PrusaSlicer-NonPlainar/xs/src/libslic3r/Fill/FillRectilinear2.cpp

#include <stdlib.h>
#include <stdint.h>

#include <algorithm>
#include <cmath>
#include <limits>

#include <boost/static_assert.hpp>

#include "../ClipperUtils.hpp"
#include "../ExPolygon.hpp"
#include "../Geometry.hpp"
#include "../Surface.hpp"

#include "FillRectilinear2.hpp"

// #define SLIC3R_DEBUG

// Make assert active if SLIC3R_DEBUG
#ifdef SLIC3R_DEBUG
    #undef NDEBUG
    #include "SVG.hpp"
#endif

#include <cassert>

// We want our version of assert.
#include "../libslic3r.h"

namespace Slic3r {

// Having a segment of a closed polygon, calculate its Euclidian length.
// The segment indices seg1 and seg2 signify an end point of an edge in the forward direction of the loop,
// therefore the point p1 lies on poly.points[seg1-1], poly.points[seg1] etc.
static inline coordf_t segment_length(const Polygon &poly, size_t seg1, const Point &p1, size_t seg2, const Point &p2)
{
#ifdef SLIC3R_DEBUG
    // Verify that p1 lies on seg1. This is difficult to verify precisely,
    // but at least verify, that p1 lies in the bounding box of seg1.
    for (size_t i = 0; i < 2; ++ i) {
        size_t seg = (i == 0) ? seg1 : seg2;
        Point  px  = (i == 0) ? p1   : p2;
        Point  pa  = poly.points[((seg == 0) ? poly.points.size() : seg) - 1];
        Point  pb  = poly.points[seg];
        if (pa(0) > pb(0))
            std::swap(pa(0), pb(0));
        if (pa(1) > pb(1))
            std::swap(pa(1), pb(1));
        assert(px(0) >= pa(0) && px(0) <= pb(0));
        assert(px(1) >= pa(1) && px(1) <= pb(1));
    }
#endif /* SLIC3R_DEBUG */
    const Point *pPrev = &p1;
    const Point *pThis = NULL;
    coordf_t len = 0;
    if (seg1 <= seg2) {
        for (size_t i = seg1; i < seg2; ++ i, pPrev = pThis)
           len += (*pPrev - *(pThis = &poly.points[i])).cast<double>().norm();
    } else {
        for (size_t i = seg1; i < poly.points.size(); ++ i, pPrev = pThis)
           len += (*pPrev - *(pThis = &poly.points[i])).cast<double>().norm();
        for (size_t i = 0; i < seg2; ++ i, pPrev = pThis)
           len += (*pPrev - *(pThis = &poly.points[i])).cast<double>().norm();
    }
    len += (*pPrev - p2).cast<double>().norm();
    return len;
}

// Append a segment of a closed polygon to a polyline.
// The segment indices seg1 and seg2 signify an end point of an edge in the forward direction of the loop.
// Only insert intermediate points between seg1 and seg2.
static inline void polygon_segment_append(Points &out, const Polygon &polygon, size_t seg1, size_t seg2)
{
    if (seg1 == seg2) {
        // Nothing to append from this segment.
    } else if (seg1 < seg2) {
        // Do not append a point pointed to by seg2.
        out.insert(out.end(), polygon.points.begin() + seg1, polygon.points.begin() + seg2);
    } else {
        out.reserve(out.size() + seg2 + polygon.points.size() - seg1);
        out.insert(out.end(), polygon.points.begin() + seg1, polygon.points.end());
        // Do not append a point pointed to by seg2.
        out.insert(out.end(), polygon.points.begin(), polygon.points.begin() + seg2);
    }
}

// Append a segment of a closed polygon to a polyline.
// The segment indices seg1 and seg2 signify an end point of an edge in the forward direction of the loop,
// but this time the segment is traversed backward.
// Only insert intermediate points between seg1 and seg2.
static inline void polygon_segment_append_reversed(Points &out, const Polygon &polygon, size_t seg1, size_t seg2)
{
    if (seg1 >= seg2) {
        out.reserve(seg1 - seg2);
        for (size_t i = seg1; i > seg2; -- i)
            out.push_back(polygon.points[i - 1]);
    } else {
        // it could be, that seg1 == seg2. In that case, append the complete loop.
        out.reserve(out.size() + seg2 + polygon.points.size() - seg1);
        for (size_t i = seg1; i > 0; -- i)
            out.push_back(polygon.points[i - 1]);
        for (size_t i = polygon.points.size(); i > seg2; -- i)
            out.push_back(polygon.points[i - 1]);
    }
}

// Intersection point of a vertical line with a polygon segment.
class SegmentIntersection
{
public:
    SegmentIntersection() : 
        iContour(0),
        iSegment(0),
        pos_p(0),
        pos_q(1),
        type(UNKNOWN),
        consumed_vertical_up(false),
        consumed_perimeter_right(false)
        {}

    // Index of a contour in ExPolygonWithOffset, with which this vertical line intersects.
    size_t      iContour;
    // Index of a segment in iContour, with which this vertical line intersects.
    size_t      iSegment;
    // y position of the intersection, ratinal number.
    int64_t     pos_p;
    uint32_t    pos_q;

    coord_t     pos() const {
        // Division rounds both positive and negative down to zero.
        // Add half of q for an arithmetic rounding effect.
        int64_t p = pos_p;
        if (p < 0)
            p -= int64_t(pos_q>>1);
        else
            p += int64_t(pos_q>>1);
        return coord_t(p / int64_t(pos_q)); 
    }

    // Kind of intersection. With the original contour, or with the inner offestted contour?
    // A vertical segment will be at least intersected by OUTER_LOW, OUTER_HIGH,
    // but it could be intersected with OUTER_LOW, INNER_LOW, INNER_HIGH, OUTER_HIGH,
    // and there may be more than one pair of INNER_LOW, INNER_HIGH between OUTER_LOW, OUTER_HIGH.
    enum SegmentIntersectionType {
        OUTER_LOW   = 0,
        OUTER_HIGH  = 1,
        INNER_LOW   = 2,
        INNER_HIGH  = 3,
        UNKNOWN     = -1
    };
    SegmentIntersectionType type;

    // Was this segment along the y axis consumed?
    // Up means up along the vertical segment.
    bool consumed_vertical_up;
    // Was a segment of the inner perimeter contour consumed?
    // Right means right from the vertical segment.
    bool consumed_perimeter_right;

    // For the INNER_LOW type, this point may be connected to another INNER_LOW point following a perimeter contour.
    // For the INNER_HIGH type, this point may be connected to another INNER_HIGH point following a perimeter contour.
    // If INNER_LOW is connected to INNER_HIGH or vice versa,
    // one has to make sure the vertical infill line does not overlap with the connecting perimeter line.
    bool is_inner() const { return type == INNER_LOW  || type == INNER_HIGH; }
    bool is_outer() const { return type == OUTER_LOW  || type == OUTER_HIGH; }
    bool is_low  () const { return type == INNER_LOW  || type == OUTER_LOW; }
    bool is_high () const { return type == INNER_HIGH || type == OUTER_HIGH; }

    // Compare two y intersection points given by rational numbers.
    // Note that the rational number is given as pos_p/pos_q, where pos_p is int64 and pos_q is uint32.
    // This function calculates pos_p * other.pos_q < other.pos_p * pos_q as a 48bit number.
    // We don't use 128bit intrinsic data types as these are usually not supported by 32bit compilers and
    // we don't need the full 128bit precision anyway.
    bool operator<(const SegmentIntersection &other) const
    {
        assert(pos_q > 0);
        assert(other.pos_q > 0);
        if (pos_p == 0 || other.pos_p == 0) {
            // Because the denominators are positive and one of the nominators is zero,
            // following simple statement holds.
            return pos_p < other.pos_p;
        } else {
            // None of the nominators is zero.
            int sign1 = (pos_p > 0) ? 1 : -1;
            int sign2 = (other.pos_p > 0) ? 1 : -1;
            int signs = sign1 * sign2;
            assert(signs == 1 || signs == -1);
            if (signs < 0) {
                // The nominators have different signs.
                return sign1 < 0;
            } else {
                // The nominators have the same sign.
                // Absolute values
                uint64_t p1, p2;
                if (sign1 > 0) {
                    p1 = uint64_t(pos_p);
                    p2 = uint64_t(other.pos_p);
                } else {
                    p1 = uint64_t(- pos_p);
                    p2 = uint64_t(- other.pos_p);
                };
                // Multiply low and high 32bit words of p1 by other_pos.q
                // 32bit x 32bit => 64bit
                // l_hi and l_lo overlap by 32 bits.
                uint64_t l_hi = (p1 >> 32) * uint64_t(other.pos_q);
                uint64_t l_lo = (p1 & 0xffffffffll) * uint64_t(other.pos_q);
                l_hi += (l_lo >> 32);
                uint64_t r_hi = (p2 >> 32) * uint64_t(pos_q);
                uint64_t r_lo = (p2 & 0xffffffffll) * uint64_t(pos_q);
                r_hi += (r_lo >> 32);
                // Compare the high 64 bits.
                if (l_hi == r_hi) {
                    // Compare the low 32 bits.
                    l_lo &= 0xffffffffll;
                    r_lo &= 0xffffffffll;
                    return (sign1 < 0) ? (l_lo > r_lo) : (l_lo < r_lo);
                }
                return (sign1 < 0) ? (l_hi > r_hi) : (l_hi < r_hi);
            }
        }
    }

    bool operator==(const SegmentIntersection &other) const 
    {
        assert(pos_q > 0);
        assert(other.pos_q > 0);
        if (pos_p == 0 || other.pos_p == 0) {
            // Because the denominators are positive and one of the nominators is zero,
            // following simple statement holds.
            return pos_p == other.pos_p;
        }

