Python: how to stack multiple array together along a new axis

Question

I have three matrices, R, B, G, which all are the same in size, say m*n. I want to stack all three of them together into a different channel of a new matrix (3*m* n or m*n*3), as implemented in Matlab:

 A(:,:,1) = R
 A(:,:,2) = G
 A(:,:,3) = B

How do I achieve this efficiently in Python?

Are you using a particular package (e.g. NumPy) for your matrices, or are they just pure lists? — Alden
– Alden, Commented Jun 3, 2017 at 1:13
@Alden: yeah. I know numpy has stack functions, but they seem only to be applicable to two matrices — jwm
– jwm, Commented Jun 3, 2017 at 1:17

Alden · Accepted Answer · 2017-06-03 01:28:17Z

4

numpy.stack should work for three arrays:

numpy.stack((R, G, B))

For example,

a = numpy.array([[1,2],[2,1]])
b = numpy.array([[3,3],[4,4]])
c = numpy.array([[5,6],[7,8]])
print(numpy.stack((a, b, c)))

prints

[[[1 2]
  [2 1]]

 [[3 3]
  [4 4]]

 [[5 6]
  [7 8]]]

edited Jun 3, 2017 at 1:28

answered Jun 3, 2017 at 1:21

Alden

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2 Comments

jwm Over a year ago

TypeError: stack() takes at most 2 arguments (3 given)

Alden Over a year ago

Make sure you are passing (R, G, B) (one argument, a tuple containing the three arrays), not R, G, B (the three arrays as individual arguments).

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