Add matrix of inertia

cadquery/occ_impl/shapes.py

@@ -631,6 +631,22 @@ def _center_of_mass(shape: "Shape") -> Vector:
 
         return Vector(Properties.CentreOfMass())
 
+    @staticmethod
+    def matrixOfInertia(obj: "Shape") -> List[List[float]]:
+        """
+        Calculates the matrix of inertia of an object.
+        :param obj: Compute the matrix of inertia of this object
+        """
+        Properties = GProp_GProps()
+        calc_function = shape_properties_LUT[shapetype(obj.wrapped)]
+
+        if calc_function:
+            calc_function(obj.wrapped, Properties)
+            moi = Properties.MatrixOfInertia()
+            return [[moi.Value(i, j) for j in range(1, 4)] for i in range(1, 4)]
+
+        raise NotImplementedError
+
     def Center(self) -> Vector:
         """
         :returns: The point of the center of mass of this Shape

tests/test_cad_objects.py

@@ -238,6 +238,27 @@ def testFaceWrapperMakePlane(self):
 
         self.assertTupleAlmostEquals((0.0, 0.0, 1.0), mplane.normalAt().toTuple(), 3)
 
+    def testMatrixOfInertia(self):
+        """
+        Tests the calculation of the matrix of inertia for a solid
+        """
+        radius = 1.0
+        height = 2.0
+        cylinder = Solid.makeCylinder(radius=radius, height=height)
+        moi = Shape.matrixOfInertia(cylinder)
+        two_pi = 2 * math.pi
+        true_moi = (
+            two_pi * (radius ** 2 / 4 + height ** 2 / 12),
+            two_pi * (radius ** 2 / 4 + height ** 2 / 12),
+            two_pi * radius ** 2 / 2,
+        )
+        self.assertTupleAlmostEquals((moi[0][0], moi[1][1], moi[2][2]), true_moi, 3)
+
+    def testVertexMatrixOfInertiaNotImplemented(self):
+        with self.assertRaises(NotImplementedError):
+            vertex = Vertex.makeVertex(1, 1, 1)
+            Shape.matrixOfInertia(vertex)
+
     def testCenterOfBoundBox(self):
         pass