A script to split a Duolingo flag sheet up into individual PNG images.

split_flags.py

from io import BytesIO
from pathlib import Path

from cairosvg import svg2png
from PIL import Image, ImageChops

# This likely only works with the old style of flags (flag-spriteXX.svg)
FLAG_SHEET = 'flags.svg'
OUTPUT_DIR = 'flags'
TARGET_FLAG_SIZE = 512


def main():
    full_flag_size = TARGET_FLAG_SIZE + (TARGET_FLAG_SIZE / 64)
    flag_sheet_size = (full_flag_size / 80) * 1000

    print('Rasterizing SVG...')
    svg = Path(FLAG_SHEET).read_text()
    png = svg2png(bytestring=svg, output_width=flag_sheet_size)

    Image.MAX_IMAGE_PIXELS = None
    image = Image.open(BytesIO(png))
    width, height = image.size

    Path(OUTPUT_DIR).mkdir(parents=True, exist_ok=True)

    flag_id = 0
    over_flag_y = False
    print('Splitting flags...')
    for y in range(height):
        if image.getpixel((0, y))[3] == 0:
            over_flag_y = False
            continue
        if over_flag_y:
            continue

        over_flag_x = False
        for x in range(width):
            if image.getpixel((x, y))[3] == 0:
                over_flag_x = False
                continue
            if over_flag_x:
                continue
            print(f'{flag_id}: {(x, y)}')

            flag = image.crop((x, y, x + full_flag_size, y + full_flag_size))
            background = Image.new(flag.mode, flag.size, (0, 0, 0, 0))
            difference = ImageChops.difference(flag, background)
            bbox = difference.getbbox()
            if bbox:
                flag = flag.crop(bbox)
            new_width, new_height = flag.size
            if new_width != new_height:
                final_size = new_width if new_width < new_height else new_height
                left = (new_width - final_size) // 2
                top = (new_height - final_size) // 2
                right = (new_width + final_size) // 2
                bottom = (new_height + final_size) // 2

                flag = flag.crop((left, top, right, bottom))

            bottom_center_pixel = flag.getpixel((flag.size[0] // 2, flag.size[1] - 1))
            if bottom_center_pixel == (0, 0, 0, 0):
                over_flag_x = True
                flag_id += 1
                continue

            flag.thumbnail((TARGET_FLAG_SIZE, TARGET_FLAG_SIZE), Image.ANTIALIAS)
            flag.save(f'{OUTPUT_DIR}/flag_{flag_id}.png')

            over_flag_x = True
            flag_id += 1
        over_flag_y = True

    print('Done!')


if __name__ == '__main__':
    main()