Monospace font solution

[ghost]

With checking if the current column is white and the next one is black, we can determine starting and ending columns of each letter. Thus we can get rid of fixed-space issue

from PIL import Image
import sys


def decoder(
        im,
        threshold=200,
        mask="letters.bmp",
        alphabet="0123456789abcdef"):

    img = Image.open(im)
    img = img.convert("RGB")
    box = (8, 8, 58, 18)
    img = img.crop(box)
    pixdata = img.load()

    # open the mask
    letters = Image.open(mask)
    ledata = letters.load()

    def test_letter(img, letter):
        A = img.load()
        B = letter.load()
        mx = 1000000
        max_x = 0
        x = 0
        for x in range(img.size[0] - letter.size[0]):
            _sum = 0
            for i in range(letter.size[0]):
                for j in range(letter.size[1]):
                    _sum = _sum + abs(A[x + i, j][0] - B[i, j][0])
            if _sum < mx:
                mx = _sum
                max_x = x
        return mx, max_x

    # Clean the background noise, if color != white, then set to black.
    for y in range(img.size[1]):
        for x in range(img.size[0]):
            if (pixdata[x, y][0] > threshold) \
                    and (pixdata[x, y][1] > threshold) \
                    and (pixdata[x, y][2] > threshold):

                pixdata[x, y] = (255, 255, 255, 255)
            else:
                pixdata[x, y] = (0, 0, 0, 255)

    counter = 0
    old_x = -1

    letterlist = []

    for x in range(letters.size[0]):
        black = True
        for y in range(letters.size[1]):
            if ledata[x, y][0] != 0:
                black = False
                break
        if black:
            box = (old_x + 1, 0, x, 10)
            letter = letters.crop(box)
            t = test_letter(img, letter)
            letterlist.append((t[0], alphabet[counter], t[1]))
            old_x = x
            counter += 1

    box = (old_x + 1, 0, 140, 10)
    letter = letters.crop(box)
    t = test_letter(img, letter)
    letterlist.append((t[0], alphabet[counter], t[1]))

    t = sorted(letterlist)
    t = t[0:5]  # 5-letter captcha

    final = sorted(t, key=lambda e: e[2])

    answer = ''.join(map(lambda l: l[1], final))
    return answer

if __name__ == '__main__':
    print(decoder(sys.argv[1]))``