# mtexdec.py
# Decode Mtex images as used by Sora no Woto.
# Usage: mtexdec.py <src> <dest> [<format>]
# Requires the Python Imaging Library.
# Copyright (C) 2013 Shiz, C_U.
# Released under the terms of the WTFPL: see http://www.wtfpl.net/txt/copying/ for details.

import sys
import os
import struct
import PIL.Image

MAGIC = 0x7865744D
COLORMAPSIZE = 0x400

def mtex_to_image(source, dest=None, format='GIF'):
    with open(source, 'rb') as f:
        magic = struct.unpack('<I', f.read(4))[0]
        if magic != MAGIC:
            raise ValueError('Not a Mtex image: {}'.format(magic))

        # ????
        f.seek(4, os.SEEK_CUR)
        # File size.
        fsize = struct.unpack('<I', f.read(4))[0]
        # File name.
        fname = (f.read(8) + b'\x00').decode('us-ascii')
        # ????
        f.seek(20, os.SEEK_CUR)
        # Image width and height.
        width, height = struct.unpack('<HH', f.read(4))
        # ????
        f.seek(8, os.SEEK_CUR)
        # Offset to color map.
        cmoffset = struct.unpack('<I', f.read(4))[0]
        # Image data size.
        datalen = struct.unpack('<I', f.read(4))[0]
        # ???
        f.seek(4, os.SEEK_CUR)

        # Image data.
        pixels = f.read(datalen)
        # Seek to color map.
        f.seek(cmoffset)
        # Color map data.
        colormap = f.read(COLORMAPSIZE)

        # Build pixel data.
        imagedata = []
        for byte in pixels:
            cmoffset = byte * 4
            color = colormap[cmoffset:cmoffset + 4]
            imagedata.append(color)
        pixeldata = b''.join(imagedata)

        # Save image.
        if dest is None:
            dest = fname.lstrip('/').replace('.tex', format.lower())
        image = PIL.Image.frombuffer('RGBA', (width, height), pixeldata, 'raw', 'RGBA', 0, 1)
        image.save(dest, format)

if __name__ == '__main__':
    if len(sys.argv) < 3:
        exit('usage: {} SRC DEST [FORMAT]'.format(sys.argv[0]))

    mtex_to_image(sys.argv[1], sys.argv[2], sys.argv[3] if len(sys.argv) > 3 else 'GIF')