python - matplotlib.pyplot saves binary mask with 4 channels(?)

Question

I'm trying to save binary masks, i.e. arrays with False and True values, as .png files.

However, when doing so with matplotlib.pyplot, it saves the image with 4 channels and I don't know why.

For example to save the image:

import numpy as np
import matplotlib.pyplot as plt
import matplotlib.cm as cm

m = np.array([[False, True],
             [False, True]])

plt.imsave("mask.png", m, cmap=cm.gray)

Then reading it again and printing the shape:

plt.imread("mask.png").shape

Gives me:

(2, 2, 4)

Any reason for that and how to just save it as plain grayscale with 0/1 values so that the shape essentially will just be (2,2)?

Since all of the three RGB layers are the same, you can extract your original mask with arr[:, :, 0], with arr being the mask loaded by imread. You can also convert it back to a Boolean array a[:, :, 0].astype(bool) — Mr. T
– Mr. T, Commented May 27, 2018 at 10:17
Okay, this workaround seem to work for me, thanks. But anyway I'm still curious why is it doing it in the first place? — TheDude
– TheDude, Commented May 27, 2018 at 10:28
I don't know the inner workings of imsave, but it is obvious that it has to create a file that adheres to the file structure defined by the extension, e.g., jpg, png, eps etc. There are other libraries like PIL that can deal with greyscale images directly: stackoverflow.com/a/26929408/8881141 If you retrieve a Boolean array in the end, you don't even have to scale it Image.fromarray(m.astype(np.uint8)).save("maskgrey.png") — Mr. T
– Mr. T, Commented May 27, 2018 at 11:14

Dragondin · Accepted Answer · 2020-05-23 14:39:17Z

1

I had the same issue. It is related to matplotlib.pyplot. Use library PIL and the png file has only 1 channel.

from PIL import Image
Image.fromarray(img).save('img.png')

If you're dealing with a float image, you also have to convert it before to uint8. For example like this:

img = 255 * img
img = img.astype(np.uint8)

edited May 23, 2020 at 14:39

answered May 23, 2020 at 14:23

Dragondin

112 bronze badges

Sign up to request clarification or add additional context in comments.

Collectives™ on Stack Overflow

python - matplotlib.pyplot saves binary mask with 4 channels(?)

1 Answer 1

Comments

Your Answer

Linked

Hot Network Questions

Collectives™ on Stack Overflow

1 Answer 1

Comments

Your Answer

Sign up or log in

Post as a guest

Linked

Related