ifcgeom: don't silently accept implausible empty opening subtractions

When an opening subtraction collapses to an empty shape, the result
passed every post-boolean sanity check (validity, manifoldness,
interference) because those checks are vacuous on an empty compound,
so the element silently disappeared from the output with no log
message. Seen in issue 5779 where hollow pipe members voided by two
non-manifold brep openings at their ends vanished from the converted
glTF.

An empty cut result is now only accepted when the subtraction operands
can plausibly consume the whole first operand, meaning their combined
bounding box encloses it and their total volume is at least as large.
Otherwise the result is discarded, which engages the existing
fuzziness retry and, ultimately, the unsubtracted fallback with the
GEO192 error, matching how other subtraction failures already behave.
Genuinely fully voided elements still resolve to empty output.

Fixes #5779

Generated with the assistance of an AI coding tool.

src/ifcgeom/kernels/opencascade/boolean_utils.cpp

@@ -10,6 +10,7 @@
 #include <TopExp.hxx>
 #include <TopoDS.hxx>
 #include <Bnd_Box.hxx>
+#include <BRepBndLib.hxx>
 #include <Extrema_ExtPC.hxx>
 #include <Geom_Plane.hxx>
 #include <Geom_BSplineCurve.hxx>
@@ -832,6 +833,34 @@ bool IfcGeom::util::points_on_planar_face_generator::operator()(gp_Pnt& p) {
 }
 
 
+namespace {
+	// An empty subtraction result is only plausible when the second operands
+	// can jointly cover the first operand, both in bounding box and in volume. #5779
+	bool empty_cut_result_plausible(double tolerance, const TopoDS_Shape& a, const NCollection_List<TopoDS_Shape>& b) {
+		Bnd_Box A, B;
+		BRepBndLib::Add(a, A);
+		double volume_b = 0.;
+		for (NCollection_List<TopoDS_Shape>::Iterator it(b); it.More(); it.Next()) {
+			BRepBndLib::Add(it.Value(), B);
+			volume_b += std::abs(IfcGeom::util::shape_volume(it.Value()));
+		}
+		if (A.IsVoid() || B.IsVoid()) {
+			return false;
+		}
+		B.Enlarge(tolerance);
+		double axmin, aymin, azmin, axmax, aymax, azmax;
+		double bxmin, bymin, bzmin, bxmax, bymax, bzmax;
+		A.Get(axmin, aymin, azmin, axmax, aymax, azmax);
+		B.Get(bxmin, bymin, bzmin, bxmax, bymax, bzmax);
+		if (!(bxmin <= axmin && bymin <= aymin && bzmin <= azmin &&
+			axmax <= bxmax && aymax <= bymax && azmax <= bzmax)) {
+			return false;
+		}
+		const double volume_a = IfcGeom::util::shape_volume(a);
+		return volume_b >= volume_a * (1. - 1.e-6);
+	}
+}
+
 bool IfcGeom::util::boolean_operation(const boolean_settings& settings, const TopoDS_Shape& a_input, const NCollection_List<TopoDS_Shape>& b_input, BOPAlgo_Operation op, TopoDS_Shape& result, double fuzziness) {
 	using namespace std::string_literals;
 
@@ -1332,7 +1361,10 @@ bool IfcGeom::util::boolean_operation(const boolean_settings& settings, const To
 					int result_n_faces = count(r, TopAbs_FACE);
 					int first_op_n_faces = count(a, TopAbs_FACE);
 
-					if (op == BOPAlgo_CUT && has_open_shells && all_faces_included_in_result && result_n_faces > first_op_n_faces) {
+					if (op == BOPAlgo_CUT && result_n_faces == 0 && first_op_n_faces > 0 && !empty_cut_result_plausible(settings.precision + fuzziness, a, b)) {
+						success = false;
+						settings.log().Notice("GEO", 405, "Empty subtraction result discarded because second operands do not enclose first operand");
+					} else if (op == BOPAlgo_CUT && has_open_shells && all_faces_included_in_result && result_n_faces > first_op_n_faces) {
 						success = false;
 						settings.log().Notice("GEO", 149, "Boolean result discarded because subtractions results in only the addition of faces");
 					} else {