == "mmu_segmented_region_max_width"
            || opt_key == "mmu_segmented_region_interlocking_depth"
            || opt_key == "raft_layers"
            || opt_key == "raft_contact_distance"
            || opt_key == "slice_closing_radius"
            || opt_key == "slicing_mode") {
            steps.emplace_back(posSlice);
		} else if (
               opt_key == "elefant_foot_compensation"
            || opt_key == "support_top_z_distance"
            || opt_key == "support_bottom_z_distance"
            || opt_key == "xy_hole_compensation"
            || opt_key == "xy_contour_compensation") {
            steps.emplace_back(posSlice);
        } else if (opt_key == "enable_support") {
            steps.emplace_back(posSupportMaterial);
            if (m_config.support_top_z_distance == 0.) {
            	// Enabling / disabling supports while soluble support interface is enabled.
            	// This changes the bridging logic (bridging enabled without supports, disabled with supports).
            	// Reset everything.
            	// See GH #1482 for details.
	            steps.emplace_back(posSlice);
	        }
        } else if (
        	   opt_key == "support_type"
            || opt_key == "support_angle"
            || opt_key == "support_on_build_plate_only"
            || opt_key == "support_critical_regions_only"
            || opt_key == "support_remove_small_overhang"
            || opt_key == "enforce_support_layers"
            || opt_key == "support_filament"
            || opt_key == "support_line_width"
            || opt_key == "support_interface_top_layers"
            || opt_key == "support_interface_bottom_layers"
            || opt_key == "support_interface_pattern"
            || opt_key == "support_interface_loop_pattern"
            || opt_key == "support_interface_filament"
            || opt_key == "support_interface_not_for_body"
            || opt_key == "support_interface_spacing"
            || opt_key == "support_bottom_interface_spacing" //BBS
            || opt_key == "support_base_pattern"
            || opt_key == "support_style"
            || opt_key == "support_object_xy_distance"
            || opt_key == "support_object_first_layer_gap"
            || opt_key == "support_base_pattern_spacing"
            || opt_key == "support_expansion"
            //|| opt_key == "independent_support_layer_height" // BBS
            || opt_key == "support_threshold_angle"
            || opt_key == "raft_expansion"
            || opt_key == "raft_first_layer_density"
            || opt_key == "raft_first_layer_expansion"
            || opt_key == "bridge_no_support"
            || opt_key == "max_bridge_length"
            || opt_key == "initial_layer_line_width"
            || opt_key == "tree_support_branch_distance"
            || opt_key == "tree_support_branch_diameter"
            || opt_key == "tree_support_branch_angle"
            || opt_key == "tree_support_wall_count") {
            steps.emplace_back(posSupportMaterial);
        } else if (
               opt_key == "bottom_shell_layers"
            || opt_key == "top_shell_layers") {

            steps.emplace_back(posSlice);
#if (0)
            const auto *old_shell_layers = old_config.option<ConfigOptionInt>(opt_key);
            const auto *new_shell_layers = new_config.option<ConfigOptionInt>(opt_key);
            assert(old_shell_layers && new_shell_layers);

            bool value_changed = (old_shell_layers->value == 0 && new_shell_layers->value > 0) ||
                                 (old_shell_layers->value > 0 && new_shell_layers->value == 0);

            if (value_changed && this->object_extruders().size() > 1) {
                steps.emplace_back(posSlice);
            }
            else if (m_print->config().spiral_mode && opt_key == "bottom_shell_layers") {
                // Changing the number of bottom layers when a spiral vase is enabled requires re-slicing the object again.
                // Otherwise, holes in the bottom layers could be filled, as is reported in GH #5528.
                steps.emplace_back(posSlice);
            }
#endif
        } else if (
               opt_key == "interface_shells"
            || opt_key == "infill_combination"
            || opt_key == "bottom_shell_thickness"
            || opt_key == "top_shell_thickness"
            || opt_key == "minimum_sparse_infill_area"
            || opt_key == "sparse_infill_filament"
            || opt_key == "solid_infill_filament"
            || opt_key == "sparse_infill_line_width"
            || opt_key == "infill_direction"
            || opt_key == "ensure_vertical_shell_thickness"
            || opt_key == "bridge_angle"
            //BBS
        ) {
            steps.emplace_back(posPrepareInfill);
        } else if (
               opt_key == "top_surface_pattern"
            || opt_key == "bottom_surface_pattern"
            || opt_key == "internal_solid_infill_pattern"
            || opt_key == "external_fill_link_max_length"
            || opt_key == "sparse_infill_anchor"
            || opt_key == "sparse_infill_anchor_max"
            || opt_key == "top_surface_line_width"
            || opt_key == "initial_layer_line_width") {
            steps.emplace_back(posInfill);
        } else if (opt_key == "sparse_infill_pattern") {
            steps.emplace_back(posPrepareInfill);
        } else if (opt_key == "sparse_infill_density") {
            // One likely wants to reslice only when switching between zero infill to simulate boolean difference (subtracting volumes),
            // normal infill and 100% (solid) infill.
            const auto *old_density = old_config.option<ConfigOptionPercent>(opt_key);
            const auto *new_density = new_config.option<ConfigOptionPercent>(opt_key);
            assert(old_density && new_density);
            //FIXME Vojtech is not quite sure about the 100% here, maybe it is not needed.
            if (is_approx(old_density->value, 0.) || is_approx(old_density->value, 100.) ||
                is_approx(new_density->value, 0.) || is_approx(new_density->value, 100.))
                steps.emplace_back(posPerimeters);
            steps.emplace_back(posPrepareInfill);
        } else if (opt_key == "internal_solid_infill_line_width") {
            // This value is used for calculating perimeter - infill overlap, thus perimeters need to be recalculated.
            steps.emplace_back(posPerimeters);
            steps.emplace_back(posPrepareInfill);
        } else if (
               opt_key == "outer_wall_line_width"
            || opt_key == "wall_filament"
            || opt_key == "fuzzy_skin"
            || opt_key == "fuzzy_skin_thickness"
            || opt_key == "fuzzy_skin_point_distance"
            || opt_key == "detect_overhang_wall"
            //BBS
            || opt_key == "enable_overhang_speed"
            || opt_key == "detect_thin_wall") {
            steps.emplace_back(posPerimeters);
            steps.emplace_back(posSupportMaterial);
        } else if (opt_key == "bridge_flow") {
            if (m_config.support_top_z_distance > 0.) {
            	// Only invalidate due to bridging if bridging is enabled.
            	// If later "support_top_z_distance" is modified, the complete PrintObject is invalidated anyway.
            	steps.emplace_back(posPerimeters);
            	steps.emplace_back(posInfill);
	            steps.emplace_back(posSupportMaterial);
	        }
        } else if (
                opt_key == "wall_generator"
            || opt_key == "wall_transition_length"
            || opt_key == "wall_transition_filter_deviation"
            || opt_key == "wall_transition_angle"
            || opt_key == "wall_distribution_count"
            || opt_key == "min_feature_size"
            || opt_key == "min_bead_width") {
            steps.emplace_back(posSlice);
        } else if (
               opt_key == "seam_position"
            || opt_key == "seam_slope_type"
            || opt_key == "seam_slope_conditional"
            || opt_key == "scarf_angle_threshold"
            || opt_key == "seam_slope_start_height"
            || opt_key == "seam_slope_entire_loop"
            || opt_key == "seam_slope_min_length"
            || opt_key == "seam_slope_steps"
            || opt_key == "seam_slope_inner_walls"
            || opt_key == "seam_gap"
            || opt_key == "wipe_speed"
            || opt_key == "support_speed"
            || opt_key == "support_interface_speed"
            || opt_key == "smooth_speed_discontinuity_area"
            || opt_key == "smooth_coefficient"
            || opt_key == "overhang_1_4_speed"
            || opt_key == "overhang_2_4_speed"
            || opt_key == "overhang_3_4_speed"
            || opt_key == "overhang_4_4_speed"
            || opt_key == "overhang_totally_speed"
            || opt_key == "bridge_speed"
            || opt_key == "outer_wall_speed"
            || opt_key == "small_perimeter_speed"
            || opt_key == "small_perimeter_threshold"
            || opt_key == "sparse_infill_speed"
            || opt_key == "inner_wall_speed"
            || opt_key == "internal_solid_infill_speed"
            || opt_key == "top_surface_speed") {
            invalidated |= m_print->invalidate_step(psGCodeExport);
        } else if (
               opt_key == "flush_into_infill"
            || opt_key == "flush_into_objects"
            || opt_key == "flush_into_support") {
            invalidated |= m_print->invalidate_step(psWipeTower);
            invalidated |= m_print->invalidate_step(psGCodeExport);
        } else {
            // for legacy, if we can't handle this option let's invalidate all steps
            this->invalidate_all_steps();
            invalidated = true;
        }
    }

    sort_remove_duplicates(steps);
    for (PrintObjectStep step : steps)
        invalidated |= this->invalidate_step(step);
    return invalidated;
}

bool PrintObject::invalidate_step(PrintObjectStep step)
{
	bool invalidated = Inherited::invalidate_step(step);

    // propagate to dependent steps
    if (step == posPerimeters) {
		invalidated |= this->invalidate_steps({ posPrepareInfill, posInfill, posIroning, posSimplifyWall, posSimplifyInfill });
        invalidated |= m_print->invalidate_steps({ psSkirtBrim });
    } else if (step == posPrepareInfill) {
        invalidated |= this->invalidate_steps({ posInfill, posIroning, posSimplifyWall, posSimplifyInfill });
    } else if (step == posInfill) {
        invalidated |= this->invalidate_steps({ posIroning, posSimplifyInfill });
        invalidated |= m_print->invalidate_steps({ psSkirtBrim });
    } else if (step == posSlice) {
		invalidated |= this->invalidate_steps({ posPerimeters, posPrepareInfill, posInfill, posIroning, posSupportMaterial, posSimplifyWall, posSimplifyInfill });
        invalidated |= m_print->invalidate_steps({ psSkirtBrim });
        m_slicing_params.valid = false;
    } else if (step == posSupportMaterial) {
        invalidated |= this->invalidate_steps({ posSimplifySupportPath });
        invalidated |= m_print->invalidate_steps({ psSkirtBrim });
        m_slicing_params.valid = false;
    }

    // Wipe tower depends on the ordering of extruders, which in turn depends on everything.
    // It also decides about what the flush_into_infill / wipe_into_object / flush_into_support features will do,
    // and that too depends on many of the settings.
    invalidated |= m_print->invalidate_step(psWipeTower);
    // Invalidate G-code export in any case.
    invalidated |= m_print->invalidate_step(psGCodeExport);
    return invalidated;
}

bool PrintObject::invalidate_all_steps()
{
	// First call the "invalidate" functions, which may cancel background processing.
    bool result = Inherited::invalidate_all_steps() | m_print->invalidate_all_steps();
	// Then reset some of the depending values.
	m_slicing_params.valid = false;
	return result;
}

// This function analyzes slices of a region (SurfaceCollection slices).
// Each region slice (instance of Surface) is analyzed, whether it is supported or whether it is the top surface.
// Initially all slices are of type stInternal.
// Slices are compared against the top / bottom slices and regions and classified to the following groups:
// stTop          - Part of a region, which is not covered by any upper layer. This surface will be filled with a top solid infill.
// stBottomBridge - Part of a region, which is not fully supported, but it hangs in the air, or it hangs losely on a support or a raft.
// stBottom       - Part of a region, which is not supported by the same region, but it is supported either by another region, or by a soluble interface layer.
// stInternal     - Part of a region, which is supported by the same region type.
// If a part of a region is of stBottom and stTop, the stBottom wins.
void PrintObject::detect_surfaces_type()
{
    BOOST_LOG_TRIVIAL(info) << "Detecting solid surfaces..." << log_memory_info();

    // Interface shells: the intersecting parts are treated as self standing objects supporting each other.
    // Each of the objects will have a full number of top / bottom layers, even if these top / bottom layers
    // are completely hidden inside a collective body of intersecting parts.
    // This is useful if one of the parts is to be dissolved, or if it is transparent and the internal shells
    // should be visible.
    bool spiral_mode      = this->print()->config().spiral_mode.value;
    bool interface_shells = ! spiral_mode && m_config.interface_shells.value;
    size_t num_layers     = spiral_mode ? std::min(size_t(this->printing_region(0).config().bottom_shell_layers), m_layers.size()) : m_layers.size();

    for (size_t region_id = 0; region_id < this->num_printing_regions(); ++ region_id) {
        BOOST_LOG_TRIVIAL(debug) << "Detecting solid surfaces for region " << region_id << " in parallel - start";
#ifdef SLIC3R_DEBUG_SLICE_PROCESSING
        for (Layer *layer : m_layers)
            layer->m_regions[region_id]->export_region_fill_surfaces_to_svg_debug("1_detect_surfaces_type-initial");
#endif /* SLIC3R_DEBUG_SLICE_PROCESSING */

        // If interface shells are allowed, the region->surfaces cannot be overwritten as they may be used by other threads.
        // Cache the result of the following parallel_loop.
        std::vector<Surfaces> surfaces_new;
        if (interface_shells)
            surfaces_new.assign(num_layers, Surfaces());

        tbb::parallel_for(
            tbb::blocked_range<size_t>(0,
            	spiral_mode ?
            		// In spiral vase mode, reserve the last layer for the top surface if more than 1 layer is planned for the vase bottom.
            		((num_layers > 1) ? num_layers - 1 : num_layers) :
            		// In non-spiral vase mode, go over all layers.
            		m_layers.size()),
            [this, spiral_mode, region_id, interface_shells, &surfaces_new](const tbb::blocked_range<size_t>& range) {
                // If we have soluble support material, don't bridge. The overhang will be squished against a soluble layer separating
                // the support from the print.
                // BBS: the above logic only applys for normal(auto) support. Complete logic:
                // 1. has support, top z distance=0 (soluble material), auto support
                // 2. for normal(auto), bridge_no_support is off
                // 3. for tree(auto), interface top layers=0, max bridge length=0, support_critical_regions_only=false (only in this way the bridge is fully supported)
                bool bottom_is_fully_supported = this->has_support() && m_config.support_top_z_distance.value == 0 && is_auto(m_config.support_type.value);
                if (m_config.support_type.value == stNormalAuto)
                    bottom_is_fully_supported &= !m_config.bridge_no_support.value;
                else if (m_config.support_type.value == stTreeAuto) {
                    bottom_is_fully_supported &= (m_config.support_interface_top_layers.value > 0 && m_config.max_bridge_length.value == 0 && m_config.support_critical_regions_only.value==false);
                }
                SurfaceType surface_type_bottom_other = bottom_is_fully_supported ? stBottom : stBottomBridge;
                for (size_t idx_layer = range.begin(); idx_layer < range.end(); ++ idx_layer) {
                    m_print->throw_if_canceled();
                    // BOOST_LOG_TRIVIAL(trace) << "Detecting solid surfaces for region " << region_id << " and layer " << layer->print_z;
                    Layer       *layer  = m_layers[idx_layer];
                    LayerRegion *layerm = layer->m_regions[region_id];
                    // comparison happens against the *full* slices (considering all regions)
                    // unless internal shells are requested
                    Layer       *upper_layer = (idx_layer + 1 < this->layer_count()) ? m_layers[idx_layer + 1] : nullptr;
                    Layer       *lower_layer = (idx_layer > 0) ? m_layers[idx_layer - 1] : nullptr;
                    // collapse very narrow parts (using the safety offset in the diff is not enough)
                    float        offset = layerm->flow(frExternalPerimeter).scaled_width() / 10.f;

                    bool detect_top = spiral_mode || layerm->region().config().top_shell_layers;
                    bool detect_bottom = spiral_mode || layerm->region().config().bottom_shell_layers;

                    // find top surfaces (difference between current surfaces
                    // of current layer and upper one)
                    Surfaces top;
                    if (detect_top) {
                        if (upper_layer) {
                            ExPolygons upper_slices = interface_shells ?
                                diff_ex(layerm->slices.surfaces, upper_layer->m_regions[region_id]->slices.surfaces, ApplySafetyOffset::Yes) :
                                diff_ex(layerm->slices.surfaces, upper_layer->lslices, ApplySafetyOffset::Yes);
                            surfaces_append(top, opening_ex(upper_slices, offset), stTop);
                        }
                        else {
                            // if no upper layer, all surfaces of this one are solid
                            // we clone surfaces because we're going to clear the slices collection
                            top = layerm->slices.surfaces;
                            for (Surface& surface : top)
                                surface.surface_type = stTop;
                        }
                    }

                    // Find bottom surfaces (difference between current surfaces of current layer and lower one).
                    Surfaces bottom;
                    if (detect_bottom) {
                        if (lower_layer) {
#if 0
                            //FIXME Why is this branch failing t\multi.t ?
                            Polygons lower_slices = interface_shells ?
                                to_polygons(lower_layer->get_region(region_id)->slices.surfaces) :
                                to_polygons(lower_layer->slices);
                            surfaces_append(bottom,
                                opening_ex(diff(layerm->slices.surfaces, lower_slices, true), offset),
                                surface_type_bottom_other);
#else
                            // Any surface lying on the void is a true bottom bridge (an overhang)
                            surfaces_append(
                                bottom,
                                opening_ex(
                                    diff_ex(layerm->slices.surfaces, lower_layer->lslices, ApplySafetyOffset::Yes),
                                    offset),
                                surface_type_bottom_other);
                            // if user requested internal shells, we need to identify surfaces
                            // lying on other slices not belonging to this region
                            if (interface_shells) {
                                // non-bridging bottom surfaces: any part of this layer lying
                                // on something else, excluding those lying on our own region
                                surfaces_append(
                                    bottom,
                                    opening_ex(
                                        diff_ex(
                                            intersection(layerm->slices.surfaces, lower_layer->lslices), // supported
                                            lower_layer->m_regions[region_id]->slices.surfaces,
                                            ApplySafetyOffset::Yes),
                                        offset),
                                    stBottom);
                            }
#endif
                        }
                        else {
                            // if no lower layer, all surfaces of this one are solid
                            // we clone surfaces because we're going to clear the slices collection
                            bottom = layerm->slices.surfaces;
                            for (Surface& surface : bottom)
                                surface.surface_type = stBottom;
                        }
                    }

                    // now, if the object contained a thin membrane, we could have overlapping bottom
                    // and top surfaces; let's do an intersection to discover them and consider them
                    // as bottom surfaces (to allow for bridge detection)
                    if (! top.empty() && ! bottom.empty()) {
        //                Polygons overlapping = intersection(to_polygons(top), to_polygons(bottom));
        //                Slic3r::debugf "  layer %d contains %d membrane(s)\n", $layerm->layer->id, scalar(@$overlapping)
        //                    if $Slic3r::debug;
                        Polygons top_polygons = to_polygons(std::move(top));
                        top.clear();
                        surfaces_append(top, diff_ex(top_polygons, bottom), stTop);
                    }

        #ifdef SLIC3R_DEBUG_SLICE_PROCESSING
                    {
                        static int iRun = 0;
                        std::vector<std::pair<Slic3r::ExPolygons, SVG::ExPolygonAttributes>> expolygons_with_attributes;
                        expolygons_with_attributes.emplace_back(std::make_pair(union_ex(top),                           SVG::ExPolygonAttributes("green")));
                        expolygons_with_attributes.emplace_back(std::make_pair(union_ex(bottom),                        SVG::ExPolygonAttributes("brown")));
                        expolygons_with_attributes.emplace_back(std::make_pair(to_expolygons(layerm->slices.surfaces),  SVG::ExPolygonAttributes("black")));
                        SVG::export_expolygons(debug_out_path("1_detect_surfaces_type_%d_region%d-layer_%f.svg", iRun ++, region_id, layer->print_z).c_str(), expolygons_with_attributes);
                    }
        #endif /* SLIC3R_DEBUG_SLICE_PROCESSING */

