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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/McpControl.cpp
+12
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@@ -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},
};
}