assimp/code/IFCBoolean.cpp

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*/

/** @file  IFCBoolean.cpp
 *  @brief Implements a subset of Ifc boolean operations
 */


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

#include <iterator>
#include <boost/tuple/tuple.hpp>


namespace Assimp {
	namespace IFC {
		
// ------------------------------------------------------------------------------------------------
// Calculates intersection between line segment and plane. To catch corner cases, specify which side you prefer.
// The function then generates a hit only if the end is beyond a certain margin in that direction, filtering out
// "very close to plane" ghost hits as long as start and end stay directly on or within the given plane side.
bool IntersectSegmentPlane(const IfcVector3& p,const IfcVector3& n, const IfcVector3& e0, 
	const IfcVector3& e1, bool assumeStartOnWhiteSide, IfcVector3& out)
{
	const IfcVector3 pdelta = e0 - p, seg = e1 - e0;
	const IfcFloat dotOne = n*seg, dotTwo = -(n*pdelta);

	// if segment ends on plane, do not report a hit. We stay on that side until a following segment starting at this
	// point leaves the plane through the other side
	if( std::abs(dotOne + dotTwo) < 1e-6 )
		return false;

	// if segment starts on the plane, report a hit only if the end lies on the *other* side
	if( std::abs(dotTwo) < 1e-6 )
	{
		if( (assumeStartOnWhiteSide && dotOne + dotTwo < 1e-6) || (!assumeStartOnWhiteSide && dotOne + dotTwo > -1e-6) )
		{
			out = e0;
			return true;
		}
		else
		{
			return false;
		}
	}

	// ignore if segment is parallel to plane and far away from it on either side
	// Warning: if there's a few thousand of such segments which slowly accumulate beyond the epsilon, no hit would be registered
	if( std::abs(dotOne) < 1e-6 )
		return false;

	// t must be in [0..1] if the intersection point is within the given segment
	const IfcFloat t = dotTwo / dotOne;
	if( t > 1.0 || t < 0.0 )
		return false;

	out = e0 + t*seg;
	return true;
}

// ------------------------------------------------------------------------------------------------
void FilterPolygon(std::vector<IfcVector3>& resultpoly)
{
	if( resultpoly.size() < 3 )
	{
		resultpoly.clear();
		return;
	}

	IfcVector3 vmin, vmax;
	ArrayBounds(resultpoly.data(), resultpoly.size(), vmin, vmax);

	// filter our IfcFloat points - those may happen if a point lies
	// directly on the intersection line or directly on the clipping plane
	const IfcFloat epsilon = (vmax - vmin).SquareLength() / 1e6f;
	FuzzyVectorCompare fz(epsilon);
	std::vector<IfcVector3>::iterator e = std::unique(resultpoly.begin(), resultpoly.end(), fz);

	if( e != resultpoly.end() )
		resultpoly.erase(e, resultpoly.end());

	if( !resultpoly.empty() && fz(resultpoly.front(), resultpoly.back()) )
		resultpoly.pop_back();
}

// ------------------------------------------------------------------------------------------------
void WritePolygon(std::vector<IfcVector3>& resultpoly, TempMesh& result)
{
	FilterPolygon(resultpoly);

	if( resultpoly.size() > 2 )
	{
		result.verts.insert(result.verts.end(), resultpoly.begin(), resultpoly.end());
		result.vertcnt.push_back(resultpoly.size());
	}
}


// ------------------------------------------------------------------------------------------------
void ProcessBooleanHalfSpaceDifference(const IfcHalfSpaceSolid* hs, TempMesh& result,
	const TempMesh& first_operand,
	ConversionData& /*conv*/)
{
	ai_assert(hs != NULL);

	const IfcPlane* const plane = hs->BaseSurface->ToPtr<IfcPlane>();
	if(!plane) {
		IFCImporter::LogError("expected IfcPlane as base surface for the IfcHalfSpaceSolid");
		return;
	}

