numpy with python: convert 3d array to 2d

You need to use np.transpose to rearrange dimensions. Now, n x m x 3 is to be converted to 3 x (n*m), so send the last axis to the front and shift right the order of the remaining axes (0,1). Finally , reshape to have 3 rows. Thus, the implementation would be –

img.transpose(2,0,1).reshape(3,-1)

Sample run –

In [16]: img
Out[16]: 
array([[[155,  33, 129],
        [161, 218,   6]],

       [[215, 142, 235],
        [143, 249, 164]],

       [[221,  71, 229],
        [ 56,  91, 120]],

       [[236,   4, 177],
        [171, 105,  40]]])

In [17]: img.transpose(2,0,1).reshape(3,-1)
Out[17]: 
array([[155, 161, 215, 143, 221,  56, 236, 171],
       [ 33, 218, 142, 249,  71,  91,   4, 105],
       [129,   6, 235, 164, 229, 120, 177,  40]])