generating two orthogonal vectors that are orthogonal to a particular direction

Question

What is the simplest and most efficient ways in numpy to generate two orthonormal vectors a and b such that the cross product of the two vectors equals another unit vector k, which is already known?

I know there are infinitely many such pairs, and it doesn't matter to me which pairs I get as long as the conditions axb=k and a.b=0 are satisfied.

Answer 1

This will do:

>>> k  # an arbitrary unit vector k is not array. k is must be numpy class. np.array
np.array([ 0.59500984,  0.09655469, -0.79789754])

To obtain the 1st one:

>>> x = np.random.randn(3)  # take a random vector
>>> x -= x.dot(k) * k       # make it orthogonal to k
>>> x /= np.linalg.norm(x)  # normalize it

To obtain the 2nd one:

>>> y = np.cross(k, x)      # cross product with k

and to verify:

>>> np.linalg.norm(x), np.linalg.norm(y)
(1.0, 1.0)
>>> np.cross(x, y)          # same as k
array([ 0.59500984,  0.09655469, -0.79789754])
>>> np.dot(x, y)            # and they are orthogonal
-1.3877787807814457e-17
>>> np.dot(x, k)
-1.1102230246251565e-16
>>> np.dot(y, k)
0.0