def png_pack(png_tag, data):
    chunk_head = (png_tag + data)
    return ((struct.pack('!I', len(data)) + chunk_head) + struct.pack('!I', (4294967295 & zlib.crc32(chunk_head))))
def png_pack(png_tag, data):
    chunk_head = (png_tag + data)
    return ((struct.pack('!I', len(data)) + chunk_head) + struct.pack('!I', (4294967295 & zlib.crc32(chunk_head))))