class Chunk(bin.Chunk, encoding='ascii'):
"""
A special chunk for PNG, which puts the size before the type
and includes a CRC field for verifying data integrity.
"""
size = bin.Integer(size=4)
id = bin.String(size=4)
payload = bin.Payload(size=size)
crc = bin.CRC32(first=id)
@property
def is_critical(self):
# Critical chunks will always have an uppercase letter for the
# first character in the type. Ancillary will always be lower.
return self.type[0].upper() == self.type[0]
@property
def is_public(self):
# Public chunks will always have an uppercase letter for the
# second character in the type. Private will always be lower.
return self.type[1].upper() == self.type[1]
class Chunk(bin.Chunk):
id = bin.String(size=4, encoding='ascii')
size = bin.Integer(size=4)
payload = bin.Payload(size=size)
class Form(bin.Chunk, encoding='ascii'):
tag = bin.FixedString('FORM')
size = bin.Integer(size=4)
id = bin.String(size=4)
payload = bin.Payload(size=size)
class ScanChunk(bin.Chunk):
id = bin.Integer(size=2)
size = bin.Integer(size=2)
payload = bin.Payload(size=size - 2)
class EmptyChunk(bin.Chunk):
id = bin.Integer(size=2)
size = bin.Bytes(size=0)
payload = bin.Payload(size=0)