Use exact topology points at parametric range bounds to guarantee welding #4487

src/ifcgeom/IfcGeomRepresentation.cpp

@@ -397,7 +397,10 @@ int IfcGeom::Representation::Triangulation::addVertex(int item_id, int material_
 	if (settings().get<ifcopenshell::geometry::settings::WeldVertices>().get()) {
 		const VertexKey key = std::make_tuple(item_id, material_index, X, Y, Z);
 		typename VertexKeyMap::const_iterator it = welds.find(key);
-		if (it != welds.end()) return it->second;
+		if (it != welds.end()) {
+			// Return index for previously encountered point
+			return it->second;
+		}
 		i = (int)(welds.size() + weld_offset_);
 		welds[key] = i;
 	}

src/ifcgeom/kernels/opencascade/OpenCascadeConversionResult.cpp

@@ -248,8 +248,34 @@ void ifcopenshell::geometry::OpenCascadeShape::Triangulate(ifcopenshell::geometr
 			int previous = -1;
 			const bool reversed = texp.Value().Orientation() == TopAbs_REVERSED;
 			bool first = true;
+
+			gp_Pnt p0, p1;
+			double u0 = std::numeric_limits<double>::quiet_NaN(), u1 = std::numeric_limits<double>::quiet_NaN();
+			if (auto crv = BRep_Tool::Curve(TopoDS::Edge(texp.Value()), u0, u1)) {
+				TopoDS_Vertex v0, v1;
+				TopExp::Vertices(TopoDS::Edge(texp.Value()), v0, v1, false);
+				if (!v0.IsNull() && !v1.IsNull()) {
+					p0 = BRep_Tool::Pnt(v0);
+					p1 = BRep_Tool::Pnt(v1);
+				} else {
+					u0 = u1 = std::numeric_limits<double>::quiet_NaN();
+				}
+			}
+
 			for (int i = (reversed ? n : 1); reversed ? (i >= 1) : (i <= n); i += reversed ? -1 : 1) {
-				gp_XYZ p = tessellater.Value(i).XYZ();
+				gp_XYZ p;
+				if (std::fabs(tessellater.Parameter(i) - u0) < 1.e-7) {
+					// Use the exact points from the topology when parameter is close to the begin or end of the parametric range
+					// This guarantees points are properly welded, because the GCPnts_QuasiUniformDeflection could otherwise introduce
+					// minor differences between the approximated points from shared vertices.
+					// @todo Using GCPnts_QuasiUniformDeflection on linear edges is pure lazyness
+					p = p0.XYZ();
+				} else if (std::fabs(tessellater.Parameter(i) - u1) < 1.e-7) {
+					p = p1.XYZ();
+				} else {
+					p = tessellater.Value(i).XYZ();
+				}
+				
 				auto p_local = p;
 				taxonomy_transform(place.components_, p);