BOOST_FOREACH(const IfcVector2& cp, temp_contour) {
        if ((cp-vv).SquareLength() < 1e-5f) {
            return true;
        }
    }

/*
Open Asset Import Library (assimp)
----------------------------------------------------------------------

All rights reserved.

----------------------------------------------------------------------
*/

// Modified by Lasse Oorni for Urho3D

/** @file  IFCOpenings.cpp
 *  @brief Implements a subset of Ifc CSG operations for pouring
  *    holes for windows and doors into walls.
 */

#ifndef ASSIMP_BUILD_NO_IFC_IMPORTER
#include "IFCUtil.h"
#include "PolyTools.h"
#include "ProcessHelper.h"

#include "../contrib/poly2tri/poly2tri/poly2tri.h"
#include "../contrib/clipper/clipper.hpp"

#include <iterator>

namespace Assimp {
    namespace IFC {

        using ClipperLib::ulong64;
        // XXX use full -+ range ...
        const ClipperLib::long64 max_ulong64 = 1518500249; // clipper.cpp / hiRange var

#define to_int64(p)  (static_cast<ulong64>(static_cast<IfcFloat>((p) ) * max_ulong64 ))
#define from_int64(p) (static_cast<IfcFloat>((p)) / max_ulong64)
#define one_vec (IfcVector2(static_cast<IfcFloat>(1.0),static_cast<IfcFloat>(1.0)))

        // fallback method to generate wall openings
        bool TryAddOpenings_Poly2Tri(const std::vector<TempOpening>& openings,const std::vector<IfcVector3>& nors,
            TempMesh& curmesh);

typedef std::pair< IfcVector2, IfcVector2 > BoundingBox;
typedef std::map<IfcVector2,size_t,XYSorter> XYSortedField;

void QuadrifyPart(const IfcVector2& pmin, const IfcVector2& pmax, XYSortedField& field,
    const std::vector< BoundingBox >& bbs,
    std::vector<IfcVector2>& out)
{
    if (!(pmin.x-pmax.x) || !(pmin.y-pmax.y)) {
        return;
    }

    IfcFloat xs = 1e10, xe = 1e10;
    bool found = false;

    // Search along the x-axis until we find an opening
    XYSortedField::iterator start = field.begin();
    for(; start != field.end(); ++start) {
        const BoundingBox& bb = bbs[(*start).second];
        if(bb.first.x >= pmax.x) {
            break;
        }

        if (bb.second.x > pmin.x && bb.second.y > pmin.y && bb.first.y < pmax.y) {
            xs = bb.first.x;
            xe = bb.second.x;
            found = true;
            break;
        }
    }

    if (!found) {
        // the rectangle [pmin,pend] is opaque, fill it
        out.push_back(pmin);
        out.push_back(IfcVector2(pmin.x,pmax.y));
        out.push_back(pmax);
        out.push_back(IfcVector2(pmax.x,pmin.y));
        return;
    }

    xs = std::max(pmin.x,xs);
    xe = std::min(pmax.x,xe);

    // see if there's an offset to fill at the top of our quad
    if (xs - pmin.x) {
        out.push_back(pmin);
        out.push_back(IfcVector2(pmin.x,pmax.y));
        out.push_back(IfcVector2(xs,pmax.y));
        out.push_back(IfcVector2(xs,pmin.y));
    }

    // search along the y-axis for all openings that overlap xs and our quad
    IfcFloat ylast = pmin.y;
    found = false;
    for(; start != field.end(); ++start) {
        const BoundingBox& bb = bbs[(*start).second];
        if (bb.first.x > xs || bb.first.y >= pmax.y) {
            break;
        }

        if (bb.second.y > ylast) {

            found = true;
            const IfcFloat ys = std::max(bb.first.y,pmin.y), ye = std::min(bb.second.y,pmax.y);
            if (ys - ylast > 0.0f) {
                QuadrifyPart( IfcVector2(xs,ylast), IfcVector2(xe,ys) ,field,bbs,out);
            }

            // the following are the window vertices

            /*wnd.push_back(IfcVector2(xs,ys));
            wnd.push_back(IfcVector2(xs,ye));
            wnd.push_back(IfcVector2(xe,ye));
            wnd.push_back(IfcVector2(xe,ys));*/
            ylast = ye;
        }
    }
    if (!found) {
        // the rectangle [pmin,pend] is opaque, fill it
        out.push_back(IfcVector2(xs,pmin.y));
        out.push_back(IfcVector2(xs,pmax.y));
        out.push_back(IfcVector2(xe,pmax.y));
        out.push_back(IfcVector2(xe,pmin.y));
        return;
    }
    if (ylast < pmax.y) {
        QuadrifyPart( IfcVector2(xs,ylast), IfcVector2(xe,pmax.y) ,field,bbs,out);
    }

    // now for the whole rest
    if (pmax.x-xe) {
        QuadrifyPart(IfcVector2(xe,pmin.y), pmax ,field,bbs,out);
    }
}

typedef std::vector<IfcVector2> Contour;
typedef std::vector<bool> SkipList; // should probably use int for performance reasons

struct ProjectedWindowContour
{
    Contour contour;
    BoundingBox bb;
    SkipList skiplist;
    bool is_rectangular;

    ProjectedWindowContour(const Contour& contour, const BoundingBox& bb, bool is_rectangular)
        : contour(contour)
        , bb(bb)
        , is_rectangular(is_rectangular)
    {}

    bool IsInvalid() const {
        return contour.empty();
    }

    void FlagInvalid() {
        contour.clear();
    }

    void PrepareSkiplist() {
        skiplist.resize(contour.size(),false);
    }
};

typedef std::vector< ProjectedWindowContour > ContourVector;

bool BoundingBoxesOverlapping( const BoundingBox &ibb, const BoundingBox &bb )
{
    // count the '=' case as non-overlapping but as adjacent to each other
    return ibb.first.x < bb.second.x && ibb.second.x > bb.first.x &&
        ibb.first.y < bb.second.y && ibb.second.y > bb.first.y;
}

bool IsDuplicateVertex(const IfcVector2& vv, const std::vector<IfcVector2>& temp_contour)
{
    // sanity check for duplicate vertices

    return false;
}

void ExtractVerticesFromClipper(const ClipperLib::Polygon& poly, std::vector<IfcVector2>& temp_contour,
    bool filter_duplicates = false)
{
    temp_contour.clear();
    BOOST_FOREACH(const ClipperLib::IntPoint& point, poly) {
        IfcVector2 vv = IfcVector2( from_int64(point.X), from_int64(point.Y));
        vv = std::max(vv,IfcVector2());
        vv = std::min(vv,one_vec);

        if (!filter_duplicates || !IsDuplicateVertex(vv, temp_contour)) {
            temp_contour.push_back(vv);
        }
    }
}

BoundingBox GetBoundingBox(const ClipperLib::Polygon& poly)
{
    IfcVector2 newbb_min, newbb_max;
    MinMaxChooser<IfcVector2>()(newbb_min, newbb_max);

    BOOST_FOREACH(const ClipperLib::IntPoint& point, poly) {
        IfcVector2 vv = IfcVector2( from_int64(point.X), from_int64(point.Y));

