How to generate a random binary image?

Question

I know that a binary image is matrix with 0 and 1 values. If I generate a matrix with numpy having 0 and 1 elements and then I use pillow library from python to read the image from the numpy array, the image is black. Why this is happen?

from PIL import Image
import numpy
matrix = numpy.random.randint(2, size=(512,512))
img =  Image.fromarray(matrix)
img.save(test.png)

By way of explanation, your matrix is probably of type uint32 - because you didn't specify the dtype when creating it - which means it uses 4 bytes per pixel rather than the 1-bit you intended. Which means that black is represented by 0 and white is represented by 4,294,967,295, so your values of 0 and 1 are all going to look pretty black on that range. — Mark Setchell
– Mark Setchell, Commented May 16, 2020 at 10:39

Juan Carlos Ramirez · Accepted Answer · 2023-09-18 16:01:40Z

3

What you want is a single bit PNG image. cv2 and PIL.Image both support this type of images.

from PIL import Image
import numpy
# boolean matrix
matrix = numpy.random.randint(2, size=(512,512)).astype(bool)
img =  Image.fromarray(matrix)
img.save("test.png", bits=1,optimize=True)

edited Sep 18, 2023 at 16:01

Juan Carlos Ramirez

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answered May 15, 2020 at 20:16

dumbPy

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2 Comments

Mark Setchell Over a year ago

Your answer works correctly, but it might be worth noting that you are creating an array at least 8x bigger in terms of RAM usage and taking around 6-7x longer than necessary in terms of time and then reducing it afterwards, with .astype() rather than creating it with the intended type up-front... matrix = numpy.random.randint(2, size=(512,512), dtype=np.uint8)

dumbPy Over a year ago

Yes, I was going to mention that, but then didn't. Tried to keep it similar to the OP's question. Also, I see that numpy.random.choice generates float64 and then converts to bool, so I guess it would have more or less same implications

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