                    // save surfaces to layer
                    Surfaces &surfaces_out = interface_shells ? surfaces_new[idx_layer] : layerm->slices.surfaces;
                    Surfaces  surfaces_backup;
                    if (! interface_shells) {
                        surfaces_backup = std::move(surfaces_out);
                        surfaces_out.clear();
                    }
                    const Surfaces &surfaces_prev = interface_shells ? layerm->slices.surfaces : surfaces_backup;

                    // find internal surfaces (difference between top/bottom surfaces and others)
                    {
                        Polygons topbottom = to_polygons(top);
                        polygons_append(topbottom, to_polygons(bottom));
                        surfaces_append(surfaces_out, diff_ex(surfaces_prev, topbottom), stInternal);
                    }

                    surfaces_append(surfaces_out, std::move(top));
                    surfaces_append(surfaces_out, std::move(bottom));

        //            Slic3r::debugf "  layer %d has %d bottom, %d top and %d internal surfaces\n",
        //                $layerm->layer->id, scalar(@bottom), scalar(@top), scalar(@internal) if $Slic3r::debug;

        #ifdef SLIC3R_DEBUG_SLICE_PROCESSING
                    layerm->export_region_slices_to_svg_debug("detect_surfaces_type-final");
        #endif /* SLIC3R_DEBUG_SLICE_PROCESSING */
                }
            }
        ); // for each layer of a region
        m_print->throw_if_canceled();

        if (interface_shells) {
            // Move surfaces_new to layerm->slices.surfaces
            for (size_t idx_layer = 0; idx_layer < num_layers; ++ idx_layer)
                m_layers[idx_layer]->m_regions[region_id]->slices.surfaces = std::move(surfaces_new[idx_layer]);
        }

        if (spiral_mode) {
        	if (num_layers > 1)
	        	// Turn the last bottom layer infill to a top infill, so it will be extruded with a proper pattern.
	        	m_layers[num_layers - 1]->m_regions[region_id]->slices.set_type(stTop);
	        for (size_t i = num_layers; i < m_layers.size(); ++ i)
	        	m_layers[i]->m_regions[region_id]->slices.set_type(stInternal);
        }

        BOOST_LOG_TRIVIAL(debug) << "Detecting solid surfaces for region " << region_id << " - clipping in parallel - start";
        // Fill in layerm->fill_surfaces by trimming the layerm->slices by the cummulative layerm->fill_surfaces.
        tbb::parallel_for(
            tbb::blocked_range<size_t>(0, m_layers.size()),
            [this, region_id](const tbb::blocked_range<size_t>& range) {
                for (size_t idx_layer = range.begin(); idx_layer < range.end(); ++ idx_layer) {
                    m_print->throw_if_canceled();
                    LayerRegion *layerm = m_layers[idx_layer]->m_regions[region_id];
                    layerm->slices_to_fill_surfaces_clipped();
#ifdef SLIC3R_DEBUG_SLICE_PROCESSING
                    layerm->export_region_fill_surfaces_to_svg_debug("1_detect_surfaces_type-final");
#endif /* SLIC3R_DEBUG_SLICE_PROCESSING */
                } // for each layer of a region
            });
        m_print->throw_if_canceled();
        BOOST_LOG_TRIVIAL(debug) << "Detecting solid surfaces for region " << region_id << " - clipping in parallel - end";
    } // for each this->print->region_count

    // Mark the object to have the region slices classified (typed, which also means they are split based on whether they are supported, bridging, top layers etc.)
    m_typed_slices = true;
}

void PrintObject::process_external_surfaces()
{
    BOOST_LOG_TRIVIAL(info) << "Processing external surfaces..." << log_memory_info();

    // Cached surfaces covered by some extrusion, defining regions, over which the from the surfaces one layer higher are allowed to expand.
    std::vector<Polygons> surfaces_covered;
    // Is there any printing region, that has zero infill? If so, then we don't want the expansion to be performed over the complete voids, but only
    // over voids, which are supported by the layer below.
    bool 				  has_voids = false;
	for (size_t region_id = 0; region_id < this->num_printing_regions(); ++ region_id)
		if (this->printing_region(region_id).config().sparse_infill_density == 0) {
			has_voids = true;
			break;
		}
	if (has_voids && m_layers.size() > 1) {
	    // All but stInternal fill surfaces will get expanded and possibly trimmed.
	    std::vector<unsigned char> layer_expansions_and_voids(m_layers.size(), false);
	    for (size_t layer_idx = 1; layer_idx < m_layers.size(); ++ layer_idx) {
	    	const Layer *layer = m_layers[layer_idx];
	    	bool expansions = false;
	    	bool voids      = false;
	    	for (const LayerRegion *layerm : layer->regions()) {
	    		for (const Surface &surface : layerm->fill_surfaces.surfaces) {
	    			if (surface.surface_type == stInternal)
	    				voids = true;
	    			else
	    				expansions = true;
	    			if (voids && expansions) {
	    				layer_expansions_and_voids[layer_idx] = true;
	    				goto end;
	    			}
	    		}
	    	}
		end:;
		}
	    BOOST_LOG_TRIVIAL(debug) << "Collecting surfaces covered with extrusions in parallel - start";
	    surfaces_covered.resize(m_layers.size() - 1, Polygons());
    	auto unsupported_width = - float(scale_(0.3 * EXTERNAL_INFILL_MARGIN));
	    tbb::parallel_for(
	        tbb::blocked_range<size_t>(0, m_layers.size() - 1),
	        [this, &surfaces_covered, &layer_expansions_and_voids, unsupported_width](const tbb::blocked_range<size_t>& range) {
	            for (size_t layer_idx = range.begin(); layer_idx < range.end(); ++ layer_idx)
	            	if (layer_expansions_and_voids[layer_idx + 1]) {
		                m_print->throw_if_canceled();
		                Polygons voids;
		                for (const LayerRegion *layerm : m_layers[layer_idx]->regions()) {
		                	if (layerm->region().config().sparse_infill_density.value == 0.)
		                		for (const Surface &surface : layerm->fill_surfaces.surfaces)
		                			// Shrink the holes, let the layer above expand slightly inside the unsupported areas.
		                			polygons_append(voids, offset(surface.expolygon, unsupported_width));
		                }
		                surfaces_covered[layer_idx] = diff(m_layers[layer_idx]->lslices, voids);
	            	}
	        }
	    );
	    m_print->throw_if_canceled();
	    BOOST_LOG_TRIVIAL(debug) << "Collecting surfaces covered with extrusions in parallel - end";
	}

	for (size_t region_id = 0; region_id < this->num_printing_regions(); ++region_id) {
        BOOST_LOG_TRIVIAL(debug) << "Processing external surfaces for region " << region_id << " in parallel - start";
        tbb::parallel_for(
            tbb::blocked_range<size_t>(0, m_layers.size()),
            [this, &surfaces_covered, region_id](const tbb::blocked_range<size_t>& range) {
                for (size_t layer_idx = range.begin(); layer_idx < range.end(); ++ layer_idx) {
                    m_print->throw_if_canceled();
                    // BOOST_LOG_TRIVIAL(trace) << "Processing external surface, layer" << m_layers[layer_idx]->print_z;
                    m_layers[layer_idx]->get_region(int(region_id))->process_external_surfaces(
                    	(layer_idx == 0) ? nullptr : m_layers[layer_idx - 1],
                    	(layer_idx == 0 || surfaces_covered.empty() || surfaces_covered[layer_idx - 1].empty()) ? nullptr : &surfaces_covered[layer_idx - 1]);
                }
            }
        );
        m_print->throw_if_canceled();
        BOOST_LOG_TRIVIAL(debug) << "Processing external surfaces for region " << region_id << " in parallel - end";
    }
}

void PrintObject::discover_vertical_shells()
{
    PROFILE_FUNC();

    BOOST_LOG_TRIVIAL(info) << "Discovering vertical shells..." << log_memory_info();

    struct DiscoverVerticalShellsCacheEntry
    {
        // Collected polygons, offsetted
        Polygons    top_surfaces;
        Polygons    bottom_surfaces;
        Polygons    holes;
    };
    bool     spiral_mode      = this->print()->config().spiral_mode.value;
    size_t   num_layers       = spiral_mode ? std::min(size_t(this->printing_region(0).config().bottom_shell_layers), m_layers.size()) : m_layers.size();

    std::vector<DiscoverVerticalShellsCacheEntry> cache_top_botom_regions(num_layers, DiscoverVerticalShellsCacheEntry());
    bool top_bottom_surfaces_all_regions = this->num_printing_regions() > 1 && ! m_config.interface_shells.value;

    static constexpr float top_bottom_expansion_coeff = 0.05f;
    if (top_bottom_surfaces_all_regions) {
        // This is a multi-material print and interface_shells are disabled, meaning that the vertical shell thickness
        // is calculated over all materials.
        bool has_extra_layers = false;
        for (size_t region_id = 0; region_id < this->num_printing_regions(); ++region_id) {
            const PrintRegionConfig &config = this->printing_region(region_id).config();
            if (config.ensure_vertical_shell_thickness.value) {
                has_extra_layers = true;
                break;
            }
        }
        if (! has_extra_layers)
            // The "ensure vertical wall thickness" feature is not applicable to any of the regions. Quit.
            return;

        BOOST_LOG_TRIVIAL(debug) << "Discovering vertical shells in parallel - start : cache top / bottom";
        //FIXME Improve the heuristics for a grain size.
        size_t grain_size = std::max(num_layers / 16, size_t(1));
        tbb::parallel_for(
            tbb::blocked_range<size_t>(0, num_layers, grain_size),
            [this, &cache_top_botom_regions](const tbb::blocked_range<size_t>& range) {
                const std::initializer_list<SurfaceType> surfaces_bottom{ stBottom, stBottomBridge };
                const size_t num_regions = this->num_printing_regions();
                for (size_t idx_layer = range.begin(); idx_layer < range.end(); ++idx_layer) {
                    m_print->throw_if_canceled();
                    const Layer& layer = *m_layers[idx_layer];
                    DiscoverVerticalShellsCacheEntry& cache = cache_top_botom_regions[idx_layer];
                    // Simulate single set of perimeters over all merged regions.
                    float                             perimeter_offset = 0.f;
                    float                             perimeter_min_spacing = FLT_MAX;
#ifdef SLIC3R_DEBUG_SLICE_PROCESSING
                    static size_t debug_idx = 0;
                    ++debug_idx;
#endif /* SLIC3R_DEBUG_SLICE_PROCESSING */
                    for (size_t region_id = 0; region_id < num_regions; ++region_id) {
                        LayerRegion& layerm = *layer.m_regions[region_id];
                        float        top_bottom_expansion = float(layerm.flow(frSolidInfill).scaled_spacing()) * top_bottom_expansion_coeff;
                        // Top surfaces.
                        append(cache.top_surfaces, offset(layerm.slices.filter_by_type(stTop), top_bottom_expansion));
                        //                        append(cache.top_surfaces, offset(layerm.fill_surfaces().filter_by_type(stTop), top_bottom_expansion));
                                                // Bottom surfaces.
                        append(cache.bottom_surfaces, offset(layerm.slices.filter_by_types(surfaces_bottom), top_bottom_expansion));
                        //                        append(cache.bottom_surfaces, offset(layerm.fill_surfaces().filter_by_types(surfaces_bottom), top_bottom_expansion));
                                                // Calculate the maximum perimeter offset as if the slice was extruded with a single extruder only.
                                                // First find the maxium number of perimeters per region slice.
                        unsigned int perimeters = 0;
                        for (const Surface& s : layerm.slices.surfaces)
                            perimeters = std::max<unsigned int>(perimeters, s.extra_perimeters);
                        perimeters += layerm.region().config().wall_loops.value;
                        // Then calculate the infill offset.
                        if (perimeters > 0) {
                            Flow extflow = layerm.flow(frExternalPerimeter);
                            Flow flow = layerm.flow(frPerimeter);
                            perimeter_offset = std::max(perimeter_offset,
                                0.5f * float(extflow.scaled_width() + extflow.scaled_spacing()) + (float(perimeters) - 1.f) * flow.scaled_spacing());
                            perimeter_min_spacing = std::min(perimeter_min_spacing, float(std::min(extflow.scaled_spacing(), flow.scaled_spacing())));
                        }
                        polygons_append(cache.holes, to_polygons(layerm.fill_expolygons));
                    }
                    // Save some computing time by reducing the number of polygons.
                    cache.top_surfaces = union_(cache.top_surfaces);
                    cache.bottom_surfaces = union_(cache.bottom_surfaces);
                    // For a multi-material print, simulate perimeter / infill split as if only a single extruder has been used for the whole print.
                    if (perimeter_offset > 0.) {
                        // The layer.lslices are forced to merge by expanding them first.
                        polygons_append(cache.holes, offset2(layer.lslices, 0.3f * perimeter_min_spacing, -perimeter_offset - 0.3f * perimeter_min_spacing));
#ifdef SLIC3R_DEBUG_SLICE_PROCESSING
                        {
                            Slic3r::SVG svg(debug_out_path("discover_vertical_shells-extra-holes-%d.svg", debug_idx), get_extents(layer.lslices));
                            svg.draw(layer.lslices, "blue");
                            svg.draw(union_ex(cache.holes), "red");
                            svg.draw_outline(union_ex(cache.holes), "black", "blue", scale_(0.05));
                            svg.Close();
                        }
#endif /* SLIC3R_DEBUG_SLICE_PROCESSING */
                    }
                    cache.holes = union_(cache.holes);
                }});
        m_print->throw_if_canceled();
        BOOST_LOG_TRIVIAL(debug) << "Discovering vertical shells in parallel - end : cache top / bottom";
    }

    for (size_t region_id = 0; region_id < this->num_printing_regions(); ++ region_id) {
        //FIXME Improve the heuristics for a grain size.
        const PrintRegion &region = this->printing_region(region_id);
        if (!region.config().ensure_vertical_shell_thickness.value)
            // This region will be handled by discover_horizontal_shells().
            continue;

        size_t grain_size = std::max(num_layers / 16, size_t(1));

        if (! top_bottom_surfaces_all_regions) {
            // This is either a single material print, or a multi-material print and interface_shells are enabled, meaning that the vertical shell thickness
            // is calculated over a single material.
            BOOST_LOG_TRIVIAL(debug) << "Discovering vertical shells for region " << region_id << " in parallel - start : cache top / bottom";
            tbb::parallel_for(
                tbb::blocked_range<size_t>(0, num_layers, grain_size),
                [this, region_id, &cache_top_botom_regions](const tbb::blocked_range<size_t>& range) {
                    const std::initializer_list<SurfaceType> surfaces_bottom { stBottom, stBottomBridge };
                    for (size_t idx_layer = range.begin(); idx_layer < range.end(); ++ idx_layer) {
                        m_print->throw_if_canceled();
                        Layer       &layer                = *m_layers[idx_layer];
                        LayerRegion &layerm               = *layer.m_regions[region_id];
                        float        top_bottom_expansion = float(layerm.flow(frSolidInfill).scaled_spacing()) * top_bottom_expansion_coeff;
                        // Top surfaces.
                        auto &cache = cache_top_botom_regions[idx_layer];
                        cache.top_surfaces = offset(layerm.slices.filter_by_type(stTop), top_bottom_expansion);
//                        append(cache.top_surfaces, offset(layerm.fill_surfaces().filter_by_type(stTop), top_bottom_expansion));
                        // Bottom surfaces.
                        cache.bottom_surfaces = offset(layerm.slices.filter_by_types(surfaces_bottom), top_bottom_expansion);
//                        append(cache.bottom_surfaces, offset(layerm.fill_surfaces().filter_by_types(surfaces_bottom), top_bottom_expansion));
                        // Holes over all regions. Only collect them once, they are valid for all region_id iterations.
                        if (cache.holes.empty()) {
                            for (size_t region_id = 0; region_id < layer.regions().size(); ++ region_id)
                                polygons_append(cache.holes, to_polygons(layer.regions()[region_id]->fill_expolygons));
                        }
                    }
                });
            m_print->throw_if_canceled();
            BOOST_LOG_TRIVIAL(debug) << "Discovering vertical shells for region " << region_id << " in parallel - end : cache top / bottom";
        }

        BOOST_LOG_TRIVIAL(debug) << "Discovering vertical shells for region " << region_id << " in parallel - start : ensure vertical wall thickness";
        grain_size = 1;
        tbb::parallel_for(
            tbb::blocked_range<size_t>(0, num_layers, grain_size),
            [this, region_id, &cache_top_botom_regions]
            (const tbb::blocked_range<size_t>& range) {
                // printf("discover_vertical_shells from %d to %d\n", range.begin(), range.end());
                for (size_t idx_layer = range.begin(); idx_layer < range.end(); ++ idx_layer) {
                    m_print->throw_if_canceled();
#ifdef SLIC3R_DEBUG_SLICE_PROCESSING
        			static size_t debug_idx = 0;
        			++ debug_idx;
#endif /* SLIC3R_DEBUG_SLICE_PROCESSING */

                    Layer       	        *layer          = m_layers[idx_layer];
                    LayerRegion 	        *layerm         = layer->m_regions[region_id];
                    const PrintRegionConfig &region_config  = layerm->region().config();

#ifdef SLIC3R_DEBUG_SLICE_PROCESSING
                    layerm->export_region_slices_to_svg_debug("3_discover_vertical_shells-initial");
                    layerm->export_region_fill_surfaces_to_svg_debug("3_discover_vertical_shells-initial");
#endif /* SLIC3R_DEBUG_SLICE_PROCESSING */

                    Flow         solid_infill_flow   = layerm->flow(frSolidInfill);
                    coord_t      infill_line_spacing = solid_infill_flow.scaled_spacing(); 
                    // Find a union of perimeters below / above this surface to guarantee a minimum shell thickness.
                    Polygons shell;
                    Polygons holes;
#ifdef SLIC3R_DEBUG_SLICE_PROCESSING
                    ExPolygons shell_ex;
#endif /* SLIC3R_DEBUG_SLICE_PROCESSING */
                    float min_perimeter_infill_spacing = float(infill_line_spacing) * 1.05f;
#if 0
// #ifdef SLIC3R_DEBUG_SLICE_PROCESSING
                    {
        				Slic3r::SVG svg_cummulative(debug_out_path("discover_vertical_shells-perimeters-before-union-run%d.svg", debug_idx), this->bounding_box());
                        for (int n = (int)idx_layer - n_extra_bottom_layers; n <= (int)idx_layer + n_extra_top_layers; ++ n) {
                            if (n < 0 || n >= (int)m_layers.size())
                                continue;
                            ExPolygons &expolys = m_layers[n]->perimeter_expolygons;
                            for (size_t i = 0; i < expolys.size(); ++ i) {
        						Slic3r::SVG svg(debug_out_path("discover_vertical_shells-perimeters-before-union-run%d-layer%d-expoly%d.svg", debug_idx, n, i), get_extents(expolys[i]));
                                svg.draw(expolys[i]);
                                svg.draw_outline(expolys[i].contour, "black", scale_(0.05));
                                svg.draw_outline(expolys[i].holes, "blue", scale_(0.05));
                                svg.Close();

