From 21d39d8a2333d513b9180495cdf2c20385bc196e Mon Sep 17 00:00:00 2001
From: Thomas Krijnen <t.krijnen@gmail.com>
Date: Fri, 18 Apr 2025 10:08:57 +0200
Subject: [PATCH] Use exact topology points at parametric range bounds to
 guarantee welding #4487

---
 src/ifcgeom/IfcGeomRepresentation.cpp         |  5 +++-
 .../OpenCascadeConversionResult.cpp           | 28 ++++++++++++++++++-
 2 files changed, 31 insertions(+), 2 deletions(-)

diff --git a/src/ifcgeom/IfcGeomRepresentation.cpp b/src/ifcgeom/IfcGeomRepresentation.cpp
index c888efba2a..2c1983108e 100644
--- a/src/ifcgeom/IfcGeomRepresentation.cpp
+++ b/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;
 	}
diff --git a/src/ifcgeom/kernels/opencascade/OpenCascadeConversionResult.cpp b/src/ifcgeom/kernels/opencascade/OpenCascadeConversionResult.cpp
index 32f90904a1..1382f1fdc9 100644
--- a/src/ifcgeom/kernels/opencascade/OpenCascadeConversionResult.cpp
+++ b/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);