bluesky_pad.py

#!/usr/bin/env python3

"""Pads an image for Bluesky."""

import argparse
import math
from pathlib import Path

from PIL import Image


def get_output_size(size, aspect):
    in_w, in_h = size
    in_aspect = size[0] / size[1]
    if in_aspect < aspect:
        out_w = int(in_h * aspect)
        out_h = in_h
    else:
        out_w = in_w
        out_h = int(in_w / aspect)
    return out_w, out_h


def main():
    parser = argparse.ArgumentParser(description=__doc__)
    parser.add_argument("image", type=Path, help="the input image")
    parser.add_argument(
        "--bg", type=int, nargs=4, default=(0, 0, 0, 0), help="the background color"
    )
    args = parser.parse_args()

    allowed_aspects = [3 / 4, 1.0, 4 / 3]
    allowed_log_aspects = [math.log(x) for x in allowed_aspects]

    image = Image.open(args.image).convert("RGBA")

    # Find the closest aspect ratio
    log_aspect = math.log(image.size[0] / image.size[1])
    aspect_diffs = [abs(log_aspect - x) for x in allowed_log_aspects]
    out_aspect = allowed_aspects[aspect_diffs.index(min(aspect_diffs))]

    # Paste into output image
    out_size = get_output_size(image.size, out_aspect)
    out_image = Image.new("RGBA", out_size, tuple(args.bg))
    paste_x = (out_size[0] - image.size[0]) // 2
    paste_y = (out_size[1] - image.size[1]) // 2
    out_image.paste(image, (paste_x, paste_y))

    # Save the output image
    out_path = args.image.with_stem(args.image.stem + "_pad").with_suffix(".png")
    out_image.save(out_path)


if __name__ == "__main__":
    main()