                                svg_cummulative.draw(expolys[i]);
                                svg_cummulative.draw_outline(expolys[i].contour, "black", scale_(0.05));
                                svg_cummulative.draw_outline(expolys[i].holes, "blue", scale_(0.05));
                            }
                        }
                    }
#endif /* SLIC3R_DEBUG_SLICE_PROCESSING */
			        polygons_append(holes, cache_top_botom_regions[idx_layer].holes);
                    auto combine_holes = [&holes](const Polygons &holes2) {
                        if (holes.empty() || holes2.empty())
                            holes.clear();
                        else
                            holes = intersection(holes, holes2);
                    };
                    auto combine_shells = [&shell](const Polygons &shells2) {
                        if (shell.empty())
                            shell = std::move(shells2);
                        else if (! shells2.empty()) {
                            polygons_append(shell, shells2);
                            // Running the union_ using the Clipper library piece by piece is cheaper 
                            // than running the union_ all at once.
                            shell = union_(shell);
                        }
                    };
                    static constexpr const bool one_more_layer_below_top_bottom_surfaces = false;
			        if (int n_top_layers = region_config.top_shell_layers.value; n_top_layers > 0) {
                        // Gather top regions projected to this layer.
                        coordf_t print_z = layer->print_z;
                        int i = int(idx_layer) + 1;
                        int itop = int(idx_layer) + n_top_layers;
                        bool at_least_one_top_projected = false;
	                    for (; i < int(cache_top_botom_regions.size()) &&
	                         (i < itop || m_layers[i]->print_z - print_z < region_config.top_shell_thickness - EPSILON);
	                        ++ i) {
                            at_least_one_top_projected = true;
	                        const DiscoverVerticalShellsCacheEntry &cache = cache_top_botom_regions[i];
                            combine_holes(cache.holes);
                            combine_shells(cache.top_surfaces);
	                    }
                        if (!at_least_one_top_projected && i < int(cache_top_botom_regions.size())) {
                            // Lets consider this a special case - with only 1 top solid and minimal shell thickness settings, the
                            // boundaries of solid layers are not anchored over/under perimeters, so lets fix it by adding at least one
                            // perimeter width of area
                            Polygons anchor_area = intersection(expand(cache_top_botom_regions[idx_layer].top_surfaces,
                                                                       layerm->flow(frExternalPerimeter).scaled_spacing()),
                                                                to_polygons(m_layers[i]->lslices));
                            combine_shells(anchor_area);
                        }

                        if (one_more_layer_below_top_bottom_surfaces)
                            if (i < int(cache_top_botom_regions.size()) &&
                                (i <= itop || m_layers[i]->bottom_z() - print_z < region_config.top_shell_thickness - EPSILON))
                                combine_holes(cache_top_botom_regions[i].holes);
	                }
	                if (int n_bottom_layers = region_config.bottom_shell_layers.value; n_bottom_layers > 0) {
                        // Gather bottom regions projected to this layer.
                        coordf_t bottom_z = layer->bottom_z();
                        int i = int(idx_layer) - 1;
                        int ibottom = int(idx_layer) - n_bottom_layers;
                        bool at_least_one_bottom_projected = false;
	                    for (; i >= 0 &&
	                         (i > ibottom || bottom_z - m_layers[i]->bottom_z() < region_config.bottom_shell_thickness - EPSILON);
	                        -- i) {
                                at_least_one_bottom_projected = true;
	                        const DiscoverVerticalShellsCacheEntry &cache = cache_top_botom_regions[i];
							combine_holes(cache.holes);
                            combine_shells(cache.bottom_surfaces);
	                    }

                        if (!at_least_one_bottom_projected && i >= 0) {
                            Polygons anchor_area = intersection(expand(cache_top_botom_regions[idx_layer].bottom_surfaces,
                                                                       layerm->flow(frExternalPerimeter).scaled_spacing()),
                                                                to_polygons(m_layers[i]->lslices));
                            combine_shells(anchor_area);
                        }

                        if (one_more_layer_below_top_bottom_surfaces)
                            if (i >= 0 &&
                                (i > ibottom || bottom_z - m_layers[i]->print_z < region_config.bottom_shell_thickness - EPSILON))
                                combine_holes(cache_top_botom_regions[i].holes);
	                }
#ifdef SLIC3R_DEBUG_SLICE_PROCESSING
                    {
        				Slic3r::SVG svg(debug_out_path("discover_vertical_shells-perimeters-before-union-%d.svg", debug_idx), get_extents(shell));
                        svg.draw(shell);
                        svg.draw_outline(shell, "black", scale_(0.05));
                        svg.Close(); 
                    }
#endif /* SLIC3R_DEBUG_SLICE_PROCESSING */
#if 0
//                    shell = union_(shell, true);
                    shell = union_(shell, false); 
#endif
#ifdef SLIC3R_DEBUG_SLICE_PROCESSING
                    shell_ex = union_safety_offset_ex(shell);
#endif /* SLIC3R_DEBUG_SLICE_PROCESSING */

                    //if (shell.empty())
                    //    continue;

#ifdef SLIC3R_DEBUG_SLICE_PROCESSING
                    {
                        Slic3r::SVG svg(debug_out_path("discover_vertical_shells-perimeters-after-union-%d.svg", debug_idx), get_extents(shell));
                        svg.draw(shell_ex);
                        svg.draw_outline(shell_ex, "black", "blue", scale_(0.05));
                        svg.Close();  
                    }
#endif /* SLIC3R_DEBUG_SLICE_PROCESSING */

#ifdef SLIC3R_DEBUG_SLICE_PROCESSING
                    {
                        Slic3r::SVG svg(debug_out_path("discover_vertical_shells-internal-wshell-%d.svg", debug_idx), get_extents(shell));
                        svg.draw(layerm->fill_surfaces().filter_by_type(stInternal), "yellow", 0.5);
                        svg.draw_outline(layerm->fill_surfaces().filter_by_type(stInternal), "black", "blue", scale_(0.05));
                        svg.draw(shell_ex, "blue", 0.5);
                        svg.draw_outline(shell_ex, "black", "blue", scale_(0.05));
                        svg.Close();
                    } 
                    {
                        Slic3r::SVG svg(debug_out_path("discover_vertical_shells-internalvoid-wshell-%d.svg", debug_idx), get_extents(shell));
                        svg.draw(layerm->fill_surfaces().filter_by_type(stInternalVoid), "yellow", 0.5);
                        svg.draw_outline(layerm->fill_surfaces().filter_by_type(stInternalVoid), "black", "blue", scale_(0.05));
                        svg.draw(shell_ex, "blue", 0.5);
                        svg.draw_outline(shell_ex, "black", "blue", scale_(0.05));
                        svg.Close();
                    } 
                    {
                        Slic3r::SVG svg(debug_out_path("discover_vertical_shells-internalsolid-wshell-%d.svg", debug_idx), get_extents(shell));
                        svg.draw(layerm->fill_surfaces().filter_by_type(stInternalSolid), "yellow", 0.5);
                        svg.draw_outline(layerm->fill_surfaces().filter_by_type(stInternalSolid), "black", "blue", scale_(0.05));
                        svg.draw(shell_ex, "blue", 0.5);
                        svg.draw_outline(shell_ex, "black", "blue", scale_(0.05));
                        svg.Close();
                    } 
#endif /* SLIC3R_DEBUG_SLICE_PROCESSING */

                    // Trim the shells region by the internal & internal void surfaces.
                    const Polygons    polygonsInternal = to_polygons(layerm->fill_surfaces.filter_by_types({ stInternal, stInternalVoid, stInternalSolid }));
                    shell = intersection(shell, polygonsInternal, ApplySafetyOffset::Yes);
                    polygons_append(shell, diff(polygonsInternal, holes));
                    if (shell.empty())
                        continue;

                    // Append the internal solids, so they will be merged with the new ones.
                    polygons_append(shell, to_polygons(layerm->fill_surfaces.filter_by_type(stInternalSolid)));

                    // These regions will be filled by a rectilinear full infill. Currently this type of infill
                    // only fills regions, which fit at least a single line. To avoid gaps in the sparse infill,
                    // make sure that this region does not contain parts narrower than the infill spacing width.
#ifdef SLIC3R_DEBUG_SLICE_PROCESSING
                    Polygons shell_before = shell;
#endif /* SLIC3R_DEBUG_SLICE_PROCESSING */
                    ExPolygons regularized_shell;
                    {
                        // Open to remove (filter out) regions narrower than a bit less than an infill extrusion line width.
                        // Such narrow regions are difficult to fill in with a gap fill algorithm (or Arachne), however they are most likely
                        // not needed for print stability / quality.
                        const float narrow_ensure_vertical_wall_thickness_region_radius = 0.5f * 0.65f * min_perimeter_infill_spacing;
                        // Then close gaps narrower than 1.2 * line width, such gaps are difficult to fill in with sparse infill,
                        // thus they will be merged into the solid infill.
                        const float narrow_sparse_infill_region_radius                  = 0.5f * 1.2f * min_perimeter_infill_spacing;
                        // Finally expand the infill a bit to remove tiny gaps between solid infill and the other regions.
                        const float tiny_overlap_radius                                 = 0.2f        * min_perimeter_infill_spacing;
                        regularized_shell = shrink_ex(offset2_ex(union_ex(shell),
                            // Open to remove (filter out) regions narrower than an infill extrusion line width.
                            -narrow_ensure_vertical_wall_thickness_region_radius,
                            // Then close gaps narrower than 1.2 * line width, such gaps are difficult to fill in with sparse infill.
                            narrow_ensure_vertical_wall_thickness_region_radius + narrow_sparse_infill_region_radius, ClipperLib::jtSquare),
                            // Finally expand the infill a bit to remove tiny gaps between solid infill and the other regions.
                            narrow_sparse_infill_region_radius - tiny_overlap_radius, ClipperLib::jtSquare);

                        Polygons object_volume;
                        Polygons internal_volume;
                        {
                            Polygons shrinked_bottom_slice = idx_layer > 0 ? to_polygons(m_layers[idx_layer - 1]->lslices) : Polygons{};
                            Polygons shrinked_upper_slice  = (idx_layer + 1) < m_layers.size() ?
                                                                 to_polygons(m_layers[idx_layer + 1]->lslices) :
                                                                 Polygons{};
                            object_volume = intersection(shrinked_bottom_slice, shrinked_upper_slice);
                            internal_volume = closing(polygonsInternal, float(SCALED_EPSILON));
                        }

                        // The regularization operation may cause scattered tiny drops on the smooth parts of the model, filter them out
                        // If the region checks both following conditions, it is removed:
                        //   1. the area is very small,
                        //      OR the area is quite small and it is fully wrapped in model (not visible)
                        //      the in-model condition is there due to small sloping surfaces, e.g. top of the hull of the benchy
                        //   2. the area does not fully cover an internal polygon
                        //         This is there mainly for a very thin parts, where the solid layers would be missing if the part area is quite small
                        regularized_shell.erase(std::remove_if(regularized_shell.begin(), regularized_shell.end(),
                                                               [&internal_volume, &min_perimeter_infill_spacing,
                                                                &object_volume](const ExPolygon &p) {
                                                                   return (p.area() < min_perimeter_infill_spacing * scaled(1.5) ||
                                                                           (p.area() < min_perimeter_infill_spacing * scaled(8.0) &&
                                                                            diff(to_polygons(p), object_volume).empty())) &&
                                                                          diff(internal_volume,
                                                                               expand(to_polygons(p), min_perimeter_infill_spacing))
                                                                                  .size() >= internal_volume.size();
                                                               }),
                                                regularized_shell.end());
                    }
                    if (regularized_shell.empty())
                        continue;

                    ExPolygons new_internal_solid = intersection_ex(polygonsInternal, regularized_shell);
#ifdef SLIC3R_DEBUG_SLICE_PROCESSING
                    {
                        Slic3r::SVG svg(debug_out_path("discover_vertical_shells-regularized-%d.svg", debug_idx), get_extents(shell_before));
                        // Source shell.
                        svg.draw(union_safety_offset_ex(shell_before));
                        // Shell trimmed to the internal surfaces.
                        svg.draw_outline(union_safety_offset_ex(shell), "black", "blue", scale_(0.05));
                        // Regularized infill region.
                        svg.draw_outline(new_internal_solid, "red", "magenta", scale_(0.05));
                        svg.Close();  
                    }
#endif /* SLIC3R_DEBUG_SLICE_PROCESSING */

                    // Trim the internal & internalvoid by the shell.
                    Slic3r::ExPolygons new_internal = diff_ex(layerm->fill_surfaces.filter_by_type(stInternal), regularized_shell);
                    Slic3r::ExPolygons new_internal_void = diff_ex(layerm->fill_surfaces.filter_by_type(stInternalVoid), regularized_shell);

#ifdef SLIC3R_DEBUG_SLICE_PROCESSING
                    {
                        SVG::export_expolygons(debug_out_path("discover_vertical_shells-new_internal-%d.svg", debug_idx), get_extents(shell), new_internal, "black", "blue", scale_(0.05));
        				SVG::export_expolygons(debug_out_path("discover_vertical_shells-new_internal_void-%d.svg", debug_idx), get_extents(shell), new_internal_void, "black", "blue", scale_(0.05));
        				SVG::export_expolygons(debug_out_path("discover_vertical_shells-new_internal_solid-%d.svg", debug_idx), get_extents(shell), new_internal_solid, "black", "blue", scale_(0.05));
                    }
#endif /* SLIC3R_DEBUG_SLICE_PROCESSING */

                    // Assign resulting internal surfaces to layer.
                    layerm->fill_surfaces.keep_types({ stTop, stBottom, stBottomBridge });
                    layerm->fill_surfaces.append(new_internal,       stInternal);
                    layerm->fill_surfaces.append(new_internal_void,  stInternalVoid);
                    layerm->fill_surfaces.append(new_internal_solid, stInternalSolid);
                } // for each layer
            });
        m_print->throw_if_canceled();
        BOOST_LOG_TRIVIAL(debug) << "Discovering vertical shells for region " << region_id << " in parallel - end";

#ifdef SLIC3R_DEBUG_SLICE_PROCESSING
		for (size_t idx_layer = 0; idx_layer < m_layers.size(); ++idx_layer) {
			LayerRegion *layerm = m_layers[idx_layer]->get_region(region_id);
			layerm->export_region_slices_to_svg_debug("4_discover_vertical_shells-final");
			layerm->export_region_fill_surfaces_to_svg_debug("4_discover_vertical_shells-final");
		}
#endif /* SLIC3R_DEBUG_SLICE_PROCESSING */
    } // for each region

    // Write the profiler measurements to file
//    PROFILE_UPDATE();
//    PROFILE_OUTPUT(debug_out_path("discover_vertical_shells-profile.txt").c_str());
}

#if 0
/* This method applies bridge flow to the first internal solid layer above
   sparse infill */
void PrintObject::bridge_over_infill()
{
    BOOST_LOG_TRIVIAL(info) << "Bridge over infill..." << log_memory_info();

    for (size_t region_id = 0; region_id < this->num_printing_regions(); ++ region_id) {
        const PrintRegion &region = this->printing_region(region_id);

        // skip bridging in case there are no voids
        if (region.config().sparse_infill_density.value == 100)
            continue;

		for (LayerPtrs::iterator layer_it = m_layers.begin(); layer_it != m_layers.end(); ++ layer_it) {
            // skip first layer
			if (layer_it == m_layers.begin())
                continue;

            Layer       *layer       = *layer_it;
            LayerRegion *layerm      = layer->m_regions[region_id];
            const PrintObjectConfig& object_config = layer->object()->config();
            //BBS: enable thick bridge for internal bridge only
            Flow         bridge_flow = layerm->bridging_flow(frSolidInfill, true);

            // extract the stInternalSolid surfaces that might be transformed into bridges
            Polygons internal_solid;
            layerm->fill_surfaces.filter_by_type(stInternalSolid, &internal_solid);

            // check whether the lower area is deep enough for absorbing the extra flow
            // (for obvious physical reasons but also for preventing the bridge extrudates
            // from overflowing in 3D preview)
            ExPolygons to_bridge;
            {
                Polygons to_bridge_pp = internal_solid;

                // iterate through lower layers spanned by bridge_flow
                double bottom_z = layer->print_z - bridge_flow.height() - EPSILON;
                for (int i = int(layer_it - m_layers.begin()) - 1; i >= 0; --i) {
                    const Layer* lower_layer = m_layers[i];

                    // stop iterating if layer is lower than bottom_z
                    if (lower_layer->print_z < bottom_z) break;

                    // iterate through regions and collect internal surfaces
                    Polygons lower_internal;
                    for (LayerRegion *lower_layerm : lower_layer->m_regions)
                        lower_layerm->fill_surfaces.filter_by_type(stInternal, &lower_internal);

                    // intersect such lower internal surfaces with the candidate solid surfaces
                    to_bridge_pp = intersection(to_bridge_pp, lower_internal);
                }

                // BBS: expand to make avoid gap between bridge and inner wall
                to_bridge_pp = expand(to_bridge_pp, bridge_flow.scaled_width());
                to_bridge_pp = intersection(to_bridge_pp, internal_solid);

                // there's no point in bridging too thin/short regions
                //FIXME Vojtech: The offset2 function is not a geometric offset,
                // therefore it may create 1) gaps, and 2) sharp corners, which are outside the original contour.
                // The gaps will be filled by a separate region, which makes the infill less stable and it takes longer.
                {
                    float min_width = float(bridge_flow.scaled_width()) * 3.f;
                    to_bridge_pp = opening(to_bridge_pp, min_width);
                }

                if (to_bridge_pp.empty()) continue;

                // convert into ExPolygons
                to_bridge = union_ex(to_bridge_pp);
            }

            #ifdef SLIC3R_DEBUG
            printf("Bridging %zu internal areas at layer %zu\n", to_bridge.size(), layer->id());
            #endif

            // compute the remaning internal solid surfaces as difference
            ExPolygons not_to_bridge = diff_ex(internal_solid, to_bridge, ApplySafetyOffset::Yes);
            to_bridge = intersection_ex(to_bridge, internal_solid, ApplySafetyOffset::Yes);
            // build the new collection of fill_surfaces
            layerm->fill_surfaces.remove_type(stInternalSolid);
            for (ExPolygon &ex : to_bridge) {
                layerm->fill_surfaces.surfaces.push_back(Surface(stInternalBridge, ex));
                // BBS: detect angle for internal bridge infill
                InternalBridgeDetector ibd(ex, layerm->fill_no_overlap_expolygons, bridge_flow.scaled_spacing());
                if (ibd.detect_angle()) {
                    (layerm->fill_surfaces.surfaces.end() - 1)->bridge_angle = ibd.angle;
                }
            }

            for (ExPolygon &ex : not_to_bridge)
                layerm->fill_surfaces.surfaces.push_back(Surface(stInternalSolid, ex));

            //BBS: modify stInternal to be stInternalWithLoop to give better support to internal bridge
            if (!to_bridge.empty()){
                float internal_loop_thickness = object_config.internal_bridge_support_thickness.value;
                double bottom_z = layer->print_z - layer->height - internal_loop_thickness + EPSILON;
                //BBS: lighting infill doesn't support this feature. Don't need to add loop when infill density is high than 50%
                if (region.config().sparse_infill_pattern != InfillPattern::ipLightning && region.config().sparse_infill_density.value < 50)
                    for (int i = int(layer_it - m_layers.begin()) - 1; i >= 0; --i) {
                        const Layer* lower_layer = m_layers[i];

                        if (lower_layer->print_z < bottom_z) break;