	// extract plane base position vector and normal vector
	IfcVector3 p,n(0.f,0.f,1.f);
	if (plane->Position->Axis) {
		ConvertDirection(n,plane->Position->Axis.Get());
	}
	ConvertCartesianPoint(p,plane->Position->Location);

	if(!IsTrue(hs->AgreementFlag)) {
		n *= -1.f;
	}

	// clip the current contents of `meshout` against the plane we obtained from the second operand
	const std::vector<IfcVector3>& in = first_operand.verts;
	std::vector<IfcVector3>& outvert = result.verts;

	std::vector<unsigned int>::const_iterator begin = first_operand.vertcnt.begin(), 
		end = first_operand.vertcnt.end(), iit;

	outvert.reserve(in.size());
	result.vertcnt.reserve(first_operand.vertcnt.size());

	unsigned int vidx = 0;
	for(iit = begin; iit != end; vidx += *iit++) {

		unsigned int newcount = 0;
		bool isAtWhiteSide = (in[vidx] - p) * n > -1e-6;
		for( unsigned int i = 0; i < *iit; ++i ) {
			const IfcVector3& e0 = in[vidx + i], e1 = in[vidx + (i + 1) % *iit];

			// does the next segment intersect the plane?
			IfcVector3 isectpos;
			if( IntersectSegmentPlane(p, n, e0, e1, isAtWhiteSide, isectpos) ) {
				if( isAtWhiteSide ) {
					// e0 is on the right side, so keep it 
					outvert.push_back(e0);
					outvert.push_back(isectpos);
					newcount += 2;
				}
				else {
					// e0 is on the wrong side, so drop it and keep e1 instead
					outvert.push_back(isectpos);
					++newcount;
				}
				isAtWhiteSide = !isAtWhiteSide;
			}
			else
			{
				if( isAtWhiteSide ) {
					outvert.push_back(e0);
					++newcount;
				}
			}
		}

		if (!newcount) {
			continue;
		}

		IfcVector3 vmin,vmax;
		ArrayBounds(&*(outvert.end()-newcount),newcount,vmin,vmax);

		// filter our IfcFloat points - those may happen if a point lies
		// directly on the intersection line. However, due to IfcFloat
		// precision a bitwise comparison is not feasible to detect
		// this case.
		const IfcFloat epsilon = (vmax-vmin).SquareLength() / 1e6f;
		FuzzyVectorCompare fz(epsilon);

		std::vector<IfcVector3>::iterator e = std::unique( outvert.end()-newcount, outvert.end(), fz );

		if (e != outvert.end()) {
			newcount -= static_cast<unsigned int>(std::distance(e,outvert.end()));
			outvert.erase(e,outvert.end());
		}
		if (fz(*( outvert.end()-newcount),outvert.back())) {
			outvert.pop_back();
			--newcount;
		}
		if(newcount > 2) {
			result.vertcnt.push_back(newcount);
		}
		else while(newcount-->0) {
			result.verts.pop_back();
		}

	}
	IFCImporter::LogDebug("generating CSG geometry by plane clipping (IfcBooleanClippingResult)");
}

// ------------------------------------------------------------------------------------------------
// Check if e0-e1 intersects a sub-segment of the given boundary line.
// note: this functions works on 3D vectors, but performs its intersection checks solely in xy.
// New version takes the supposed inside/outside state as a parameter and treats corner cases as if
// the line stays on that side. This should make corner cases more stable.
// Two million assumptions! Boundary should have all z at 0.0, will be treated as closed, should not have
// segments with length <1e-6, self-intersecting might break the corner case handling... just don't go there, ok?
bool IntersectsBoundaryProfile(const IfcVector3& e0, const IfcVector3& e1, const std::vector<IfcVector3>& boundary,
	const bool isStartAssumedInside, std::vector<std::pair<size_t, IfcVector3> >& intersect_results,
	const bool halfOpen = false)
{
	ai_assert(intersect_results.empty());

	// determine winding order - necessary to detect segments going "inwards" or "outwards" from a point directly on the border
	// positive sum of angles means clockwise order when looking down the -Z axis
	IfcFloat windingOrder = 0.0;
	for( size_t i = 0, bcount = boundary.size(); i < bcount; ++i ) {
		IfcVector3 b01 = boundary[(i + 1) % bcount] - boundary[i];
		IfcVector3 b12 = boundary[(i + 2) % bcount] - boundary[(i + 1) % bcount];
		IfcVector3 b1_side = IfcVector3(b01.y, -b01.x, 0.0); // rotated 90<39> clockwise in Z plane
		// Warning: rough estimate only. A concave poly with lots of small segments each featuring a small counter rotation
		// could fool the accumulation. Correct implementation would be sum( acos( b01 * b2) * sign( b12 * b1_side))
		windingOrder += (b1_side.x*b12.x + b1_side.y*b12.y);
	}
	windingOrder = windingOrder > 0.0 ? 1.0 : -1.0;

	const IfcVector3 e = e1 - e0;

	for( size_t i = 0, bcount = boundary.size(); i < bcount; ++i ) {
		// boundary segment i: b0-b1
		const IfcVector3& b0 = boundary[i];
		const IfcVector3& b1 = boundary[(i + 1) % bcount];
		IfcVector3 b = b1 - b0;
		IfcFloat b_sqlen_inv = 1.0 / b.SquareLength();

