Saving NumPy RGB array as an image fails because of the array shape

Question

I have a bunch of arrays of frames with the shape of (3, 225, 400) that were obtained from the screen buffer. I have trouble while saving them as RGB png images because of the shapes.

> arr = np.load("frame134.npy") 
> print(arr)

[[[63 47 63 ... 47 47 27]
  [63 47 39 ... 47 55 35]
  [63 47 39 ... 55 55 47]
  ...
  [91 35 35 ... 79 79 79]
  [91 35 63 ... 79 79 79]
  [55 63 63 ... 79 79 79]]

 [[71 47 47 ... 55 55 27]
  [71 47 39 ... 55 63 43]
  [71 47 39 ... 63 63 55]
  ...
  [99 35 35 ... 79 79 79]
  [99 35 71 ... 79 79 79]
  [63 71 71 ... 79 79 79]]

 [[43 47 23 ... 31 31 27]
  [43 47 39 ... 31 39 15]
  [43 47 39 ... 39 39 31]
  ...
  [71 35 35 ... 79 79 79]
  [71 35 43 ... 79 79 79]
  [39 43 43 ... 79 79 79]]]

When I try to save it by using PIL:

im = Image.fromarray(arr, "RGB")
im.save("tmp.png")

The result is as follows:

or, if I use matplotlib:

> plt.imsave('tmp.png', arr)
ValueError: Third dimension must be 3 or 4

How can I resize my array to make it able to be saved properly? Thanks!

Often the size 3 color channel is the last dimension, so the array would have shape (225, 400,3). Your array needs a transpose. — hpaulj
– hpaulj, Commented Dec 19, 2020 at 22:55
Yes, I agree with you about the cause but I did not know how to transpose. The answer below solved the problem! Thanks! — bbasaran
– bbasaran, Commented Dec 19, 2020 at 23:37

Lior Cohen · Accepted Answer · 2020-12-19 23:01:24Z

2

You will have to move your first axis to be the third. This can be done by

arr = np.moveaxis(arr , 0, -1)

answered Dec 19, 2020 at 23:01

Lior Cohen

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3 Comments

bbasaran Over a year ago

I was not aware of the "moveaxis" method. It solved my problem. Thanks! I have only one more question for this case. When I try to save it with OpenCV ( cv2.imwrite('cv2.png', arr) ), colors differ somehow. Do you have any idea about that?

Lior Cohen Over a year ago

sure. opencv defalt colors order is BGR. Yours are probably RGB so you need to swap them by arr[:,:,(2,1,0)]

bbasaran Over a year ago

My arrays are in RGB as you said. It was good information. Thank you for your clear explanations!

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