Python PIL bitmap/png from array with mode=1

Question

Playing with PIL (and numpy) for the first time ever. I was trying to generate a black and white checkerboard image through mode='1', but it doesn't work.

from PIL import Image
import numpy as np

if __name__ == '__main__':
    g = np.asarray(dtype=np.dtype('uint8'), a=[
        [0, 1, 0, 1, 0, 1, 0, 1, ],
        [1, 0, 1, 0, 1, 0, 1, 0, ],
        [0, 1, 0, 1, 0, 1, 0, 1, ],
        [1, 0, 1, 0, 1, 0, 1, 0, ],
        [0, 1, 0, 1, 0, 1, 0, 1, ],
        [1, 0, 1, 0, 1, 0, 1, 0, ],
        [0, 1, 0, 1, 0, 1, 0, 1, ],
        [1, 0, 1, 0, 1, 0, 1, 0, ],
    ])
    print(g)

    i = Image.fromarray(g, mode='1')
    i.save('checker.png')

Sorry browser is probably going to try to interpolate this, but it is an 8x8 PNG.

What am I missing?

Relevant PIL docs: https://pillow.readthedocs.org/handbook/concepts.html#concept-modes

$ pip freeze
numpy==1.9.2
Pillow==2.9.0
wheel==0.24.0

Answer 1

There seem to be issues when using mode 1 with numpy arrays. As a workaround you could use mode L and convert to mode 1 before saving. The below snippet produces the expected checkerboard.

from PIL import Image
import numpy as np

if __name__ == '__main__':
    g = np.asarray(dtype=np.dtype('uint8'), a=[
        [0, 255, 0, 255, 0, 255, 0, 255],
        [255, 0, 255, 0, 255, 0, 255, 0],
        [0, 255, 0, 255, 0, 255, 0, 255],
        [255, 0, 255, 0, 255, 0, 255, 0],
        [0, 255, 0, 255, 0, 255, 0, 255],
        [255, 0, 255, 0, 255, 0, 255, 0],
        [0, 255, 0, 255, 0, 255, 0, 255],
        [255, 0, 255, 0, 255, 0, 255, 0]
    ])
    print(g)

    i = Image.fromarray(g, mode='L').convert('1')
    i.save('checker.png')