        // sanity rounding
        vv = std::max(vv,IfcVector2());
        vv = std::min(vv,one_vec);

        newbb_min = std::min(newbb_min,vv);
        newbb_max = std::max(newbb_max,vv);
    }
    return BoundingBox(newbb_min, newbb_max);
}

void InsertWindowContours(const ContourVector& contours,
    const std::vector<TempOpening>& /*openings*/,
    TempMesh& curmesh)
{
    // fix windows - we need to insert the real, polygonal shapes into the quadratic holes that we have now
    for(size_t i = 0; i < contours.size();++i) {
        const BoundingBox& bb = contours[i].bb;
        const std::vector<IfcVector2>& contour = contours[i].contour;
        if(contour.empty()) {
            continue;
        }

        // check if we need to do it at all - many windows just fit perfectly into their quadratic holes,
        // i.e. their contours *are* already their bounding boxes.
        if (contour.size() == 4) {
            std::set<IfcVector2,XYSorter> verts;
            for(size_t n = 0; n < 4; ++n) {
                verts.insert(contour[n]);
            }
            const std::set<IfcVector2,XYSorter>::const_iterator end = verts.end();
            if (verts.find(bb.first)!=end && verts.find(bb.second)!=end
                && verts.find(IfcVector2(bb.first.x,bb.second.y))!=end
                && verts.find(IfcVector2(bb.second.x,bb.first.y))!=end
                ) {
                    continue;
            }
        }

        const IfcFloat diag = (bb.first-bb.second).Length();
        const IfcFloat epsilon = diag/1000.f;

        // walk through all contour points and find those that lie on the BB corner
        size_t last_hit = -1, very_first_hit = -1;
        IfcVector2 edge;
        for(size_t n = 0, e=0, size = contour.size();; n=(n+1)%size, ++e) {

            // sanity checking
            if (e == size*2) {
                IFCImporter::LogError("encountered unexpected topology while generating window contour");
                break;
            }

            const IfcVector2& v = contour[n];

            bool hit = false;
            if (std::fabs(v.x-bb.first.x)<epsilon) {
                edge.x = bb.first.x;
                hit = true;
            }
            else if (std::fabs(v.x-bb.second.x)<epsilon) {
                edge.x = bb.second.x;
                hit = true;
            }

            if (std::fabs(v.y-bb.first.y)<epsilon) {
                edge.y = bb.first.y;
                hit = true;
            }
            else if (std::fabs(v.y-bb.second.y)<epsilon) {
                edge.y = bb.second.y;
                hit = true;
            }

            if (hit) {
                if (last_hit != (size_t)-1) {

                    const size_t old = curmesh.verts.size();
                    size_t cnt = last_hit > n ? size-(last_hit-n) : n-last_hit;
                    for(size_t a = last_hit, e = 0; e <= cnt; a=(a+1)%size, ++e) {
                        // hack: this is to fix cases where opening contours are self-intersecting.
                        // Clipper doesn't produce such polygons, but as soon as we're back in
                        // our brave new floating-point world, very small distances are consumed
                        // by the maximum available precision, leading to self-intersecting
                        // polygons. This fix makes concave windows fail even worse, but
                        // anyway, fail is fail.
                        if ((contour[a] - edge).SquareLength() > diag*diag*0.7) {
                            continue;
                        }
                        curmesh.verts.push_back(IfcVector3(contour[a].x, contour[a].y, 0.0f));
                    }

                    if (edge != contour[last_hit]) {

                        IfcVector2 corner = edge;

                        if (std::fabs(contour[last_hit].x-bb.first.x)<epsilon) {
                            corner.x = bb.first.x;
                        }
                        else if (std::fabs(contour[last_hit].x-bb.second.x)<epsilon) {
                            corner.x = bb.second.x;
                        }

                        if (std::fabs(contour[last_hit].y-bb.first.y)<epsilon) {
                            corner.y = bb.first.y;
                        }
                        else if (std::fabs(contour[last_hit].y-bb.second.y)<epsilon) {
                            corner.y = bb.second.y;
                        }

                        curmesh.verts.push_back(IfcVector3(corner.x, corner.y, 0.0f));
                    }
                    else if (cnt == 1) {
                        // avoid degenerate polygons (also known as lines or points)
                        curmesh.verts.erase(curmesh.verts.begin()+old,curmesh.verts.end());
                    }

                    if (const size_t d = curmesh.verts.size()-old) {
                        curmesh.vertcnt.push_back(d);
                        std::reverse(curmesh.verts.rbegin(),curmesh.verts.rbegin()+d);
                    }
                    if (n == very_first_hit) {
                        break;
                    }
                }
                else {
                    very_first_hit = n;
                }

                last_hit = n;
            }
        }
    }
}

void MergeWindowContours (const std::vector<IfcVector2>& a,
    const std::vector<IfcVector2>& b,
    ClipperLib::ExPolygons& out)
{
    out.clear();

    ClipperLib::Clipper clipper;
    ClipperLib::Polygon clip;

    BOOST_FOREACH(const IfcVector2& pip, a) {
        clip.push_back(ClipperLib::IntPoint(  to_int64(pip.x), to_int64(pip.y) ));
    }

    if (ClipperLib::Orientation(clip)) {
        std::reverse(clip.begin(), clip.end());
    }

    clipper.AddPolygon(clip, ClipperLib::ptSubject);
    clip.clear();

    BOOST_FOREACH(const IfcVector2& pip, b) {
        clip.push_back(ClipperLib::IntPoint(  to_int64(pip.x), to_int64(pip.y) ));
    }

    if (ClipperLib::Orientation(clip)) {
        std::reverse(clip.begin(), clip.end());
    }

    clipper.AddPolygon(clip, ClipperLib::ptSubject);
    clipper.Execute(ClipperLib::ctUnion, out,ClipperLib::pftNonZero,ClipperLib::pftNonZero);
}

// Subtract a from b
void MakeDisjunctWindowContours (const std::vector<IfcVector2>& a,
    const std::vector<IfcVector2>& b,
    ClipperLib::ExPolygons& out)
{
    out.clear();

    ClipperLib::Clipper clipper;
    ClipperLib::Polygon clip;

    BOOST_FOREACH(const IfcVector2& pip, a) {
        clip.push_back(ClipperLib::IntPoint(  to_int64(pip.x), to_int64(pip.y) ));
    }

    if (ClipperLib::Orientation(clip)) {
        std::reverse(clip.begin(), clip.end());
    }

    clipper.AddPolygon(clip, ClipperLib::ptClip);
    clip.clear();

    BOOST_FOREACH(const IfcVector2& pip, b) {
        clip.push_back(ClipperLib::IntPoint(  to_int64(pip.x), to_int64(pip.y) ));
    }

    if (ClipperLib::Orientation(clip)) {
        std::reverse(clip.begin(), clip.end());
    }

    clipper.AddPolygon(clip, ClipperLib::ptSubject);
    clipper.Execute(ClipperLib::ctDifference, out,ClipperLib::pftNonZero,ClipperLib::pftNonZero);
}

void CleanupWindowContour(ProjectedWindowContour& window)
{
    std::vector<IfcVector2> scratch;
    std::vector<IfcVector2>& contour = window.contour;

    ClipperLib::Polygon subject;
    ClipperLib::Clipper clipper;
    ClipperLib::ExPolygons clipped;

    BOOST_FOREACH(const IfcVector2& pip, contour) {
        subject.push_back(ClipperLib::IntPoint(  to_int64(pip.x), to_int64(pip.y) ));
    }

    clipper.AddPolygon(subject,ClipperLib::ptSubject);
    clipper.Execute(ClipperLib::ctUnion,clipped,ClipperLib::pftNonZero,ClipperLib::pftNonZero);