                        for (LayerRegion* lower_layerm : lower_layer->m_regions) {
                            Polygons lower_internal;
                            lower_layerm->fill_surfaces.filter_by_type(stInternal, &lower_internal);
                            ExPolygons internal_with_loop = intersection_ex(lower_internal, to_bridge);
                            ExPolygons internal = diff_ex(lower_internal, to_bridge);
                            if (internal_with_loop.empty()) {
                                //BBS: don't need to do anything
                            }
                            else if (internal.empty()) {
                                lower_layerm->fill_surfaces.change_to_new_type(stInternal, stInternalWithLoop);
                            }
                            else {
                                lower_layerm->fill_surfaces.remove_type(stInternal);
                                for (ExPolygon& ex : internal_with_loop)
                                    lower_layerm->fill_surfaces.surfaces.push_back(Surface(stInternalWithLoop, ex));
                                for (ExPolygon& ex : internal)
                                    lower_layerm->fill_surfaces.surfaces.push_back(Surface(stInternal, ex));
                            }
                        }
                    }
            }

            /*
            # exclude infill from the layers below if needed
            # see discussion at https://github.com/alexrj/Slic3r/issues/240
            # Update: do not exclude any infill. Sparse infill is able to absorb the excess material.
            if (0) {
                my $excess = $layerm->extruders->{infill}->bridge_flow->width - $layerm->height;
                for (my $i = $layer_id-1; $excess >= $self->get_layer($i)->height; $i--) {
                    Slic3r::debugf "  skipping infill below those areas at layer %d\n", $i;
                    foreach my $lower_layerm (@{$self->get_layer($i)->regions}) {
                        my @new_surfaces = ();
                        # subtract the area from all types of surfaces
                        foreach my $group (@{$lower_layerm->fill_surfaces->group}) {
                            push @new_surfaces, map $group->[0]->clone(expolygon => $_),
                                @{diff_ex(
                                    [ map $_->p, @$group ],
                                    [ map @$_, @$to_bridge ],
                                )};
                            push @new_surfaces, map Slic3r::Surface->new(
                                expolygon       => $_,
                                surface_type    => stInternalVoid,
                            ), @{intersection_ex(
                                [ map $_->p, @$group ],
                                [ map @$_, @$to_bridge ],
                            )};
                        }
                        $lower_layerm->fill_surfaces->clear;
                        $lower_layerm->fill_surfaces->append($_) for @new_surfaces;
                    }

                    $excess -= $self->get_layer($i)->height;
                }
            }
            */

#ifdef SLIC3R_DEBUG_SLICE_PROCESSING
            layerm->export_region_slices_to_svg_debug("7_bridge_over_infill");
            layerm->export_region_fill_surfaces_to_svg_debug("7_bridge_over_infill");
#endif /* SLIC3R_DEBUG_SLICE_PROCESSING */
            m_print->throw_if_canceled();
        }
    }
}
#else
// This method applies bridge flow to the first internal solid layer above sparse infill.
// This method applies bridge flow to the first internal solid layer above sparse infill.
//该方法将桥流应用于稀疏填充物上方的第一个内部固体层。
void PrintObject::bridge_over_infill()
{
    BOOST_LOG_TRIVIAL(info) << "Bridge over infill - Start" << log_memory_info();

    struct CandidateSurface
    {
        CandidateSurface(const Surface     *original_surface,
            int                layer_index,
            Polygons           new_polys,
            const LayerRegion *region,
            double             bridge_angle)
            : original_surface(original_surface)
            , layer_index(layer_index)
            , new_polys(new_polys)
            , region(region)
            , bridge_angle(bridge_angle)
        {}
        const Surface     *original_surface;
        int                layer_index;
        Polygons           new_polys;
        const LayerRegion *region;
        double             bridge_angle;
    };

    std::map<size_t, std::vector<CandidateSurface>> surfaces_by_layer;

    // SECTION to gather and filter surfaces for expanding, and then cluster them by layer
    {
        tbb::concurrent_vector<CandidateSurface> candidate_surfaces;
        tbb::parallel_for(tbb::blocked_range<size_t>(0, this->layers().size()), [po = static_cast<const PrintObject *>(this),
            &candidate_surfaces](tbb::blocked_range<size_t> r) {
                for (size_t lidx = r.begin(); lidx < r.end(); lidx++) {
                    const Layer *layer = po->get_layer(lidx);
                    if (layer->lower_layer == nullptr) {
                        continue;
                    }
                    double spacing = layer->regions().front()->flow(frSolidInfill).scaled_spacing();
                    // unsupported area will serve as a filter for polygons worth bridging.
                    Polygons   unsupported_area;
                    Polygons   lower_layer_solids;
                    for (const LayerRegion *region : layer->lower_layer->regions()) {
                        Polygons fill_polys = to_polygons(region->fill_expolygons);
                        // initially consider the whole layer unsupported, but also gather solid layers to later cut off supported parts
                        unsupported_area.insert(unsupported_area.end(), fill_polys.begin(), fill_polys.end());
                        for (const Surface &surface : region->fill_surfaces.surfaces) {
                            if (surface.surface_type != stInternal || region->region().config().sparse_infill_density.value == 100) {
                                Polygons p = to_polygons(surface.expolygon);
                                lower_layer_solids.insert(lower_layer_solids.end(), p.begin(), p.end());
                            }
                        }
                    }
                    unsupported_area = closing(unsupported_area, float(SCALED_EPSILON));
                    // By expanding the lower layer solids, we avoid making bridges from the tiny internal overhangs that are (very likely) supported by previous layer solids
                    // NOTE that we cannot filter out polygons worth bridging by their area, because sometimes there is a very small internal island that will grow into large hole
                    lower_layer_solids = shrink(lower_layer_solids, 1 * spacing); // first remove thin regions that will not support anything
                    lower_layer_solids = expand(lower_layer_solids, (1 + 3) * spacing); // then expand back (opening), and further for parts supported by internal solids
                    // By shrinking the unsupported area, we avoid making bridges from narrow ensuring region along perimeters.
                    unsupported_area   = shrink(unsupported_area, 3 * spacing);
                    unsupported_area   = diff(unsupported_area, lower_layer_solids);

                    for (LayerRegion *region : layer->regions()) {
                        SurfacesPtr region_internal_solids = region->fill_surfaces.filter_by_type(stInternalSolid);
                        for (const Surface *s : region_internal_solids) {
                            Polygons unsupported         = intersection(to_polygons(s->expolygon), unsupported_area);
                            // The following flag marks those surfaces, which overlap with unuspported area, but at least part of them is supported. 
                            // These regions can be filtered by area, because they for sure are touching solids on lower layers, and it does not make sense to bridge their tiny overhangs 
                            bool     partially_supported = area(unsupported) < area(to_polygons(s->expolygon)) - EPSILON;
                            if (!unsupported.empty() && (!partially_supported || area(unsupported) > 3 * 3 * spacing * spacing)) {
                                Polygons worth_bridging = intersection(to_polygons(s->expolygon), expand(unsupported, 4 * spacing));
                                // after we extracted the part worth briding, we go over the leftovers and merge the tiny ones back, to not brake the surface too much
                                for (const Polygon& p : diff(to_polygons(s->expolygon), expand(worth_bridging, spacing))) {
                                    double area = p.area();
                                    if (area < spacing * scale_(12.0) && area > spacing * spacing) {
                                        worth_bridging.push_back(p);
                                    }
                                }
                                worth_bridging = intersection(closing(worth_bridging, float(SCALED_EPSILON)), s->expolygon);
                                candidate_surfaces.push_back(CandidateSurface(s, lidx, worth_bridging, region, 0));

#ifdef DEBUG_BRIDGE_OVER_INFILL
                                debug_draw(std::to_string(lidx) + "_candidate_surface_" + std::to_string(area(s->expolygon)),
                                    to_lines(region->layer()->lslices), to_lines(s->expolygon), to_lines(worth_bridging),
                                    to_lines(unsupported_area));
#endif
#ifdef DEBUG_BRIDGE_OVER_INFILL
                                debug_draw(std::to_string(lidx) + "_candidate_processing_" + std::to_string(area(unsupported)),
                                    to_lines(unsupported), to_lines(intersection(to_polygons(s->expolygon), expand(unsupported, 5 * spacing))), 
                                    to_lines(diff(to_polygons(s->expolygon), expand(worth_bridging, spacing))),
                                    to_lines(unsupported_area));
#endif
                            }
                        }
                    }
                }
            });

        for (const CandidateSurface &c : candidate_surfaces) {
            surfaces_by_layer[c.layer_index].push_back(c);
        }
    }

    // LIGHTNING INFILL SECTION - If lightning infill is used somewhere, we check the areas that are going to be bridges, and those that rely on the 
    // lightning infill under them get expanded. This somewhat helps to ensure that most of the extrusions are anchored to the lightning infill at the ends.
    // It requires modifying this instance of print object in a specific way, so that we do not invalidate the pointers in our surfaces_by_layer structure.
    bool has_lightning_infill = false;
    for (size_t i = 0; i < this->num_printing_regions(); i++) {
        if (this->printing_region(i).config().sparse_infill_pattern == ipLightning) {
            has_lightning_infill = true;
            break;
        }
    }
    if (has_lightning_infill) {
        // Prepare backup data for the Layer Region infills. Before modfiyng the layer region, we backup its fill surfaces by moving! them into this map.
        // then a copy is created, modifiyed and passed to lightning infill generator. After generator is created, we restore the original state of the fills
        // again by moving the data from this map back to the layer regions. This ensures that pointers to surfaces stay valid.
        std::map<size_t, std::map<const LayerRegion *, SurfaceCollection>> backup_surfaces;
        for (size_t lidx = 0; lidx < this->layer_count(); lidx++) {
            backup_surfaces[lidx] = {};
        }

        tbb::parallel_for(tbb::blocked_range<size_t>(0, this->layers().size()), [po = this, &backup_surfaces,
            &surfaces_by_layer](tbb::blocked_range<size_t> r) {
                for (size_t lidx = r.begin(); lidx < r.end(); lidx++) {
                    if (surfaces_by_layer.find(lidx) == surfaces_by_layer.end())
                        continue;

                    Layer       *layer       = po->get_layer(lidx);
                    const Layer *lower_layer = layer->lower_layer;
                    if (lower_layer == nullptr)
                        continue;

                    Polygons lightning_fill;
                    for (LayerRegion *region : lower_layer->regions()) {
                        if (region->region().config().sparse_infill_pattern == ipLightning) {
                            Polygons lf = to_polygons(region->fill_surfaces.filter_by_type(stInternal));
                            lightning_fill.insert(lightning_fill.end(), lf.begin(), lf.end());
                        }
                    }

                    if (lightning_fill.empty())
                        continue;

                    for (LayerRegion *region : layer->regions()) {
                        backup_surfaces[lidx][region] = std::move(
                            region->fill_surfaces); // Make backup copy by move!! so that pointers in candidate surfaces stay valid
                        // Copy the surfaces back, this will make copy, but we will later discard it anyway
                        region->fill_surfaces = backup_surfaces[lidx][region];
                    }

                    for (LayerRegion *region : layer->regions()) {
                        ExPolygons sparse_infill = to_expolygons(region->fill_surfaces.filter_by_type(stInternal));
                        ExPolygons solid_infill  = to_expolygons(region->fill_surfaces.filter_by_type(stInternalSolid));

                        if (sparse_infill.empty()) {
                            break;
                        }
                        for (const auto &surface : surfaces_by_layer[lidx]) {
                            if (surface.region != region)
                                continue;
                            ExPolygons expansion = intersection_ex(sparse_infill, expand(surface.new_polys, scaled<float>(3.0)));
                            solid_infill.insert(solid_infill.end(), expansion.begin(), expansion.end());
                        }

                        solid_infill  = union_safety_offset_ex(solid_infill);
                        sparse_infill = diff_ex(sparse_infill, solid_infill);

                        region->fill_surfaces.remove_types({stInternalSolid, stInternal});
                        for (const ExPolygon &ep : solid_infill) {
                            region->fill_surfaces.surfaces.emplace_back(stInternalSolid, ep);
                        }
                        for (const ExPolygon &ep : sparse_infill) {
                            region->fill_surfaces.surfaces.emplace_back(stInternal, ep);
                        }
                    }
                }
            });

        // Use the modified surfaces to generate expanded lightning anchors
        this->m_lightning_generator = this->prepare_lightning_infill_data();

        // And now restore carefully the original surfaces, again using move to avoid reallocation and preserving the validity of the
        // pointers in surface candidates
        for (size_t lidx = 0; lidx < this->layer_count(); lidx++) {
            Layer *layer = this->get_layer(lidx);
            for (LayerRegion *region : layer->regions()) {
                if (backup_surfaces[lidx].find(region) != backup_surfaces[lidx].end()) {
                    region->fill_surfaces = std::move(backup_surfaces[lidx][region]);
                }
            }
        }
    }

    std::map<size_t, Polylines> infill_lines;
    // SECTION to generate infill polylines
    {
        std::vector<std::pair<const Surface *, float>> surfaces_w_bottom_z;
        for (const auto &pair : surfaces_by_layer) {
            for (const CandidateSurface &c : pair.second) {
                surfaces_w_bottom_z.emplace_back(c.original_surface, c.region->m_layer->bottom_z());
            }
        }

        this->m_adaptive_fill_octrees = this->prepare_adaptive_infill_data(surfaces_w_bottom_z);

        std::vector<size_t> layers_to_generate_infill;
        for (const auto &pair : surfaces_by_layer) {
            assert(pair.first > 0);
            infill_lines[pair.first - 1] = {};
            layers_to_generate_infill.push_back(pair.first - 1);
        }

        tbb::parallel_for(tbb::blocked_range<size_t>(0, layers_to_generate_infill.size()), [po = static_cast<const PrintObject *>(this),
            &layers_to_generate_infill,
            &infill_lines](tbb::blocked_range<size_t> r) {
                for (size_t job_idx = r.begin(); job_idx < r.end(); job_idx++) {
                    size_t lidx = layers_to_generate_infill[job_idx];
                    infill_lines.at(
                        lidx) = po->get_layer(lidx)->generate_sparse_infill_polylines_for_anchoring(po->m_adaptive_fill_octrees.first.get(),
                            po->m_adaptive_fill_octrees.second.get(),
                            po->m_lightning_generator.get());
                }
            });
#ifdef DEBUG_BRIDGE_OVER_INFILL
        for (const auto &il : infill_lines) {
            debug_draw(std::to_string(il.first) + "_infill_lines", to_lines(get_layer(il.first)->lslices), to_lines(il.second), {}, {});
        }
#endif
    }

    // cluster layers by depth needed for thick bridges. Each cluster is to be processed by single thread sequentially, so that bridges cannot appear one on another
    std::vector<std::vector<size_t>> clustered_layers_for_threads;
    float target_flow_height_factor = 0.9f;
    {
        std::vector<size_t> layers_with_candidates;
        std::map<size_t, Polygons> layer_area_covered_by_candidates;
        for (const auto& pair : surfaces_by_layer) {
            layers_with_candidates.push_back(pair.first);
            layer_area_covered_by_candidates[pair.first] = {};
        }

        // prepare inflated filter for each candidate on each layer. layers will be put into single thread cluster if they are close to each other (z-axis-wise)
        // and if the inflated AABB polygons overlap somewhere
        tbb::parallel_for(tbb::blocked_range<size_t>(0, layers_with_candidates.size()), [&layers_with_candidates, &surfaces_by_layer,
            &layer_area_covered_by_candidates](
                tbb::blocked_range<size_t> r) {
                    for (size_t job_idx = r.begin(); job_idx < r.end(); job_idx++) {
                        size_t lidx = layers_with_candidates[job_idx];
                        for (const auto &candidate : surfaces_by_layer.at(lidx)) {
                            Polygon candiate_inflated_aabb = get_extents(candidate.new_polys).inflated(scale_(7)).polygon();
                            layer_area_covered_by_candidates.at(lidx) = union_(layer_area_covered_by_candidates.at(lidx),
                                Polygons{candiate_inflated_aabb});
                        }
                    }
            });

        // note: surfaces_by_layer is ordered map
        for (auto pair : surfaces_by_layer) {
            if (clustered_layers_for_threads.empty() ||
                this->get_layer(clustered_layers_for_threads.back().back())->print_z <
                this->get_layer(pair.first)->print_z -
                this->get_layer(pair.first)->regions()[0]->bridging_flow(frSolidInfill, true).height() * target_flow_height_factor -
                EPSILON ||
                intersection(layer_area_covered_by_candidates[clustered_layers_for_threads.back().back()],
                    layer_area_covered_by_candidates[pair.first])
                .empty()) {
                clustered_layers_for_threads.push_back({pair.first});
            } else {
                clustered_layers_for_threads.back().push_back(pair.first);
            }
        }

#ifdef DEBUG_BRIDGE_OVER_INFILL
        std::cout << "BRIDGE OVER INFILL CLUSTERED LAYERS FOR SINGLE THREAD" << std::endl;
        for (auto cluster : clustered_layers_for_threads) {
            std::cout << "CLUSTER: ";
            for (auto l : cluster) {
                std::cout << l << "  ";
            }
            std::cout << std::endl;
        }
#endif
    }

    // LAMBDA to gather areas with sparse infill deep enough that we can fit thick bridges there.
    auto gather_areas_w_depth = [target_flow_height_factor](const PrintObject *po, int lidx, float target_flow_height) {
        // Gather layers sparse infill areas, to depth defined by used bridge flow
        ExPolygons layers_sparse_infill{};
        ExPolygons not_sparse_infill{};
        double   bottom_z = po->get_layer(lidx)->print_z - target_flow_height * target_flow_height_factor - EPSILON;
        for (int i = int(lidx) - 1; i >= 0; --i) {
            // Stop iterating if layer is lower than bottom_z and at least one iteration was made
            const Layer *layer = po->get_layer(i);
            if (layer->print_z < bottom_z && i < int(lidx) - 1)
                break;

            for (const LayerRegion *region : layer->regions()) {
                bool has_low_density = region->region().config().sparse_infill_density.value < 100;
                for (const Surface &surface : region->fill_surfaces.surfaces) {
                    if ((surface.surface_type == stInternal && has_low_density) || surface.surface_type == stInternalVoid ) {
                        layers_sparse_infill.push_back(surface.expolygon);
                    } else {
                        not_sparse_infill.push_back(surface.expolygon);
                    }
                }
            }
        }
        layers_sparse_infill = union_ex(layers_sparse_infill);
        layers_sparse_infill = closing_ex(layers_sparse_infill, float(SCALED_EPSILON));
        not_sparse_infill    = union_ex(not_sparse_infill);
        not_sparse_infill    = closing_ex(not_sparse_infill, float(SCALED_EPSILON));
        return diff(layers_sparse_infill, not_sparse_infill);
        };

    // LAMBDA do determine optimal bridging angle
    auto determine_bridging_angle = [](const Polygons &bridged_area, const Lines &anchors, InfillPattern dominant_pattern, double infill_direction) {
        AABBTreeLines::LinesDistancer<Line> lines_tree(anchors);

