Just something I have to do for a class. We read a paper on OBB-tree construction, and there's this equation on how to compute the covariance matrix for computing the OBB principal axes. Like so many papers that are published, they only give the result without the derivation (wouldn't it be nice if the derivation is given also?) To verify the correctness, I have to sit down and work out the math ... derivation of covariance matrix for OBB construction.