    // This should yield only one polygon or something went wrong
    if (clipped.size() != 1) {

        // Empty polygon? drop the contour altogether
        if(clipped.empty()) {
            IFCImporter::LogError("error during polygon clipping, window contour is degenerate");
            window.FlagInvalid();
            return;
        }

        // Else: take the first only
        IFCImporter::LogError("error during polygon clipping, window contour is not convex");
    }

    ExtractVerticesFromClipper(clipped[0].outer, scratch);
    // Assume the bounding box doesn't change during this operation
}

void CleanupWindowContours(ContourVector& contours)
{
    // Use PolyClipper to clean up window contours
    try {
        BOOST_FOREACH(ProjectedWindowContour& window, contours) {
            CleanupWindowContour(window);
        }
    }
    catch (const char* sx) {
        IFCImporter::LogError("error during polygon clipping, window shape may be wrong: (Clipper: "
            + std::string(sx) + ")");
    }
}

void CleanupOuterContour(const std::vector<IfcVector2>& contour_flat, TempMesh& curmesh)
{
    std::vector<IfcVector3> vold;
    std::vector<unsigned int> iold;

    vold.reserve(curmesh.verts.size());
    iold.reserve(curmesh.vertcnt.size());

    // Fix the outer contour using polyclipper
    try {

        ClipperLib::Polygon subject;
        ClipperLib::Clipper clipper;
        ClipperLib::ExPolygons clipped;

        ClipperLib::Polygon clip;
        clip.reserve(contour_flat.size());
        BOOST_FOREACH(const IfcVector2& pip, contour_flat) {
            clip.push_back(ClipperLib::IntPoint(  to_int64(pip.x), to_int64(pip.y) ));
        }

        if (!ClipperLib::Orientation(clip)) {
            std::reverse(clip.begin(), clip.end());
        }

        // We need to run polyclipper on every single polygon -- we can't run it one all
        // of them at once or it would merge them all together which would undo all
        // previous steps
        subject.reserve(4);
        size_t index = 0;
        size_t countdown = 0;
        BOOST_FOREACH(const IfcVector3& pip, curmesh.verts) {
            if (!countdown) {
                countdown = curmesh.vertcnt[index++];
                if (!countdown) {
                    continue;
                }
            }
            subject.push_back(ClipperLib::IntPoint(  to_int64(pip.x), to_int64(pip.y) ));
            if (--countdown == 0) {
                if (!ClipperLib::Orientation(subject)) {
                    std::reverse(subject.begin(), subject.end());
                }

                clipper.AddPolygon(subject,ClipperLib::ptSubject);
                clipper.AddPolygon(clip,ClipperLib::ptClip);

                clipper.Execute(ClipperLib::ctIntersection,clipped,ClipperLib::pftNonZero,ClipperLib::pftNonZero);

                BOOST_FOREACH(const ClipperLib::ExPolygon& ex, clipped) {
                    iold.push_back(ex.outer.size());
                    BOOST_FOREACH(const ClipperLib::IntPoint& point, ex.outer) {
                        vold.push_back(IfcVector3(
                            from_int64(point.X),
                            from_int64(point.Y),
                            0.0f));
                    }
                }

                subject.clear();
                clipped.clear();
                clipper.Clear();
            }
        }
    }
    catch (const char* sx) {
        IFCImporter::LogError("Ifc: error during polygon clipping, wall contour line may be wrong: (Clipper: "
            + std::string(sx) + ")");

        return;
    }

    // swap data arrays
    std::swap(vold,curmesh.verts);
    std::swap(iold,curmesh.vertcnt);
}

typedef std::vector<TempOpening*> OpeningRefs;
typedef std::vector<OpeningRefs > OpeningRefVector;

typedef std::vector<std::pair<
    ContourVector::const_iterator,
    Contour::const_iterator>
> ContourRefVector;

bool BoundingBoxesAdjacent(const BoundingBox& bb, const BoundingBox& ibb)
{
    // TODO: I'm pretty sure there is a much more compact way to check this
    const IfcFloat epsilon = 1e-5f;
    return  (std::fabs(bb.second.x - ibb.first.x) < epsilon && bb.first.y <= ibb.second.y && bb.second.y >= ibb.first.y) ||
        (std::fabs(bb.first.x - ibb.second.x) < epsilon && ibb.first.y <= bb.second.y && ibb.second.y >= bb.first.y) ||
        (std::fabs(bb.second.y - ibb.first.y) < epsilon && bb.first.x <= ibb.second.x && bb.second.x >= ibb.first.x) ||
        (std::fabs(bb.first.y - ibb.second.y) < epsilon && ibb.first.x <= bb.second.x && ibb.second.x >= bb.first.x);
}

// Check if m0,m1 intersects n0,n1 assuming same ordering of the points in the line segments
// output the intersection points on n0,n1
bool IntersectingLineSegments(const IfcVector2& n0, const IfcVector2& n1,
    const IfcVector2& m0, const IfcVector2& m1,
    IfcVector2& out0, IfcVector2& out1)
{
    const IfcVector2 n0_to_n1 = n1 - n0;

    const IfcVector2 n0_to_m0 = m0 - n0;
    const IfcVector2 n1_to_m1 = m1 - n1;

    const IfcVector2 n0_to_m1 = m1 - n0;

    const IfcFloat e = 1e-5f;
    const IfcFloat smalle = 1e-9f;

    static const IfcFloat inf = std::numeric_limits<IfcFloat>::infinity();

    if (!(n0_to_m0.SquareLength() < e*e || std::fabs(n0_to_m0 * n0_to_n1) / (n0_to_m0.Length() * n0_to_n1.Length()) > 1-1e-5 )) {
        return false;
    }

    if (!(n1_to_m1.SquareLength() < e*e || std::fabs(n1_to_m1 * n0_to_n1) / (n1_to_m1.Length() * n0_to_n1.Length()) > 1-1e-5 )) {
        return false;
    }

    IfcFloat s0;
    IfcFloat s1;

    // pick the axis with the higher absolute difference so the result
    // is more accurate. Since we cannot guarantee that the axis with
    // the higher absolute difference is big enough as to avoid
    // divisions by zero, the case 0/0 ~ infinity is detected and
    // handled separately.
    if(std::fabs(n0_to_n1.x) > std::fabs(n0_to_n1.y)) {
        s0 = n0_to_m0.x / n0_to_n1.x;
        s1 = n0_to_m1.x / n0_to_n1.x;

        if (std::fabs(s0) == inf && std::fabs(n0_to_m0.x) < smalle) {
            s0 = 0.;
        }
        if (std::fabs(s1) == inf && std::fabs(n0_to_m1.x) < smalle) {
            s1 = 0.;
        }
    }
    else {
        s0 = n0_to_m0.y / n0_to_n1.y;
        s1 = n0_to_m1.y / n0_to_n1.y;

        if (std::fabs(s0) == inf && std::fabs(n0_to_m0.y) < smalle) {
            s0 = 0.;
        }
        if (std::fabs(s1) == inf && std::fabs(n0_to_m1.y) < smalle) {
            s1 = 0.;
        }
    }

    if (s1 < s0) {
        std::swap(s1,s0);
    }

    s0 = std::max(0.0,s0);
    s1 = std::max(0.0,s1);

    s0 = std::min(1.0,s0);
    s1 = std::min(1.0,s1);

    if (std::fabs(s1-s0) < e) {
        return false;
    }

    out0 = n0 + s0 * n0_to_n1;
    out1 = n0 + s1 * n0_to_n1;

    return true;
}

void FindAdjacentContours(ContourVector::iterator current, const ContourVector& contours)
{
    const IfcFloat sqlen_epsilon = static_cast<IfcFloat>(1e-8);
    const BoundingBox& bb = (*current).bb;

