Project wires onto a plane if non-planar. Fixes #57.

src/ifcgeom/IfcGeom.h

@@ -128,6 +128,8 @@ public:
 	void remove_collinear_points_from_loop(TColgp_SequenceOfPnt& polygon, bool closed, double tol=-1.);
 	bool wire_to_sequence_of_point(const TopoDS_Wire&, TColgp_SequenceOfPnt&);
 	void sequence_of_point_to_wire(const TColgp_SequenceOfPnt&, TopoDS_Wire&, bool closed);
+	bool approximate_plane_through_wire(const TopoDS_Wire&, gp_Pln&);
+	bool flatten_wire(TopoDS_Wire&);
 
 	std::pair<std::string, double> initializeUnits(IfcSchema::IfcUnitAssignment*);
 

src/ifcgeom/IfcGeomFaces.cpp

@@ -202,7 +202,11 @@ bool IfcGeom::Kernel::convert(const IfcSchema::IfcFace* l, TopoDS_Shape& face) {
 			ShapeFix_ShapeTolerance FTol;
 			FTol.SetTolerance(wire, getValue(GV_PRECISION), TopAbs_WIRE);
 
+			bool flattened_wire = false;
+
 			if (!mf) {
+			process_wire:
+
 				if (face_surface.IsNull()) {
 					mf = new BRepBuilderAPI_MakeFace(wire);
 				} else {
@@ -264,9 +268,16 @@ bool IfcGeom::Kernel::convert(const IfcSchema::IfcFace* l, TopoDS_Shape& face) {
 						success = true;
 					}
 				} else {
-					Logger::Message(Logger::LOG_ERROR, "Failed to process face boundary", bound->entity);
+					const bool non_planar = mf->Error() == BRepBuilderAPI_NotPlanar;
 					delete mf;
-					return false;
+					if (!non_planar || flattened_wire || !flatten_wire(wire)) {
+						Logger::Message(Logger::LOG_ERROR, "Failed to process face boundary", bound->entity);
+						return false;
+					} else {
+						Logger::Message(Logger::LOG_ERROR, "Flattening face boundary", bound->entity);
+						flattened_wire = true;
+						goto process_wire;
+					}
 				}
 
 			} else {

src/ifcgeom/IfcGeomFunctions.cpp

@@ -75,6 +75,8 @@
 #include <BRepAlgoAPI_Cut.hxx>
 #include <BRepAlgoAPI_Fuse.hxx>
 
+#include <BRepAlgo_NormalProjection.hxx>
+
 #include <ShapeFix_Shape.hxx>
 #include <ShapeFix_ShapeTolerance.hxx>
 #include <ShapeFix_Solid.hxx>
@@ -94,6 +96,7 @@
 
 #include <BRepMesh_IncrementalMesh.hxx>
 #include <BRepTools.hxx>
+#include <BRepTools_WireExplorer.hxx>
 
 #include <Poly_Triangulation.hxx>
 #include <Poly_Array1OfTriangle.hxx>
@@ -1301,3 +1304,74 @@ const IfcSchema::IfcRepresentationItem* IfcGeom::Kernel::find_item_carrying_styl
 
 	return item;
 }
+
+bool IfcGeom::Kernel::approximate_plane_through_wire(const TopoDS_Wire& wire, gp_Pln& plane) {
+	// Newell's Method is used for the normal calculation
+	// as a simple edge cross product can give opposite results
+	// for a concave face boundary.
+	// Reference: Graphics Gems III p. 231
+
+	double x = 0, y = 0, z = 0;
+	gp_Pnt current, previous, first;
+	gp_XYZ center;
+	int n = 0;
+	
+	BRepTools_WireExplorer exp(wire);
+
+	for (;; exp.Next()) {
+		const bool has_more = exp.More();
+		if (has_more) {
+			const TopoDS_Vertex& v = exp.CurrentVertex();
+			current = BRep_Tool::Pnt(v);
+			center += current.XYZ();
+		} else {
+			current = first;
+		}
+		if (n) {
+			const double& xn = previous.X();
+			const double& yn = previous.Y();
+			const double& zn = previous.Z();
+			const double& xn1 = current.X();
+			const double& yn1 = current.Y();
+			const double& zn1 = current.Z();
+			x += (yn - yn1)*(zn + zn1);
+			y += (xn + xn1)*(zn - zn1);
+			z += (xn - xn1)*(yn + yn1);
+		} else {
+			first = current;
+		}
+		if (!has_more) {
+			break;
+		}
+		previous = current;
+		++n;
+	}
+
+	if (n < 3) {
+		return false;
+	}
+
+	plane = gp_Pln(center / n, gp_Dir(x, y, z));
+	return true;
+}
+
+bool IfcGeom::Kernel::flatten_wire(TopoDS_Wire& wire) {
+	gp_Pln pln;
+	if (!approximate_plane_through_wire(wire, pln)) {
+		return false;
+	}
+	TopoDS_Face face = BRepBuilderAPI_MakeFace(pln).Face();
+	BRepAlgo_NormalProjection proj(face);
+	proj.Add(wire);
+	proj.Build();
+	if (!proj.IsDone()) {
+		return false;
+	}
+	TopTools_ListOfShape list;
+	proj.BuildWire(list);
+	if (list.Extent() != 1) {
+		return false;
+	}
+	wire = TopoDS::Wire(list.First());
+	return true;
+}