pillow: why does mode “1” give a weird image?

Question

inspired by this post (plot an image from an array with pillow.)

I tried this

diamond = np.array([[  0,   0,   0,   0, 255,   0,   0,   0,   0],
       [  0,   0,   0, 255,   0, 255,   0,   0,   0],
       [  0,   0, 255,   0,   0,   0, 255,   0,   0],
       [  0, 255,   0,   0,   0,   0,   0, 255,   0],
       [255,   0,   0,   0,   0,   0,   0,   0, 255],
       [  0, 255,   0,   0,   0,   0,   0, 255,   0],
       [  0,   0, 255,   0,   0,   0, 255,   0,   0],
       [  0,   0,   0, 255,   0, 255,   0,   0,   0],
       [  0,   0,   0,   0, 255,   0,   0,   0,   0]], dtype=uint8)

size = 36
my_dpi = mpl.rcParams['figure.dpi']
plt.subplots(figsize=(size/my_dpi, size/my_dpi))
plt.axis('off')
img = Image.fromarray(diamond, 'L')
img.resize((size,size)).save('diamond.ppm')
plt.imshow(img)

so far, everything work well.

per doc "1" represents (1-bit pixels, black and white, stored with one pixel per byte)

using mode "1" to replace "L" outputs this image

the expected output should look like

why is that?

Answer 1

convert to mode 1 is a workaround

size = 36
my_dpi = mpl.rcParams['figure.dpi']
plt.subplots(figsize=(size/my_dpi, size/my_dpi))
plt.axis('off')
img = Image.fromarray(diamond, 'L').convert('1')
img.resize((size,size)).save('diamond.ppm')
plt.imshow(img)