        // None of the nominators is zero, none of the denominators is zero.
        bool positive = pos_p > 0;
        if (positive != (other.pos_p > 0))
            return false;
        // The nominators have the same sign.
        // Absolute values
        uint64_t p1 = positive ? uint64_t(pos_p) : uint64_t(- pos_p);
        uint64_t p2 = positive ? uint64_t(other.pos_p) : uint64_t(- other.pos_p);
        // Multiply low and high 32bit words of p1 by other_pos.q
        // 32bit x 32bit => 64bit
        // l_hi and l_lo overlap by 32 bits.
        uint64_t l_lo = (p1 & 0xffffffffll) * uint64_t(other.pos_q);
        uint64_t r_lo = (p2 & 0xffffffffll) * uint64_t(pos_q);
        if (l_lo != r_lo)
            return false;
        uint64_t l_hi = (p1 >> 32) * uint64_t(other.pos_q);
        uint64_t r_hi = (p2 >> 32) * uint64_t(pos_q);
        return l_hi + (l_lo >> 32) == r_hi + (r_lo >> 32);
    }
};

// A vertical line with intersection points with polygons.
class SegmentedIntersectionLine
{
public:
    // Index of this vertical intersection line.
    size_t                              idx;
    // x position of this vertical intersection line.
    coord_t                             pos;
    // List of intersection points with polygons, sorted increasingly by the y axis.
    std::vector<SegmentIntersection>    intersections;
};

// A container maintaining an expolygon with its inner offsetted polygon.
// The purpose of the inner offsetted polygon is to provide segments to connect the infill lines.
struct ExPolygonWithOffset
{
public:
    ExPolygonWithOffset(
        const ExPolygon &expolygon,
        float   angle,
        coord_t aoffset1,
        coord_t aoffset2)
    {
        // Copy and rotate the source polygons.
        polygons_src = expolygon;
        polygons_src.contour.rotate(angle);
        for (Polygons::iterator it = polygons_src.holes.begin(); it != polygons_src.holes.end(); ++ it)
            it->rotate(angle);

        double mitterLimit = 3.;
        // for the infill pattern, don't cut the corners.
        // default miterLimt = 3
        //double mitterLimit = 10.;
        assert(aoffset1 < 0);
        assert(aoffset2 < 0);
        assert(aoffset2 < aoffset1);
        bool sticks_removed = remove_sticks(polygons_src);
//        if (sticks_removed) printf("Sticks removed!\n");
        polygons_outer = offset(polygons_src, aoffset1,
            ClipperLib::jtMiter,
            mitterLimit);
        polygons_inner = offset(polygons_outer, aoffset2 - aoffset1,
            ClipperLib::jtMiter,
            mitterLimit);
		// Filter out contours with zero area or small area, contours with 2 points only.
        const double min_area_threshold = 0.01 * aoffset2 * aoffset2;
        remove_small(polygons_outer, min_area_threshold);
        remove_small(polygons_inner, min_area_threshold);
        remove_sticks(polygons_outer);
        remove_sticks(polygons_inner);
		n_contours_outer = polygons_outer.size();
        n_contours_inner = polygons_inner.size();
        n_contours = n_contours_outer + n_contours_inner;
        polygons_ccw.assign(n_contours, false);
        for (size_t i = 0; i < n_contours; ++ i) {
            contour(i).remove_duplicate_points();
            assert(! contour(i).has_duplicate_points());
            polygons_ccw[i] = Slic3r::Geometry::is_ccw(contour(i));
        }
    }

    // Any contour with offset1
    bool             is_contour_outer(size_t idx) const { return idx < n_contours_outer; }
    // Any contour with offset2
    bool             is_contour_inner(size_t idx) const { return idx >= n_contours_outer; }

    const Polygon&   contour(size_t idx) const 
        { return is_contour_outer(idx) ? polygons_outer[idx] : polygons_inner[idx - n_contours_outer]; }

    Polygon&         contour(size_t idx)
        { return is_contour_outer(idx) ? polygons_outer[idx] : polygons_inner[idx - n_contours_outer]; }

    bool             is_contour_ccw(size_t idx) const { return polygons_ccw[idx]; }

    BoundingBox      bounding_box_src() const 
        { return get_extents(polygons_src); }
    BoundingBox      bounding_box_outer() const 
        { return get_extents(polygons_outer); }
    BoundingBox      bounding_box_inner() const 
        { return get_extents(polygons_inner); }

#ifdef SLIC3R_DEBUG
    void export_to_svg(Slic3r::SVG &svg) {
        svg.draw_outline(polygons_src,   "black");
        svg.draw_outline(polygons_outer, "green");
        svg.draw_outline(polygons_inner, "brown");
    }
#endif /* SLIC3R_DEBUG */

    ExPolygon        polygons_src;
    Polygons         polygons_outer;
    Polygons         polygons_inner;

    size_t           n_contours_outer;
    size_t           n_contours_inner;
    size_t           n_contours;

protected:
    // For each polygon of polygons_inner, remember its orientation.
    std::vector<unsigned char> polygons_ccw;
};

static inline int distance_of_segmens(const Polygon &poly, size_t seg1, size_t seg2, bool forward)
{
    int d = int(seg2) - int(seg1);
    if (! forward)
        d = - d;
    if (d < 0)
        d += int(poly.points.size());
    return d;
}

// For a vertical line, an inner contour and an intersection point,
// find an intersection point on the previous resp. next vertical line.
// The intersection point is connected with the prev resp. next intersection point with iInnerContour.
// Return -1 if there is no such point on the previous resp. next vertical line.
static inline int intersection_on_prev_next_vertical_line(
    const ExPolygonWithOffset                     &poly_with_offset,
    const std::vector<SegmentedIntersectionLine>  &segs,
    size_t                                         iVerticalLine,
    size_t                                         iInnerContour,
    size_t                                         iIntersection,
    bool                                           dir_is_next)
{
    size_t iVerticalLineOther = iVerticalLine;
    if (dir_is_next) {
        if (++ iVerticalLineOther == segs.size())
            // No successive vertical line.
            return -1;
    } else if (iVerticalLineOther -- == 0) {
        // No preceding vertical line.
        return -1;
    }

    const SegmentedIntersectionLine &il    = segs[iVerticalLine];
    const SegmentIntersection       &itsct = il.intersections[iIntersection];
    const SegmentedIntersectionLine &il2   = segs[iVerticalLineOther];
    const Polygon                   &poly  = poly_with_offset.contour(iInnerContour);
//    const bool                       ccw   = poly_with_offset.is_contour_ccw(iInnerContour);
    const bool                       forward = itsct.is_low() == dir_is_next;
    // Resulting index of an intersection point on il2.
    int                              out   = -1;
    // Find an intersection point on iVerticalLineOther, intersecting iInnerContour
    // at the same orientation as iIntersection, and being closest to iIntersection
    // in the number of contour segments, when following the direction of the contour.
    int                              dmin  = std::numeric_limits<int>::max();
    for (size_t i = 0; i < il2.intersections.size(); ++ i) {
        const SegmentIntersection &itsct2 = il2.intersections[i];
        if (itsct.iContour == itsct2.iContour && itsct.type == itsct2.type) {
            /*
            if (itsct.is_low()) {
                assert(itsct.type == SegmentIntersection::INNER_LOW);
                assert(iIntersection > 0);
                assert(il.intersections[iIntersection-1].type == SegmentIntersection::OUTER_LOW);                
                assert(i > 0);
                if (il2.intersections[i-1].is_inner())
                    // Take only the lowest inner intersection point.
                    continue;
                assert(il2.intersections[i-1].type == SegmentIntersection::OUTER_LOW);
            } else {
                assert(itsct.type == SegmentIntersection::INNER_HIGH);
                assert(iIntersection+1 < il.intersections.size());
                assert(il.intersections[iIntersection+1].type == SegmentIntersection::OUTER_HIGH);
                assert(i+1 < il2.intersections.size());
                if (il2.intersections[i+1].is_inner())
                    // Take only the highest inner intersection point.
                    continue;
                assert(il2.intersections[i+1].type == SegmentIntersection::OUTER_HIGH);
            }
            */
            // The intersection points lie on the same contour and have the same orientation.
            // Find the intersection point with a shortest path in the direction of the contour.
            int d = distance_of_segmens(poly, itsct.iSegment, itsct2.iSegment, forward);
            if (d < dmin) {
                out = i;
                dmin = d;
            }
        }
    }
    //FIXME this routine is not asymptotic optimal, it will be slow if there are many intersection points along the line.
    return out;
}