		// segment-segment intersection
		// solve b0 + b*s = e0 + e*t for (s,t)
		const IfcFloat det = (-b.x * e.y + e.x * b.y);
		if( std::abs(det) < 1e-6 ) {
			// no solutions (parallel lines)
			continue;
		}

		const IfcFloat x = b0.x - e0.x;
		const IfcFloat y = b0.y - e0.y;
		const IfcFloat s = (x*e.y - e.x*y) / det; // scale along boundary edge
		const IfcFloat t = (x*b.y - b.x*y) / det; // scale along given segment
		const IfcVector3 p = e0 + e*t;
#ifdef ASSIMP_BUILD_DEBUG
		const IfcVector3 check = b0 + b*s - p;
		ai_assert((IfcVector2(check.x, check.y)).SquareLength() < 1e-5);
#endif

		// also calculate the distance of e0 and e1 to the segment. We need to detect the "starts directly on segment" 
		// and "ends directly at segment" cases
		bool startsAtSegment, endsAtSegment;
		{
			// calculate closest point to each end on the segment, clamp that point to the segment's length, then check
			// distance to that point. This approach is like testing if e0 is inside a capped cylinder.
			IfcFloat et0 = (b.x*(e0.x - b0.x) + b.y*(e0.y - b0.y)) * b_sqlen_inv;
			IfcVector3 closestPosToE0OnBoundary = b0 + std::max(IfcFloat(0.0), std::min(IfcFloat(1.0), et0)) * b;
			startsAtSegment = (closestPosToE0OnBoundary - IfcVector3(e0.x, e0.y, 0.0)).SquareLength() < 1e-12;
			IfcFloat et1 = (b.x*(e1.x - b0.x) + b.y*(e1.y - b0.y)) * b_sqlen_inv;
			IfcVector3 closestPosToE1OnBoundary = b0 + std::max(IfcFloat(0.0), std::min(IfcFloat(1.0), et1)) * b;
			endsAtSegment = (closestPosToE1OnBoundary - IfcVector3(e1.x, e1.y, 0.0)).SquareLength() < 1e-12;
		}

		// Line segment ends at boundary -> ignore any hit, it will be handled by possibly following segments
		if( endsAtSegment && !halfOpen )
			continue;

		// Line segment starts at boundary -> generate a hit only if following that line would change the INSIDE/OUTSIDE
		// state. This should catch the case where a connected set of segments has a point directly on the boundary,
		// one segment not hitting it because it ends there and the next segment not hitting it because it starts there
		// Should NOT generate a hit if the segment only touches the boundary but turns around and stays inside.
		if( startsAtSegment )
		{
			IfcVector3 inside_dir = IfcVector3(b.y, -b.x, 0.0) * windingOrder;
			bool isGoingInside = (inside_dir * e) > 0.0;
			if( isGoingInside == isStartAssumedInside )
				continue;

			// only insert the point into the list if it is sufficiently far away from the previous intersection point. 
			// This way, we avoid duplicate detection if the intersection is directly on the vertex between two segments.
			if( !intersect_results.empty() && intersect_results.back().first == i - 1 )
			{
				const IfcVector3 diff = intersect_results.back().second - e0;
				if( IfcVector2(diff.x, diff.y).SquareLength() < 1e-10 )
					continue;
			}
			intersect_results.push_back(std::make_pair(i, e0));
			continue;
		}

		// for a valid intersection, s and t should be in range [0,1]. Including a bit of epsilon on s, potential double
		// hits on two consecutive boundary segments are filtered
		if( s >= -1e-6 * b_sqlen_inv && s <= 1.0 + 1e-6*b_sqlen_inv && t >= 0.0 && (t <= 1.0 || halfOpen) )
		{
			// only insert the point into the list if it is sufficiently far away from the previous intersection point. 
			// This way, we avoid duplicate detection if the intersection is directly on the vertex between two segments.
			if( !intersect_results.empty() && intersect_results.back().first == i - 1 )
			{
				const IfcVector3 diff = intersect_results.back().second - p;
				if( IfcVector2(diff.x, diff.y).SquareLength() < 1e-10 )
					continue;
			}
			intersect_results.push_back(std::make_pair(i, p));
		}
	}

	return !intersect_results.empty();
}


// ------------------------------------------------------------------------------------------------
// note: this functions works on 3D vectors, but performs its intersection checks solely in xy.
bool PointInPoly(const IfcVector3& p, const std::vector<IfcVector3>& boundary)
{
	// even-odd algorithm: take a random vector that extends from p to infinite
	// and counts how many times it intersects edges of the boundary.
	// because checking for segment intersections is prone to numeric inaccuracies
	// or double detections (i.e. when hitting multiple adjacent segments at their
	// shared vertices) we do it thrice with different rays and vote on it.

