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How to rotate a mesh having a rotation matrix?

Hello everyone. This is my second post here, I have recently been using the Qt framework and it's my first time working with 3D graphics.
How can I rotate a mesh having a 3x3 rotation matrix?
I am using Qt for Python and C ++, I haven't used QML yet.Thanks in advance!

You just need to multiply each point of the mesh with the matrix.
Let R be your rotation matrix and P your point, then P_rot = R * P will be the point in the rotated coordinate system.