blob: 6c942194ecbfaa89dda8c350c8b568fff80131a0 (
plain)
1
2
3
4
5
6
7
8
9
10
11
12
13
14
15
16
17
18
19
20
21
22
23
24
25
26
27
28
29
30
31
32
33
34
35
36
37
38
39
40
41
42
43
|
"""PNG image manipulation helpers."""
from __future__ import annotations
import binascii
import struct
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: str) -> int | None:
"""Read the special tEXt chunk indicating the depth from a PNG file."""
with open(filename, 'rb') as f:
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
else:
return struct.unpack('!i', depthchunk[14:18])[0]
def write_png_depth(filename: str, depth: int) -> None:
"""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)
with open(filename, 'r+b') as f:
# 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)
|