        // Check it the infill that require a fixed infill angle.
        switch (dominant_pattern) {
        case ip3DHoneycomb:
        case ipCrossHatch:
            return (infill_direction + 45.0) * 2.0 * M_PI / 360.;
        default: break;
        }

        std::map<double, int> counted_directions;
        for (const Polygon &p : bridged_area) {
            double acc_distance = 0;
            for (int point_idx = 0; point_idx < int(p.points.size()) - 1; ++point_idx) {
                Vec2d  start        = p.points[point_idx].cast<double>();
                Vec2d  next         = p.points[point_idx + 1].cast<double>();
                Vec2d  v            = next - start; // vector from next to current
                double dist_to_next = v.norm();
                acc_distance += dist_to_next;
                if (acc_distance > scaled(2.0)) {
                    acc_distance = 0.0;
                    v.normalize();
                    int   lines_count = int(std::ceil(dist_to_next / scaled(2.0)));
                    float step_size   = dist_to_next / lines_count;
                    for (int i = 0; i < lines_count; ++i) {
                        Point a                   = (start + v * (i * step_size)).cast<coord_t>();
                        auto [distance, index, p] = lines_tree.distance_from_lines_extra<false>(a);
                        double angle = lines_tree.get_line(index).orientation();
                        if (angle > PI) {
                            angle -= PI;
                        }
                        angle += PI * 0.5;
                        counted_directions[angle]++;
                    }
                }
            }
        }

        std::pair<double, int> best_dir{0, 0};
        // sliding window accumulation
        for (const auto &dir : counted_directions) {
            int    score_acc          = 0;
            double dir_acc            = 0;
            double window_start_angle = dir.first - PI * 0.1;
            double window_end_angle   = dir.first + PI * 0.1;
            for (auto dirs_window = counted_directions.lower_bound(window_start_angle);
                dirs_window != counted_directions.upper_bound(window_end_angle); dirs_window++) {
                dir_acc += dirs_window->first * dirs_window->second;
                score_acc += dirs_window->second;
            }
            // current span of directions is 0.5 PI to 1.5 PI (due to the aproach.). Edge values should also account for the
            //  opposite direction.
            if (window_start_angle < 0.5 * PI) {
                for (auto dirs_window = counted_directions.lower_bound(1.5 * PI - (0.5 * PI - window_start_angle));
                    dirs_window != counted_directions.end(); dirs_window++) {
                    dir_acc += dirs_window->first * dirs_window->second;
                    score_acc += dirs_window->second;
                }
            }
            if (window_start_angle > 1.5 * PI) {
                for (auto dirs_window = counted_directions.begin();
                    dirs_window != counted_directions.upper_bound(window_start_angle - 1.5 * PI); dirs_window++) {
                    dir_acc += dirs_window->first * dirs_window->second;
                    score_acc += dirs_window->second;
                }
            }

            if (score_acc > best_dir.second) {
                best_dir = {dir_acc / score_acc, score_acc};
            }
        }
        double bridging_angle = best_dir.first;
        if (bridging_angle == 0) {
            bridging_angle = 0.001;
        }
        switch (dominant_pattern) {
        case ipHilbertCurve: bridging_angle += 0.25 * PI; break;
        case ipOctagramSpiral: bridging_angle += (1.0 / 16.0) * PI; break;
        default: break;
        }

        return bridging_angle;
        };

    // LAMBDA that will fill given polygons with lines, exapand the lines to the nearest anchor, and reconstruct polygons from the newly
    // generated lines
    auto construct_anchored_polygon = [](Polygons bridged_area, Lines anchors, const Flow &bridging_flow, double bridging_angle) {
        auto lines_rotate = [](Lines &lines, double cos_angle, double sin_angle) {
            for (Line &l : lines) {
                double ax = double(l.a.x());
                double ay = double(l.a.y());
                l.a.x()   = coord_t(round(cos_angle * ax - sin_angle * ay));
                l.a.y()   = coord_t(round(cos_angle * ay + sin_angle * ax));
                double bx = double(l.b.x());
                double by = double(l.b.y());
                l.b.x()   = coord_t(round(cos_angle * bx - sin_angle * by));
                l.b.y()   = coord_t(round(cos_angle * by + sin_angle * bx));
            }
            };

        auto segments_overlap = [](coord_t alow, coord_t ahigh, coord_t blow, coord_t bhigh) {
            return (alow >= blow && alow <= bhigh) || (ahigh >= blow && ahigh <= bhigh) || (blow >= alow && blow <= ahigh) ||
                (bhigh >= alow && bhigh <= ahigh);
            };

        Polygons expanded_bridged_area{};
        double   aligning_angle = -bridging_angle + PI * 0.5;
        {
            polygons_rotate(bridged_area, aligning_angle);
            lines_rotate(anchors, cos(aligning_angle), sin(aligning_angle));
            BoundingBox bb_x = get_extents(bridged_area);
            BoundingBox bb_y = get_extents(anchors);

            const size_t n_vlines = (bb_x.max.x() - bb_x.min.x() + bridging_flow.scaled_spacing() - 1) / bridging_flow.scaled_spacing();
            std::vector<Line> vertical_lines(n_vlines);
            for (size_t i = 0; i < n_vlines; i++) {
                coord_t x           = bb_x.min.x() + i * bridging_flow.scaled_spacing();
                coord_t y_min       = bb_y.min.y() - bridging_flow.scaled_spacing();
                coord_t y_max       = bb_y.max.y() + bridging_flow.scaled_spacing();
                vertical_lines[i].a = Point{x, y_min};
                vertical_lines[i].b = Point{x, y_max};
            }

            auto anchors_and_walls_tree = AABBTreeLines::LinesDistancer<Line>{std::move(anchors)};
            auto bridged_area_tree      = AABBTreeLines::LinesDistancer<Line>{to_lines(bridged_area)};

            std::vector<std::vector<Line>> polygon_sections(n_vlines);
            for (size_t i = 0; i < n_vlines; i++) {
                auto area_intersections = bridged_area_tree.intersections_with_line<true>(vertical_lines[i]);
                for (int intersection_idx = 0; intersection_idx < int(area_intersections.size()) - 1; intersection_idx++) {
                    if (bridged_area_tree.outside(
                        (area_intersections[intersection_idx].first + area_intersections[intersection_idx + 1].first) / 2) < 0) {
                        polygon_sections[i].emplace_back(area_intersections[intersection_idx].first,
                            area_intersections[intersection_idx + 1].first);
                    }
                }
                auto anchors_intersections = anchors_and_walls_tree.intersections_with_line<true>(vertical_lines[i]);

                for (Line &section : polygon_sections[i]) {
                    auto maybe_below_anchor = std::upper_bound(anchors_intersections.rbegin(), anchors_intersections.rend(), section.a,
                        [](const Point &a, const std::pair<Point, size_t> &b) {
                            return a.y() > b.first.y();
                        });
                    if (maybe_below_anchor != anchors_intersections.rend()) {
                        section.a = maybe_below_anchor->first;
                        section.a.y() -= bridging_flow.scaled_width() * (0.5 + 0.5);
                    }

                    auto maybe_upper_anchor = std::upper_bound(anchors_intersections.begin(), anchors_intersections.end(), section.b,
                        [](const Point &a, const std::pair<Point, size_t> &b) {
                            return a.y() < b.first.y();
                        });
                    if (maybe_upper_anchor != anchors_intersections.end()) {
                        section.b = maybe_upper_anchor->first;
                        section.b.y() += bridging_flow.scaled_width() * (0.5 + 0.5);
                    }
                }

                for (int section_idx = 0; section_idx < int(polygon_sections[i].size()) - 1; section_idx++) {
                    Line &section_a = polygon_sections[i][section_idx];
                    Line &section_b = polygon_sections[i][section_idx + 1];
                    if (segments_overlap(section_a.a.y(), section_a.b.y(), section_b.a.y(), section_b.b.y())) {
                        section_b.a = section_a.a.y() < section_b.a.y() ? section_a.a : section_b.a;
                        section_b.b = section_a.b.y() < section_b.b.y() ? section_b.b : section_a.b;
                        section_a.a = section_a.b;
                    }
                }

                polygon_sections[i].erase(std::remove_if(polygon_sections[i].begin(), polygon_sections[i].end(),
                    [](const Line &s) { return s.a == s.b; }),
                    polygon_sections[i].end());
                std::sort(polygon_sections[i].begin(), polygon_sections[i].end(),
                    [](const Line &a, const Line &b) { return a.a.y() < b.b.y(); });
            }

            // reconstruct polygon from polygon sections
            struct TracedPoly
            {
                Points lows;
                Points highs;
            };

            std::vector<TracedPoly> current_traced_polys;
            for (const auto &polygon_slice : polygon_sections) {
                std::unordered_set<const Line *> used_segments;
                for (TracedPoly &traced_poly : current_traced_polys) {
                    auto candidates_begin = std::upper_bound(polygon_slice.begin(), polygon_slice.end(), traced_poly.lows.back(),
                        [](const Point &low, const Line &seg) { return seg.b.y() > low.y(); });
                    auto candidates_end   = std::upper_bound(polygon_slice.begin(), polygon_slice.end(), traced_poly.highs.back(),
                        [](const Point &high, const Line &seg) { return seg.a.y() > high.y(); });

                    bool segment_added = false;
                    for (auto candidate = candidates_begin; candidate != candidates_end && !segment_added; candidate++) {
                        if (used_segments.find(&(*candidate)) != used_segments.end()) {
                            continue;
                        }

                        if ((traced_poly.lows.back() - candidate->a).cast<double>().squaredNorm() <
                            36.0 * double(bridging_flow.scaled_spacing()) * bridging_flow.scaled_spacing()) {
                            traced_poly.lows.push_back(candidate->a);
                        } else {
                            traced_poly.lows.push_back(traced_poly.lows.back() + Point{bridging_flow.scaled_spacing() / 2, 0});
                            traced_poly.lows.push_back(candidate->a - Point{bridging_flow.scaled_spacing() / 2, 0});
                            traced_poly.lows.push_back(candidate->a);
                        }

                        if ((traced_poly.highs.back() - candidate->b).cast<double>().squaredNorm() <
                            36.0 * double(bridging_flow.scaled_spacing()) * bridging_flow.scaled_spacing()) {
                            traced_poly.highs.push_back(candidate->b);
                        } else {
                            traced_poly.highs.push_back(traced_poly.highs.back() + Point{bridging_flow.scaled_spacing() / 2, 0});
                            traced_poly.highs.push_back(candidate->b - Point{bridging_flow.scaled_spacing() / 2, 0});
                            traced_poly.highs.push_back(candidate->b);
                        }
                        segment_added = true;
                        used_segments.insert(&(*candidate));
                    }

                    if (!segment_added) {
                        // Zero overlapping segments, we just close this polygon
                        traced_poly.lows.push_back(traced_poly.lows.back() + Point{bridging_flow.scaled_spacing() / 2, 0});
                        traced_poly.highs.push_back(traced_poly.highs.back() + Point{bridging_flow.scaled_spacing() / 2, 0});
                        Polygon &new_poly = expanded_bridged_area.emplace_back(std::move(traced_poly.lows));
                        new_poly.points.insert(new_poly.points.end(), traced_poly.highs.rbegin(), traced_poly.highs.rend());
                        traced_poly.lows.clear();
                        traced_poly.highs.clear();
                    }
                }

                current_traced_polys.erase(std::remove_if(current_traced_polys.begin(), current_traced_polys.end(),
                    [](const TracedPoly &tp) { return tp.lows.empty(); }),
                    current_traced_polys.end());

                for (const auto &segment : polygon_slice) {
                    if (used_segments.find(&segment) == used_segments.end()) {
                        TracedPoly &new_tp = current_traced_polys.emplace_back();
                        new_tp.lows.push_back(segment.a - Point{bridging_flow.scaled_spacing() / 2, 0});
                        new_tp.lows.push_back(segment.a);
                        new_tp.highs.push_back(segment.b - Point{bridging_flow.scaled_spacing() / 2, 0});
                        new_tp.highs.push_back(segment.b);
                    }
                }
            }

            // add not closed polys
            for (TracedPoly &traced_poly : current_traced_polys) {
                Polygon &new_poly = expanded_bridged_area.emplace_back(std::move(traced_poly.lows));
                new_poly.points.insert(new_poly.points.end(), traced_poly.highs.rbegin(), traced_poly.highs.rend());
            }
            expanded_bridged_area = union_safety_offset(expanded_bridged_area);
        }

        polygons_rotate(expanded_bridged_area, -aligning_angle);
        return expanded_bridged_area;
        };

    tbb::parallel_for(tbb::blocked_range<size_t>(0, clustered_layers_for_threads.size()), [po = static_cast<const PrintObject *>(this),
        target_flow_height_factor, &surfaces_by_layer,
        &clustered_layers_for_threads,
        gather_areas_w_depth, &infill_lines,
        determine_bridging_angle,
        construct_anchored_polygon](
            tbb::blocked_range<size_t> r) {
                for (size_t cluster_idx = r.begin(); cluster_idx < r.end(); cluster_idx++) {
                    for (size_t job_idx = 0; job_idx < clustered_layers_for_threads[cluster_idx].size(); job_idx++) {
                        size_t       lidx  = clustered_layers_for_threads[cluster_idx][job_idx];
                        const Layer *layer = po->get_layer(lidx);
                        // this thread has exclusive access to all surfaces in layers enumerated in
                        // clustered_layers_for_threads[cluster_idx]

                        // Presort the candidate polygons. This will help choose the same angle for neighbournig surfaces, that
                        // would otherwise compete over anchoring sparse infill lines, leaving one area unachored
                        std::sort(surfaces_by_layer[lidx].begin(), surfaces_by_layer[lidx].end(),
                            [](const CandidateSurface &left, const CandidateSurface &right) {
                                auto a = get_extents(left.new_polys);
                                auto b = get_extents(right.new_polys);

                                if (a.min.x() == b.min.x()) {
                                    return a.min.y() < b.min.y();
                                };
                                return a.min.x() < b.min.x();
                            });
                        if (surfaces_by_layer[lidx].size() > 2) {
                            Vec2d origin = get_extents(surfaces_by_layer[lidx].front().new_polys).max.cast<double>();
                            std::stable_sort(surfaces_by_layer[lidx].begin() + 1, surfaces_by_layer[lidx].end(),
                                [origin](const CandidateSurface &left, const CandidateSurface &right) {
                                    auto a = get_extents(left.new_polys);
                                    auto b = get_extents(right.new_polys);

                                    return (origin - a.min.cast<double>()).squaredNorm() <
                                        (origin - b.min.cast<double>()).squaredNorm();
                                });
                        }

                        // Gather deep infill areas, where thick bridges fit
                        coordf_t spacing            = surfaces_by_layer[lidx].front().region->bridging_flow(frSolidInfill, true).scaled_spacing();
                        coordf_t target_flow_height = surfaces_by_layer[lidx].front().region->bridging_flow(frSolidInfill, true).height() *
                            target_flow_height_factor;
                        Polygons deep_infill_area = gather_areas_w_depth(po, lidx, target_flow_height);

                        {
                            // Now also remove area that has been already filled on lower layers by bridging expansion - For this
                            // reason we did the clustering of layers per thread.
                            Polygons filled_polyons_on_lower_layers;
                            double   bottom_z = layer->print_z - target_flow_height - EPSILON;
                            if (job_idx > 0) {
                                for (int lower_job_idx = job_idx - 1; lower_job_idx >= 0; lower_job_idx--) {
                                    size_t       lower_layer_idx = clustered_layers_for_threads[cluster_idx][lower_job_idx];
                                    const Layer *lower_layer     = po->get_layer(lower_layer_idx);
                                    if (lower_layer->print_z >= bottom_z) {
                                        for (const auto &c : surfaces_by_layer[lower_layer_idx]) {
                                            filled_polyons_on_lower_layers.insert(filled_polyons_on_lower_layers.end(), c.new_polys.begin(),
                                                c.new_polys.end());
                                        }
                                    } else {
                                        break;
                                    }
                                }
                            }
                            deep_infill_area = diff(deep_infill_area, filled_polyons_on_lower_layers);
                        }

                        deep_infill_area = expand(deep_infill_area, spacing * 1.5);

                        // Now gather expansion polygons - internal infill on current layer, from which we can cut off anchors
                        Polygons lightning_area;
                        Polygons expansion_area;
                        Polygons total_fill_area;
                        Polygons top_area;

                        for (LayerRegion *region : layer->regions()) {
                            Polygons internal_polys = to_polygons(region->fill_surfaces.filter_by_types({stInternal, stInternalSolid}));
                            expansion_area.insert(expansion_area.end(), internal_polys.begin(), internal_polys.end());
                            Polygons fill_polys = to_polygons(region->fill_expolygons);
                            total_fill_area.insert(total_fill_area.end(), fill_polys.begin(), fill_polys.end());
                            Polygons top_polys = to_polygons(region->fill_surfaces.filter_by_type(stTop));
                            top_area.insert(top_area.end(), top_polys.begin(), top_polys.end());
                            if (region->region().config().sparse_infill_pattern == ipLightning) {
                                Polygons l = to_polygons(region->fill_surfaces.filter_by_type(stInternal));
                                lightning_area.insert(lightning_area.end(), l.begin(), l.end());
                            }
                        }
                        total_fill_area   = closing(total_fill_area, float(SCALED_EPSILON));
                        expansion_area    = closing(expansion_area, float(SCALED_EPSILON));
                        expansion_area    = intersection(expansion_area, deep_infill_area);
                        Polylines anchors = intersection_pl(infill_lines[lidx - 1], shrink(expansion_area, spacing));
                        Polygons internal_unsupported_area = shrink(deep_infill_area, spacing * 4.5);

#ifdef DEBUG_BRIDGE_OVER_INFILL
                        debug_draw(std::to_string(lidx) + "_" + std::to_string(cluster_idx) + "_" + std::to_string(job_idx) + "_" + "_total_area",
                            to_lines(total_fill_area), to_lines(expansion_area), to_lines(deep_infill_area), to_lines(anchors));
#endif

                        std::vector<CandidateSurface> expanded_surfaces;
                        expanded_surfaces.reserve(surfaces_by_layer[lidx].size());
                        for (const CandidateSurface &candidate : surfaces_by_layer[lidx]) {
                            const Flow &flow              = candidate.region->bridging_flow(frSolidInfill, true);
                            Polygons    area_to_be_bridge = expand(candidate.new_polys, flow.scaled_spacing());
                            area_to_be_bridge             = intersection(area_to_be_bridge, deep_infill_area);

                            area_to_be_bridge.erase(std::remove_if(area_to_be_bridge.begin(), area_to_be_bridge.end(),
                                [internal_unsupported_area](const Polygon &p) {
                                    return intersection({p}, internal_unsupported_area).empty();
                                }),
                                area_to_be_bridge.end());

                            Polygons limiting_area = union_(area_to_be_bridge, expansion_area);

                            if (area_to_be_bridge.empty())
                                continue;

                            Polylines boundary_plines = to_polylines(expand(total_fill_area, 1.3 * flow.scaled_spacing()));
                            {
                                Polylines limiting_plines = to_polylines(expand(limiting_area, 0.3*flow.spacing()));
                                boundary_plines.insert(boundary_plines.end(), limiting_plines.begin(), limiting_plines.end());
                            }

#ifdef DEBUG_BRIDGE_OVER_INFILL
                            int r = rand();
                            debug_draw(std::to_string(lidx) + "_" + std::to_string(cluster_idx) + "_" + std::to_string(job_idx) + "_" +
                                "_anchors_" + std::to_string(r),
                                to_lines(area_to_be_bridge), to_lines(boundary_plines), to_lines(anchors), to_lines(expansion_area));
#endif

                            double bridging_angle = 0;
                            if (!anchors.empty()) {
                                bridging_angle = determine_bridging_angle(area_to_be_bridge, to_lines(anchors),
                                                                            candidate.region->region().config().sparse_infill_pattern.value,
                                                                            candidate.region->region().config().infill_direction.value);
                            } else {
                                // use expansion boundaries as anchors.
                                // Also, use Infill pattern that is neutral for angle determination, since there are no infill lines.
                                bridging_angle = determine_bridging_angle(area_to_be_bridge, to_lines(boundary_plines), InfillPattern::ipLine, 0);
                            }

                            boundary_plines.insert(boundary_plines.end(), anchors.begin(), anchors.end());
                            if (!lightning_area.empty() && !intersection(area_to_be_bridge, lightning_area).empty()) {
                                boundary_plines = intersection_pl(boundary_plines, expand(area_to_be_bridge, scale_(10)));
                            }
                            Polygons bridging_area = construct_anchored_polygon(area_to_be_bridge, to_lines(boundary_plines), flow, bridging_angle);