static inline int intersection_on_prev_vertical_line(
    const ExPolygonWithOffset                     &poly_with_offset, 
    const std::vector<SegmentedIntersectionLine>  &segs,
    size_t                                         iVerticalLine,
    size_t                                         iInnerContour,
    size_t                                         iIntersection)
{
    return intersection_on_prev_next_vertical_line(poly_with_offset, segs, iVerticalLine, iInnerContour, iIntersection, false);
}

static inline int intersection_on_next_vertical_line(
    const ExPolygonWithOffset                     &poly_with_offset, 
    const std::vector<SegmentedIntersectionLine>  &segs, 
    size_t                                         iVerticalLine, 
    size_t                                         iInnerContour, 
    size_t                                         iIntersection)
{
    return intersection_on_prev_next_vertical_line(poly_with_offset, segs, iVerticalLine, iInnerContour, iIntersection, true);
}

enum IntersectionTypeOtherVLine {
    // There is no connection point on the other vertical line.
    INTERSECTION_TYPE_OTHER_VLINE_UNDEFINED = -1,
    // Connection point on the other vertical segment was found
    // and it could be followed.
    INTERSECTION_TYPE_OTHER_VLINE_OK = 0,
    // The connection segment connects to a middle of a vertical segment.
    // Cannot follow.
    INTERSECTION_TYPE_OTHER_VLINE_INNER,
    // Cannot extend the contor to this intersection point as either the connection segment
    // or the succeeding vertical segment were already consumed.
    INTERSECTION_TYPE_OTHER_VLINE_CONSUMED,
    // Not the first intersection along the contor. This intersection point
    // has been preceded by an intersection point along the vertical line.
    INTERSECTION_TYPE_OTHER_VLINE_NOT_FIRST,
};

// Find an intersection on a previous line, but return -1, if the connecting segment of a perimeter was already extruded.
static inline IntersectionTypeOtherVLine intersection_type_on_prev_next_vertical_line(
    const std::vector<SegmentedIntersectionLine>  &segs,
    size_t                                         iVerticalLine,
    size_t                                         iIntersection,
    size_t                                         iIntersectionOther,
    bool                                           dir_is_next)
{
    // This routine will propose a connecting line even if the connecting perimeter segment intersects 
    // iVertical line multiple times before reaching iIntersectionOther.
    if (iIntersectionOther == -1)
        return INTERSECTION_TYPE_OTHER_VLINE_UNDEFINED;
    assert(dir_is_next ? (iVerticalLine + 1 < segs.size()) : (iVerticalLine > 0));
    const SegmentedIntersectionLine &il_this      = segs[iVerticalLine];
    const SegmentIntersection       &itsct_this   = il_this.intersections[iIntersection];
    const SegmentedIntersectionLine &il_other     = segs[dir_is_next ? (iVerticalLine+1) : (iVerticalLine-1)];
    const SegmentIntersection       &itsct_other  = il_other.intersections[iIntersectionOther];
    assert(itsct_other.is_inner());
    assert(iIntersectionOther > 0);
    assert(iIntersectionOther + 1 < il_other.intersections.size());
    // Is iIntersectionOther at the boundary of a vertical segment?
    const SegmentIntersection       &itsct_other2 = il_other.intersections[itsct_other.is_low() ? iIntersectionOther - 1 : iIntersectionOther + 1];
    if (itsct_other2.is_inner())
        // Cannot follow a perimeter segment into the middle of another vertical segment.
        // Only perimeter segments connecting to the end of a vertical segment are followed.
        return INTERSECTION_TYPE_OTHER_VLINE_INNER;
    assert(itsct_other.is_low() == itsct_other2.is_low());
    if (dir_is_next ? itsct_this.consumed_perimeter_right : itsct_other.consumed_perimeter_right)
        // This perimeter segment was already consumed.
        return INTERSECTION_TYPE_OTHER_VLINE_CONSUMED;
    if (itsct_other.is_low() ? itsct_other.consumed_vertical_up : il_other.intersections[iIntersectionOther-1].consumed_vertical_up)
        // This vertical segment was already consumed.
        return INTERSECTION_TYPE_OTHER_VLINE_CONSUMED;
    return INTERSECTION_TYPE_OTHER_VLINE_OK;
}

static inline IntersectionTypeOtherVLine intersection_type_on_prev_vertical_line(
    const std::vector<SegmentedIntersectionLine>  &segs, 
    size_t                                         iVerticalLine, 
    size_t                                         iIntersection,
    size_t                                         iIntersectionPrev)
{
    return intersection_type_on_prev_next_vertical_line(segs, iVerticalLine, iIntersection, iIntersectionPrev, false);
}

static inline IntersectionTypeOtherVLine intersection_type_on_next_vertical_line(
    const std::vector<SegmentedIntersectionLine>  &segs, 
    size_t                                         iVerticalLine, 
    size_t                                         iIntersection,
    size_t                                         iIntersectionNext)
{
    return intersection_type_on_prev_next_vertical_line(segs, iVerticalLine, iIntersection, iIntersectionNext, true);
}

// Measure an Euclidian length of a perimeter segment when going from iIntersection to iIntersection2.
static inline coordf_t measure_perimeter_prev_next_segment_length(
    const ExPolygonWithOffset                     &poly_with_offset, 
    const std::vector<SegmentedIntersectionLine>  &segs,
    size_t                                         iVerticalLine,
    size_t                                         iInnerContour,
    size_t                                         iIntersection,
    size_t                                         iIntersection2,
    bool                                           dir_is_next)
{
    size_t iVerticalLineOther = iVerticalLine;
    if (dir_is_next) {
        if (++ iVerticalLineOther == segs.size())
            // No successive vertical line.
            return coordf_t(-1);
    } else if (iVerticalLineOther -- == 0) {
        // No preceding vertical line.
        return coordf_t(-1);
    }

    const SegmentedIntersectionLine &il     = segs[iVerticalLine];
    const SegmentIntersection       &itsct  = il.intersections[iIntersection];
    const SegmentedIntersectionLine &il2    = segs[iVerticalLineOther];
    const SegmentIntersection       &itsct2 = il2.intersections[iIntersection2];
    const Polygon                   &poly   = poly_with_offset.contour(iInnerContour);
//    const bool                       ccw    = poly_with_offset.is_contour_ccw(iInnerContour);
    assert(itsct.type == itsct2.type);
    assert(itsct.iContour == itsct2.iContour);
    assert(itsct.is_inner());
    const bool                       forward = itsct.is_low() == dir_is_next;

    Point p1(il.pos, itsct.pos());
    Point p2(il2.pos, itsct2.pos());
    return forward ?
        segment_length(poly, itsct .iSegment, p1, itsct2.iSegment, p2) :
        segment_length(poly, itsct2.iSegment, p2, itsct .iSegment, p1);
}

static inline coordf_t measure_perimeter_prev_segment_length(
    const ExPolygonWithOffset                     &poly_with_offset,
    const std::vector<SegmentedIntersectionLine>  &segs,
    size_t                                         iVerticalLine,
    size_t                                         iInnerContour,
    size_t                                         iIntersection,
    size_t                                         iIntersection2)
{
    return measure_perimeter_prev_next_segment_length(poly_with_offset, segs, iVerticalLine, iInnerContour, iIntersection, iIntersection2, false);
}

static inline coordf_t measure_perimeter_next_segment_length(
    const ExPolygonWithOffset                     &poly_with_offset,
    const std::vector<SegmentedIntersectionLine>  &segs,
    size_t                                         iVerticalLine,
    size_t                                         iInnerContour,
    size_t                                         iIntersection,
    size_t                                         iIntersection2)
{
    return measure_perimeter_prev_next_segment_length(poly_with_offset, segs, iVerticalLine, iInnerContour, iIntersection, iIntersection2, true);
}

