Use OpenCV to draw grid lines on an image.

draw-grid-lines-on-image.py

"""Draw grid lines on an image.

Usage:
    # To draw a 3x4 grid on an image
    $ python draw_grid_lines.py <path/to/image> <num_rows> <num_cols> -o <output/dir-or-path>
    $ python draw_grid_lines.py image.png 3 4 -o output.png

    # for more options, see
    $ python draw_grid_lines.py --help

"""

import cv2 as cv  # tested with version 4.5.3.56 (pip install opencv-python)
import numpy as np
from pathlib import Path


def draw_grid(img, grid_shape, color=(0, 255, 0), thickness=1):
    h, w, _ = img.shape
    rows, cols = grid_shape
    dy, dx = h / rows, w / cols

    # draw vertical lines
    for x in np.linspace(start=dx, stop=w-dx, num=cols-1):
        x = int(round(x))
        cv.line(img, (x, 0), (x, h), color=color, thickness=thickness)

    # draw horizontal lines
    for y in np.linspace(start=dy, stop=h-dy, num=rows-1):
        y = int(round(y))
        cv.line(img, (0, y), (w, y), color=color, thickness=thickness)

    return img


def main(args):
    img = cv.imread(str(args.input_image_path))
    img = draw_grid(img=img,
                    grid_shape=(args.rows, args.cols),
                    color=args.color,
                    thickness=args.thickness)
    cv.imwrite(str(args.output_image_path), img)


def get_user_args():
    import argparse

    parser = argparse.ArgumentParser()
    parser.add_argument(
        'input_image_path', type=Path, help='Path to image.'
    )
    parser.add_argument(
        'rows', type=int, default=None, help='Number of grid rows.'
    )
    parser.add_argument(
        'cols', type=int, default=None, help='Number of grid columns.'
    )
    parser.add_argument(
        '--output_image_path', '-o', type=Path, default=None, help='Where to store output image (full path or directory).'
    )
    parser.add_argument(
        '--overwrite_ok', '-f', action='store_true', help='Allow overwriting existing output path.'
    )
    parser.add_argument(
        '--color', '-c', default=(0, 255, 0), nargs=3, type=int, help='Line color as BGR triplet.'
    )
    parser.add_argument(
        '--thickness', '-t', default=1, type=int, help='Line thickness.'
    )
    args = parser.parse_args()

    # parse output path
    input_image_dir = args.input_image_path.resolve().parent
    if args.output_image_path is None:
        # set default output path to same dir as original, with filename prefix
        args.output_image_path = \
            input_image_dir / f"{args.rows}x{args.cols}_grid_{args.input_image_path.name}"
    elif args.output_image_path.is_dir():
        # set output path filename to "<output_dir>/<original_filename>"
        args.output_image_path = args.output_image_path / args.input_image_path.name

    if not args.overwrite_ok:
        assert not args.output_image_path.exists()

    return args


if __name__ == '__main__':
    main(get_user_args())