    // What is to be done here is to populate the skip lists for the contour
    // and to add necessary padding points when needed.
    SkipList& skiplist = (*current).skiplist;

    // First step to find possible adjacent contours is to check for adjacent bounding
    // boxes. If the bounding boxes are not adjacent, the contours lines cannot possibly be.
    for (ContourVector::const_iterator it = contours.begin(), end = contours.end(); it != end; ++it) {
        if ((*it).IsInvalid()) {
            continue;
        }

        // this left here to make clear we also run on the current contour
        // to check for overlapping contour segments (which can happen due
        // to projection artifacts).

        const bool is_me = it == current;

        const BoundingBox& ibb = (*it).bb;

        // Assumption: the bounding boxes are pairwise disjoint or identical
        ai_assert(is_me || !BoundingBoxesOverlapping(bb, ibb));

        if (is_me || BoundingBoxesAdjacent(bb, ibb)) {

            // Now do a each-against-everyone check for intersecting contour
            // lines. This obviously scales terribly, but in typical real
            // world Ifc files it will not matter since most windows that
            // are adjacent to each others are rectangular anyway.

            Contour& ncontour = (*current).contour;
            const Contour& mcontour = (*it).contour;

            for (size_t n = 0; n < ncontour.size(); ++n) {
                const IfcVector2& n0 = ncontour[n];
                const IfcVector2& n1 = ncontour[(n+1) % ncontour.size()];

                for (size_t m = 0, mend = (is_me ? n : mcontour.size()); m < mend; ++m) {
                    ai_assert(&mcontour != &ncontour || m < n);

                    const IfcVector2& m0 = mcontour[m];
                    const IfcVector2& m1 = mcontour[(m+1) % mcontour.size()];

                    IfcVector2 isect0, isect1;
                    if (IntersectingLineSegments(n0,n1, m0, m1, isect0, isect1)) {

                        if ((isect0 - n0).SquareLength() > sqlen_epsilon) {
                            ++n;

                            ncontour.insert(ncontour.begin() + n, isect0);
                            skiplist.insert(skiplist.begin() + n, true);
                        }
                        else {
                            skiplist[n] = true;
                        }

                        if ((isect1 - n1).SquareLength() > sqlen_epsilon) {
                            ++n;

                            ncontour.insert(ncontour.begin() + n, isect1);
                            skiplist.insert(skiplist.begin() + n, false);
                        }
                    }
                }
            }
        }
    }
}

AI_FORCE_INLINE bool LikelyBorder(const IfcVector2& vdelta)
{
    const IfcFloat dot_point_epsilon = static_cast<IfcFloat>(1e-5);
    return std::fabs(vdelta.x * vdelta.y) < dot_point_epsilon;
}

void FindBorderContours(ContourVector::iterator current)
{
    const IfcFloat border_epsilon_upper = static_cast<IfcFloat>(1-1e-4);
    const IfcFloat border_epsilon_lower = static_cast<IfcFloat>(1e-4);

    bool outer_border = false;
    bool start_on_outer_border = false;

    SkipList& skiplist = (*current).skiplist;
    IfcVector2 last_proj_point;

    const Contour::const_iterator cbegin = (*current).contour.begin(), cend = (*current).contour.end();

    for (Contour::const_iterator cit = cbegin; cit != cend; ++cit) {
        const IfcVector2& proj_point = *cit;

        // Check if this connection is along the outer boundary of the projection
        // plane. In such a case we better drop it because such 'edges' should
        // not have any geometry to close them (think of door openings).
        if (proj_point.x <= border_epsilon_lower || proj_point.x >= border_epsilon_upper ||
            proj_point.y <= border_epsilon_lower || proj_point.y >= border_epsilon_upper) {

                if (outer_border) {
                    ai_assert(cit != cbegin);
                    if (LikelyBorder(proj_point - last_proj_point)) {
                        skiplist[std::distance(cbegin, cit) - 1] = true;
                    }
                }
                else if (cit == cbegin) {
                    start_on_outer_border = true;
                }

                outer_border = true;
        }
        else {
            outer_border = false;
        }

        last_proj_point = proj_point;
    }

    // handle last segment
    if (outer_border && start_on_outer_border) {
        const IfcVector2& proj_point = *cbegin;
        if (LikelyBorder(proj_point - last_proj_point)) {
            skiplist[skiplist.size()-1] = true;
        }
    }
}

AI_FORCE_INLINE bool LikelyDiagonal(IfcVector2 vdelta)
{
    vdelta.x = std::fabs(vdelta.x);
    vdelta.y = std::fabs(vdelta.y);
    return (std::fabs(vdelta.x-vdelta.y) < 0.8 * std::max(vdelta.x, vdelta.y));
}

void FindLikelyCrossingLines(ContourVector::iterator current)
{
    SkipList& skiplist = (*current).skiplist;
    IfcVector2 last_proj_point;

    const Contour::const_iterator cbegin = (*current).contour.begin(), cend = (*current).contour.end();
    for (Contour::const_iterator cit = cbegin; cit != cend; ++cit) {
        const IfcVector2& proj_point = *cit;

        if (cit != cbegin) {
            IfcVector2 vdelta = proj_point - last_proj_point;
            if (LikelyDiagonal(vdelta)) {
                skiplist[std::distance(cbegin, cit) - 1] = true;
            }
        }

        last_proj_point = proj_point;
    }

    // handle last segment
    if (LikelyDiagonal(*cbegin - last_proj_point)) {
        skiplist[skiplist.size()-1] = true;
    }
}

size_t CloseWindows(ContourVector& contours,
    const IfcMatrix4& minv,
    OpeningRefVector& contours_to_openings,
    TempMesh& curmesh)
{
    size_t closed = 0;
    // For all contour points, check if one of the assigned openings does
    // already have points assigned to it. In this case, assume this is
    // the other side of the wall and generate connections between
    // the two holes in order to close the window.

    // All this gets complicated by the fact that contours may pertain to
    // multiple openings(due to merging of adjacent or overlapping openings).
    // The code is based on the assumption that this happens symmetrically
    // on both sides of the wall. If it doesn't (which would be a bug anyway)
    // wrong geometry may be generated.
    for (ContourVector::iterator it = contours.begin(), end = contours.end(); it != end; ++it) {
        if ((*it).IsInvalid()) {
            continue;
        }
        OpeningRefs& refs = contours_to_openings[std::distance(contours.begin(), it)];

        bool has_other_side = false;
        BOOST_FOREACH(const TempOpening* opening, refs) {
            if(!opening->wallPoints.empty()) {
                has_other_side = true;
                break;
            }
        }

        if (has_other_side) {

            ContourRefVector adjacent_contours;

            // prepare a skiplist for this contour. The skiplist is used to
            // eliminate unwanted contour lines for adjacent windows and
            // those bordering the outer frame.
            (*it).PrepareSkiplist();

            FindAdjacentContours(it, contours);
            FindBorderContours(it);

