CAD: a sheet body reports no volume, instead of a confident wrong one

mass_properties on an open shell returned volume 96000 with an inertia diagonal
of [-4.2e7, -4.2e7, -6.9e7] for a 60x60x40 four-walled box — negative principal
moments, which no real body can have. BRepGProp::VolumeProperties integrates the
divergence theorem over whatever faces exist; on an open shell that is not a
volume at all, and the old code hid the only obvious tell by taking std::abs()
of the mass. "valid: true" then asserted the number was trustworthy.

This matters because mass_properties is what an agent or a user reaches for to
confirm a cut removed the right material. Silent nonsense there means the check
passes on garbage.

MassProps gains is_solid. For a sheet we compute surface area only — that stays
exact — and report volume 0 with the inertia left zeroed. The MCP verb returns
is_solid plus a note saying volume and inertia are not defined for an open
shell; the GUI's Mass command says "sheet body — N cm² of surface, no volume"
rather than quoting material that is not there.

Verified on the rig: the sheet now returns volume 0.0, surface_area 9600.0
(exactly 4 x 60 x 40), is_solid false. The solid controls are unchanged and
exact — a 60 mm cube reports 216000.0 and 21600.0.

Co-Authored-By: Claude Opus 5 (1M context) <noreply@anthropic.com>

src/libslic3r/GeometryEngine.cpp

@@ -470,6 +470,21 @@ GeometryEngine::MassProps GeometryEngine::mass_properties(const TopoDS_Shape& sh
     MassProps p;
     if (shape.IsNull()) return p;
     try {
+        // A sheet body (open shell, no solid) encloses nothing, and BRepGProp::VolumeProperties
+        // integrates the divergence theorem over whatever faces exist — on an open shell that is
+        // not a volume at all. It came back as 96000 with an inertia diagonal of
+        // [-4.2e7, -4.2e7, -6.9e7] for a 60x60x40 four-walled box: negative principal moments,
+        // which no real body can have. The old code then hid the only obvious tell by taking
+        // std::abs() of the mass. Report the honest answer instead — surface area is still
+        // meaningful, so this is not a failure, just not a solid.
+        p.is_solid = TopExp_Explorer(shape, TopAbs_SOLID).More();
+        if (!p.is_solid) {
+            GProp_GProps sonly;
+            BRepGProp::SurfaceProperties(shape, sonly);
+            p.surface_area = sonly.Mass();
+            p.valid = true;          // the area IS trustworthy; volume/inertia stay zero
+            return p;
+        }
         GProp_GProps vprops;
         BRepGProp::VolumeProperties(shape, vprops);
         double mass = vprops.Mass();

src/libslic3r/GeometryEngine.hpp

@@ -100,6 +100,9 @@ public:
         Vec3d  center_of_mass{Vec3d::Zero()};
         std::array<double, 9> inertia{};
         bool   valid{false};
+        // False for a sheet body (an open shell with no solid). Volume and inertia are then
+        // meaningless and are reported as zero; surface_area stays meaningful. See the .cpp.
+        bool   is_solid{false};
     };
     static MassProps mass_properties(const TopoDS_Shape& shape);
 

src/slic3r/GUI/DesignPanel.cpp

@@ -5239,6 +5239,19 @@ void DesignPanel::on_mass_properties()
     wxString name = wxString::Format(_L("Body %d"), m_sel_solid_body + 1);
     if (!m_doc.bodies[m_sel_solid_body].name.empty())
         name = wxString::FromUTF8(m_doc.bodies[m_sel_solid_body].name);
+    if (!mp.is_solid) {
+        // Sheet body: quoting a volume here would be inventing material that is not there.
+        m_status->SetForegroundColour(wxNullColour);
+        set_status(wxString::Format(_L("%s: sheet body — %.2f cm² of surface, no volume"),
+                                    name, mp.surface_area / 100.0));
+        m_status->Refresh();
+        wxMessageBox(wxString::Format(_L("%s\n\nSheet body (open shell)\nSurface area: %.2f cm²\n\n"
+                                         "A sheet encloses no material, so it has no volume. "
+                                         "Thicken it into a solid to get one."),
+                                      name, mp.surface_area / 100.0),
+                     _L("Mass properties"), wxOK, this);
+        return;
+    }
     m_status->SetForegroundColour(wxNullColour);
     set_status(wxString::Format(_L("%s: %.3f cm³, %.2f cm²"),
                                         name, mp.volume / 1000.0, mp.surface_area / 100.0));

src/slic3r/GUI/McpControl.cpp

@@ -556,11 +556,23 @@ json mass_properties(DesignPanel* panel, const json& params)
     const TopoDS_Shape& shape = body_shape(panel, params);
     auto mp = GeometryEngine::mass_properties(shape);
     if (!mp.valid) throw std::runtime_error("mass properties could not be computed (null/empty shape)");
+    // A sheet body has no volume and no inertia. Report the area and say so, rather than
+    // returning numbers a caller would reasonably treat as a material check.
+    if (!mp.is_solid)
+        return json{
+            {"surface_area", mp.surface_area},
+            {"volume", 0.0},
+            {"is_solid", false},
+            {"valid", mp.valid},
+            {"note", "sheet body (open shell): it encloses no material, so volume and inertia "
+                     "are not defined; surface_area is exact"},
+        };
     return json{
         {"volume", mp.volume},
         {"surface_area", mp.surface_area},
         {"center_of_mass", json::array({mp.center_of_mass.x(), mp.center_of_mass.y(), mp.center_of_mass.z()})},
         {"inertia", mp.inertia},
+        {"is_solid", true},
         {"valid", mp.valid},
     };
 }