Image.fromarray is poorly defined with floating-point input; it’s not well documented but the function assumes the input is laid-out as unsigned 8-bit integers.
To produce the output you’re trying to get, multiply by 255 and convert to
z = (z * 255).astype(np.uint8)
The reason it seems to work with the random array is that the bytes in this array, when interpreted as unsigned 8-bit integers, also look random. But the output is not the same random array as the input, which you can check by doing the above conversion on the random input:
np.random.seed(0) zz = np.random.rand(size, size) Image.fromarray(zz, mode="L").save('pic1.png')
Image.fromarray((zz * 255).astype('uint8'), mode="L").save('pic2.png')
Since the issue doesn’t seem to be reported anywhere, I reported it on github: https://github.com/python-pillow/Pillow/issues/2856