            // if the window is the result of a finite union or intersection of rectangles,
            // there shouldn't be any crossing or diagonal lines in it. Such lines would
            // be artifacts caused by numerical inaccuracies or other bugs in polyclipper
            // and our own code. Since rectangular openings are by far the most frequent
            // case, it is worth filtering for this corner case.
            if((*it).is_rectangular) {
                FindLikelyCrossingLines(it);
            }

            ai_assert((*it).skiplist.size() == (*it).contour.size());

            SkipList::const_iterator skipbegin = (*it).skiplist.begin();

            curmesh.verts.reserve(curmesh.verts.size() + (*it).contour.size() * 4);
            curmesh.vertcnt.reserve(curmesh.vertcnt.size() + (*it).contour.size());

            // compare base poly normal and contour normal to detect if we need to reverse the face winding
            // Urho3D: modified to not use C++11
            IfcVector3 basePolyNormal = TempMesh::ComputePolygonNormal( &curmesh.verts[0], curmesh.vertcnt.front());
            std::vector<IfcVector3> worldSpaceContourVtx( it->contour.size());
            for( size_t a = 0; a < it->contour.size(); ++a )
                worldSpaceContourVtx[a] = minv * IfcVector3( it->contour[a].x, it->contour[a].y, 0.0);
            IfcVector3 contourNormal = TempMesh::ComputePolygonNormal( &worldSpaceContourVtx[0], worldSpaceContourVtx.size());
            bool reverseCountourFaces = (contourNormal * basePolyNormal) > 0.0;

            // XXX this algorithm is really a bit inefficient - both in terms
            // of constant factor and of asymptotic runtime.
            std::vector<bool>::const_iterator skipit = skipbegin;

            IfcVector3 start0;
            IfcVector3 start1;

            const Contour::const_iterator cbegin = (*it).contour.begin(), cend = (*it).contour.end();

            bool drop_this_edge = false;
            for (Contour::const_iterator cit = cbegin; cit != cend; ++cit, drop_this_edge = *skipit++) {
                const IfcVector2& proj_point = *cit;

                // Locate the closest opposite point. This should be a good heuristic to
                // connect only the points that are really intended to be connected.
                IfcFloat best = static_cast<IfcFloat>(1e10);
                IfcVector3 bestv;

                const IfcVector3 world_point = minv * IfcVector3(proj_point.x,proj_point.y,0.0f);

                BOOST_FOREACH(const TempOpening* opening, refs) {
                    BOOST_FOREACH(const IfcVector3& other, opening->wallPoints) {
                        const IfcFloat sqdist = (world_point - other).SquareLength();

                        if (sqdist < best) {
                            // avoid self-connections
                            if(sqdist < 1e-5) {
                                continue;
                            }

                            bestv = other;
                            best = sqdist;
                        }
                    }
                }

                if (drop_this_edge) {
                    curmesh.verts.pop_back();
                    curmesh.verts.pop_back();
                }
                else {
                    curmesh.verts.push_back(((cit == cbegin) != reverseCountourFaces) ? world_point : bestv);
                    curmesh.verts.push_back(((cit == cbegin) != reverseCountourFaces) ? bestv : world_point);

                    curmesh.vertcnt.push_back(4);
                    ++closed;
                }

                if (cit == cbegin) {
                    start0 = world_point;
                    start1 = bestv;
                    continue;
                }

                curmesh.verts.push_back(reverseCountourFaces ? bestv : world_point);
                curmesh.verts.push_back(reverseCountourFaces ? world_point : bestv);

                if (cit == cend - 1) {
                    drop_this_edge = *skipit;

                    // Check if the final connection (last to first element) is itself
                    // a border edge that needs to be dropped.
                    if (drop_this_edge) {
                        --closed;
                        curmesh.vertcnt.pop_back();
                        curmesh.verts.pop_back();
                        curmesh.verts.pop_back();
                    }
                    else {
                        curmesh.verts.push_back(reverseCountourFaces ? start0 : start1);
                        curmesh.verts.push_back(reverseCountourFaces ? start1 : start0);
                    }
                }
            }
        }
        else {

            const Contour::const_iterator cbegin = (*it).contour.begin(), cend = (*it).contour.end();
            BOOST_FOREACH(TempOpening* opening, refs) {
                ai_assert(opening->wallPoints.empty());
                opening->wallPoints.reserve(opening->wallPoints.capacity() + (*it).contour.size());
                for (Contour::const_iterator cit = cbegin; cit != cend; ++cit) {

                    const IfcVector2& proj_point = *cit;
                    opening->wallPoints.push_back(minv * IfcVector3(proj_point.x,proj_point.y,0.0f));
                }
            }
        }
    }
    return closed;
}

void Quadrify(const std::vector< BoundingBox >& bbs, TempMesh& curmesh)
{
    ai_assert(curmesh.IsEmpty());

    std::vector<IfcVector2> quads;
    quads.reserve(bbs.size()*4);

    // sort openings by x and y axis as a preliminiary to the QuadrifyPart() algorithm
    XYSortedField field;
    for (std::vector<BoundingBox>::const_iterator it = bbs.begin(); it != bbs.end(); ++it) {
        if (field.find((*it).first) != field.end()) {
            IFCImporter::LogWarn("constraint failure during generation of wall openings, results may be faulty");
        }
        field[(*it).first] = std::distance(bbs.begin(),it);
    }

    QuadrifyPart(IfcVector2(),one_vec,field,bbs,quads);
    ai_assert(!(quads.size() % 4));

    curmesh.vertcnt.resize(quads.size()/4,4);
    curmesh.verts.reserve(quads.size());
    BOOST_FOREACH(const IfcVector2& v2, quads) {
        curmesh.verts.push_back(IfcVector3(v2.x, v2.y, static_cast<IfcFloat>(0.0)));
    }
}

void Quadrify(const ContourVector& contours, TempMesh& curmesh)
{
    std::vector<BoundingBox> bbs;
    bbs.reserve(contours.size());

    BOOST_FOREACH(const ContourVector::value_type& val, contours) {
        bbs.push_back(val.bb);
    }

    Quadrify(bbs, curmesh);
}

IfcMatrix4 ProjectOntoPlane(std::vector<IfcVector2>& out_contour, const TempMesh& in_mesh,
    bool &ok, IfcVector3& nor_out)
{
    const std::vector<IfcVector3>& in_verts = in_mesh.verts;
    ok = true;

    IfcMatrix4 m = IfcMatrix4(DerivePlaneCoordinateSpace(in_mesh, ok, nor_out));
    if(!ok) {
        return IfcMatrix4();
    }
#ifdef ASSIMP_BUILD_DEBUG
    const IfcFloat det = m.Determinant();
    ai_assert(std::fabs(det-1) < 1e-5);
#endif

    IfcFloat zcoord = 0;
    out_contour.reserve(in_verts.size());

    IfcVector3 vmin, vmax;
    MinMaxChooser<IfcVector3>()(vmin, vmax);

    // Project all points into the new coordinate system, collect min/max verts on the way
    BOOST_FOREACH(const IfcVector3& x, in_verts) {
        const IfcVector3 vv = m * x;
        // keep Z offset in the plane coordinate system. Ignoring precision issues
        // (which  are present, of course), this should be the same value for
        // all polygon vertices (assuming the polygon is planar).

