Odd behaviour of ImageDraw.Draw.polygon

[jmsteitz]

What did you do?

I drew two polygons, each specified by only two vertices that are both on a vertical or horizontal line.

What did you expect to happen?

Expected to get a one pixel wide vertical and a one pixel wide horizontal line.

What actually happened?

The vertical line shows but not the horizontal one.

What are your OS, Python and Pillow versions?

• OS: Ubuntu 18.04.4
• Python: 3.7.6
• Pillow: 7.1.1

import numpy as np
from PIL import Image, ImageDraw
img = Image.new("L", size=(2,2), color=0)
ImageDraw.Draw(img).polygon([(0,0),(0,1)], outline=1, fill=1)
np.array(img)

gives

array([[1, 0],
       [1, 0]], dtype=uint8)

but

import numpy as np
from PIL import Image, ImageDraw
img = Image.new("L", size=(2,2), color=0)
ImageDraw.Draw(img).polygon([(0,0),(1,0)], outline=1, fill=1)
np.array(img)

results in

array([[0, 0],
       [0, 0]], dtype=uint8)

[radarhere]

I understand that you might be trying to just raise a general inconsistency with polygon, but, on a practical level, if you change polygon to line, and remove the outline argument, the following works.

import numpy as np
from PIL import Image, ImageDraw
img = Image.new("L", size=(2,2), color=0)
ImageDraw.Draw(img).line([(0,0),(1,0)], fill=1)
print(np.array(img))

[[1 1]
 [0 0]]

[radarhere]

I've created PR #4675 to fix this.

[jmsteitz]

Thanks for the PR and the suggested workaround! We encountered the problem while generating mask images with arbitrary polygons that sometimes happen to be degenerated, i.e. falling onto a horizontal line.