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

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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>
This commit is contained in:
Tommaso Bianchi
2026-08-14 13:35:03 +02:00
co-authored by Claude Opus 5
parent 1545fb7946
commit 9013f530fa
4 changed files with 43 additions and 0 deletions
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src/slic3r/GUI/DesignPanel.cpp
+13
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@@ -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));