64c from C64 ROM font binary with some chars X'd out

romfont_to_64c.py

import numpy as np
import sys

# C64 ROM font:
# https://github.com/nurpax/petmate/raw/master/assets/system-charset.bin

unallowed_pattern = np.array([
    [1, 0, 0, 0, 0, 0, 1, 0],
    [1, 1, 0, 0, 0, 1, 1, 0],
    [0, 1, 1, 0, 1, 1, 0, 0],
    [0, 0, 1, 1, 1, 0, 0, 0],
    [0, 0, 1, 1, 1, 0, 0, 0],
    [0, 1, 1, 0, 1, 1, 0, 0],
    [1, 1, 0, 0, 0, 1, 1, 0],
    [1, 0, 0, 0, 0, 0, 1, 0],
], dtype=np.uint8)

def main():
    if len(sys.argv) < 3:
        print (f'Usage: {sys.argv[0]} <input rom font> <output.64c>')
        sys.exit(1)
    rom_font = np.fromfile(sys.argv[1], dtype=np.uint8)
    rom_font = np.unpackbits(rom_font).reshape([-1,8,8])
    num_chars = rom_font.shape[0]
    print (f'Loaded ROM font: {sys.argv[1]}, (size: {rom_font.shape[0]} chars)')

    # Mark some characters as unallowed in DirArt -- overwrite
    # their character bitmaps with an X pattern.
    unallowed_chars = np.array([0, 1, 2, 10])
    rom_font[unallowed_chars] = unallowed_pattern

    # Write a .64c file out.  The format is basically just the character
    # bits plus two bytes of header that says how many characters the
    # font contains.
    out_font = np.packbits(rom_font.reshape(-1))
    header = np.array([num_chars >> 8, num_chars & 255], dtype=np.uint8)
    out_font = np.concatenate([header, out_font])
    out_font.tofile(sys.argv[2])

if __name__ == "__main__":
    main()