I'm trying to write my own little PNG reader in Python. There is something in the documentation I don't quite understand. In chapter 3.3 (where chunks are handled) it says:
Four bits of the type code, namely bit 5 (value 32) of each byte, are used to convey chunk properties. This choice means that a human can read off the assigned properties according to whether each letter of the type code is uppercase (bit 5 is 0) or lowercase (bit 5 is 1). However, decoders should test the properties of an unknown chunk by numerically testing the specified bits; testing whether a character is uppercase or lowercase is inefficient, and even incorrect if a locale-specific case definition is used.
Ok, so it explicitly denotes one should not test whether a byte is uppercase or lowercase. Then, how do I check that bit 5?
Furthermore, the documentation states
Ancillary bit: bit 5 of first byte
0 (uppercase) = critical, 1 (lowercase) = ancillary.
I have the following function to convert an integer to a bit-stream:
def bits(x, n):
""" Convert an integer value *x* to a sequence of *n* bits as a string. """
return ''.join(str([0, 1][x >> i & 1]) for i in xrange(n - 1, -1, -1))
Just for example, take the sRGB chunk. The lowercase s denotes the chunk is ancillary. But comparing the bit-streams of an uppercase S and lowercase s
01110011
01010011
we can see that bit #5 is zero in both cases.
I think I do have a wrong understanding of counting the bits. As the only bit that changes is the third one (i.e. indexed with 2), i assume this is the bit I'm searching for? It is also the 6th bit from the right and indexed with 5 (from the right of course). Is this what I'm searching for?
Python does have bitwise manipulation. You are doing it the hard way, when they already gave you the bitmask (32 or 0x20).
is_critical = (type_code & 0x20) == 0
or, equivalently:
is_critical = (type_code & (0x1 << 5)) == 0
(with extra parentheses for clarity)