SVD computing different result in Matlab and OpenCV

Question

I wonder why there is sign difference in result for SVD computing in Matlab and OpenCV. I input the same matrix

          3.65E+06  -2.09E+06   0
 YY =    -         


        

Answer 1

Which version of OpenCV are you using?

From http://code.opencv.org/issues/1498 it seems recent versions of OpenCV no longer use LAPACK to do SVD (as used by Matlab, I think). So the assumption that the same algorithm is being used might not be correct.

Of course YY=USV'

If you negate the first columns of U and V:

U(:,1)=-U(:,1);
V(:,1)=-V(:,1)

You will find USV' still equals YY. This works for your particular case because YY is symmetric (YY=YY').