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# -*- coding: utf-8 -*-
"""
sphinx.util.png
~~~~~~~~~~~~~~~
PNG image manipulation helpers.
:copyright: Copyright 2007-2013 by the Sphinx team, see AUTHORS.
:license: BSD, see LICENSE for details.
"""
import struct
import binascii
from sphinx.util.pycompat import b
LEN_IEND = 12
LEN_DEPTH = 22
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:
f.seek(- (LEN_IEND + LEN_DEPTH), 2)
depthchunk = f.read(LEN_DEPTH)
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]
finally:
f.close()
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')
try:
# seek to the beginning of the IEND chunk
f.seek(-LEN_IEND, 2)
# overwrite it with the depth chunk
f.write(DEPTH_CHUNK_LEN + DEPTH_CHUNK_START + data)
# calculate the checksum over chunk name and data
crc = binascii.crc32(DEPTH_CHUNK_START + data) & 0xffffffff
f.write(struct.pack('!I', crc))
# replace the IEND chunk
f.write(IEND_CHUNK)
finally:
f.close()
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