// Append the points of a perimeter segment when going from iIntersection to iIntersection2.
// The first point (the point of iIntersection) will not be inserted,
// the last point will be inserted.
static inline void emit_perimeter_prev_next_segment(
    const ExPolygonWithOffset                     &poly_with_offset,
    const std::vector<SegmentedIntersectionLine>  &segs,
    size_t                                         iVerticalLine,
    size_t                                         iInnerContour,
    size_t                                         iIntersection,
    size_t                                         iIntersection2,
    Polyline                                      &out,
    bool                                           dir_is_next)
{
    size_t iVerticalLineOther = iVerticalLine;
    if (dir_is_next) {
        ++ iVerticalLineOther;
        assert(iVerticalLineOther < segs.size());
    } else {
        assert(iVerticalLineOther > 0);
        -- iVerticalLineOther;
    }

    const SegmentedIntersectionLine &il     = segs[iVerticalLine];
    const SegmentIntersection       &itsct  = il.intersections[iIntersection];
    const SegmentedIntersectionLine &il2    = segs[iVerticalLineOther];
    const SegmentIntersection       &itsct2 = il2.intersections[iIntersection2];
    const Polygon                   &poly   = poly_with_offset.contour(iInnerContour);
//    const bool                       ccw    = poly_with_offset.is_contour_ccw(iInnerContour);
    assert(itsct.type == itsct2.type);
    assert(itsct.iContour == itsct2.iContour);
    assert(itsct.is_inner());
    const bool                       forward = itsct.is_low() == dir_is_next;
    // Do not append the first point.
    // out.points.push_back(Point(il.pos, itsct.pos));
    if (forward)
        polygon_segment_append(out.points, poly, itsct.iSegment, itsct2.iSegment);
    else
        polygon_segment_append_reversed(out.points, poly, itsct.iSegment, itsct2.iSegment);
    // Append the last point.
    out.points.push_back(Point(il2.pos, itsct2.pos()));
}

static inline coordf_t measure_perimeter_segment_on_vertical_line_length(
    const ExPolygonWithOffset                     &poly_with_offset,
    const std::vector<SegmentedIntersectionLine>  &segs,
    size_t                                         iVerticalLine,
    size_t                                         iInnerContour,
    size_t                                         iIntersection,
    size_t                                         iIntersection2,
    bool                                           forward)
{
    const SegmentedIntersectionLine &il = segs[iVerticalLine];
    const SegmentIntersection       &itsct = il.intersections[iIntersection];
    const SegmentIntersection       &itsct2 = il.intersections[iIntersection2];
    const Polygon                   &poly = poly_with_offset.contour(iInnerContour);
    assert(itsct.is_inner());
    assert(itsct2.is_inner());
    assert(itsct.type != itsct2.type);
    assert(itsct.iContour == iInnerContour);
    assert(itsct.iContour == itsct2.iContour);
    Point p1(il.pos, itsct.pos());
    Point p2(il.pos, itsct2.pos());
    return forward ?
        segment_length(poly, itsct .iSegment, p1, itsct2.iSegment, p2) :
        segment_length(poly, itsct2.iSegment, p2, itsct .iSegment, p1);
}

// Append the points of a perimeter segment when going from iIntersection to iIntersection2.
// The first point (the point of iIntersection) will not be inserted,
// the last point will be inserted.
static inline void emit_perimeter_segment_on_vertical_line(
	const ExPolygonWithOffset                     &poly_with_offset,
	const std::vector<SegmentedIntersectionLine>  &segs,
	size_t                                         iVerticalLine,
	size_t                                         iInnerContour,
	size_t                                         iIntersection,
	size_t                                         iIntersection2,
	Polyline                                      &out,
	bool                                           forward)
{
	const SegmentedIntersectionLine &il = segs[iVerticalLine];
	const SegmentIntersection       &itsct = il.intersections[iIntersection];
	const SegmentIntersection       &itsct2 = il.intersections[iIntersection2];
	const Polygon                   &poly = poly_with_offset.contour(iInnerContour);
	assert(itsct.is_inner());
	assert(itsct2.is_inner());
	assert(itsct.type != itsct2.type);
    assert(itsct.iContour == iInnerContour);
	assert(itsct.iContour == itsct2.iContour);
	// Do not append the first point.
	// out.points.push_back(Point(il.pos, itsct.pos));
	if (forward)
		polygon_segment_append(out.points, poly, itsct.iSegment, itsct2.iSegment);
	else
		polygon_segment_append_reversed(out.points, poly, itsct.iSegment, itsct2.iSegment);
	// Append the last point.
	out.points.push_back(Point(il.pos, itsct2.pos()));
}

//TBD: For precise infill, measure the area of a slab spanned by an infill line.
/*
static inline float measure_outer_contour_slab(
    const ExPolygonWithOffset                     &poly_with_offset,
    const std::vector<SegmentedIntersectionLine>  &segs,
    size_t                                         i_vline,
    size_t                                         iIntersection)
{
    const SegmentedIntersectionLine &il     = segs[i_vline];
    const SegmentIntersection       &itsct  = il.intersections[i_vline];
    const SegmentIntersection       &itsct2 = il.intersections[iIntersection2];
    const Polygon                   &poly   = poly_with_offset.contour((itsct.iContour);
    assert(itsct.is_outer());
    assert(itsct2.is_outer());
    assert(itsct.type != itsct2.type);
    assert(itsct.iContour == itsct2.iContour);
    if (! itsct.is_outer() || ! itsct2.is_outer() || itsct.type == itsct2.type || itsct.iContour != itsct2.iContour)
        // Error, return zero area.
        return 0.f;

    // Find possible connection points on the previous / next vertical line.
    int iPrev = intersection_on_prev_vertical_line(poly_with_offset, segs, i_vline, itsct.iContour, i_intersection);
    int iNext = intersection_on_next_vertical_line(poly_with_offset, segs, i_vline, itsct.iContour, i_intersection);
    // Find possible connection points on the same vertical line.
    int iAbove = iBelow = -1;
    // Does the perimeter intersect the current vertical line above intrsctn?
    for (size_t i = i_intersection + 1; i + 1 < seg.intersections.size(); ++ i)
        if (seg.intersections[i].iContour == itsct.iContour)
            { iAbove = i; break; }
    // Does the perimeter intersect the current vertical line below intrsctn?
    for (int i = int(i_intersection) - 1; i > 0; -- i)
        if (seg.intersections[i].iContour == itsct.iContour)
            { iBelow = i; break; }

    if (iSegAbove != -1 && seg.intersections[iAbove].type == SegmentIntersection::OUTER_HIGH) {
        // Invalidate iPrev resp. iNext, if the perimeter crosses the current vertical line earlier than iPrev resp. iNext.
        // The perimeter contour orientation.
        const Polygon &poly = poly_with_offset.contour(itsct.iContour);
        {
            int d_horiz = (iPrev  == -1) ? std::numeric_limits<int>::max() :
                distance_of_segmens(poly, segs[i_vline-1].intersections[iPrev].iSegment, itsct.iSegment, true);
            int d_down  = (iBelow == -1) ? std::numeric_limits<int>::max() :
                distance_of_segmens(poly, iSegBelow, itsct.iSegment, true);
            int d_up    = (iAbove == -1) ? std::numeric_limits<int>::max() :
                distance_of_segmens(poly, iSegAbove, itsct.iSegment, true);
            if (intrsctn_type_prev == INTERSECTION_TYPE_OTHER_VLINE_OK && d_horiz > std::min(d_down, d_up))
                // The vertical crossing comes eralier than the prev crossing.
                // Disable the perimeter going back.
                intrsctn_type_prev = INTERSECTION_TYPE_OTHER_VLINE_NOT_FIRST;
            if (d_up > std::min(d_horiz, d_down))
                // The horizontal crossing comes earlier than the vertical crossing.
                vert_seg_dir_valid_mask &= ~DIR_BACKWARD;
        }
        {
            int d_horiz = (iNext     == -1) ? std::numeric_limits<int>::max() :
                distance_of_segmens(poly, itsct.iSegment, segs[i_vline+1].intersections[iNext].iSegment, true);
            int d_down  = (iSegBelow == -1) ? std::numeric_limits<int>::max() :
                distance_of_segmens(poly, itsct.iSegment, iSegBelow, true);
            int d_up    = (iSegAbove == -1) ? std::numeric_limits<int>::max() :
                distance_of_segmens(poly, itsct.iSegment, iSegAbove, true);
            if (d_up > std::min(d_horiz, d_down))
                // The horizontal crossing comes earlier than the vertical crossing.
                vert_seg_dir_valid_mask &= ~DIR_FORWARD;
        }
    }
}
*/

enum DirectionMask
{
    DIR_FORWARD  = 1,
    DIR_BACKWARD = 2
};

bool FillRectilinear2::fill_surface_by_lines(const Surface *surface, const FillParams &params, float angleBase, float pattern_shift, Polylines &polylines_out)
{
    // At the end, only the new polylines will be rotated back.
    size_t n_polylines_out_initial = polylines_out.size();

    // Shrink the input polygon a bit first to not push the infill lines out of the perimeters.
//    const float INFILL_OVERLAP_OVER_SPACING = 0.3f;
    const float INFILL_OVERLAP_OVER_SPACING = 0.45f;
    assert(INFILL_OVERLAP_OVER_SPACING > 0 && INFILL_OVERLAP_OVER_SPACING < 0.5f);

    // Rotate polygons so that we can work with vertical lines here
    std::pair<float, Point> rotate_vector = this->_infill_direction(surface);
    rotate_vector.first += angleBase;

    assert(params.density > 0.0001f && params.density <= 1.f);
    coord_t line_spacing = coord_t(scale_(this->spacing) / params.density);