	// the even-odd algorithm doesn't work for points which lie directly on
	// the border of the polygon. If any of our attempts produces this result,
	// we return false immediately.

	std::vector<std::pair<size_t, IfcVector3> > intersected_boundary;
	size_t votes = 0;

	IntersectsBoundaryProfile(p, p + IfcVector3(1.0, 0, 0), boundary, true, intersected_boundary, true);
	votes += intersected_boundary.size() % 2;

	intersected_boundary.clear();
	IntersectsBoundaryProfile(p, p + IfcVector3(0, 1.0, 0), boundary, true, intersected_boundary, true);
	votes += intersected_boundary.size() % 2;

	intersected_boundary.clear();
	IntersectsBoundaryProfile(p, p + IfcVector3(0.6, -0.6, 0.0), boundary, true, intersected_boundary, true);
	votes += intersected_boundary.size() % 2;

//	ai_assert(votes == 3 || votes == 0);
	return votes > 1;
}


// ------------------------------------------------------------------------------------------------
void ProcessPolygonalBoundedBooleanHalfSpaceDifference(const IfcPolygonalBoundedHalfSpace* hs, TempMesh& result, 
													   const TempMesh& first_operand, 
													   ConversionData& conv)
{
	ai_assert(hs != NULL);

	const IfcPlane* const plane = hs->BaseSurface->ToPtr<IfcPlane>();
	if(!plane) {
		IFCImporter::LogError("expected IfcPlane as base surface for the IfcHalfSpaceSolid");
		return;
	}

	// extract plane base position vector and normal vector
	IfcVector3 p,n(0.f,0.f,1.f);
	if (plane->Position->Axis) {
		ConvertDirection(n,plane->Position->Axis.Get());
	}
	ConvertCartesianPoint(p,plane->Position->Location);

	if(!IsTrue(hs->AgreementFlag)) {
		n *= -1.f;
	}

	n.Normalize();

	// obtain the polygonal bounding volume
	boost::shared_ptr<TempMesh> profile = boost::shared_ptr<TempMesh>(new TempMesh());
	if(!ProcessCurve(hs->PolygonalBoundary, *profile.get(), conv)) {
		IFCImporter::LogError("expected valid polyline for boundary of boolean halfspace");
		return;
	}

	// determine winding order by calculating the normal.
	IfcVector3 profileNormal = TempMesh::ComputePolygonNormal(profile->verts.data(), profile->verts.size());

	IfcMatrix4 proj_inv;
	ConvertAxisPlacement(proj_inv,hs->Position);

	// and map everything into a plane coordinate space so all intersection
	// tests can be done in 2D space.
	IfcMatrix4 proj = proj_inv;
	proj.Inverse();

	// clip the current contents of `meshout` against the plane we obtained from the second operand
	const std::vector<IfcVector3>& in = first_operand.verts;
	std::vector<IfcVector3>& outvert = result.verts;
	std::vector<unsigned int>& outvertcnt = result.vertcnt;

	outvert.reserve(in.size());
	outvertcnt.reserve(first_operand.vertcnt.size());

	unsigned int vidx = 0;
	std::vector<unsigned int>::const_iterator begin = first_operand.vertcnt.begin();
	std::vector<unsigned int>::const_iterator end = first_operand.vertcnt.end();
	std::vector<unsigned int>::const_iterator iit;
	for( iit = begin; iit != end; vidx += *iit++ )
	{
		// Our new approach: we cut the poly along the plane, then we intersect the part on the black side of the plane 
		// against the bounding polygon. All the white parts, and the black part outside the boundary polygon, are kept.
		std::vector<IfcVector3> whiteside, blackside;

		{
			const IfcVector3* srcVertices = &in[vidx];
			const size_t srcVtxCount = *iit;
			if( srcVtxCount == 0 )
				continue;

			IfcVector3 polyNormal = TempMesh::ComputePolygonNormal(srcVertices, srcVtxCount, true);

