Add black border to match ratio using PIL/Pillow

blackratio.py

#! /usr/bin/env python3

import argparse
import os
from PIL import Image, ImageOps


def add_border(image_path: str, desired_ratio: float, output_file: str) -> None:
    img = Image.open(image_path)
    width, height = img.size
    if width / height < desired_ratio:
        new_width = int(height * desired_ratio)
        border_width = (new_width - width) // 2
        border = border_width, 0
    else:
        new_height = int(width / desired_ratio)
        border_height = (new_height - height) // 2
        border = 0, border_height
    ImageOps.expand(img, border=border, fill="black").save(output_file)


def get_aspect_ratio(image_path: str) -> float:
    width, height = Image.open(image_path).size
    return width / height


def main():
    parser = argparse.ArgumentParser(
        description="Add black border to images to match a ratio."
    )
    parser.add_argument(
        "reference", help="The image file from which to take the target aspect ratio."
    )
    parser.add_argument("images", nargs="+", help="List of image files to process.")
    parser.add_argument(
        "-o", "--output", dest="output", required=True, help="The output folder."
    )
    args = parser.parse_args()
    ratio = get_aspect_ratio(args.reference)
    print(f"Target ratio is: {ratio}")
    for img_path in args.images:
        print(f"Processing file {img_path}")
        add_border(
            img_path, ratio, os.path.join(args.output, os.path.basename(img_path))
        )


if __name__ == "__main__":
    main()