#!/usr/bin/python
# -*- coding: utf-8 -*-
"""Generate image masks for opacity holograms."""
from __future__ import division

import sys
import os

import Image                            # python-imaging aka PIL

def main(argv):
    import cgitb
    cgitb.enable(format='text')

    xw, yw = map(int, argv[2:])
    output_image = Image.new('RGBA', Image.open(argv[1]).size)
    output_filename = 'interleave-mask-%dx%d.png' % output_image.size
    if os.path.exists(output_filename):
        sys.stderr.write("%s already exists; delete it if wrong"
                         % output_filename)
        sys.exit(1)

    # PIL’s α=0 is transparent.  Contrary to PIL’s documentation, the
    # image is initially filled not with black, but with transparent
    # (transparent black, okay, but transparent.)
    output_image.paste((0, 0, 0, 255), (0, 0) + output_image.size)

    transparent = (255, 0, 0, 0)        # transparent red
    for ii in range((output_image.size[1]-1) // yw):
        for jj in range((output_image.size[0]-1) // xw):
            output_image.putpixel((jj*xw, ii*yw), transparent)
    
    output_image.save(output_filename)

if __name__ == '__main__':
    main(sys.argv)