[FIXED] Python valueError using hstack() (ValueError: all the input array dimensions except for the concatenation axis must match exactly)

Issue

I got the following error:

Traceback (most recent call last):
File "/home/odroid/trackAndFollow/getPositions.py", line 34, in 
<module>
tfVeloToCamera = np.hstack((np.vstack((rmVeloToCamera, 
zero_array)),np.transpose(translationVector_veloToCamera)))
File "/usr/lib/python2.7/dist-packages/numpy/core/shape_base.py", line 
280, in hstack
return _nx.concatenate(arrs, 1)
ValueError: all the input array dimensions except for the 
concatenation axis must match exactly

Code:

rotationVector_veloToCamera = 
np.array([[[-1.77611954,0.30024612,0.76069987]]])
translationVector_veloToCamera = np.array([[ 
0.0146381,0.02553223,0.16231193]])

rmVeloToCamera,jac = cv2.Rodrigues(rotationVector_veloToCamera)
tfVeloToCamera = np.hstack((np.vstack((rmVeloToCamera, 
zero_array)),np.transpose(translationVector_veloToCamera)))

I read somewhere that the reason is the shape or dtype. The shape of the 2 variables inside the hstack is (4,3) (3,1) and both have dtype=float64.

Any idea whats causing this problem?

Solution

If the shapes of the arrays you're trying to stack are (4, 3) and (3, 1), then they don't have any matching array dimensions, as 4 != 3 and 3 != 1. If you were to take the transpose of the first, giving dimensions (3, 4) and (3, 1), they should stack.

np.hstack((np.zeros((4, 3)), np.zeros((3,1))))
Traceback (most recent call last):
...
ValueError: all the input array dimensions except for the concatenation axis must match exactly

np.hstack((np.zeros((4, 3)).T, np.zeros((3,1))))
Out[56]: 
array([[ 0.,  0.,  0.,  0.,  0.],
       [ 0.,  0.,  0.,  0.,  0.],
       [ 0.,  0.,  0.,  0.,  0.]])

As an aside, you might want to look at shifting over to using np.concatenate and specifying an axis, instead of np.vstack and np.hstack.

Ex:

np.concatenate((np.zeros((4, 3)), np.zeros((3,1)).T), axis=0)
Out[20]: 
array([[ 0.,  0.,  0.],
       [ 0.,  0.,  0.],
       [ 0.,  0.,  0.],
       [ 0.,  0.,  0.],
       [ 0.,  0.,  0.]])

Answered By - EFT