                            // Check collision with other expanded surfaces
                            {
                                bool     reconstruct       = false;
                                Polygons tmp_expanded_area = expand(bridging_area, 3.0 * flow.scaled_spacing());
                                for (const CandidateSurface &s : expanded_surfaces) {
                                    if (!intersection(s.new_polys, tmp_expanded_area).empty()) {
                                        bridging_angle = s.bridge_angle;
                                        reconstruct    = true;
                                        break;
                                    }
                                }
                                if (reconstruct) {
                                    bridging_area = construct_anchored_polygon(area_to_be_bridge, to_lines(boundary_plines), flow, bridging_angle);
                                }
                            }

                            bridging_area          = opening(bridging_area, flow.scaled_spacing());
                            bridging_area          = closing(bridging_area, flow.scaled_spacing());
                            bridging_area          = intersection(bridging_area, limiting_area);
                            bridging_area          = intersection(bridging_area, total_fill_area);
                            // BBS: substract top area
                            bridging_area          = diff(bridging_area, top_area);
                            // BBS: open and close again to filter some narrow parts
                            bridging_area          = opening(bridging_area, flow.scaled_spacing());
                            bridging_area          = closing(bridging_area, flow.scaled_spacing());
                            expansion_area         = diff(expansion_area, bridging_area);

#ifdef DEBUG_BRIDGE_OVER_INFILL
                            debug_draw(std::to_string(lidx) + "_" + std::to_string(cluster_idx) + "_" + std::to_string(job_idx) + "_" + "_expanded_bridging" +  std::to_string(r),
                                to_lines(layer->lslices), to_lines(boundary_plines), to_lines(candidate.new_polys), to_lines(bridging_area));
#endif

                            expanded_surfaces.push_back(CandidateSurface(candidate.original_surface, candidate.layer_index, bridging_area,
                                candidate.region, bridging_angle));
                        }
                        surfaces_by_layer[lidx].swap(expanded_surfaces);
                        expanded_surfaces.clear();
                    }
                }
        });

    BOOST_LOG_TRIVIAL(info) << "Bridge over infill - Directions and expanded surfaces computed" << log_memory_info();

    tbb::parallel_for(tbb::blocked_range<size_t>(0, this->layers().size()), [po = this, &surfaces_by_layer](tbb::blocked_range<size_t> r) {
        for (size_t lidx = r.begin(); lidx < r.end(); lidx++) {
            if (surfaces_by_layer.find(lidx) == surfaces_by_layer.end() && surfaces_by_layer.find(lidx + 1) == surfaces_by_layer.end())
                continue;
            Layer *layer = po->get_layer(lidx);

            Polygons cut_from_infill{};
            if (surfaces_by_layer.find(lidx) != surfaces_by_layer.end()) {
                for (const auto &surface : surfaces_by_layer.at(lidx)) {
                    cut_from_infill.insert(cut_from_infill.end(), surface.new_polys.begin(), surface.new_polys.end());
                }
            }

            Polygons additional_ensuring_areas{};
            if (surfaces_by_layer.find(lidx + 1) != surfaces_by_layer.end()) {
                for (const auto &surface : surfaces_by_layer.at(lidx + 1)) {
                    auto additional_area = diff(surface.new_polys,
                        shrink(surface.new_polys, surface.region->flow(frSolidInfill).scaled_spacing()));
                    additional_ensuring_areas.insert(additional_ensuring_areas.end(), additional_area.begin(), additional_area.end());
                }
            }

            for (LayerRegion *region : layer->regions()) {
                Surfaces new_surfaces;

                Polygons near_perimeters = to_polygons(union_safety_offset_ex(to_polygons(region->fill_surfaces.surfaces)));
                near_perimeters          = diff(near_perimeters, shrink(near_perimeters, region->flow(frSolidInfill).scaled_spacing()));
                ExPolygons additional_ensuring = intersection_ex(additional_ensuring_areas, near_perimeters);

                SurfacesPtr internal_infills = region->fill_surfaces.filter_by_type(stInternal);
                ExPolygons new_internal_infills = diff_ex(internal_infills, cut_from_infill);
                new_internal_infills            = diff_ex(new_internal_infills, additional_ensuring);
                for (const ExPolygon &ep : new_internal_infills) {
                    new_surfaces.emplace_back(stInternal, ep);
                }

                SurfacesPtr internal_solids = region->fill_surfaces.filter_by_type(stInternalSolid);
                if (surfaces_by_layer.find(lidx) != surfaces_by_layer.end()) {
                    for (const CandidateSurface &cs : surfaces_by_layer.at(lidx)) {
                        for (const Surface *surface : internal_solids) {
                            if (cs.original_surface == surface) {
                                Surface tmp{*surface, {}};
                                tmp.surface_type = stInternalBridge;
                                tmp.bridge_angle = cs.bridge_angle;
                                for (const ExPolygon &ep : union_ex(cs.new_polys)) {
                                    new_surfaces.emplace_back(tmp, ep);
                                }
                                break;
                            }
                        }
                    }
                }
                ExPolygons new_internal_solids = to_expolygons(internal_solids);
                new_internal_solids.insert(new_internal_solids.end(), additional_ensuring.begin(), additional_ensuring.end());
                new_internal_solids = diff_ex(new_internal_solids, cut_from_infill);
                new_internal_solids = union_safety_offset_ex(new_internal_solids);
                for (const ExPolygon &ep : new_internal_solids) {
                    new_surfaces.emplace_back(stInternalSolid, ep);
                }

#ifdef DEBUG_BRIDGE_OVER_INFILL
                debug_draw("Aensuring_" + std::to_string(reinterpret_cast<uint64_t>(&region)), to_polylines(additional_ensuring),
                    to_polylines(near_perimeters), to_polylines(to_polygons(internal_infills)),
                    to_polylines(to_polygons(internal_solids)));
                debug_draw("Aensuring_" + std::to_string(reinterpret_cast<uint64_t>(&region)) + "_new", to_polylines(additional_ensuring),
                    to_polylines(near_perimeters), to_polylines(to_polygons(new_internal_infills)),
                    to_polylines(to_polygons(new_internal_solids)));
#endif

                region->fill_surfaces.remove_types({stInternalSolid, stInternal});
                region->fill_surfaces.append(new_surfaces);
            }
        }
        });

    BOOST_LOG_TRIVIAL(info) << "Bridge over infill - End" << log_memory_info();

} // void PrintObject::bridge_over_infill()




#endif
static void clamp_exturder_to_default(ConfigOptionInt &opt, size_t num_extruders)
{
    if (opt.value > (int)num_extruders)
        // assign the default extruder
        opt.value = 1;
}

PrintObjectConfig PrintObject::object_config_from_model_object(const PrintObjectConfig &default_object_config, const ModelObject &object, size_t num_extruders)
{
    PrintObjectConfig config = default_object_config;
    {
        DynamicPrintConfig src_normalized(object.config.get());
        src_normalized.normalize_fdm();
        config.apply(src_normalized, true);
    }
    // Clamp invalid extruders to the default extruder (with index 1).
    clamp_exturder_to_default(config.support_filament,           num_extruders);
    clamp_exturder_to_default(config.support_interface_filament, num_extruders);
    return config;
}

const std::string                                                    key_extruder { "extruder" };
static constexpr const std::initializer_list<const std::string_view> keys_extruders { "sparse_infill_filament"sv, "solid_infill_filament"sv, "wall_filament"sv };

static void apply_to_print_region_config(PrintRegionConfig &out, const DynamicPrintConfig &in)
{
    // 1) Copy the "extruder key to sparse_infill_filament and wall_filament.
    auto *opt_extruder = in.opt<ConfigOptionInt>(key_extruder);
    if (opt_extruder)
        if (int extruder = opt_extruder->value; extruder != 0) {
            // Not a default extruder.
            out.sparse_infill_filament      .value = extruder;
            out.solid_infill_filament.value = extruder;
            out.wall_filament   .value = extruder;
        }
    // 2) Copy the rest of the values.
    for (auto it = in.cbegin(); it != in.cend(); ++ it)
        if (it->first != key_extruder)
            if (ConfigOption* my_opt = out.option(it->first, false); my_opt != nullptr) {
                if (one_of(it->first, keys_extruders)) {
                    // Ignore "default" extruders.
                    int extruder = static_cast<const ConfigOptionInt*>(it->second.get())->value;
                    if (extruder > 0)
                        my_opt->setInt(extruder);
                } else
                    my_opt->set(it->second.get());
            }
}

PrintRegionConfig region_config_from_model_volume(const PrintRegionConfig &default_or_parent_region_config, const DynamicPrintConfig *layer_range_config, const ModelVolume &volume, size_t num_extruders)
{
    PrintRegionConfig config = default_or_parent_region_config;
    if (volume.is_model_part()) {
        // default_or_parent_region_config contains the Print's PrintRegionConfig.
        // Override with ModelObject's PrintRegionConfig values.
        apply_to_print_region_config(config, volume.get_object()->config.get());
    } else {
        // default_or_parent_region_config contains parent PrintRegion config, which already contains ModelVolume's config.
    }
    apply_to_print_region_config(config, volume.config.get());
    if (! volume.material_id().empty())
        apply_to_print_region_config(config, volume.material()->config.get());
    if (layer_range_config != nullptr) {
        // Not applicable to modifiers.
        assert(volume.is_model_part());
    	apply_to_print_region_config(config, *layer_range_config);
    }
    // Clamp invalid extruders to the default extruder (with index 1).
    clamp_exturder_to_default(config.sparse_infill_filament,       num_extruders);
    clamp_exturder_to_default(config.wall_filament,    num_extruders);
    clamp_exturder_to_default(config.solid_infill_filament, num_extruders);
    if (config.sparse_infill_density.value < 0.00011f)
        // Switch of infill for very low infill rates, also avoid division by zero in infill generator for these very low rates.
        // See GH issue #5910.
        config.sparse_infill_density.value = 0;
    else
        config.sparse_infill_density.value = std::min(config.sparse_infill_density.value, 100.);
    if (config.fuzzy_skin.value != FuzzySkinType::None && (config.fuzzy_skin_point_distance.value < 0.01 || config.fuzzy_skin_thickness.value < 0.001))
        config.fuzzy_skin.value = FuzzySkinType::None;
    return config;
}

struct POProfiler
{
    uint32_t duration1;
    uint32_t duration2;
};

void PrintObject::generate_support_preview()
{
    POProfiler profiler;

    boost::posix_time::ptime ts1 = boost::posix_time::microsec_clock::local_time();
    this->slice();
    boost::posix_time::ptime ts2 = boost::posix_time::microsec_clock::local_time();
    profiler.duration1 = (ts2 - ts1).total_milliseconds();

    this->generate_support_material();
    boost::posix_time::ptime ts3 = boost::posix_time::microsec_clock::local_time();
    profiler.duration2 = (ts3 - ts2).total_milliseconds();
}

void PrintObject::update_slicing_parameters()
{
    if (!m_slicing_params.valid)
        m_slicing_params = SlicingParameters::create_from_config(
            this->print()->config(), m_config, this->model_object()->bounding_box().max.z(), this->object_extruders());
}

SlicingParameters PrintObject::slicing_parameters(const DynamicPrintConfig& full_config, const ModelObject& model_object, float object_max_z)
{
	PrintConfig         print_config;
	PrintObjectConfig   object_config;
	PrintRegionConfig   default_region_config;
	print_config.apply(full_config, true);
	object_config.apply(full_config, true);
	default_region_config.apply(full_config, true);
    // BBS
	size_t              filament_extruders = print_config.filament_diameter.size();
	object_config = object_config_from_model_object(object_config, model_object, filament_extruders);

	std::vector<unsigned int> object_extruders;
	for (const ModelVolume* model_volume : model_object.volumes)
		if (model_volume->is_model_part()) {
			PrintRegion::collect_object_printing_extruders(
				print_config,
				region_config_from_model_volume(default_region_config, nullptr, *model_volume, filament_extruders),
                object_config.brim_type != btNoBrim && object_config.brim_width > 0.,
				object_extruders);
			for (const std::pair<const t_layer_height_range, ModelConfig> &range_and_config : model_object.layer_config_ranges)
				if (range_and_config.second.has("wall_filament") ||
					range_and_config.second.has("sparse_infill_filament") ||
					range_and_config.second.has("solid_infill_filament"))
					PrintRegion::collect_object_printing_extruders(
						print_config,
						region_config_from_model_volume(default_region_config, &range_and_config.second.get(), *model_volume, filament_extruders),
                        object_config.brim_type != btNoBrim && object_config.brim_width > 0.,
						object_extruders);
		}
    sort_remove_duplicates(object_extruders);
    //FIXME add painting extruders

    if (object_max_z <= 0.f)
        object_max_z = (float)model_object.raw_bounding_box().size().z();
    return SlicingParameters::create_from_config(print_config, object_config, object_max_z, object_extruders);
}

// returns 0-based indices of extruders used to print the object (without brim, support and other helper extrusions)
std::vector<unsigned int> PrintObject::object_extruders() const
{
    std::vector<unsigned int> extruders;
    extruders.reserve(this->all_regions().size() * 3);
#if 0
    for (const PrintRegion &region : this->all_regions())
        region.collect_object_printing_extruders(*this->print(), extruders);
#else
    const ModelObject* mo = this->model_object();
    for (const ModelVolume* mv : mo->volumes) {
        std::vector<int> volume_extruders = mv->get_extruders();
        for (int extruder : volume_extruders) {
            assert(extruder > 0);
            extruders.push_back(extruder - 1);
        }
    }
#endif
    sort_remove_duplicates(extruders);
    return extruders;
}

bool PrintObject::update_layer_height_profile(const ModelObject &model_object, const SlicingParameters &slicing_parameters, std::vector<coordf_t> &layer_height_profile)
{
    bool updated = false;

    if (layer_height_profile.empty()) {
        // use the constructor because the assignement is crashing on ASAN OsX
        layer_height_profile = std::vector<coordf_t>(model_object.layer_height_profile.get());
//        layer_height_profile = model_object.layer_height_profile;
        updated = true;
    }

    // Verify the layer_height_profile.
    if (!layer_height_profile.empty() &&
        // Must not be of even length.
        ((layer_height_profile.size() & 1) != 0 ||
            // Last entry must be at the top of the object.
            std::abs(layer_height_profile[layer_height_profile.size() - 2] - slicing_parameters.object_print_z_max + slicing_parameters.object_print_z_min) > 1e-3))
        layer_height_profile.clear();

    if (layer_height_profile.empty() || layer_height_profile[1] != slicing_parameters.first_object_layer_height) {
        //layer_height_profile = layer_height_profile_adaptive(slicing_parameters, model_object.layer_config_ranges, model_object.volumes);
        layer_height_profile = layer_height_profile_from_ranges(slicing_parameters, model_object.layer_config_ranges);
        updated = true;
    }

    return updated;
}
//BBS:
void PrintObject::get_certain_layers(float start, float end, std::vector<LayerPtrs> &out, std::vector<BoundingBox> &boundingbox_objects)
{
    BoundingBox temp;
    LayerPtrs   out_temp;
    for (const auto &layer : layers()) {
        if (layer->print_z < start) continue;

        if (layer->print_z > end + EPSILON) break;
        temp.merge(layer->loverhangs_bbox);
        out_temp.emplace_back(layer);
    }
    boundingbox_objects.emplace_back(std::move(temp));
    out.emplace_back(std::move(out_temp));
};

std::vector<Point> PrintObject::get_instances_shift_without_plate_offset()
{
    std::vector<Point> out;
    out.reserve(m_instances.size());
    for (const auto& instance : m_instances)
        out.push_back(instance.shift_without_plate_offset());

    return out;
}

// Only active if config->infill_only_where_needed. This step trims the sparse infill,
// so it acts as an internal support. It maintains all other infill types intact.
// Here the internal surfaces and perimeters have to be supported by the sparse infill.
//FIXME The surfaces are supported by a sparse infill, but the sparse infill is only as large as the area to support.
// Likely the sparse infill will not be anchored correctly, so it will not work as intended.
// Also one wishes the perimeters to be supported by a full infill.
// Idempotence of this method is guaranteed by the fact that we don't remove things from
// fill_surfaces but we only turn them into VOID surfaces, thus preserving the boundaries.
void PrintObject::clip_fill_surfaces()
{
    if (! PrintObject::infill_only_where_needed)
        return;
    bool has_infill = false;
    for (size_t i = 0; i < this->num_printing_regions(); ++ i)
        if (this->printing_region(i).config().sparse_infill_density > 0) {
            has_infill = true;
            break;
        }
    if (! has_infill)
        return;

