Script to create ICO files from PNGs while preserving compresison

pngs-to-ico.py

#!/usr/bin/env python3
#
# Create an .ico file from a bunch of PNGs.
# Unlike ImageMagick's convert, this does not convert PNGs into bitmaps or
# otherwise modify the source PNGs; in particular, their compression is
# preserved.
#
# ICO format is actually very simple:
# https://en.wikipedia.org/wiki/ICO_(file_format)
#
# Embedded PNGs as the only content are supported since Windows Vista.
#

import argparse
import struct
import io
import PIL.Image


ap = argparse.ArgumentParser()
ap.add_argument('inputs', nargs='+')
ap.add_argument('-o', '--output', required=True)
args = ap.parse_args()

png_data = []
for path in args.inputs:
    with open(path, 'rb') as fp:
        png_data.append(fp.read())

icons = []
for data in png_data:
    with PIL.Image.open(io.BytesIO(data)) as im:
        w, h = im.size
    icons.append((w, h, data))
icons.sort()

with open(args.output, 'wb') as f:
    f.truncate()
    f.write(struct.pack('<HHH', 0, 1, len(icons)))
    offset = 6 + 16 * len(png_data)
    for w, h, data in icons:
        if w > 255:
            w = 0
        if h > 255:
            h = 0
        f.write(struct.pack('<BBBBHHII', w, h, 0, 0, 1, 0, len(data), offset))
        offset += len(data)
    for _, _, data in icons:
        f.write(data)

Inspecting what the ICO file contains:

$ magick identify Poedit.ico
Poedit.ico[0] PNG 16x16 16x16+0+0 8-bit sRGB 372B 0.000u 0:00.000
Poedit.ico[1] PNG 24x24 24x24+0+0 8-bit sRGB 624B 0.000u 0:00.000
Poedit.ico[2] PNG 32x32 32x32+0+0 8-bit sRGB 932B 0.000u 0:00.000
Poedit.ico[3] PNG 48x48 48x48+0+0 8-bit sRGB 1534B 0.000u 0:00.000
Poedit.ico[4] PNG 64x64 64x64+0+0 8-bit sRGB 3148B 0.000u 0:00.000
Poedit.ico[5] PNG 256x256 256x256+0+0 8-bit sRGB 25619B 0.000u 0:00.000

$ magick identify FilePO.ico 
FilePO.ico[0] ICO 16x16 16x16+0+0 8-bit sRGB 0.000u 0:00.022
FilePO.ico[1] ICO 32x32 32x32+0+0 8-bit sRGB 0.000u 0:00.022
FilePO.ico[2] ICO 48x48 48x48+0+0 8-bit sRGB 0.000u 0:00.022
FilePO.ico[1] PNG 256x256 256x256+0+0 8-bit sRGB 31246B 0.000u 0:00.001

Extracting all sub-images from ICO file:

$ magick convert -set filename:f "%t-%wpx" "ICO:FilePO.ico" "%[filename:f].png"