    // On the polygons of poly_with_offset, the infill lines will be connected.
    ExPolygonWithOffset poly_with_offset(
        surface->expolygon, 
        - rotate_vector.first, 
        scale_(this->overlap - (0.5 - INFILL_OVERLAP_OVER_SPACING) * this->spacing),
        scale_(this->overlap - 0.5 * this->spacing));
    if (poly_with_offset.n_contours_inner == 0) {
        // Not a single infill line fits.
        //FIXME maybe one shall trigger the gap fill here?
        return true;
    }

    BoundingBox bounding_box = poly_with_offset.bounding_box_src();

    // define flow spacing according to requested density
    if (params.full_infill() && !params.dont_adjust) {
        line_spacing = this->_adjust_solid_spacing(bounding_box.size()(0), line_spacing);
        this->spacing = unscale(line_spacing);
    } else {
        // extend bounding box so that our pattern will be aligned with other layers
        // Transform the reference point to the rotated coordinate system.
        Point refpt = rotate_vector.second.rotated(- rotate_vector.first);
        // _align_to_grid will not work correctly with positive pattern_shift.
        coord_t pattern_shift_scaled = coord_t(scale_(pattern_shift)) % line_spacing;
        refpt(0) -= (pattern_shift_scaled >= 0) ? pattern_shift_scaled : (line_spacing + pattern_shift_scaled);
        bounding_box.merge(_align_to_grid(
            bounding_box.min, 
            Point(line_spacing, line_spacing), 
            refpt));
    }

    // Intersect a set of euqally spaced vertical lines wiht expolygon.
    // n_vlines = ceil(bbox_width / line_spacing)
    size_t  n_vlines = (bounding_box.max(0) - bounding_box.min(0) + line_spacing - 1) / line_spacing;
	coord_t x0 = bounding_box.min(0);
	if (params.full_infill())
		x0 += (line_spacing + SCALED_EPSILON) / 2;

#ifdef SLIC3R_DEBUG
    static int iRun = 0;
    BoundingBox bbox_svg = poly_with_offset.bounding_box_outer();
    ::Slic3r::SVG svg(debug_out_path("FillRectilinear2-%d.svg", iRun), bbox_svg); // , scale_(1.));
    poly_with_offset.export_to_svg(svg);
    {
        ::Slic3r::SVG svg(debug_out_path("FillRectilinear2-initial-%d.svg", iRun), bbox_svg); // , scale_(1.));
        poly_with_offset.export_to_svg(svg);
    }
    iRun ++;
#endif /* SLIC3R_DEBUG */

    // For each contour
    // Allocate storage for the segments.
    std::vector<SegmentedIntersectionLine> segs(n_vlines, SegmentedIntersectionLine());
    for (size_t i = 0; i < n_vlines; ++ i) {
        segs[i].idx = i;
        segs[i].pos = x0 + i * line_spacing;
    }
    for (size_t iContour = 0; iContour < poly_with_offset.n_contours; ++ iContour) {
        const Points &contour = poly_with_offset.contour(iContour).points;
        if (contour.size() < 2)
            continue;
        // For each segment
        for (size_t iSegment = 0; iSegment < contour.size(); ++ iSegment) {
            size_t iPrev = ((iSegment == 0) ? contour.size() : iSegment) - 1;
            const Point &p1 = contour[iPrev];
            const Point &p2 = contour[iSegment];
            // Which of the equally spaced vertical lines is intersected by this segment?
            coord_t l = p1(0);
            coord_t r = p2(0);
            if (l > r)
                std::swap(l, r);
            // il, ir are the left / right indices of vertical lines intersecting a segment
            int il = (l - x0) / line_spacing;
            while (il * line_spacing + x0 < l)
                ++ il;
            il = std::max(int(0), il);
            int ir = (r - x0 + line_spacing) / line_spacing;
            while (ir * line_spacing + x0 > r)
                -- ir;
            ir = std::min(int(segs.size()) - 1, ir);
            if (il > ir)
                // No vertical line intersects this segment.
                continue;
            assert(il >= 0 && il < segs.size());
            assert(ir >= 0 && ir < segs.size());
            for (int i = il; i <= ir; ++ i) {
                coord_t this_x = segs[i].pos;
				assert(this_x == i * line_spacing + x0);
                SegmentIntersection is;
                is.iContour = iContour;
                is.iSegment = iSegment;
                assert(l <= this_x);
                assert(r >= this_x);
                // Calculate the intersection position in y axis. x is known.
                if (p1(0) == this_x) {
                    if (p2(0) == this_x) {
                        // Ignore strictly vertical segments.
                        continue;
                    }
                    is.pos_p = p1(1);
                    is.pos_q = 1;
                } else if (p2(0) == this_x) {
                    is.pos_p = p2(1);
                    is.pos_q = 1;
                } else {
                    // First calculate the intersection parameter 't' as a rational number with non negative denominator.
                    if (p2(0) > p1(0)) {
                        is.pos_p = this_x - p1(0);
                        is.pos_q = p2(0) - p1(0);
                    } else {
                        is.pos_p = p1(0) - this_x;
                        is.pos_q = p1(0) - p2(0);
                    }
                    assert(is.pos_p >= 0 && is.pos_p <= is.pos_q);
                    // Make an intersection point from the 't'.
                    is.pos_p *= int64_t(p2(1) - p1(1));
                    is.pos_p += p1(1) * int64_t(is.pos_q);
                }
                // +-1 to take rounding into account.
                assert(is.pos() + 1 >= std::min(p1(1), p2(1)));
                assert(is.pos() <= std::max(p1(1), p2(1)) + 1);
                segs[i].intersections.push_back(is);
            }
        }
    }

    // Sort the intersections along their segments, specify the intersection types.
    for (size_t i_seg = 0; i_seg < segs.size(); ++ i_seg) {
        SegmentedIntersectionLine &sil = segs[i_seg];
        // Sort the intersection points using exact rational arithmetic.
        std::sort(sil.intersections.begin(), sil.intersections.end());
        // Assign the intersection types, remove duplicate or overlapping intersection points.
        // When a loop vertex touches a vertical line, intersection point is generated for both segments.
        // If such two segments are oriented equally, then one of them is removed.
        // Otherwise the vertex is tangential to the vertical line and both segments are removed.
        // The same rule applies, if the loop is pinched into a single point and this point touches the vertical line:
        // The loop has a zero vertical size at the vertical line, therefore the intersection point is removed.
        size_t j = 0;
        for (size_t i = 0; i < sil.intersections.size(); ++ i) {
            // What is the orientation of the segment at the intersection point?
            size_t iContour = sil.intersections[i].iContour;
            const Points &contour = poly_with_offset.contour(iContour).points;
            size_t iSegment = sil.intersections[i].iSegment;
            size_t iPrev    = ((iSegment == 0) ? contour.size() : iSegment) - 1;
            coord_t dir = contour[iSegment](0) - contour[iPrev](0);
            bool low = dir > 0;
            sil.intersections[i].type = poly_with_offset.is_contour_outer(iContour) ? 
                (low ? SegmentIntersection::OUTER_LOW : SegmentIntersection::OUTER_HIGH) :
                (low ? SegmentIntersection::INNER_LOW : SegmentIntersection::INNER_HIGH);
            if (j > 0 && sil.intersections[i].iContour == sil.intersections[j-1].iContour) {
                // Two successive intersection points on a vertical line with the same contour. This may be a special case.
                if (sil.intersections[i].pos() == sil.intersections[j-1].pos()) {
                    // Two successive segments meet exactly at the vertical line.
        #ifdef SLIC3R_DEBUG
                    // Verify that the segments of sil.intersections[i] and sil.intersections[j-1] are adjoint.
                    size_t iSegment2 = sil.intersections[j-1].iSegment;
                    size_t iPrev2    = ((iSegment2 == 0) ? contour.size() : iSegment2) - 1;
                    assert(iSegment == iPrev2 || iSegment2 == iPrev);
        #endif /* SLIC3R_DEBUG */
                    if (sil.intersections[i].type == sil.intersections[j-1].type) {
                        // Two successive segments of the same direction (both to the right or both to the left)
                        // meet exactly at the vertical line.
                        // Remove the second intersection point.
                    } else {
                        // This is a loop returning to the same point.
                        // It may as well be a vertex of a loop touching this vertical line.
                        // Remove both the lines.
                        -- j;
                    }
                } else if (sil.intersections[i].type == sil.intersections[j-1].type) {
                    // Two non successive segments of the same direction (both to the right or both to the left)
                    // meet exactly at the vertical line. That means there is a Z shaped path, where the center segment
                    // of the Z shaped path is aligned with this vertical line.
                    // Remove one of the intersection points while maximizing the vertical segment length.
                    if (low) {
                        // Remove the second intersection point, keep the first intersection point.
                    } else {
                        // Remove the first intersection point, keep the second intersection point.
                        sil.intersections[j-1] = sil.intersections[i];
                    }
                } else {
                    // Vertical line intersects a contour segment at a general position (not at one of its end points).
                    // or the contour just touches this vertical line with a vertical segment or a sequence of vertical segments.
                    // Keep both intersection points.
                    if (j < i)
                        sil.intersections[j] = sil.intersections[i];
                    ++ j;
                }
            } else {
                // Vertical line intersects a contour segment at a general position (not at one of its end points).
                if (j < i)
                    sil.intersections[j] = sil.intersections[i];
                ++ j;
            }
        }
        // Shrink the list of intersections, if any of the intersection was removed during the classification.
        if (j < sil.intersections.size())
            sil.intersections.erase(sil.intersections.begin() + j, sil.intersections.end());
    }