    // We only want infill under ceilings; this is almost like an
    // internal support material.
    // Proceed top-down, skipping the bottom layer.
    Polygons upper_internal;
    for (int layer_id = int(m_layers.size()) - 1; layer_id > 0; -- layer_id) {
        Layer *layer       = m_layers[layer_id];
        Layer *lower_layer = m_layers[layer_id - 1];
        // Detect things that we need to support.
        // Cummulative fill surfaces.
        Polygons fill_surfaces;
        // Solid surfaces to be supported.
        Polygons overhangs;
        for (const LayerRegion *layerm : layer->m_regions)
            for (const Surface &surface : layerm->fill_surfaces.surfaces) {
                Polygons polygons = to_polygons(surface.expolygon);
                if (surface.is_solid())
                    polygons_append(overhangs, polygons);
                polygons_append(fill_surfaces, std::move(polygons));
            }
        Polygons lower_layer_fill_surfaces;
        Polygons lower_layer_internal_surfaces;
        for (const LayerRegion *layerm : lower_layer->m_regions)
            for (const Surface &surface : layerm->fill_surfaces.surfaces) {
                Polygons polygons = to_polygons(surface.expolygon);
                if (surface.surface_type == stInternal || surface.surface_type == stInternalVoid)
                    polygons_append(lower_layer_internal_surfaces, polygons);
                polygons_append(lower_layer_fill_surfaces, std::move(polygons));
            }
        // We also need to support perimeters when there's at least one full unsupported loop
        {
            // Get perimeters area as the difference between slices and fill_surfaces
            // Only consider the area that is not supported by lower perimeters
            Polygons perimeters = intersection(diff(layer->lslices, fill_surfaces), lower_layer_fill_surfaces);
            // Only consider perimeter areas that are at least one extrusion width thick.
            //FIXME Offset2 eats out from both sides, while the perimeters are create outside in.
            //Should the pw not be half of the current value?
            float pw = FLT_MAX;
            for (const LayerRegion *layerm : layer->m_regions)
                pw = std::min(pw, (float)layerm->flow(frPerimeter).scaled_width());
            // Append such thick perimeters to the areas that need support
            polygons_append(overhangs, opening(perimeters, pw));
        }
        // Merge the new overhangs, find new internal infill.
        polygons_append(upper_internal, std::move(overhangs));
        static constexpr const auto closing_radius = scaled<float>(2.f);
        upper_internal = intersection(
            // Regularize the overhang regions, so that the infill areas will not become excessively jagged.
            smooth_outward(
                closing(upper_internal, closing_radius, ClipperLib::jtSquare, 0.),
                scaled<coord_t>(0.1)),
            lower_layer_internal_surfaces);
        // Apply new internal infill to regions.
        for (LayerRegion *layerm : lower_layer->m_regions) {
            if (layerm->region().config().sparse_infill_density.value == 0)
                continue;
            Polygons internal;
            for (Surface &surface : layerm->fill_surfaces.surfaces)
                if (surface.surface_type == stInternal || surface.surface_type == stInternalVoid)
                    polygons_append(internal, std::move(surface.expolygon));
            layerm->fill_surfaces.remove_types({ stInternal, stInternalVoid });
            layerm->fill_surfaces.append(intersection_ex(internal, upper_internal, ApplySafetyOffset::Yes), stInternal);
            layerm->fill_surfaces.append(diff_ex        (internal, upper_internal, ApplySafetyOffset::Yes), stInternalVoid);
            // If there are voids it means that our internal infill is not adjacent to
            // perimeters. In this case it would be nice to add a loop around infill to
            // make it more robust and nicer. TODO.
#ifdef SLIC3R_DEBUG_SLICE_PROCESSING
            layerm->export_region_fill_surfaces_to_svg_debug("6_clip_fill_surfaces");
#endif
        }
        m_print->throw_if_canceled();
    }
}

void PrintObject::discover_horizontal_shells()
{
    BOOST_LOG_TRIVIAL(trace) << "discover_horizontal_shells()";

    for (size_t region_id = 0; region_id < this->num_printing_regions(); ++region_id) {
        for (size_t i = 0; i < m_layers.size(); ++i) {
            m_print->throw_if_canceled();
            Layer* layer = m_layers[i];
            LayerRegion* layerm = layer->regions()[region_id];
            const PrintRegionConfig& region_config = layerm->region().config();
#if 0
            if (region_config.solid_infill_every_layers.value > 0 && region_config.sparse_infill_density.value > 0 &&
                (i % region_config.solid_infill_every_layers) == 0) {
                // Insert a solid internal layer. Mark stInternal surfaces as stInternalSolid or stInternalBridge.
                SurfaceType type = (region_config.sparse_infill_density == 100 || region_config.solid_infill_every_layers == 1) ? stInternalSolid : stInternalBridge;
                for (Surface& surface : layerm->fill_surfaces.surfaces)
                    if (surface.surface_type == stInternal)
                        surface.surface_type = type;
            }
#endif
            // If ensure_vertical_shell_thickness, then the rest has already been performed by discover_vertical_shells().
            if (region_config.ensure_vertical_shell_thickness.value)
                continue;

            coordf_t print_z = layer->print_z;
            coordf_t bottom_z = layer->bottom_z();
            for (size_t idx_surface_type = 0; idx_surface_type < 3; ++idx_surface_type) {
                m_print->throw_if_canceled();
                SurfaceType type = (idx_surface_type == 0) ? stTop : (idx_surface_type == 1) ? stBottom : stBottomBridge;
                int num_solid_layers = (type == stTop) ? region_config.top_shell_layers.value : region_config.bottom_shell_layers.value;
                if (num_solid_layers == 0)
                    continue;
                // Find slices of current type for current layer.
                // Use slices instead of fill_surfaces, because they also include the perimeter area,
                // which needs to be propagated in shells; we need to grow slices like we did for
                // fill_surfaces though. Using both ungrown slices and grown fill_surfaces will
                // not work in some situations, as there won't be any grown region in the perimeter
                // area (this was seen in a model where the top layer had one extra perimeter, thus
                // its fill_surfaces were thinner than the lower layer's infill), however it's the best
                // solution so far. Growing the external slices by EXTERNAL_INFILL_MARGIN will put
                // too much solid infill inside nearly-vertical slopes.

                // Surfaces including the area of perimeters. Everything, that is visible from the top / bottom
                // (not covered by a layer above / below).
                // This does not contain the areas covered by perimeters!
                Polygons solid;
                for (const Surface& surface : layerm->slices.surfaces)
                    if (surface.surface_type == type)
                        polygons_append(solid, to_polygons(surface.expolygon));
                // Infill areas (slices without the perimeters).
                for (const Surface& surface : layerm->fill_surfaces.surfaces)
                    if (surface.surface_type == type)
                        polygons_append(solid, to_polygons(surface.expolygon));
                if (solid.empty())
                    continue;
                //                Slic3r::debugf "Layer %d has %s surfaces\n", $i, ($type == stTop) ? 'top' : 'bottom';

                                // Scatter top / bottom regions to other layers. Scattering process is inherently serial, it is difficult to parallelize without locking.
                for (int n = (type == stTop) ? int(i) - 1 : int(i) + 1;
                    (type == stTop) ?
                    (n >= 0 && (int(i) - n < num_solid_layers ||
                        print_z - m_layers[n]->print_z < region_config.top_shell_thickness.value - EPSILON)) :
                    (n < int(m_layers.size()) && (n - int(i) < num_solid_layers ||
                        m_layers[n]->bottom_z() - bottom_z < region_config.bottom_shell_thickness.value - EPSILON));
                    (type == stTop) ? --n : ++n)
                {
                    //                    Slic3r::debugf "  looking for neighbors on layer %d...\n", $n;
                                        // Reference to the lower layer of a TOP surface, or an upper layer of a BOTTOM surface.
                    LayerRegion* neighbor_layerm = m_layers[n]->regions()[region_id];

                    // find intersection between neighbor and current layer's surfaces
                    // intersections have contours and holes
                    // we update $solid so that we limit the next neighbor layer to the areas that were
                    // found on this one - in other words, solid shells on one layer (for a given external surface)
                    // are always a subset of the shells found on the previous shell layer
                    // this approach allows for DWIM in hollow sloping vases, where we want bottom
                    // shells to be generated in the base but not in the walls (where there are many
                    // narrow bottom surfaces): reassigning $solid will consider the 'shadow' of the
                    // upper perimeter as an obstacle and shell will not be propagated to more upper layers
                    //FIXME How does it work for stInternalBRIDGE? This is set for sparse infill. Likely this does not work.
                    Polygons new_internal_solid;
                    {
                        Polygons internal;
                        for (const Surface& surface : neighbor_layerm->fill_surfaces.surfaces)
                            if (surface.surface_type == stInternal || surface.surface_type == stInternalSolid)
                                polygons_append(internal, to_polygons(surface.expolygon));
                        new_internal_solid = intersection(solid, internal, ApplySafetyOffset::Yes);
                    }
                    if (new_internal_solid.empty()) {
                        // No internal solid needed on this layer. In order to decide whether to continue
                        // searching on the next neighbor (thus enforcing the configured number of solid
                        // layers, use different strategies according to configured infill density:
                        if (region_config.sparse_infill_density.value == 0) {
                            // If user expects the object to be void (for example a hollow sloping vase),
                            // don't continue the search. In this case, we only generate the external solid
                            // shell if the object would otherwise show a hole (gap between perimeters of
                            // the two layers), and internal solid shells are a subset of the shells found
                            // on each previous layer.
                            goto EXTERNAL;
                        }
                        else {
                            // If we have internal infill, we can generate internal solid shells freely.
                            continue;
                        }
                    }

                    if (region_config.sparse_infill_density.value == 0) {
                        // if we're printing a hollow object we discard any solid shell thinner
                        // than a perimeter width, since it's probably just crossing a sloping wall
                        // and it's not wanted in a hollow print even if it would make sense when
                        // obeying the solid shell count option strictly (DWIM!)
                        float margin = float(neighbor_layerm->flow(frExternalPerimeter).scaled_width());
                        Polygons too_narrow = diff(
                            new_internal_solid,
                            opening(new_internal_solid, margin, margin + ClipperSafetyOffset, jtMiter, 5));
                        // Trim the regularized region by the original region.
                        if (!too_narrow.empty())
                            new_internal_solid = solid = diff(new_internal_solid, too_narrow);
                    }

                    // make sure the new internal solid is wide enough, as it might get collapsed
                    // when spacing is added in Fill.pm
                    {
                        //FIXME Vojtech: Disable this and you will be sorry.
                        float margin = 3.f * layerm->flow(frSolidInfill).scaled_width(); // require at least this size
                        // we use a higher miterLimit here to handle areas with acute angles
                        // in those cases, the default miterLimit would cut the corner and we'd
                        // get a triangle in $too_narrow; if we grow it below then the shell
                        // would have a different shape from the external surface and we'd still
                        // have the same angle, so the next shell would be grown even more and so on.
                        Polygons too_narrow = diff(
                            new_internal_solid,
                            opening(new_internal_solid, margin, margin + ClipperSafetyOffset, ClipperLib::jtMiter, 5));
                        if (!too_narrow.empty()) {
                            // grow the collapsing parts and add the extra area to  the neighbor layer
                            // as well as to our original surfaces so that we support this
                            // additional area in the next shell too
                            // make sure our grown surfaces don't exceed the fill area
                            Polygons internal;
                            for (const Surface& surface : neighbor_layerm->fill_surfaces.surfaces)
                                if (surface.is_internal() && !surface.is_bridge())
                                    polygons_append(internal, to_polygons(surface.expolygon));
                            polygons_append(new_internal_solid,
                                intersection(
                                    expand(too_narrow, +margin),
                                    // Discard bridges as they are grown for anchoring and we can't
                                    // remove such anchors. (This may happen when a bridge is being
                                    // anchored onto a wall where little space remains after the bridge
                                    // is grown, and that little space is an internal solid shell so
                                    // it triggers this too_narrow logic.)
                                    internal));
                            // solid = new_internal_solid;
                        }
                    }

                    // internal-solid are the union of the existing internal-solid surfaces
                    // and new ones
                    SurfaceCollection backup = std::move(neighbor_layerm->fill_surfaces);
                    polygons_append(new_internal_solid, to_polygons(backup.filter_by_type(stInternalSolid)));
                    ExPolygons internal_solid = union_ex(new_internal_solid);
                    // assign new internal-solid surfaces to layer
                    neighbor_layerm->fill_surfaces.set(internal_solid, stInternalSolid);
                    // subtract intersections from layer surfaces to get resulting internal surfaces
                    Polygons polygons_internal = to_polygons(std::move(internal_solid));
                    ExPolygons internal = diff_ex(backup.filter_by_type(stInternal), polygons_internal, ApplySafetyOffset::Yes);
                    // assign resulting internal surfaces to layer
                    neighbor_layerm->fill_surfaces.append(internal, stInternal);
                    polygons_append(polygons_internal, to_polygons(std::move(internal)));
                    // assign top and bottom surfaces to layer
                    backup.keep_types({ stTop, stBottom, stBottomBridge });
                    std::vector<SurfacesPtr> top_bottom_groups;
                    backup.group(&top_bottom_groups);
                    for (SurfacesPtr& group : top_bottom_groups)
                        neighbor_layerm->fill_surfaces.append(
                            diff_ex(group, polygons_internal),
                            // Use an existing surface as a template, it carries the bridge angle etc.
                            *group.front());
                }
            EXTERNAL:;
            } // foreach type (stTop, stBottom, stBottomBridge)
        } // for each layer
    } // for each region

#ifdef SLIC3R_DEBUG_SLICE_PROCESSING
    for (size_t region_id = 0; region_id < this->num_printing_regions(); ++region_id) {
        for (const Layer* layer : m_layers) {
            const LayerRegion* layerm = layer->m_regions[region_id];
            layerm->export_region_slices_to_svg_debug("5_discover_horizontal_shells");
            layerm->export_region_fill_surfaces_to_svg_debug("5_discover_horizontal_shells");
        } // for each layer
    } // for each region
#endif /* SLIC3R_DEBUG_SLICE_PROCESSING */
}

// combine fill surfaces across layers to honor the "infill every N layers" option
// Idempotence of this method is guaranteed by the fact that we don't remove things from
// fill_surfaces but we only turn them into VOID surfaces, thus preserving the boundaries.
void PrintObject::combine_infill()
{
    // Work on each region separately.
    for (size_t region_id = 0; region_id < this->num_printing_regions(); ++ region_id) {
        const PrintRegion &region = this->printing_region(region_id);
        //BBS
        const bool enable_combine_infill = region.config().infill_combination.value;
        if (enable_combine_infill == false || region.config().sparse_infill_density == 0.)
            continue;
        // Limit the number of combined layers to the maximum height allowed by this regions' nozzle.
        //FIXME limit the layer height to max_layer_height
        double nozzle_diameter = std::min(
            this->print()->config().nozzle_diameter.get_at(region.config().sparse_infill_filament.value - 1),
            this->print()->config().nozzle_diameter.get_at(region.config().solid_infill_filament.value - 1));
        // define the combinations
        std::vector<size_t> combine(m_layers.size(), 0);
        {
            double current_height = 0.;
            size_t num_layers = 0;
            for (size_t layer_idx = 0; layer_idx < m_layers.size(); ++ layer_idx) {
                m_print->throw_if_canceled();
                const Layer *layer = m_layers[layer_idx];
                if (layer->id() == 0)
                    // Skip first print layer (which may not be first layer in array because of raft).
                    continue;
                // Check whether the combination of this layer with the lower layers' buffer
                // would exceed max layer height or max combined layer count.
                // BBS: automatically calculate how many layers should be combined
                if (current_height + layer->height >= nozzle_diameter + EPSILON) {
                    // Append combination to lower layer.
                    combine[layer_idx - 1] = num_layers;
                    current_height = 0.;
                    num_layers = 0;
                }
                current_height += layer->height;
                ++ num_layers;
            }

            // Append lower layers (if any) to uppermost layer.
            combine[m_layers.size() - 1] = num_layers;
        }

        // loop through layers to which we have assigned layers to combine
        for (size_t layer_idx = 0; layer_idx < m_layers.size(); ++ layer_idx) {
            m_print->throw_if_canceled();
            size_t num_layers = combine[layer_idx];
			if (num_layers <= 1)
                continue;
            // Get all the LayerRegion objects to be combined.
            std::vector<LayerRegion*> layerms;
            layerms.reserve(num_layers);
			for (size_t i = layer_idx + 1 - num_layers; i <= layer_idx; ++ i)
                layerms.emplace_back(m_layers[i]->regions()[region_id]);
            // We need to perform a multi-layer intersection, so let's split it in pairs.
            // Initialize the intersection with the candidates of the lowest layer.
            ExPolygons intersection = to_expolygons(layerms.front()->fill_surfaces.filter_by_type(stInternal));
            // Start looping from the second layer and intersect the current intersection with it.
            for (size_t i = 1; i < layerms.size(); ++ i)
                intersection = intersection_ex(layerms[i]->fill_surfaces.filter_by_type(stInternal), intersection);
            double area_threshold = layerms.front()->infill_area_threshold();
            if (! intersection.empty() && area_threshold > 0.)
                intersection.erase(std::remove_if(intersection.begin(), intersection.end(),
                    [area_threshold](const ExPolygon &expoly) { return expoly.area() <= area_threshold; }),
                    intersection.end());
            if (intersection.empty())
                continue;
//            Slic3r::debugf "  combining %d %s regions from layers %d-%d\n",
//                scalar(@$intersection),
//                ($type == stInternal ? 'internal' : 'internal-solid'),
//                $layer_idx-($every-1), $layer_idx;
            // intersection now contains the regions that can be combined across the full amount of layers,
            // so let's remove those areas from all layers.
            Polygons intersection_with_clearance;
            intersection_with_clearance.reserve(intersection.size());
            float clearance_offset =
                0.5f * layerms.back()->flow(frPerimeter).scaled_width() +
             // Because fill areas for rectilinear and honeycomb are grown
             // later to overlap perimeters, we need to counteract that too.
                ((region.config().sparse_infill_pattern == ipRectilinear   ||
                  region.config().sparse_infill_pattern == ipMonotonic     ||
                  region.config().sparse_infill_pattern == ipGrid          ||
                  region.config().sparse_infill_pattern == ipLine          ||
                  region.config().sparse_infill_pattern == ipHoneycomb) ? 1.5f : 0.5f) *
                    layerms.back()->flow(frSolidInfill).scaled_width();
            for (ExPolygon &expoly : intersection)
                polygons_append(intersection_with_clearance, offset(expoly, clearance_offset));
            for (LayerRegion *layerm : layerms) {
                Polygons internal = to_polygons(std::move(layerm->fill_surfaces.filter_by_type(stInternal)));
                layerm->fill_surfaces.remove_type(stInternal);
                layerm->fill_surfaces.append(diff_ex(internal, intersection_with_clearance), stInternal);
                if (layerm == layerms.back()) {
                    // Apply surfaces back with adjusted depth to the uppermost layer.
                    Surface templ(stInternal, ExPolygon());
                    templ.thickness = 0.;
                    for (LayerRegion *layerm2 : layerms)
                        templ.thickness += layerm2->layer()->height;
                    templ.thickness_layers = (unsigned short)layerms.size();
                    layerm->fill_surfaces.append(intersection, templ);
                } else {
                    // Save void surfaces.
                    layerm->fill_surfaces.append(
                        intersection_ex(internal, intersection_with_clearance),
                        stInternalVoid);
                }
            }
        }
    }
}

void PrintObject::_generate_support_material()
{
    if (is_tree(m_config.support_type.value)) {
        TreeSupport tree_support(*this, m_slicing_params);
        tree_support.throw_on_cancel = [this]() { this->throw_if_canceled(); };
        tree_support.generate();
    }
    else {
        PrintObjectSupportMaterial support_material(this, m_slicing_params);
        support_material.generate(*this);
    }
}

// BBS
#define SUPPORT_SURFACES_OFFSET_PARAMETERS ClipperLib::jtSquare, 0.
#define SUPPORT_MATERIAL_MARGIN 1.2
template<typename PolysType>
void PrintObject::remove_bridges_from_contacts(
    const Layer* lower_layer,
    const Layer* current_layer,
    float extrusion_width,
    PolysType* overhang_regions,
    float max_bridge_length,
    bool break_bridge)
{
    // Extrusion width accounts for the roundings of the extrudates.
    // It is the maximum widh of the extrudate.
    float fw = extrusion_width;
    Lines overhang_perimeters = to_lines(*overhang_regions);
    auto layer_regions = current_layer->regions();
    Polygons lower_layer_polygons = to_polygons(lower_layer->lslices);
    const PrintObjectConfig& object_config = current_layer->object()->config();

    Polygons all_bridges;
    for (LayerRegion* layerm : layer_regions)
    {
        Polygons bridges;
        // Surface supporting this layer, expanded by 0.5 * nozzle_diameter, as we consider this kind of overhang to be sufficiently supported.
        Polygons lower_grown_slices = offset(lower_layer_polygons,
            //FIXME to mimic the decision in the perimeter generator, we should use half the external perimeter width.
            0.5f * fw, SUPPORT_SURFACES_OFFSET_PARAMETERS);
        Polylines overhang_perimeters = diff_pl(layerm->perimeters.as_polylines(), lower_grown_slices);
        // only consider straight overhangs
            // only consider overhangs having endpoints inside layer's slices
            // convert bridging polylines into polygons by inflating them with their thickness
            // since we're dealing with bridges, we can't assume width is larger than spacing,
            // so we take the largest value and also apply safety offset to be ensure no gaps
            // are left in between
        Flow bridge_flow = layerm->bridging_flow(frPerimeter, object_config.thick_bridges);
        float w = float(std::max(bridge_flow.scaled_width(), bridge_flow.scaled_spacing()));
        for (Polyline& polyline : overhang_perimeters)
            if (polyline.is_straight()) {
                // This is a bridge
                polyline.extend_start(fw);
                polyline.extend_end(fw);
                // Is the straight perimeter segment supported at both sides?
                Point pts[2] = { polyline.first_point(), polyline.last_point() };
                bool  supported[2] = { false, false };
                for (size_t i = 0; i < lower_layer->lslices.size() && !(supported[0] && supported[1]); ++i)
                    for (int j = 0; j < 2; ++j)
                        if (!supported[j] && lower_layer->lslices_bboxes[i].contains(pts[j]) && lower_layer->lslices[i].contains(pts[j]))
                            supported[j] = true;
                if (supported[0] && supported[1]) {
                    Polylines lines;
                    if (polyline.length() > max_bridge_length + 10) {
                        if (break_bridge) {
                            // equally divide the polyline
                            float len = polyline.length() / ceil(polyline.length() / max_bridge_length);
                            lines = polyline.equally_spaced_lines(len);
                            for (auto& line : lines) {
                                if (line.is_valid())
                                    line.clip_start(fw);
                                if (line.is_valid())
                                    line.clip_end(fw);
                            }
                        }
                    }
                    else
                        lines.push_back(polyline);
                    // Offset a polyline into a thick line.
                    polygons_append(bridges, offset(lines, 0.5f * w + 10.f));
                }
            }
        bridges = union_(bridges);

