how to save an array as a grayscale image with matplotlib/numpy?

Question

I am trying to save a numpy array of dimensions 128x128 pixels into a grayscale image. I simply thought that the pyplot.imsave function would do the job but it\'s not, it so

Answer 1

I didn't want to use PIL in my code and as noted in the question I ran into the same problem with pyplot, where even in grayscale, the file is saved in MxNx3 matrix.

Since the actual image on disk wasn't important to me, I ended up writing the matrix as is and reading it back "as-is" using numpy's save and load methods:

np.save("filename", image_matrix)

And:

np.load("filename.npy")

Answer 2

There is also a possibility to use scikit-image, then there is no need to convert numpy array into a PIL object.

from skimage import io
io.imsave('output.tiff', I.astype(np.uint16))

Answer 3

With PIL it should work like this

import Image

I8 = (((I - I.min()) / (I.max() - I.min())) * 255.9).astype(np.uint8)

img = Image.fromarray(I8)
img.save("file.png")