			// if the poly is parallel to the plane, put it completely on the black or white side
			if( std::abs(polyNormal * n) > 0.9999 )
			{
				bool isOnWhiteSide = (srcVertices[0] - p) * n > -1e-6;
				std::vector<IfcVector3>& targetSide = isOnWhiteSide ? whiteside : blackside;
				targetSide.insert(targetSide.end(), srcVertices, srcVertices + srcVtxCount);
			}
			else
			{
				// otherwise start building one polygon for each side. Whenever the current line segment intersects the plane
				// we put a point there as an end of the current segment. Then we switch to the other side, put a point there, too,
				// as a beginning of the current segment, and simply continue accumulating vertices. 
				bool isCurrentlyOnWhiteSide = ((srcVertices[0]) - p) * n > -1e-6;
				for( size_t a = 0; a < srcVtxCount; ++a )
				{
					IfcVector3 e0 = srcVertices[a];
					IfcVector3 e1 = srcVertices[(a + 1) % srcVtxCount];
					IfcVector3 ei;

					// put starting point to the current mesh
					std::vector<IfcVector3>& trgt = isCurrentlyOnWhiteSide ? whiteside : blackside;
					trgt.push_back(srcVertices[a]);

					// if there's an intersection, put an end vertex there, switch to the other side's mesh,
					// and add a starting vertex there, too
					bool isPlaneHit = IntersectSegmentPlane(p, n, e0, e1, isCurrentlyOnWhiteSide, ei);
					if( isPlaneHit )
					{
						if( trgt.empty() || (trgt.back() - ei).SquareLength() > 1e-12 )
							trgt.push_back(ei);
						isCurrentlyOnWhiteSide = !isCurrentlyOnWhiteSide;
						std::vector<IfcVector3>& newtrgt = isCurrentlyOnWhiteSide ? whiteside : blackside;
						newtrgt.push_back(ei);
					}
				}
			}
		}

		// the part on the white side can be written into the target mesh right away
		WritePolygon(whiteside, result);

		// The black part is the piece we need to get rid of, but only the part of it within the boundary polygon.
		// So we now need to construct all the polygons that result from BlackSidePoly minus BoundaryPoly.
		FilterPolygon(blackside);

		// Complicated, II. We run along the polygon. a) When we're inside the boundary, we run on until we hit an 
		// intersection, which means we're leaving it. We then start a new out poly there. b) When we're outside the 
		// boundary, we start collecting vertices until we hit an intersection, then we run along the boundary until we hit 
		// an intersection, then we switch back to the poly and run on on this one again, and so on until we got a closed 
		// loop. Then we continue with the path we left to catch potential additional polys on the other side of the 
		// boundary as described in a)
		if( !blackside.empty() )
		{
			// poly edge index, intersection point, edge index in boundary poly
			std::vector<boost::tuple<size_t, IfcVector3, size_t> > intersections;
			bool startedInside = PointInPoly(proj * blackside.front(), profile->verts);
			bool isCurrentlyInside = startedInside;

			std::vector<std::pair<size_t, IfcVector3> > intersected_boundary;

			for( size_t a = 0; a < blackside.size(); ++a )
			{
				const IfcVector3 e0 = proj * blackside[a];
				const IfcVector3 e1 = proj * blackside[(a + 1) % blackside.size()];

				intersected_boundary.clear();
				IntersectsBoundaryProfile(e0, e1, profile->verts, isCurrentlyInside, intersected_boundary);
				// sort the hits by distance from e0 to get the correct in/out/in sequence. Manually :-( I miss you, C++11.
				if( intersected_boundary.size() > 1 )
				{
					bool keepSorting = true;
					while( keepSorting )
					{
						keepSorting = false;
						for( size_t b = 0; b < intersected_boundary.size() - 1; ++b )
						{
							if( (intersected_boundary[b + 1].second - e0).SquareLength() < (intersected_boundary[b].second - e0).SquareLength() )
							{
								keepSorting = true;
								std::swap(intersected_boundary[b + 1], intersected_boundary[b]);
							}
						}
					}
				}
				// now add them to the list of intersections
				for( size_t b = 0; b < intersected_boundary.size(); ++b )
					intersections.push_back(boost::make_tuple(a, proj_inv * intersected_boundary[b].second, intersected_boundary[b].first));

				// and calculate our new inside/outside state
				if( intersected_boundary.size() & 1 )
					isCurrentlyInside = !isCurrentlyInside;
			}

