Re: CALL for DISCUSSION: cOMp/cOMP/coMp/coMP: compression wrapper chunks

Thanks for putting together the proposal.

On Sun, Jan 29, 2017 at 3:10 PM, John Bowler <
[email protected]> wrote:

>
>    Thus the chunks have the following layout, following the description
>    given in section 5.3 and using cOMp as an example:
>
>       length [4 bytes, length of the cOMp data]
>       cOMp [4 bytes, 99, 79, 77, 112]
>          original chunk length
>          original chunk type
>          compressed original chunk data
>      crc
>
>    The compressed original data shall have the format:
>
>        | compression method byte | compressed data |
>
>    Where the compression method byte is as defined in ISO-PNG
>
[...]

Please specify whether the "compressed original chunk data" does or does
not include the original chunk CRC.  I have been stripping it off, but it
seems that it may be worthwhile keeping it.  Without it, the decoder has
to construct one, so leaving it out doesn't really reduce the burden on
decoders.  Keeping it adds some measure of validating the integrity
of the comp/decomp process, although there's also the ADLER32
checksum within the ZLIB datastream so it's still fairly safe without it.

Also specify whether the "original chunk length" includes the
compression method byte (I don't think it does).

Glenn

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