    // Verify the segments. If something is wrong, give up.
#define ASSERT_OR_RETURN(CONDITION) do { assert(CONDITION); if (! (CONDITION)) return false; } while (0)
    for (size_t i_seg = 0; i_seg < segs.size(); ++ i_seg) {
        SegmentedIntersectionLine &sil = segs[i_seg];
        // The intersection points have to be even.
        ASSERT_OR_RETURN((sil.intersections.size() & 1) == 0);
        for (size_t i = 0; i < sil.intersections.size();) {
            // An intersection segment crossing the bigger contour may cross the inner offsetted contour even number of times.
            ASSERT_OR_RETURN(sil.intersections[i].type == SegmentIntersection::OUTER_LOW);
            size_t j = i + 1;
            ASSERT_OR_RETURN(j < sil.intersections.size());
            ASSERT_OR_RETURN(sil.intersections[j].type == SegmentIntersection::INNER_LOW || sil.intersections[j].type == SegmentIntersection::OUTER_HIGH);
            for (; j < sil.intersections.size() && sil.intersections[j].is_inner(); ++ j) ;
            ASSERT_OR_RETURN(j < sil.intersections.size());
            ASSERT_OR_RETURN((j & 1) == 1);
            ASSERT_OR_RETURN(sil.intersections[j].type == SegmentIntersection::OUTER_HIGH);
            ASSERT_OR_RETURN(i + 1 == j || sil.intersections[j - 1].type == SegmentIntersection::INNER_HIGH);
            i = j + 1;
        }
    }
#undef ASSERT_OR_RETURN

#ifdef SLIC3R_DEBUG
    // Paint the segments and finalize the SVG file.
    for (size_t i_seg = 0; i_seg < segs.size(); ++ i_seg) {
        SegmentedIntersectionLine &sil = segs[i_seg];
        for (size_t i = 0; i < sil.intersections.size();) {
            size_t j = i + 1;
            for (; j < sil.intersections.size() && sil.intersections[j].is_inner(); ++ j) ;
            if (i + 1 == j) {
                svg.draw(Line(Point(sil.pos, sil.intersections[i].pos()), Point(sil.pos, sil.intersections[j].pos())), "blue");
            } else {
                svg.draw(Line(Point(sil.pos, sil.intersections[i].pos()), Point(sil.pos, sil.intersections[i+1].pos())), "green");
                svg.draw(Line(Point(sil.pos, sil.intersections[i+1].pos()), Point(sil.pos, sil.intersections[j-1].pos())), (j - i + 1 > 4) ? "yellow" : "magenta");
                svg.draw(Line(Point(sil.pos, sil.intersections[j-1].pos()), Point(sil.pos, sil.intersections[j].pos())), "green");
            }
            i = j + 1;
        }
    }
    svg.Close();
#endif /* SLIC3R_DEBUG */

    // For each outer only chords, measure their maximum distance to the bow of the outer contour.
    // Mark an outer only chord as consumed, if the distance is low.
    for (size_t i_vline = 0; i_vline < segs.size(); ++ i_vline) {
        SegmentedIntersectionLine &seg = segs[i_vline];
        for (size_t i_intersection = 0; i_intersection + 1 < seg.intersections.size(); ++ i_intersection) {
            if (seg.intersections[i_intersection].type == SegmentIntersection::OUTER_LOW &&
                seg.intersections[i_intersection+1].type == SegmentIntersection::OUTER_HIGH) {
                bool consumed = false;
//                if (params.full_infill()) {
//                        measure_outer_contour_slab(poly_with_offset, segs, i_vline, i_ntersection);
//                } else
                    consumed = true;
                seg.intersections[i_intersection].consumed_vertical_up = consumed;
            }
        }
    }

    // Now construct a graph.
    // Find the first point.
    // Naively one would expect to achieve best results by chaining the paths by the shortest distance,
    // but that procedure does not create the longest continuous paths.
    // A simple "sweep left to right" procedure achieves better results.
    size_t    i_vline = 0;
    size_t    i_intersection = size_t(-1);
    // Follow the line, connect the lines into a graph.
    // Until no new line could be added to the output path:
    Point     pointLast;
    Polyline *polyline_current = NULL;
    if (! polylines_out.empty())
        pointLast = polylines_out.back().points.back();
    for (;;) {
        if (i_intersection == size_t(-1)) {
            // The path has been interrupted. Find a next starting point, closest to the previous extruder position.
            coordf_t dist2min = std::numeric_limits<coordf_t>().max();
            for (size_t i_vline2 = 0; i_vline2 < segs.size(); ++ i_vline2) {
                const SegmentedIntersectionLine &seg = segs[i_vline2];
                if (! seg.intersections.empty()) {
                    assert(seg.intersections.size() > 1);
                    // Even number of intersections with the loops.
                    assert((seg.intersections.size() & 1) == 0);
                    assert(seg.intersections.front().type == SegmentIntersection::OUTER_LOW);
                    for (size_t i = 0; i < seg.intersections.size(); ++ i) {
                        const SegmentIntersection &intrsctn = seg.intersections[i];
                        if (intrsctn.is_outer()) {
                            assert(intrsctn.is_low() || i > 0);
                            bool consumed = intrsctn.is_low() ? 
                                intrsctn.consumed_vertical_up : 
                                seg.intersections[i-1].consumed_vertical_up;
                            if (! consumed) {
                                coordf_t dist2 = sqr(coordf_t(pointLast(0) - seg.pos)) + sqr(coordf_t(pointLast(1) - intrsctn.pos()));
                                if (dist2 < dist2min) {
                                    dist2min = dist2;
                                    i_vline = i_vline2;
                                    i_intersection = i;
                                    //FIXME We are taking the first left point always. Verify, that the caller chains the paths
                                    // by a shortest distance, while reversing the paths if needed.
                                    //if (polylines_out.empty())
                                        // Initial state, take the first line, which is the first from the left.
                                        goto found;
                                }
                            }
                        }
                    }
                }
            }
            if (i_intersection == size_t(-1))
                // We are finished.
                break;
        found:
            // Start a new path.
            polylines_out.push_back(Polyline());
            polyline_current = &polylines_out.back();
            // Emit the first point of a path.
            pointLast = Point(segs[i_vline].pos, segs[i_vline].intersections[i_intersection].pos());
            polyline_current->points.push_back(pointLast);
        }

        // From the initial point (i_vline, i_intersection), follow a path.
        SegmentedIntersectionLine &seg      = segs[i_vline];
        SegmentIntersection       *intrsctn = &seg.intersections[i_intersection];
        bool going_up = intrsctn->is_low();
        bool try_connect = false;
        if (going_up) {
            assert(! intrsctn->consumed_vertical_up);
            assert(i_intersection + 1 < seg.intersections.size());
            // Step back to the beginning of the vertical segment to mark it as consumed.
            if (intrsctn->is_inner()) {
                assert(i_intersection > 0);
                -- intrsctn;
                -- i_intersection;
            }
            // Consume the complete vertical segment up to the outer contour.
            do {
                intrsctn->consumed_vertical_up = true;
                ++ intrsctn;
                ++ i_intersection;
                assert(i_intersection < seg.intersections.size());
            } while (intrsctn->type != SegmentIntersection::OUTER_HIGH);
            if ((intrsctn - 1)->is_inner()) {
                // Step back.
                -- intrsctn;
                -- i_intersection;
                assert(intrsctn->type == SegmentIntersection::INNER_HIGH);
                try_connect = true;
            }
        } else {
            // Going down.
            assert(intrsctn->is_high());
            assert(i_intersection > 0);
            assert(! (intrsctn - 1)->consumed_vertical_up);
            // Consume the complete vertical segment up to the outer contour.
            if (intrsctn->is_inner())
                intrsctn->consumed_vertical_up = true;
            do {
                assert(i_intersection > 0);
                -- intrsctn;
                -- i_intersection;
                intrsctn->consumed_vertical_up = true;
            } while (intrsctn->type != SegmentIntersection::OUTER_LOW);
            if ((intrsctn + 1)->is_inner()) {
                // Step back.
                ++ intrsctn;
                ++ i_intersection;
                assert(intrsctn->type == SegmentIntersection::INNER_LOW);
                try_connect = true;
            }
        }
        if (try_connect) {
            // Decide, whether to finish the segment, or whether to follow the perimeter.

