   orthogonal basis of matrix covariance
  hi everyone,I'm implementing some OBB in my physics engine.When i compute a sphere OBB, or a cube OBB, the basis is orthogonal, but when the body has another shape, the basis is not orthogonal.Is there a problem in my computing ?or is it normal ?(to resolve this pb i've taken the cross product of the 2 others axis, but is there another way ?)thanks   
well i found 5 seconds later an idea:Must i calculate the eigenvectors of the matrix ?;  well, i think i've found the solution:i've computed the jacobi matrix of the covariance matrix, and the results are good.if someone can give me a confirmation, or a proposition ?