Divide RGB image by grayscale elementwise in numpy

Question

Suppose we have two arrays of shape (480, 640, 3) and (480, 640), say RGB and grayscale image. How you divide elementwise first array by the second? So far I use the following code, but is there a better snippet for it?

arr1[:, :, 0] /= arr2
arr1[:, :, 1] /= arr2
arr1[:, :, 2] /= arr2

Answer 1

You can add another axis to arr2 so that it will broadcast.

>>> a = np.ones((2,2,3))
>>> b = np.ones((2,2)) * 2
>>> a
array([[[ 1.,  1.,  1.],
        [ 1.,  1.,  1.]],

       [[ 1.,  1.,  1.],
        [ 1.,  1.,  1.]]])
>>> b
array([[ 2.,  2.],
       [ 2.,  2.]])
>>> a = a / b[:, :, np.newaxis]
>>> a
array([[[ 0.5,  0.5,  0.5],
        [ 0.5,  0.5,  0.5]],

       [[ 0.5,  0.5,  0.5],
        [ 0.5,  0.5,  0.5]]])
>>>

Answer 2

Hmm... you could perhaps duplicate your grayscale array and just do one division?

arr1 /= arr2.repeat(3).reshape(np.shape(arr1))