            // 1) Find possible connection points on the previous / next vertical line.
            int iPrev = intersection_on_prev_vertical_line(poly_with_offset, segs, i_vline, intrsctn->iContour, i_intersection);
            int iNext = intersection_on_next_vertical_line(poly_with_offset, segs, i_vline, intrsctn->iContour, i_intersection);
            IntersectionTypeOtherVLine intrsctn_type_prev = intersection_type_on_prev_vertical_line(segs, i_vline, i_intersection, iPrev);
            IntersectionTypeOtherVLine intrsctn_type_next = intersection_type_on_next_vertical_line(segs, i_vline, i_intersection, iNext);

            // 2) Find possible connection points on the same vertical line.
            int iAbove = -1;
            int iBelow = -1;
            int iSegAbove = -1;
            int iSegBelow = -1;
            {
                SegmentIntersection::SegmentIntersectionType type_crossing = (intrsctn->type == SegmentIntersection::INNER_LOW) ?
                    SegmentIntersection::INNER_HIGH : SegmentIntersection::INNER_LOW;
                // Does the perimeter intersect the current vertical line above intrsctn?
                for (size_t i = i_intersection + 1; i + 1 < seg.intersections.size(); ++ i)
//                    if (seg.intersections[i].iContour == intrsctn->iContour && seg.intersections[i].type == type_crossing) {
                    if (seg.intersections[i].iContour == intrsctn->iContour) {
                        iAbove = i;
                        iSegAbove = seg.intersections[i].iSegment;
                        break;
                    }
                // Does the perimeter intersect the current vertical line below intrsctn?
                for (size_t i = i_intersection - 1; i > 0; -- i)
//                    if (seg.intersections[i].iContour == intrsctn->iContour && seg.intersections[i].type == type_crossing) {
                    if (seg.intersections[i].iContour == intrsctn->iContour) {
                        iBelow = i;
                        iSegBelow = seg.intersections[i].iSegment;
                        break;
                    }
            }

            // 3) Sort the intersection points, clear iPrev / iNext / iSegBelow / iSegAbove,
            // if it is preceded by any other intersection point along the contour.
			unsigned int vert_seg_dir_valid_mask = 
                (going_up ? 
                    (iSegAbove != -1 && seg.intersections[iAbove].type == SegmentIntersection::INNER_LOW) :
                    (iSegBelow != -1 && seg.intersections[iBelow].type == SegmentIntersection::INNER_HIGH)) ?
                (DIR_FORWARD | DIR_BACKWARD) :
                0;
            {
                // Invalidate iPrev resp. iNext, if the perimeter crosses the current vertical line earlier than iPrev resp. iNext.
                // The perimeter contour orientation.
                const bool forward = intrsctn->is_low(); // == poly_with_offset.is_contour_ccw(intrsctn->iContour);
                const Polygon &poly = poly_with_offset.contour(intrsctn->iContour);
                {
                    int d_horiz = (iPrev     == -1) ? std::numeric_limits<int>::max() :
                        distance_of_segmens(poly, segs[i_vline-1].intersections[iPrev].iSegment, intrsctn->iSegment, forward);
                    int d_down  = (iSegBelow == -1) ? std::numeric_limits<int>::max() :
                        distance_of_segmens(poly, iSegBelow, intrsctn->iSegment, forward);
                    int d_up    = (iSegAbove == -1) ? std::numeric_limits<int>::max() :
                        distance_of_segmens(poly, iSegAbove, intrsctn->iSegment, forward);
                    if (intrsctn_type_prev == INTERSECTION_TYPE_OTHER_VLINE_OK && d_horiz > std::min(d_down, d_up))
                        // The vertical crossing comes eralier than the prev crossing.
                        // Disable the perimeter going back.
                        intrsctn_type_prev = INTERSECTION_TYPE_OTHER_VLINE_NOT_FIRST;
                    if (going_up ? (d_up > std::min(d_horiz, d_down)) : (d_down > std::min(d_horiz, d_up)))
                        // The horizontal crossing comes earlier than the vertical crossing.
                        vert_seg_dir_valid_mask &= ~(forward ? DIR_BACKWARD : DIR_FORWARD);
                }
                {
                    int d_horiz = (iNext     == -1) ? std::numeric_limits<int>::max() :
                        distance_of_segmens(poly, intrsctn->iSegment, segs[i_vline+1].intersections[iNext].iSegment, forward);
                    int d_down  = (iSegBelow == -1) ? std::numeric_limits<int>::max() :
                        distance_of_segmens(poly, intrsctn->iSegment, iSegBelow, forward);
                    int d_up    = (iSegAbove == -1) ? std::numeric_limits<int>::max() :
                        distance_of_segmens(poly, intrsctn->iSegment, iSegAbove, forward);
                    if (intrsctn_type_next == INTERSECTION_TYPE_OTHER_VLINE_OK && d_horiz > std::min(d_down, d_up))
                        // The vertical crossing comes eralier than the prev crossing.
                        // Disable the perimeter going forward.
                        intrsctn_type_next = INTERSECTION_TYPE_OTHER_VLINE_NOT_FIRST;
                    if (going_up ? (d_up > std::min(d_horiz, d_down)) : (d_down > std::min(d_horiz, d_up)))
                        // The horizontal crossing comes earlier than the vertical crossing.
                        vert_seg_dir_valid_mask &= ~(forward ? DIR_FORWARD : DIR_BACKWARD);
                }
            }

            // 4) Try to connect to a previous or next vertical line, making a zig-zag pattern.
            if (intrsctn_type_prev == INTERSECTION_TYPE_OTHER_VLINE_OK || intrsctn_type_next == INTERSECTION_TYPE_OTHER_VLINE_OK) {
                coordf_t distPrev = (intrsctn_type_prev != INTERSECTION_TYPE_OTHER_VLINE_OK) ? std::numeric_limits<coord_t>::max() :
                    measure_perimeter_prev_segment_length(poly_with_offset, segs, i_vline, intrsctn->iContour, i_intersection, iPrev);
                coordf_t distNext = (intrsctn_type_next != INTERSECTION_TYPE_OTHER_VLINE_OK) ? std::numeric_limits<coord_t>::max() :
                    measure_perimeter_next_segment_length(poly_with_offset, segs, i_vline, intrsctn->iContour, i_intersection, iNext);
                // Take the shorter path.
                //FIXME this may not be always the best strategy to take the shortest connection line now.
                bool take_next = (intrsctn_type_prev == INTERSECTION_TYPE_OTHER_VLINE_OK && intrsctn_type_next == INTERSECTION_TYPE_OTHER_VLINE_OK) ?
                    (distNext < distPrev) : 
                    intrsctn_type_next == INTERSECTION_TYPE_OTHER_VLINE_OK;
                assert(intrsctn->is_inner());
                bool skip = params.dont_connect || (link_max_length > 0 && (take_next ? distNext : distPrev) > link_max_length);
                if (skip) {
                    // Just skip the connecting contour and start a new path.
                    goto dont_connect;
                    polyline_current->points.push_back(Point(seg.pos, intrsctn->pos()));
                    polylines_out.push_back(Polyline()); 
                    polyline_current = &polylines_out.back(); 
                    const SegmentedIntersectionLine &il2 = segs[take_next ? (i_vline + 1) : (i_vline - 1)];
                    polyline_current->points.push_back(Point(il2.pos, il2.intersections[take_next ? iNext : iPrev].pos()));
                } else {
                    polyline_current->points.push_back(Point(seg.pos, intrsctn->pos()));
                    emit_perimeter_prev_next_segment(poly_with_offset, segs, i_vline, intrsctn->iContour, i_intersection, take_next ? iNext : iPrev, *polyline_current, take_next);
                }
                // Mark both the left and right connecting segment as consumed, because one cannot go to this intersection point as it has been consumed.
                if (iPrev != -1)
                    segs[i_vline-1].intersections[iPrev].consumed_perimeter_right = true;
                if (iNext != -1)
                    intrsctn->consumed_perimeter_right = true;
                //FIXME consume the left / right connecting segments at the other end of this line? Currently it is not critical because a perimeter segment is not followed if the vertical segment at the other side has already been consumed.
                // Advance to the neighbor line.
                if (take_next) {
                    ++ i_vline;
                    i_intersection = iNext;
                } else {
                    -- i_vline;
                    i_intersection = iPrev;
                }
                continue;
            } 