			// we got a list of in-out-combinations of intersections. That should be an even number of intersections, or
			// we're fucked.
			if( (intersections.size() & 1) != 0 )
			{
				IFCImporter::LogWarn("Odd number of intersections, can't work with that. Omitting half space boundary check.");
				continue;
			}

			if( intersections.size() > 1 )
			{
				// If we started outside, the first intersection is a out->in intersection. Cycle them so that it
				// starts with an intersection leaving the boundary
				if( !startedInside )
				for( size_t b = 0; b < intersections.size() - 1; ++b )
					std::swap(intersections[b], intersections[(b + intersections.size() - 1) % intersections.size()]);

				// Filter pairs of out->in->out that lie too close to each other. 
				for( size_t a = 0; intersections.size() > 0 && a < intersections.size() - 1; /**/ )
				{
					if( (intersections[a].get<1>() - intersections[(a + 1) % intersections.size()].get<1>()).SquareLength() < 1e-10 )
						intersections.erase(intersections.begin() + a, intersections.begin() + a + 2);
					else
						a++;
				}
				if( intersections.size() > 1 && (intersections.back().get<1>() - intersections.front().get<1>()).SquareLength() < 1e-10 )
				{
					intersections.pop_back(); intersections.erase(intersections.begin());
				}
			}


			// no intersections at all: either completely inside the boundary, so everything gets discarded, or completely outside.
			// in the latter case we're implementional lost. I'm simply going to ignore this, so a large poly will not get any
			// holes if the boundary is smaller and does not touch it anywhere.
			if( intersections.empty() )
			{
				// starting point was outside -> everything is outside the boundary -> nothing is clipped -> add black side 
				// to result mesh unchanged
				if( !startedInside )
				{
					outvertcnt.push_back(blackside.size());
					outvert.insert(outvert.end(), blackside.begin(), blackside.end());
					continue;
				}
				else
				{
					// starting point was inside the boundary -> everything is inside the boundary -> nothing is spared from the 
					// clipping -> nothing left to add to the result mesh
					continue;
				}
			}

			// determine the direction in which we're marching along the boundary polygon. If the src poly is faced upwards
			// and the boundary is also winded this way, we need to march *backwards* on the boundary.
			const IfcVector3 polyNormal = IfcMatrix3(proj) * TempMesh::ComputePolygonNormal(blackside.data(), blackside.size());
			bool marchBackwardsOnBoundary = (profileNormal * polyNormal) >= 0.0;

			// Build closed loops from these intersections. Starting from an intersection leaving the boundary we
			// walk along the polygon to the next intersection (which should be an IS entering the boundary poly).
			// From there we walk along the boundary until we hit another intersection leaving the boundary,
			// walk along the poly to the next IS and so on until we're back at the starting point.
			// We remove every intersection we "used up", so any remaining intersection is the start of a new loop.
			while( !intersections.empty() )
			{
				std::vector<IfcVector3> resultpoly;
				size_t currentIntersecIdx = 0;

				while( true )
				{
					ai_assert(intersections.size() > currentIntersecIdx + 1);
					boost::tuple<size_t, IfcVector3, size_t> currintsec = intersections[currentIntersecIdx + 0];
					boost::tuple<size_t, IfcVector3, size_t> nextintsec = intersections[currentIntersecIdx + 1];
					intersections.erase(intersections.begin() + currentIntersecIdx, intersections.begin() + currentIntersecIdx + 2);

					// we start with an in->out intersection
					resultpoly.push_back(currintsec.get<1>());
					// climb along the polygon to the next intersection, which should be an out->in
					size_t numPolyPoints = (currintsec.get<0>() > nextintsec.get<0>() ? blackside.size() : 0)
						+ nextintsec.get<0>() - currintsec.get<0>();
					for( size_t a = 1; a <= numPolyPoints; ++a )
						resultpoly.push_back(blackside[(currintsec.get<0>() + a) % blackside.size()]);
					// put the out->in intersection 
					resultpoly.push_back(nextintsec.get<1>());

