Re: EXIF support in PNG

On Thu, Jan 5, 2017 at 12:32 PM, John Bowler <
[email protected]> wrote:

> On Thu, Jan 5, 2017 at 7:09 AM, Glenn Randers-Pehrson <[email protected]>
> wrote:
> > glenn.rp> cat dunflate.f
>
> Ok.  There's something I don't understand about this FORTRAN.  You
> seem to be trying to write 65540 data blocks, each containing 65535
> bytes of the original EXIF data, but there is no explicit FOR loop.
>

It's only in my mind right now but of course there will be a DO loop or two.


>
> Now I've used langauges with implicit loops (LISP MAPCAR style) and,
> indeed, I've designed such languages.  Are you saying that the
> followng FORTRAN iterates over the input (EXIF) data to construct
> "blocks"?
>
> c     if (not last block i.e., remaining_length > 64k-1)
> c         block header:
> c           128 (1, 00, xxxxx) [check on position of the "1"]
> * * *
> c         some data
>           remaining_data -= 64k-1
> c     else
> c         block header:
> c           0 (0, 00, xxxxx)
> * * *
> c         remaining data
>
> Because later you use the data object "blocks":
>
> c     calculate length and crc32 over
> c         "iCCP", zero, cmf, blocks, and adler32
> * * *
> c     write blocks
> c     write ADLER32
>
> Sorry, it's just unclear to me.  Even if it is implicit I think there
> has to be a FOR loop and an intermediate buffer allocation somewhere.
>

Of course there will be a DO loop to iterate over the input bytes.

You wanted to see the code before the auditors were done with it,
so there it is.

Glenn

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