This script accepts labelme's json filepath, and creates a segmentation mask.

create_mask.py

import math
import sys
import json
import numpy as np
import PIL.Image
import PIL.ImageDraw
import cv2

def shape_to_mask(
    img_shape, points, shape_type=None, line_width=10, point_size=5, mask=None
):
    if mask is None:
        mask = np.zeros(img_shape[:2], dtype=np.uint8)

    mask = PIL.Image.fromarray(mask)
    draw = PIL.ImageDraw.Draw(mask)
    xy = [tuple(point) for point in points]
    if shape_type == "circle":
        assert len(xy) == 2, "Shape of shape_type=circle must have 2 points"
        (cx, cy), (px, py) = xy
        d = math.sqrt((cx - px) ** 2 + (cy - py) ** 2)
        draw.ellipse([cx - d, cy - d, cx + d, cy + d], outline=1, fill=1)
    elif shape_type == "rectangle":
        assert len(xy) == 2, "Shape of shape_type=rectangle must have 2 points"
        draw.rectangle(xy, outline=1, fill=1)
    elif shape_type == "line":
        assert len(xy) == 2, "Shape of shape_type=line must have 2 points"
        draw.line(xy=xy, fill=1, width=line_width)
    elif shape_type == "linestrip":
        draw.line(xy=xy, fill=1, width=line_width)
    elif shape_type == "point":
        assert len(xy) == 1, "Shape of shape_type=point must have 1 points"
        cx, cy = xy[0]
        r = point_size
        draw.ellipse([cx - r, cy - r, cx + r, cy + r], outline=1, fill=1)
    else:
        assert len(xy) > 2, "Polygon must have points more than 2"
        draw.polygon(xy=xy, outline=1, fill=1)
    mask = np.array(mask, dtype=bool)
    mask = mask.astype(np.uint8)#boolean to 0,Convert to 1
    mask[mask>0] = 255
    return mask

with open( sys.argv[1], "r",encoding="utf-8") as f:
    dj = json.load(f)
    mask = np.zeros((dj['imageHeight'],dj['imageWidth']), dtype=np.uint8)
    for i, _ in enumerate(dj['shapes']):
        mask = shape_to_mask( (dj['imageHeight'],dj['imageWidth']), dj['shapes'][i]['points'], shape_type=None,line_width= 0.2, point_size=0.5, mask=mask)
        #mask_img = mask.astype(np.uint8)#boolean to 0,Convert to 1
        #mask_img[mask_img>0] = 255

cv2.imwrite(f'{sys.argv[1].split(".")[0]}_mask.png', mask)