					// generate segments along the boundary polygon that lie in the poly's plane until we hit another intersection
					IfcVector3 startingPoint = proj * nextintsec.get<1>();
					size_t currentBoundaryEdgeIdx = (nextintsec.get<2>() + (marchBackwardsOnBoundary ? 1 : 0)) % profile->verts.size();
					size_t nextIntsecIdx = SIZE_MAX;
					while( nextIntsecIdx == SIZE_MAX )
					{
						IfcFloat t = 1e10;

						size_t nextBoundaryEdgeIdx = marchBackwardsOnBoundary ? (currentBoundaryEdgeIdx + profile->verts.size() - 1) : currentBoundaryEdgeIdx + 1;
						nextBoundaryEdgeIdx %= profile->verts.size();
						// vertices of the current boundary segments
						IfcVector3 currBoundaryPoint = profile->verts[currentBoundaryEdgeIdx];
						IfcVector3 nextBoundaryPoint = profile->verts[nextBoundaryEdgeIdx];
						// project the two onto the polygon
						if( std::abs(polyNormal.z) > 1e-5 )
						{
							currBoundaryPoint.z = startingPoint.z + (currBoundaryPoint.x - startingPoint.x) * polyNormal.x/polyNormal.z + (currBoundaryPoint.y - startingPoint.y) * polyNormal.y/polyNormal.z;
							nextBoundaryPoint.z = startingPoint.z + (nextBoundaryPoint.x - startingPoint.x) * polyNormal.x/polyNormal.z + (nextBoundaryPoint.y - startingPoint.y) * polyNormal.y/polyNormal.z;
						}

						// build a direction that goes along the boundary border but lies in the poly plane
						IfcVector3 boundaryPlaneNormal = ((nextBoundaryPoint - currBoundaryPoint) ^ profileNormal).Normalize();
						IfcVector3 dirAtPolyPlane = (boundaryPlaneNormal ^ polyNormal).Normalize() * (marchBackwardsOnBoundary ? -1.0 : 1.0);
						// if we can project the direction to the plane, we can calculate a maximum marching distance along that dir
						// until we finish that boundary segment and continue on the next 
						if( std::abs(polyNormal.z) > 1e-5 )
						{
							t = std::min(t, (nextBoundaryPoint - startingPoint).Length());
						}

						// check if the direction hits the loop start - if yes, we got a poly to output
						IfcVector3 dirToThatPoint = proj * resultpoly.front() - startingPoint;
						IfcFloat tpt = dirToThatPoint * dirAtPolyPlane;
						if( tpt > -1e-6 && tpt <= t && (dirToThatPoint - tpt * dirAtPolyPlane).SquareLength() < 1e-10 )
						{
							nextIntsecIdx = intersections.size(); // dirty hack to end marching along the boundary and signal the end of the loop
							t = tpt;
						}

						// also check if the direction hits any in->out intersections earlier. If we hit one, we can switch back
						// to marching along the poly border from that intersection point
						for( size_t a = 0; a < intersections.size(); a += 2 )
						{
							dirToThatPoint = proj * intersections[a].get<1>() - startingPoint;
							tpt = dirToThatPoint * dirAtPolyPlane;
							if( tpt > -1e-6 && tpt <= t && (dirToThatPoint - tpt * dirAtPolyPlane).SquareLength() < 1e-10 )
							{
								nextIntsecIdx = a; // switch back to poly and march on from this in->out intersection
								t = tpt;
							}
						}

						// if we keep marching on the boundary, put the segment end point to the result poly and well... keep marching
						if( nextIntsecIdx == SIZE_MAX )
						{
							resultpoly.push_back(proj_inv * nextBoundaryPoint);
							currentBoundaryEdgeIdx = nextBoundaryEdgeIdx;
							startingPoint = nextBoundaryPoint;
						}

						// quick endless loop check
						if( resultpoly.size() > blackside.size() + profile->verts.size() )
						{
							IFCImporter::LogError("Encountered endless loop while clipping polygon against poly-bounded half space.");
							break;
						}
					}

					// we're back on the poly - if this is the intersection we started from, we got a closed loop.
					if( nextIntsecIdx >= intersections.size() )
					{
						break;
					}

					// otherwise it's another intersection. Continue marching from there.
					currentIntersecIdx = nextIntsecIdx;
				}

				WritePolygon(resultpoly, result);
			}
		}
	}
	IFCImporter::LogDebug("generating CSG geometry by plane clipping with polygonal bounding (IfcBooleanClippingResult)");	
}

// ------------------------------------------------------------------------------------------------
void ProcessBooleanExtrudedAreaSolidDifference(const IfcExtrudedAreaSolid* as, TempMesh& result, 
											   const TempMesh& first_operand, 
											   ConversionData& conv)
{
	ai_assert(as != NULL);

	// This case is handled by reduction to an instance of the quadrify() algorithm.
	// Obviously, this won't work for arbitrarily complex cases. In fact, the first
	// operand should be near-planar. Luckily, this is usually the case in Ifc 
	// buildings.