        // XXX this should be guarded, but we somehow need to pick a suitable
        // epsilon
        zcoord += vv.z;
        vmin = std::min(vv, vmin);
        vmax = std::max(vv, vmax);

        out_contour.push_back(IfcVector2(vv.x,vv.y));
    }

    zcoord /= in_verts.size();

    // Further improve the projection by mapping the entire working set into
    // [0,1] range. This gives us a consistent data range so all epsilons
    // used below can be constants.
    vmax -= vmin;
    BOOST_FOREACH(IfcVector2& vv, out_contour) {
        vv.x  = (vv.x - vmin.x) / vmax.x;
        vv.y  = (vv.y - vmin.y) / vmax.y;

        // sanity rounding
        vv = std::max(vv,IfcVector2());
        vv = std::min(vv,one_vec);
    }

    IfcMatrix4 mult;
    mult.a1 = static_cast<IfcFloat>(1.0) / vmax.x;
    mult.b2 = static_cast<IfcFloat>(1.0) / vmax.y;

    mult.a4 = -vmin.x * mult.a1;
    mult.b4 = -vmin.y * mult.b2;
    mult.c4 = -zcoord;
    m = mult * m;

    // debug code to verify correctness
#ifdef ASSIMP_BUILD_DEBUG
    std::vector<IfcVector2> out_contour2;
    BOOST_FOREACH(const IfcVector3& x, in_verts) {
        const IfcVector3& vv = m * x;

        out_contour2.push_back(IfcVector2(vv.x,vv.y));
        ai_assert(std::fabs(vv.z) < vmax.z + 1e-8);
    }

    for(size_t i = 0; i < out_contour.size(); ++i) {
        ai_assert((out_contour[i]-out_contour2[i]).SquareLength() < 1e-6);
    }
#endif

    return m;
}

bool GenerateOpenings(std::vector<TempOpening>& openings,
    const std::vector<IfcVector3>& nors,
    TempMesh& curmesh,
    bool check_intersection,
    bool generate_connection_geometry,
    const IfcVector3& wall_extrusion_axis)
{
    OpeningRefVector contours_to_openings;

    // Try to derive a solid base plane within the current surface for use as
    // working coordinate system. Map all vertices onto this plane and
    // rescale them to [0,1] range. This normalization means all further
    // epsilons need not be scaled.
    bool ok = true;

    std::vector<IfcVector2> contour_flat;

    IfcVector3 nor;
    const IfcMatrix4 m = ProjectOntoPlane(contour_flat, curmesh,  ok, nor);
    if(!ok) {
        return false;
    }

    // Obtain inverse transform for getting back to world space later on
    const IfcMatrix4 minv = IfcMatrix4(m).Inverse();

    // Compute bounding boxes for all 2D openings in projection space
    ContourVector contours;

    std::vector<IfcVector2> temp_contour;
    std::vector<IfcVector2> temp_contour2;

    IfcVector3 wall_extrusion_axis_norm = wall_extrusion_axis;
    wall_extrusion_axis_norm.Normalize();

    BOOST_FOREACH(TempOpening& opening,openings) {

        // extrusionDir may be 0,0,0 on case where the opening mesh is not an
        // IfcExtrudedAreaSolid but something else (i.e. a brep)
        IfcVector3 norm_extrusion_dir = opening.extrusionDir;
        if (norm_extrusion_dir.SquareLength() > 1e-10) {
            norm_extrusion_dir.Normalize();
        }
        else {
            norm_extrusion_dir = IfcVector3();
        }

        TempMesh* profile_data =  opening.profileMesh.get();
        bool is_2d_source = false;
        if (opening.profileMesh2D && norm_extrusion_dir.SquareLength() > 0) {

            if(std::fabs(norm_extrusion_dir * wall_extrusion_axis_norm) < 0.1) {
                // horizontal extrusion
                if (std::fabs(norm_extrusion_dir * nor) > 0.9) {
                    profile_data = opening.profileMesh2D.get();
                    is_2d_source = true;
                }
            }
            else {
                // vertical extrusion
                if (std::fabs(norm_extrusion_dir * nor) > 0.9) {
                    profile_data = opening.profileMesh2D.get();
                    is_2d_source = true;
                }
            }
        }
        std::vector<IfcVector3> profile_verts = profile_data->verts;
        std::vector<unsigned int> profile_vertcnts = profile_data->vertcnt;
        if(profile_verts.size() <= 2) {
            continue;
        }

        // The opening meshes are real 3D meshes so skip over all faces
        // clearly facing into the wrong direction. Also, we need to check
        // whether the meshes do actually intersect the base surface plane.
        // This is done by recording minimum and maximum values for the
        // d component of the plane equation for all polys and checking
        // against surface d.

        // Use the sign of the dot product of the face normal to the plane
        // normal to determine to which side of the difference mesh a
        // triangle belongs. Get independent bounding boxes and vertex
        // sets for both sides and take the better one (we can't just
        // take both - this would likely cause major screwup of vertex
        // winding, producing errors as late as in CloseWindows()).
        IfcFloat dmin, dmax;
        MinMaxChooser<IfcFloat>()(dmin,dmax);

        temp_contour.clear();
        temp_contour2.clear();

        IfcVector2 vpmin,vpmax;
        MinMaxChooser<IfcVector2>()(vpmin,vpmax);

        IfcVector2 vpmin2,vpmax2;
        MinMaxChooser<IfcVector2>()(vpmin2,vpmax2);

        for (size_t f = 0, vi_total = 0, fend = profile_vertcnts.size(); f < fend; ++f) {

            bool side_flag = true;
            if (!is_2d_source) {
                const IfcVector3 face_nor = ((profile_verts[vi_total+2] - profile_verts[vi_total]) ^
                    (profile_verts[vi_total+1] - profile_verts[vi_total])).Normalize();

                const IfcFloat abs_dot_face_nor = std::abs(nor * face_nor);
                if (abs_dot_face_nor < 0.9) {
                    vi_total += profile_vertcnts[f];
                    continue;
                }

                side_flag = nor * face_nor > 0;
            }

            for (unsigned int vi = 0, vend = profile_vertcnts[f]; vi < vend; ++vi, ++vi_total) {
                const IfcVector3& x = profile_verts[vi_total];

                const IfcVector3 v = m * x;
                IfcVector2 vv(v.x, v.y);

                    dmin = std::min(dmin, v.z);
                    dmax = std::max(dmax, v.z);

                // sanity rounding
                vv = std::max(vv,IfcVector2());
                vv = std::min(vv,one_vec);

                if(side_flag) {
                    vpmin = std::min(vpmin,vv);
                    vpmax = std::max(vpmax,vv);
                }
                else {
                    vpmin2 = std::min(vpmin2,vv);
                    vpmax2 = std::max(vpmax2,vv);
                }

                std::vector<IfcVector2>& store = side_flag ? temp_contour : temp_contour2;

                if (!IsDuplicateVertex(vv, store)) {
                    store.push_back(vv);
                }
            }
        }

        if (temp_contour2.size() > 2) {
            ai_assert(!is_2d_source);
            const IfcVector2 area = vpmax-vpmin;
            const IfcVector2 area2 = vpmax2-vpmin2;
            if (temp_contour.size() <= 2 || std::fabs(area2.x * area2.y) > std::fabs(area.x * area.y)) {
                temp_contour.swap(temp_contour2);

                vpmax = vpmax2;
                vpmin = vpmin2;
            }
        }
        if(temp_contour.size() <= 2) {
            continue;
        }

