Broadcast one channel in Numpy array into three channels

Question

I have a RGB image img which is of shape (2560L, 1920L, 3L) and another single channel image mask which is of shape (2560L, 1920L). Now, I want to make this mask of shape (2560L, 1920L, 3L) i.e. I want to copy this single channel data into all the three channels.

I'm doing it as follows.

np.array([[[j,j,j] for j in i] for i in mask])

Is there a faster way of doing this using numpy?

Answer 1

np.repeat(mask.reshape(2560L, 1920L, 1L), 3, axis=2)