# Inertia matrix

Hi,

I want to calculate the inertia matrix of a cube. It is calculated this way :

// Cube creation
gp_Pnt Pnt1(-5,-5,-5);
gp_Pnt Pnt2(5,5,5);

BRepPrimAPI_MakeBox MakeBox(Pnt1, Pnt2);
TopoDS_Shell myBlock = MakeBox.Shell();
// The cube is created

// We get back the barycenter of the cube
gp_Pnt origin;

GProp_GProps Props;
BRepGProp::VolumeProperties(myBlock, Props);
origin = Props.CentreOfMass();
// We have the barycenter

// We get back the Inertia matrix
gp_Mat Mat;

GProp_GProps VProps(origin);
BRepGProp::VolumeProperties(myBlock, VProps);
Mat = VProps.MatrixOfInertia();

The matrix I get back is diagonal. But it never changes even if I apply a rotation on the cube. The matrix is always diagonal. Is it accurate ?

Unlike the cube, the inertia matrix of a cylinder is not always diagonal. It changes if the cylinder rotates.

Have I forget something ? Do I an error somewhere ?

Patrick.

I think you are calculating the Inertia about the center of mass of the cube and this is always the same, even if the cube is rotated. Try the next code:

GProp_GProps Props;
BRepGProp::VolumeProperties(myBlock, Props);
origin = Props.CentreOfMass();

gp_Mat Mat;

Mat = Props.MatrixOfInertia();

Good luck

Thank you very much for your answer. It helps me a lot.

best regards,

Patrick Resch.

Hello
The inertia matrix of a cube never changes when you apply rotations around the center of mass because the terms on the diagonal are equal.
For a cylinder terms are differents on the diagonal and when you compute R(transpose)*I*R the result is a non diagonal matrix.
It is not an error

jerome