        // remove the entire bridges and only support the unsupported edges
        //FIXME the brided regions are already collected as layerm->bridged. Use it?
        for (const Surface& surface : layerm->fill_surfaces.surfaces)
            if (surface.surface_type == stBottomBridge && surface.bridge_angle != -1) {
                auto bbox      = get_extents(surface.expolygon);
                auto bbox_size = bbox.size();
                if (bbox_size[0] < max_bridge_length && bbox_size[1] < max_bridge_length)
                    polygons_append(bridges, surface.expolygon);
                else {
                    if (break_bridge) {
                        Polygons holes;
                        coord_t  x0 = bbox.min.x();
                        coord_t  x1 = bbox.max.x();
                        coord_t  y0 = bbox.min.y();
                        coord_t  y1 = bbox.max.y();
                        const int grid_lw = int(w/2); // grid line width

                        Vec2f bridge_direction{ cos(surface.bridge_angle),sin(surface.bridge_angle) };
                        if (fabs(bridge_direction(0)) > fabs(bridge_direction(1)))
                        {   // cut bridge along x-axis if bridge direction is aligned to x-axis more than to y-axis
                            // Note: surface.bridge_angle may be pi, so we can't compare it to 0 & pi/2.
                            int step = bbox_size(0) / ceil(bbox_size(0) / max_bridge_length);
                            for (int x = x0 + step; x < x1; x += step) {
                                Polygon poly;
                                poly.points = {Point(x - grid_lw, y0), Point(x + grid_lw, y0), Point(x + grid_lw, y1), Point(x - grid_lw, y1)};
                                holes.emplace_back(poly);
                            }
                        } else {
                            int step = bbox_size(1) / ceil(bbox_size(1) / max_bridge_length);
                            for (int y = y0 + step; y < y1; y += step) {
                                Polygon poly;
                                poly.points = {Point(x0, y - grid_lw), Point(x0, y + grid_lw), Point(x1, y + grid_lw), Point(x1, y - grid_lw)};
                                holes.emplace_back(poly);
                            }
                        }
                        auto expoly = diff_ex(surface.expolygon, holes);
                        polygons_append(bridges, expoly);
                    }
                }
            }
        //FIXME add the gap filled areas. Extrude the gaps with a bridge flow?
        // Remove the unsupported ends of the bridges from the bridged areas.
        //FIXME add supports at regular intervals to support long bridges!
        bridges = diff(bridges,
            // Offset unsupported edges into polygons.
            offset(layerm->unsupported_bridge_edges, scale_(SUPPORT_MATERIAL_MARGIN), SUPPORT_SURFACES_OFFSET_PARAMETERS));
        append(all_bridges, bridges);
    }
    if (typeid(overhang_regions) == typeid(ExPolygons*)) {
        *(ExPolygons*)overhang_regions = diff_ex(*overhang_regions, all_bridges, ApplySafetyOffset::Yes);
    }
    else if (typeid(overhang_regions) == typeid(Polygons*)) {
        *(Polygons*)overhang_regions = diff(*overhang_regions, all_bridges, ApplySafetyOffset::Yes);
    }
}

template void PrintObject::remove_bridges_from_contacts<ExPolygons>(
    const Layer* lower_layer,
    const Layer* current_layer,
    float extrusion_width,
    ExPolygons* overhang_regions,
    float max_bridge_length, bool break_bridge);
template void PrintObject::remove_bridges_from_contacts<Polygons>(
    const Layer* lower_layer,
    const Layer* current_layer,
    float extrusion_width,
    Polygons* overhang_regions,
    float max_bridge_length, bool break_bridge);


SupportNecessaryType PrintObject::is_support_necessary()
{
    static const double super_overhang_area_threshold = SQ(scale_(5.0));
    const double cantilevel_dist_thresh = scale_(6);
#if 0
    double threshold_rad = (m_config.support_threshold_angle.value < EPSILON ? 30 : m_config.support_threshold_angle.value + 1) * M_PI / 180.;
    int enforce_support_layers = m_config.enforce_support_layers;
    const coordf_t extrusion_width = m_config.line_width.value;
    const coordf_t extrusion_width_scaled = scale_(extrusion_width);
    float max_bridge_length = scale_(m_config.max_bridge_length.value);
    const bool bridge_no_support = max_bridge_length > 0;// config.bridge_no_support.value;

    for (size_t layer_nr = enforce_support_layers + 1; layer_nr < this->layer_count(); layer_nr++) {
        Layer* layer = m_layers[layer_nr];
        Layer* lower_layer = layer->lower_layer;

        coordf_t support_offset_scaled = extrusion_width_scaled * 0.9;
        ExPolygons lower_layer_offseted = offset_ex(lower_layer->lslices, support_offset_scaled, SUPPORT_SURFACES_OFFSET_PARAMETERS);

        // 1. check sharp tail
        for (const LayerRegion* layerm : layer->regions()) {
            for (const ExPolygon& expoly : layerm->raw_slices) {
                // detect sharp tail
                if (intersection_ex({ expoly }, lower_layer_offseted).empty())
                    return SharpTail;
            }
        }

        // 2. check overhang area
        ExPolygons super_overhang_expolys = std::move(diff_ex(layer->lslices, lower_layer_offseted));
        super_overhang_expolys.erase(std::remove_if(
            super_overhang_expolys.begin(),
            super_overhang_expolys.end(),
            [extrusion_width_scaled](ExPolygon& area) {
                return offset_ex(area, -0.1 * extrusion_width_scaled).empty();
            }),
            super_overhang_expolys.end());

        // remove bridge
        if (bridge_no_support)
            remove_bridges_from_contacts(lower_layer, layer, extrusion_width_scaled, &super_overhang_expolys, max_bridge_length);

        Polygons super_overhang_polys = to_polygons(super_overhang_expolys);


        super_overhang_polys.erase(std::remove_if(
            super_overhang_polys.begin(),
            super_overhang_polys.end(),
            [extrusion_width_scaled](Polygon& area) {
                return offset_ex(area, -0.1 * extrusion_width_scaled).empty();
            }),
            super_overhang_polys.end());

        double super_overhang_area = 0.0;
        for (Polygon& poly : super_overhang_polys) {
            bool is_ccw = poly.is_counter_clockwise();
            double area_  = poly.area();
            if (is_ccw) {
                if (area_ > super_overhang_area_threshold)
                    return LargeOverhang;
                super_overhang_area += area_;
            }
            else {
                super_overhang_area -= area_;
            }
        }

        //if (super_overhang_area > super_overhang_area_threshold)
        //    return LargeOverhang;

        // 3. check overhang distance
        const double distance_threshold_scaled = extrusion_width_scaled * 2;
        ExPolygons lower_layer_offseted_2 = offset_ex(lower_layer->lslices, distance_threshold_scaled, SUPPORT_SURFACES_OFFSET_PARAMETERS);
        ExPolygons exceed_overhang = std::move(diff_ex(super_overhang_polys, lower_layer_offseted_2));
        exceed_overhang.erase(std::remove_if(
            exceed_overhang.begin(),
            exceed_overhang.end(),
            [extrusion_width_scaled](ExPolygon& area) {
                // tolerance for 1 extrusion width offset
                return offset_ex(area, -0.5 * extrusion_width_scaled).empty();
            }),
            exceed_overhang.end());
        if (!exceed_overhang.empty())
            return LargeOverhang;
    }
#else
    TreeSupport tree_support(*this, m_slicing_params);
    tree_support.support_type = SupportType::stTreeAuto; // need to set support type to fully utilize the power of feature detection
    tree_support.detect_overhangs(true);
    this->clear_support_layers();
    if (tree_support.has_sharp_tails)
        return SharpTail;
    else if (tree_support.has_cantilever && tree_support.max_cantilever_dist > cantilevel_dist_thresh)
        return Cantilever;
#endif
    return NoNeedSupp;
}

static void project_triangles_to_slabs(ConstLayerPtrsAdaptor layers, const indexed_triangle_set &custom_facets, const Transform3f &tr, bool seam, std::vector<Polygons> &out)
{
    if (custom_facets.indices.empty())
        return;

    const float tr_det_sign = (tr.matrix().determinant() > 0. ? 1.f : -1.f);

    // The projection will be at most a pentagon. Let's minimize heap
    // reallocations by saving in in the following struct.
    // Points are used so that scaling can be done in parallel
    // and they can be moved from to create an ExPolygon later.
    struct LightPolygon {
        LightPolygon() { pts.reserve(5); }
        LightPolygon(const std::array<Vec2f, 3>& tri) {
            pts.reserve(3);
            pts.emplace_back(scaled<coord_t>(tri.front()));
            pts.emplace_back(scaled<coord_t>(tri[1]));
            pts.emplace_back(scaled<coord_t>(tri.back()));
        }

        Points pts;

        void add(const Vec2f& pt) {
            pts.emplace_back(scaled<coord_t>(pt));
            assert(pts.size() <= 5);
        }
    };

    // Structure to collect projected polygons. One element for each triangle.
    // Saves vector of polygons and layer_id of the first one.
    struct TriangleProjections {
        size_t first_layer_id;
        std::vector<LightPolygon> polygons;
    };

    // Vector to collect resulting projections from each triangle.
    std::vector<TriangleProjections> projections_of_triangles(custom_facets.indices.size());

    // Iterate over all triangles.
    tbb::parallel_for(
        tbb::blocked_range<size_t>(0, custom_facets.indices.size()),
        [&custom_facets, &tr, tr_det_sign, seam, layers, &projections_of_triangles](const tbb::blocked_range<size_t>& range) {
        for (size_t idx = range.begin(); idx < range.end(); ++ idx) {

        std::array<Vec3f, 3> facet;

        // Transform the triangle into worlds coords.
        for (int i=0; i<3; ++i)
            facet[i] = tr * custom_facets.vertices[custom_facets.indices[idx](i)];

        // Ignore triangles with upward-pointing normal. Don't forget about mirroring.
        float z_comp = (facet[1]-facet[0]).cross(facet[2]-facet[0]).z();
        if (! seam && tr_det_sign * z_comp > 0.)
            continue;

        // The algorithm does not process vertical triangles, but it should for seam.
        // In that case, tilt the triangle a bit so the projection does not degenerate.
        if (seam && z_comp == 0.f)
            facet[0].x() += float(EPSILON);

        // Sort the three vertices according to z-coordinate.
        std::sort(facet.begin(), facet.end(),
                  [](const Vec3f& pt1, const Vec3f&pt2) {
                      return pt1.z() < pt2.z();
                  });

        std::array<Vec2f, 3> trianglef;
        for (int i=0; i<3; ++i)
            trianglef[i] = to_2d(facet[i]);

        // Find lowest slice not below the triangle.
        auto it = std::lower_bound(layers.begin(), layers.end(), facet[0].z()+EPSILON,
                      [](const Layer* l1, float z) {
                           return l1->slice_z < z;
                      });

        // Count how many projections will be generated for this triangle
        // and allocate respective amount in projections_of_triangles.
        size_t first_layer_id = projections_of_triangles[idx].first_layer_id = it - layers.begin();
        size_t last_layer_id  = first_layer_id;
        // The cast in the condition below is important. The comparison must
        // be an exact opposite of the one lower in the code where
        // the polygons are appended. And that one is on floats.
        while (last_layer_id + 1 < layers.size()
            && float(layers[last_layer_id]->slice_z) <= facet[2].z())
            ++last_layer_id;

        if (first_layer_id == last_layer_id) {
            // The triangle fits just a single slab, just project it. This also avoids division by zero for horizontal triangles.
            float dz = facet[2].z() - facet[0].z();
            assert(dz >= 0);
            // The face is nearly horizontal and it crosses the slicing plane at first_layer_id - 1.
            // Rather add this face to both the planes.
            bool add_below = dz < float(2. * EPSILON) && first_layer_id > 0 && layers[first_layer_id - 1]->slice_z > facet[0].z() - EPSILON;
            projections_of_triangles[idx].polygons.reserve(add_below ? 2 : 1);
            projections_of_triangles[idx].polygons.emplace_back(trianglef);
            if (add_below) {
                -- projections_of_triangles[idx].first_layer_id;
                projections_of_triangles[idx].polygons.emplace_back(trianglef);
            }
            continue;
        }

        projections_of_triangles[idx].polygons.resize(last_layer_id - first_layer_id + 1);

        // Calculate how to move points on triangle sides per unit z increment.
        Vec2f ta(trianglef[1] - trianglef[0]);
        Vec2f tb(trianglef[2] - trianglef[0]);
        ta *= 1.f/(facet[1].z() - facet[0].z());
        tb *= 1.f/(facet[2].z() - facet[0].z());

        // Projection on current slice will be built directly in place.
        LightPolygon* proj = &projections_of_triangles[idx].polygons[0];
        proj->add(trianglef[0]);

        bool passed_first = false;
        bool stop = false;

        // Project a sub-polygon on all slices intersecting the triangle.
        while (it != layers.end()) {
            const float z = float((*it)->slice_z);

            // Projections of triangle sides intersections with slices.
            // a moves along one side, b tracks the other.
            Vec2f a;
            Vec2f b;

            // If the middle vertex was already passed, append the vertex
            // and use ta for tracking the remaining side.
            if (z > facet[1].z() && ! passed_first) {
                proj->add(trianglef[1]);
                ta = trianglef[2]-trianglef[1];
                ta *= 1.f/(facet[2].z() - facet[1].z());
                passed_first = true;
            }

            // This slice is above the triangle already.
            if (z > facet[2].z() || it+1 == layers.end()) {
                proj->add(trianglef[2]);
                stop = true;
            }
            else {
                // Move a, b along the side it currently tracks to get
                // projected intersection with current slice.
                a = passed_first ? (trianglef[1]+ta*(z-facet[1].z()))
                                 : (trianglef[0]+ta*(z-facet[0].z()));
                b = trianglef[0]+tb*(z-facet[0].z());
                proj->add(a);
                proj->add(b);
            }

           if (stop)
                break;

            // Advance to the next layer.
            ++it;
            ++proj;
            assert(proj <= &projections_of_triangles[idx].polygons.back() );

            // a, b are first two points of the polygon for the next layer.
            proj->add(b);
            proj->add(a);
        }
    }
    }); // end of parallel_for

    // Make sure that the output vector can be used.
    out.resize(layers.size());

    // Now append the collected polygons to respective layers.
    for (auto& trg : projections_of_triangles) {
        int layer_id = int(trg.first_layer_id);
        for (LightPolygon &poly : trg.polygons) {
            if (layer_id >= int(out.size()))
                break; // part of triangle could be projected above top layer
            assert(! poly.pts.empty());
            // The resulting triangles are fed to the Clipper library, which seem to handle flipped triangles well.
//                if (cross2(Vec2d((poly.pts[1] - poly.pts[0]).cast<double>()), Vec2d((poly.pts[2] - poly.pts[1]).cast<double>())) < 0)
//                    std::swap(poly.pts.front(), poly.pts.back());

            out[layer_id].emplace_back(std::move(poly.pts));
            ++layer_id;
        }
    }
}

void PrintObject::project_and_append_custom_facets(
        bool seam, EnforcerBlockerType type, std::vector<Polygons>& out) const
{
    for (const ModelVolume* mv : this->model_object()->volumes)
        if (mv->is_model_part()) {
            const indexed_triangle_set custom_facets = seam
                    ? mv->seam_facets.get_facets_strict(*mv, type)
                    : mv->supported_facets.get_facets_strict(*mv, type);
            if (! custom_facets.indices.empty()) {
                if (seam)
                    project_triangles_to_slabs(this->layers(), custom_facets,
                        (this->trafo_centered() * mv->get_matrix()).cast<float>(),
                        seam, out);
                else {
                    std::vector<Polygons> projected;
                    // Support blockers or enforcers. Project downward facing painted areas upwards to their respective slicing plane.
                    slice_mesh_slabs(custom_facets, zs_from_layers(this->layers()), this->trafo_centered() * mv->get_matrix(), nullptr, &projected, [](){});
                    // Merge these projections with the output, layer by layer.
                    assert(! projected.empty());
                    assert(out.empty() || out.size() == projected.size());
                    if (out.empty())
                        out = std::move(projected);
                    else
                        for (size_t i = 0; i < out.size(); ++ i)
                            append(out[i], std::move(projected[i]));
                }
            }
        }
}

const Layer* PrintObject::get_layer_at_printz(coordf_t print_z) const {
    auto it = Slic3r::lower_bound_by_predicate(m_layers.begin(), m_layers.end(), [print_z](const Layer *layer) { return layer->print_z < print_z; });
    return (it == m_layers.end() || (*it)->print_z != print_z) ? nullptr : *it;
}



Layer* PrintObject::get_layer_at_printz(coordf_t print_z) { return const_cast<Layer*>(std::as_const(*this).get_layer_at_printz(print_z)); }



// Get a layer approximately at print_z.
const Layer* PrintObject::get_layer_at_printz(coordf_t print_z, coordf_t epsilon) const {
    coordf_t limit = print_z - epsilon;
    auto it = Slic3r::lower_bound_by_predicate(m_layers.begin(), m_layers.end(), [limit](const Layer *layer) { return layer->print_z < limit; });
    return (it == m_layers.end() || (*it)->print_z > print_z + epsilon) ? nullptr : *it;
}



Layer* PrintObject::get_layer_at_printz(coordf_t print_z, coordf_t epsilon) { return const_cast<Layer*>(std::as_const(*this).get_layer_at_printz(print_z, epsilon)); }

const Layer *PrintObject::get_first_layer_bellow_printz(coordf_t print_z, coordf_t epsilon) const
{
    coordf_t limit = print_z + epsilon;
    auto it = Slic3r::lower_bound_by_predicate(m_layers.begin(), m_layers.end(), [limit](const Layer *layer) { return layer->print_z < limit; });
    return (it == m_layers.begin()) ? nullptr : *(--it);
}
int PrintObject::get_layer_idx_get_printz(coordf_t print_z, coordf_t epsilon) {
    coordf_t limit = print_z + epsilon;
    auto     it    = Slic3r::lower_bound_by_predicate(m_layers.begin(), m_layers.end(), [limit](const Layer *layer) { return layer->print_z < limit; });
    return (it == m_layers.begin()) ? -1 : std::distance(m_layers.begin(), it);
}
// BBS
const Layer* PrintObject::get_layer_at_bottomz(coordf_t bottom_z, coordf_t epsilon) const {
    coordf_t limit_upper = bottom_z + epsilon;
    coordf_t limit_lower = bottom_z - epsilon;

    for (const Layer* layer : m_layers) {
        if (layer->bottom_z() > limit_lower)
            return layer->bottom_z() < limit_upper ? layer : nullptr;
    }

    return nullptr;
}

Layer* PrintObject::get_layer_at_bottomz(coordf_t bottom_z, coordf_t epsilon) { return const_cast<Layer*>(std::as_const(*this).get_layer_at_bottomz(bottom_z, epsilon)); }


} // namespace Slic3r