	boost::shared_ptr<TempMesh> meshtmp = boost::shared_ptr<TempMesh>(new TempMesh());
	ProcessExtrudedAreaSolid(*as,*meshtmp,conv,false);

	std::vector<TempOpening> openings(1, TempOpening(as,IfcVector3(0,0,0),meshtmp,boost::shared_ptr<TempMesh>()));

	result = first_operand;

	TempMesh temp;

	std::vector<IfcVector3>::const_iterator vit = first_operand.verts.begin();
	BOOST_FOREACH(unsigned int pcount, first_operand.vertcnt) {
		temp.Clear();

		temp.verts.insert(temp.verts.end(), vit, vit + pcount);
		temp.vertcnt.push_back(pcount);

		// The algorithms used to generate mesh geometry sometimes
		// spit out lines or other degenerates which must be
		// filtered to avoid running into assertions later on.

		// ComputePolygonNormal returns the Newell normal, so the
		// length of the normal is the area of the polygon.
		const IfcVector3& normal = temp.ComputeLastPolygonNormal(false);
		if (normal.SquareLength() < static_cast<IfcFloat>(1e-5)) {
			IFCImporter::LogWarn("skipping degenerate polygon (ProcessBooleanExtrudedAreaSolidDifference)");
			continue;
		}

		GenerateOpenings(openings, std::vector<IfcVector3>(1,IfcVector3(1,0,0)), temp, false, true);
		result.Append(temp);

		vit += pcount;
	}

	IFCImporter::LogDebug("generating CSG geometry by geometric difference to a solid (IfcExtrudedAreaSolid)");
}

// ------------------------------------------------------------------------------------------------
void ProcessBoolean(const IfcBooleanResult& boolean, TempMesh& result, ConversionData& conv)
{
	// supported CSG operations:
	//   DIFFERENCE
	if(const IfcBooleanResult* const clip = boolean.ToPtr<IfcBooleanResult>()) {
		if(clip->Operator != "DIFFERENCE") {
			IFCImporter::LogWarn("encountered unsupported boolean operator: " + (std::string)clip->Operator);
			return;
		}

		// supported cases (1st operand):
		//  IfcBooleanResult -- call ProcessBoolean recursively
		//  IfcSweptAreaSolid -- obtain polygonal geometry first

		// supported cases (2nd operand):
		//  IfcHalfSpaceSolid -- easy, clip against plane
		//  IfcExtrudedAreaSolid -- reduce to an instance of the quadrify() algorithm


		const IfcHalfSpaceSolid* const hs = clip->SecondOperand->ResolveSelectPtr<IfcHalfSpaceSolid>(conv.db);
		const IfcExtrudedAreaSolid* const as = clip->SecondOperand->ResolveSelectPtr<IfcExtrudedAreaSolid>(conv.db);
		if(!hs && !as) {
			IFCImporter::LogError("expected IfcHalfSpaceSolid or IfcExtrudedAreaSolid as second clipping operand");
			return;
		}

		TempMesh first_operand;
		if(const IfcBooleanResult* const op0 = clip->FirstOperand->ResolveSelectPtr<IfcBooleanResult>(conv.db)) {
			ProcessBoolean(*op0,first_operand,conv);
		}
		else if (const IfcSweptAreaSolid* const swept = clip->FirstOperand->ResolveSelectPtr<IfcSweptAreaSolid>(conv.db)) {
			ProcessSweptAreaSolid(*swept,first_operand,conv);
		}
		else {
			IFCImporter::LogError("expected IfcSweptAreaSolid or IfcBooleanResult as first clipping operand");
			return;
		}

		if(hs) {

			const IfcPolygonalBoundedHalfSpace* const hs_bounded = clip->SecondOperand->ResolveSelectPtr<IfcPolygonalBoundedHalfSpace>(conv.db);
			if (hs_bounded) {
				ProcessPolygonalBoundedBooleanHalfSpaceDifference(hs_bounded, result, first_operand, conv);
			}
			else {
				ProcessBooleanHalfSpaceDifference(hs, result, first_operand, conv);
			}
		}
		else {
			ProcessBooleanExtrudedAreaSolidDifference(as, result, first_operand, conv);
		}
	}
	else {
		IFCImporter::LogWarn("skipping unknown IfcBooleanResult entity, type is " + boolean.GetClassName());
	}
}

} // ! IFC
} // ! Assimp

#endif