Decode COCO RLE images to PNG masks

coco.py

#!/usr/bin/env python3

import sys
import re
import numpy as np
from PIL import Image

def decode_rle(rle, width, height):
    img = np.zeros((height, width), dtype=np.uint8)
    seq = img.ravel()
    idx = 0
    pattern = re.compile(r"(\d+)([FT])")
    for match in pattern.finditer(rle):
        length = int(match.group(1))
        bit = match.group(2)
        if bit == "T":
            seq[idx:idx + length] = 1
        idx += length
    assert idx == width * height, "RLE does not decode to expected image size"
    return Image.fromarray(img * 255, mode="L").convert("1")

if __name__ == "__main__":
    if len(sys.argv) != 5:
        print("Usage: python3 coco.py <width> <height> <in.rle> <out.png>")
        sys.exit(1)

    width = int(sys.argv[1])
    height = int(sys.argv[2])
    with open(sys.argv[3], "r") as f:
        rle = f.read().strip()

    img = decode_rle(rle, width, height)
    img.save(sys.argv[4], format="PNG")

License: CC0-1.0