Re: CALL for DISCUSSION: eXIf 20170115

On Sat, Jan 28, 2017 at 4:51 AM, Glenn Randers-Pehrson
<[email protected]> wrote:
> Back to the drawing board for zxIf.  It needs an "uncompressed length" byte
> at the beginning.

Well, it's off topic, but that's why I was suggesting "cOMp":

<chunk-length>: compressed length + 9
'comp': chunk tag (case matches case of the embedded chunk)
<compression-method>: 1 byte, typically 0 for deflate
<original-length>: 4 bytes
'data': original chunk tag (see below)
<compressed-chunk-data>
<crc32>

Regardless of debates about where the compression byte goes it is
possible to write a simple, streaming, filter for this which simply
copies a PNG from input to output except for replacing every cOMp
chunk, as above, with:

<original-length>: 4 bytes
'data': original chunk tag (see below)
<original-chunk-data>
<generated-crc32>

There is no non-trivial buffering requirement.  Neither is there a
buffering requirement to handle cOMp transparently above a library
like libpng; the read callback identifies 'cOMp' and returns the
original header to libpng then when asked for the data it runs a zlib
inflate engine; libpng actually provides the output buffer!

John Bowler

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