ENH: making the svd tol matching MATLAB's tol

Author: jbpoline

nibabel/orientations.py

@@ -37,7 +37,7 @@ def io_orientation(affine, tol=None):
        threshold below which SVD values of the affine are considered zero. If
        `tol` is None, and ``S`` is an array with singular values for `affine`,
        and ``eps`` is the epsilon value for datatype of ``S``, then `tol` set to
-       ``S.max() * eps``.
+       ``S.max() * max(S.shape) * eps``.
 
     Returns
     -------
@@ -62,7 +62,7 @@ def io_orientation(affine, tol=None):
     P, S, Qs = npl.svd(RS)
     # Threshold the singular values to determine the rank.
     if tol is None:
-        tol = S.max() * np.finfo(S.dtype).eps
+        tol = S.max() * max(S.shape) * np.finfo(S.dtype).eps
     keep = (S > tol)
     R = np.dot(P[:, keep], Qs[keep])
     # the matrix R is such that np.dot(R,R.T) is projection onto the