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import struct
class PNGFilter:
"""Skips non-critical chunks in a PNG file."""
magic = b"\x89PNG\r\n\x1a\n"
def __init__(self):
self.inbuffer = b""
self.critchunk = False
self.chunkleft = None
def filter(self, data):
self.inbuffer += data
if self.chunkleft is None:
if len(self.inbuffer) < 8:
return b""
if not self.inbuffer.startswith(self.magic):
raise ValueError("PNG file magic not found")
self.inbuffer = self.inbuffer[8:]
self.chunkleft = 0
ret = b""
while True:
if self.chunkleft == 0:
if len(self.inbuffer) < 8:
break
self.chunkleft, chunktype = struct.unpack(">I4s",
self.inbuffer[:8])
self.chunkleft += 12 # len, type, crc
self.critchunk = chunktype[0].isupper()
if self.critchunk:
print("critical chunk %s %r" % (chunktype, self.inbuffer[8:16]))
n = min(self.chunkleft, len(self.inbuffer))
if self.critchunk:
ret += self.inbuffer[:n]
self.inbuffer = self.inbuffer[n:]
self.chunkleft -= n
if self.chunkleft:
break
return ret
def flush(self):
ret = self.inbuffer
self.inbuffer = b""
return ret
def copy(self):
new = PNGFilter()
new.inbuffer = self.inbuffer
new.critchunk = self.critchunk
new.chunkleft = self.chunkleft
return new
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