        // TODO: This epsilon may be too large
        const IfcFloat epsilon = std::fabs(dmax-dmin) * 0.0001;
        if (!is_2d_source && check_intersection && (0 < dmin-epsilon || 0 > dmax+epsilon)) {
            continue;
        }

        BoundingBox bb = BoundingBox(vpmin,vpmax);

        // Skip over very small openings - these are likely projection errors
        // (i.e. they don't belong to this side of the wall)
        if(std::fabs(vpmax.x - vpmin.x) * std::fabs(vpmax.y - vpmin.y) < static_cast<IfcFloat>(1e-10)) {
            continue;
        }
        std::vector<TempOpening*> joined_openings(1, &opening);

        bool is_rectangle = temp_contour.size() == 4;

        // See if this BB intersects or is in close adjacency to any other BB we have so far.
        for (ContourVector::iterator it = contours.begin(); it != contours.end(); ) {
            const BoundingBox& ibb = (*it).bb;

            if (BoundingBoxesOverlapping(ibb, bb)) {

                if (!(*it).is_rectangular) {
                    is_rectangle = false;
                }

                const std::vector<IfcVector2>& other = (*it).contour;
                ClipperLib::ExPolygons poly;

                // First check whether subtracting the old contour (to which ibb belongs)
                // from the new contour (to which bb belongs) yields an updated bb which
                // no longer overlaps ibb
                MakeDisjunctWindowContours(other, temp_contour, poly);
                if(poly.size() == 1) {

                    const BoundingBox newbb = GetBoundingBox(poly[0].outer);
                    if (!BoundingBoxesOverlapping(ibb, newbb )) {
                         // Good guy bounding box
                         bb = newbb ;

                         ExtractVerticesFromClipper(poly[0].outer, temp_contour, false);
                         continue;
                    }
                }

                // Take these two overlapping contours and try to merge them. If they
                // overlap (which should not happen, but in fact happens-in-the-real-
                // world [tm] ), resume using a single contour and a single bounding box.
                MergeWindowContours(temp_contour, other, poly);

                if (poly.size() > 1) {
                    return TryAddOpenings_Poly2Tri(openings, nors, curmesh);
                }
                else if (poly.size() == 0) {
                    IFCImporter::LogWarn("ignoring duplicate opening");
                    temp_contour.clear();
                    break;
                }
                else {
                    IFCImporter::LogDebug("merging overlapping openings");
                    ExtractVerticesFromClipper(poly[0].outer, temp_contour, false);

                    // Generate the union of the bounding boxes
                    bb.first = std::min(bb.first, ibb.first);
                    bb.second = std::max(bb.second, ibb.second);

                    // Update contour-to-opening tables accordingly
                    if (generate_connection_geometry) {
                        std::vector<TempOpening*>& t = contours_to_openings[std::distance(contours.begin(),it)];
                        joined_openings.insert(joined_openings.end(), t.begin(), t.end());

                        contours_to_openings.erase(contours_to_openings.begin() + std::distance(contours.begin(),it));
                    }

                    contours.erase(it);

                    // Restart from scratch because the newly formed BB might now
                    // overlap any other BB which its constituent BBs didn't
                    // previously overlap.
                    it = contours.begin();
                    continue;
                }
            }
            ++it;
        }

        if(!temp_contour.empty()) {
            if (generate_connection_geometry) {
                contours_to_openings.push_back(std::vector<TempOpening*>(
                    joined_openings.begin(),
                    joined_openings.end()));
            }

            contours.push_back(ProjectedWindowContour(temp_contour, bb, is_rectangle));
        }
    }

    // Check if we still have any openings left - it may well be that this is
    // not the cause, for example if all the opening candidates don't intersect
    // this surface or point into a direction perpendicular to it.
    if (contours.empty()) {
        return false;
    }

    curmesh.Clear();

    // Generate a base subdivision into quads to accommodate the given list
    // of window bounding boxes.
    Quadrify(contours,curmesh);

    // Run a sanity cleanup pass on the window contours to avoid generating
    // artifacts during the contour generation phase later on.
    CleanupWindowContours(contours);

    // Previously we reduced all windows to rectangular AABBs in projection
    // space, now it is time to fill the gaps between the BBs and the real
    // window openings.
    InsertWindowContours(contours,openings, curmesh);

    // Clip the entire outer contour of our current result against the real
    // outer contour of the surface. This is necessary because the result
    // of the Quadrify() algorithm is always a square area spanning
    // over [0,1]^2 (i.e. entire projection space).
    CleanupOuterContour(contour_flat, curmesh);

    // Undo the projection and get back to world (or local object) space
    BOOST_FOREACH(IfcVector3& v3, curmesh.verts) {
        v3 = minv * v3;
    }

    // Generate window caps to connect the symmetric openings on both sides
    // of the wall.
    if (generate_connection_geometry) {
        CloseWindows(contours, minv, contours_to_openings, curmesh);
    }
    return true;
}

bool TryAddOpenings_Poly2Tri(const std::vector<TempOpening>& openings,const std::vector<IfcVector3>& nors,
    TempMesh& curmesh)
{
    IFCImporter::LogWarn("forced to use poly2tri fallback method to generate wall openings");
    std::vector<IfcVector3>& out = curmesh.verts;

    bool result = false;

    // Try to derive a solid base plane within the current surface for use as
    // working coordinate system.
    bool ok;
    IfcVector3 nor;
    const IfcMatrix3 m = DerivePlaneCoordinateSpace(curmesh, ok, nor);
    if (!ok) {
        return false;
    }

    const IfcMatrix3 minv = IfcMatrix3(m).Inverse();

    IfcFloat coord = -1;

    std::vector<IfcVector2> contour_flat;
    contour_flat.reserve(out.size());

    IfcVector2 vmin, vmax;
    MinMaxChooser<IfcVector2>()(vmin, vmax);

    // Move all points into the new coordinate system, collecting min/max verts on the way
    BOOST_FOREACH(IfcVector3& x, out) {
        const IfcVector3 vv = m * x;

        // keep Z offset in the plane coordinate system. Ignoring precision issues
        // (which  are present, of course), this should be the same value for
        // all polygon vertices (assuming the polygon is planar).

        // XXX this should be guarded, but we somehow need to pick a suitable
        // epsilon

        coord = vv.z;

        vmin = std::min(IfcVector2(vv.x, vv.y), vmin);
        vmax = std::max(IfcVector2(vv.x, vv.y), vmax);

        contour_flat.push_back(IfcVector2(vv.x,vv.y));
    }

    // With the current code in DerivePlaneCoordinateSpace,
    // vmin,vmax should always be the 0...1 rectangle (+- numeric inaccuracies)
    // but here we won't rely on this.

    vmax -= vmin;

    // If this happens then the projection must have been wrong.
    assert(vmax.Length());

    ClipperLib::ExPolygons clipped;
    ClipperLib::Polygons holes_union;

    IfcVector3 wall_extrusion;
    bool do_connections = false, first = true;

    try {

        ClipperLib::Clipper clipper_holes;
        size_t c = 0;

        BOOST_FOREACH(const TempOpening& t,openings) {
            const IfcVector3& outernor = nors[c++];
            const IfcFloat dot = nor * outernor;
            if (std::fabs(dot)<1.f-1e-6f) {
                continue;
            }

            const std::vector<IfcVector3>& va = t.profileMesh->verts;
            if(va.size() <= 2) {
                continue;
            }

            std::vector<IfcVector2> contour;

