sphinx / sphinx / util /

# -*- coding: utf-8 -*-

    PNG image manipulation helpers.

    :copyright: Copyright 2007-2011 by the Sphinx team, see AUTHORS.
    :license: BSD, see LICENSE for details.

import struct
import binascii

from sphinx.util.pycompat import b


DEPTH_CHUNK_LEN = struct.pack('!i', 10)
DEPTH_CHUNK_START = b('tEXtDepth\x00')
IEND_CHUNK = b('\x00\x00\x00\x00IEND\xAE\x42\x60\x82')

def read_png_depth(filename):
    """Read the special tEXt chunk indicating the depth from a PNG file."""
    result = None
    f = open(filename, 'rb')
    try: (LEN_IEND + LEN_DEPTH), 2)
        depthchunk =
        if not depthchunk.startswith(DEPTH_CHUNK_LEN + DEPTH_CHUNK_START):
            # either not a PNG file or not containing the depth chunk
            return None
        result = struct.unpack('!i', depthchunk[14:18])[0]
    return result

def write_png_depth(filename, depth):
    """Write the special tEXt chunk indicating the depth to a PNG file.

    The chunk is placed immediately before the special IEND chunk.
    data = struct.pack('!i', depth)
    f = open(filename, 'r+b')
        # seek to the beginning of the IEND chunk, 2)
        # overwrite it with the depth chunk
        f.write(DEPTH_CHUNK_LEN + DEPTH_CHUNK_START + data)
        # calculate the checksum over chunk name and data
        f.write(struct.pack('!i', binascii.crc32(DEPTH_CHUNK_START + data)))
        # replace the IEND chunk
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