            // 5) Try to connect to a previous or next point on the same vertical line.
            if (vert_seg_dir_valid_mask) {
                bool valid = true;
                // Verify, that there is no intersection with the inner contour up to the end of the contour segment.
				// Verify, that the successive segment has not been consumed yet.
				if (going_up) {
					if (seg.intersections[iAbove].consumed_vertical_up) {
						valid = false;
					} else {
						for (int i = (int)i_intersection + 1; i < iAbove && valid; ++i)
							if (seg.intersections[i].is_inner()) 
								valid = false;
					}
                } else {
					if (seg.intersections[iBelow-1].consumed_vertical_up) {
						valid = false;
					} else {
						for (int i = iBelow + 1; i < (int)i_intersection && valid; ++i)
							if (seg.intersections[i].is_inner()) 
								valid = false;
					}
                }
                if (valid) {
                    const Polygon &poly = poly_with_offset.contour(intrsctn->iContour);
                    int iNext    = going_up ? iAbove : iBelow;
                    int iSegNext = going_up ? iSegAbove : iSegBelow;
                    bool dir_forward = (vert_seg_dir_valid_mask == (DIR_FORWARD | DIR_BACKWARD)) ?
                        // Take the shorter length between the current and the next intersection point.
                        (distance_of_segmens(poly, intrsctn->iSegment, iSegNext, true) <
                         distance_of_segmens(poly, intrsctn->iSegment, iSegNext, false)) :
                        (vert_seg_dir_valid_mask == DIR_FORWARD);
                    // Skip this perimeter line?
                    bool skip = params.dont_connect;
                    if (! skip && link_max_length > 0) {
                        coordf_t link_length = measure_perimeter_segment_on_vertical_line_length(
                            poly_with_offset, segs, i_vline, intrsctn->iContour, i_intersection, iNext, dir_forward);
                        skip = link_length > link_max_length;
                    }
                    polyline_current->points.push_back(Point(seg.pos, intrsctn->pos()));
                    if (skip) {
                        // Just skip the connecting contour and start a new path.
                        polylines_out.push_back(Polyline()); 
                        polyline_current = &polylines_out.back();
                        polyline_current->points.push_back(Point(seg.pos, seg.intersections[iNext].pos()));
                    } else {
                        // Consume the connecting contour and the next segment.
                        emit_perimeter_segment_on_vertical_line(poly_with_offset, segs, i_vline, intrsctn->iContour, i_intersection, iNext, *polyline_current, dir_forward);
                    }
                    // Mark both the left and right connecting segment as consumed, because one cannot go to this intersection point as it has been consumed.
                    // If there are any outer intersection points skipped (bypassed) by the contour,
                    // mark them as processed.
                    if (going_up) {
                        for (int i = (int)i_intersection; i < iAbove; ++ i)
                            seg.intersections[i].consumed_vertical_up = true;
                    } else {
                        for (int i = iBelow; i < (int)i_intersection; ++ i)
                            seg.intersections[i].consumed_vertical_up = true;
                    }
//                    seg.intersections[going_up ? i_intersection : i_intersection - 1].consumed_vertical_up = true;
                    intrsctn->consumed_perimeter_right = true;
                    i_intersection = iNext;
                    if (going_up)
                        ++ intrsctn;
                    else
                        -- intrsctn;
                    intrsctn->consumed_perimeter_right = true;
                    continue;
                }
            }
        dont_connect: 
            // No way to continue the current polyline. Take the rest of the line up to the outer contour.
            // This will finish the polyline, starting another polyline at a new point.
            if (going_up)
                ++ intrsctn;
            else
                -- intrsctn;
        }

        // Finish the current vertical line,
        // reset the current vertical line to pick a new starting point in the next round.
        assert(intrsctn->is_outer());
        assert(intrsctn->is_high() == going_up);
        pointLast = Point(seg.pos, intrsctn->pos());
        polyline_current->points.push_back(pointLast);
        // Handle duplicate points and zero length segments.
        polyline_current->remove_duplicate_points();
        assert(! polyline_current->has_duplicate_points());
        // Handle nearly zero length edges.
        if (polyline_current->points.size() <= 1 ||
        	(polyline_current->points.size() == 2 &&
        		std::abs(polyline_current->points.front()(0) - polyline_current->points.back()(0)) < SCALED_EPSILON &&
				std::abs(polyline_current->points.front()(1) - polyline_current->points.back()(1)) < SCALED_EPSILON))
            polylines_out.pop_back();
        intrsctn = NULL;
        i_intersection = -1;
        polyline_current = NULL;
    }

#ifdef SLIC3R_DEBUG
    {
        {
            ::Slic3r::SVG svg(debug_out_path("FillRectilinear2-final-%03d.svg", iRun), bbox_svg); // , scale_(1.));
            poly_with_offset.export_to_svg(svg);
            for (size_t i = n_polylines_out_initial; i < polylines_out.size(); ++ i)
                svg.draw(polylines_out[i].lines(), "black");
        }
        // Paint a picture per polyline. This makes it easier to discover the order of the polylines and their overlap.
        for (size_t i_polyline = n_polylines_out_initial; i_polyline < polylines_out.size(); ++ i_polyline) {
            ::Slic3r::SVG svg(debug_out_path("FillRectilinear2-final-%03d-%03d.svg", iRun, i_polyline), bbox_svg); // , scale_(1.));
            svg.draw(polylines_out[i_polyline].lines(), "black");
        }
    }
#endif /* SLIC3R_DEBUG */

    // paths must be rotated back
    for (Polylines::iterator it = polylines_out.begin() + n_polylines_out_initial; it != polylines_out.end(); ++ it) {
        // No need to translate, the absolute position is irrelevant.
        // it->translate(- rotate_vector.second(0), - rotate_vector.second(1));
        assert(! it->has_duplicate_points());
        it->rotate(rotate_vector.first);
        //FIXME rather simplify the paths to avoid very short edges?
        //assert(! it->has_duplicate_points());
        it->remove_duplicate_points();
    }

#ifdef SLIC3R_DEBUG
    // Verify, that there are no duplicate points in the sequence.
    for (Polyline &polyline : polylines_out)
        assert(! polyline.has_duplicate_points());
#endif /* SLIC3R_DEBUG */

    return true;
}

Polylines FillRectilinear2::fill_surface(const Surface *surface, const FillParams &params)
{
    Polylines polylines_out;
    if (! fill_surface_by_lines(surface, params, 0.f, 0.f, polylines_out)) {
        printf("FillRectilinear2::fill_surface() failed to fill a region.\n");
    }
    return polylines_out;
}

Polylines FillGrid2::fill_surface(const Surface *surface, const FillParams &params)
{
    // Each linear fill covers half of the target coverage.
    FillParams params2 = params;
    params2.density *= 0.5f;
    Polylines polylines_out;
    if (! fill_surface_by_lines(surface, params2, 0.f, 0.f, polylines_out) ||
        ! fill_surface_by_lines(surface, params2, float(M_PI / 2.), 0.f, polylines_out)) {
        printf("FillGrid2::fill_surface() failed to fill a region.\n");
    }
    return polylines_out;
}

Polylines FillTriangles::fill_surface(const Surface *surface, const FillParams &params)
{
    // Each linear fill covers 1/3 of the target coverage.
    FillParams params2 = params;
    params2.density *= 0.333333333f;
    FillParams params3 = params2;
    params3.dont_connect = true;
    Polylines polylines_out;
    if (! fill_surface_by_lines(surface, params2, 0.f, 0., polylines_out) ||
        ! fill_surface_by_lines(surface, params2, float(M_PI / 3.), 0., polylines_out) ||
        ! fill_surface_by_lines(surface, params3, float(2. * M_PI / 3.), 0., polylines_out)) {
        printf("FillTriangles::fill_surface() failed to fill a region.\n");
    }
    return polylines_out;
}

Polylines FillStars::fill_surface(const Surface *surface, const FillParams &params)
{
    // Each linear fill covers 1/3 of the target coverage.
    FillParams params2 = params;
    params2.density *= 0.333333333f;
    FillParams params3 = params2;
    params3.dont_connect = true;
    Polylines polylines_out;
    if (! fill_surface_by_lines(surface, params2, 0.f, 0., polylines_out) ||
        ! fill_surface_by_lines(surface, params2, float(M_PI / 3.), 0., polylines_out) ||
        ! fill_surface_by_lines(surface, params3, float(2. * M_PI / 3.), 0.5 * this->spacing / params2.density, polylines_out)) {
        printf("FillStars::fill_surface() failed to fill a region.\n");
    }
    return polylines_out;
}

Polylines FillCubic::fill_surface(const Surface *surface, const FillParams &params)
{
    // Each linear fill covers 1/3 of the target coverage.
    FillParams params2 = params;
    params2.density *= 0.333333333f;
    FillParams params3 = params2;
    params3.dont_connect = true;
    Polylines polylines_out;
    coordf_t dx = sqrt(0.5) * z;
    if (! fill_surface_by_lines(surface, params2, 0.f, dx, polylines_out) ||
        ! fill_surface_by_lines(surface, params2, float(M_PI / 3.), - dx, polylines_out) ||
        // Rotated by PI*2/3 + PI to achieve reverse sloping wall.
        ! fill_surface_by_lines(surface, params3, float(M_PI * 2. / 3.), dx, polylines_out)) {
        printf("FillCubic::fill_surface() failed to fill a region.\n");
    } 
    return polylines_out; 
}

} // namespace Slic3r