            BOOST_FOREACH(const IfcVector3& xx, t.profileMesh->verts) {
                IfcVector3 vv = m *  xx, vv_extr = m * (xx + t.extrusionDir);

                const bool is_extruded_side = std::fabs(vv.z - coord) > std::fabs(vv_extr.z - coord);
                if (first) {
                    first = false;
                    if (dot > 0.f) {
                        do_connections = true;
                        wall_extrusion = t.extrusionDir;
                        if (is_extruded_side) {
                            wall_extrusion = - wall_extrusion;
                        }
                    }
                }

                // XXX should not be necessary - but it is. Why? For precision reasons?
                vv = is_extruded_side ? vv_extr : vv;
                contour.push_back(IfcVector2(vv.x,vv.y));
            }

            ClipperLib::Polygon hole;
            BOOST_FOREACH(IfcVector2& pip, contour) {
                pip.x  = (pip.x - vmin.x) / vmax.x;
                pip.y  = (pip.y - vmin.y) / vmax.y;

                hole.push_back(ClipperLib::IntPoint(  to_int64(pip.x), to_int64(pip.y) ));
            }

            if (!ClipperLib::Orientation(hole)) {
                std::reverse(hole.begin(), hole.end());
            }

            /*ClipperLib::Polygons pol_temp(1), pol_temp2(1);
            pol_temp[0] = hole;

            ClipperLib::OffsetPolygons(pol_temp,pol_temp2,5.0);
            hole = pol_temp2[0];*/

            clipper_holes.AddPolygon(hole,ClipperLib::ptSubject);
        }

        clipper_holes.Execute(ClipperLib::ctUnion,holes_union,
            ClipperLib::pftNonZero,
            ClipperLib::pftNonZero);

        if (holes_union.empty()) {
            return false;
        }

        // Now that we have the big union of all holes, subtract it from the outer contour
        // to obtain the final polygon to feed into the triangulator.
        {
            ClipperLib::Polygon poly;
            BOOST_FOREACH(IfcVector2& pip, contour_flat) {
                pip.x  = (pip.x - vmin.x) / vmax.x;
                pip.y  = (pip.y - vmin.y) / vmax.y;

                poly.push_back(ClipperLib::IntPoint( to_int64(pip.x), to_int64(pip.y) ));
            }

            if (ClipperLib::Orientation(poly)) {
                std::reverse(poly.begin(), poly.end());
            }
            clipper_holes.Clear();
            clipper_holes.AddPolygon(poly,ClipperLib::ptSubject);

            clipper_holes.AddPolygons(holes_union,ClipperLib::ptClip);
            clipper_holes.Execute(ClipperLib::ctDifference,clipped,
                ClipperLib::pftNonZero,
                ClipperLib::pftNonZero);
        }

    }
    catch (const char* sx) {
        IFCImporter::LogError("Ifc: error during polygon clipping, skipping openings for this face: (Clipper: "
            + std::string(sx) + ")");

        return false;
    }

    std::vector<IfcVector3> old_verts;
    std::vector<unsigned int> old_vertcnt;

    old_verts.swap(curmesh.verts);
    old_vertcnt.swap(curmesh.vertcnt);

    // add connection geometry to close the adjacent 'holes' for the openings
    // this should only be done from one side of the wall or the polygons
    // would be emitted twice.
    if (false && do_connections) {

        std::vector<IfcVector3> tmpvec;
        BOOST_FOREACH(ClipperLib::Polygon& opening, holes_union) {

            assert(ClipperLib::Orientation(opening));

            tmpvec.clear();

            BOOST_FOREACH(ClipperLib::IntPoint& point, opening) {

                tmpvec.push_back( minv * IfcVector3(
                    vmin.x + from_int64(point.X) * vmax.x,
                    vmin.y + from_int64(point.Y) * vmax.y,
                    coord));
            }

            for(size_t i = 0, size = tmpvec.size(); i < size; ++i) {
                const size_t next = (i+1)%size;

                curmesh.vertcnt.push_back(4);

                const IfcVector3& in_world = tmpvec[i];
                const IfcVector3& next_world = tmpvec[next];

                // Assumptions: no 'partial' openings, wall thickness roughly the same across the wall
                curmesh.verts.push_back(in_world);
                curmesh.verts.push_back(in_world+wall_extrusion);
                curmesh.verts.push_back(next_world+wall_extrusion);
                curmesh.verts.push_back(next_world);
            }
        }
    }

    std::vector< std::vector<p2t::Point*> > contours;
    BOOST_FOREACH(ClipperLib::ExPolygon& clip, clipped) {

        contours.clear();

        // Build the outer polygon contour line for feeding into poly2tri
        std::vector<p2t::Point*> contour_points;
        BOOST_FOREACH(ClipperLib::IntPoint& point, clip.outer) {
            contour_points.push_back( new p2t::Point(from_int64(point.X), from_int64(point.Y)) );
        }

        p2t::CDT* cdt ;
        try {
            // Note: this relies on custom modifications in poly2tri to raise runtime_error's
            // instead if assertions. These failures are not debug only, they can actually
            // happen in production use if the input data is broken. An assertion would be
            cdt = new p2t::CDT(contour_points);
        }
        catch(const std::exception& e) {
            IFCImporter::LogError("Ifc: error during polygon triangulation, skipping some openings: (poly2tri: "
                + std::string(e.what()) + ")");
            continue;
        }

        // Build the poly2tri inner contours for all holes we got from ClipperLib
        BOOST_FOREACH(ClipperLib::Polygon& opening, clip.holes) {

            contours.push_back(std::vector<p2t::Point*>());
            std::vector<p2t::Point*>& contour = contours.back();

            BOOST_FOREACH(ClipperLib::IntPoint& point, opening) {
                contour.push_back( new p2t::Point(from_int64(point.X), from_int64(point.Y)) );
            }

            cdt->AddHole(contour);
        }

        try {
            // Note: See above
            cdt->Triangulate();
        }
        catch(const std::exception& e) {
            IFCImporter::LogError("Ifc: error during polygon triangulation, skipping some openings: (poly2tri: "
                + std::string(e.what()) + ")");
            continue;
        }

        const std::vector<p2t::Triangle*> tris = cdt->GetTriangles();

        // Collect the triangles we just produced
        BOOST_FOREACH(p2t::Triangle* tri, tris) {
            for(int i = 0; i < 3; ++i) {

                const IfcVector2 v = IfcVector2(
                    static_cast<IfcFloat>( tri->GetPoint(i)->x ),
                    static_cast<IfcFloat>( tri->GetPoint(i)->y )
                );

                assert(v.x <= 1.0 && v.x >= 0.0 && v.y <= 1.0 && v.y >= 0.0);
                const IfcVector3 v3 = minv * IfcVector3(vmin.x + v.x * vmax.x, vmin.y + v.y * vmax.y,coord) ;

                curmesh.verts.push_back(v3);
            }
            curmesh.vertcnt.push_back(3);
        }

        result = true;
    }

    if (!result) {
        // revert -- it's a shame, but better than nothing
        curmesh.verts.insert(curmesh.verts.end(),old_verts.begin(), old_verts.end());
        curmesh.vertcnt.insert(curmesh.vertcnt.end(),old_vertcnt.begin(), old_vertcnt.end());

        IFCImporter::LogError("Ifc: revert, could not generate openings for this wall");
    }

    return result;
}

    } // ! IFC
} // ! Assimp

#undef to_int64
#undef from_int64
#undef one_vec

#endif