Re: EXIF support in PNG [was: Modern compressor]

I like John's eXIf + cOMp proposal.  The only things I'd add are

    a requirement that only one eXIf chunk be present in a PNG.
    otherwise there are no ordering constraints beyond those imposed
    by the PNG spec

    The cOMp chunk may appear multiple times, however, it must
     obey the numbering and positioning rules for the chunk that it
contains (e.g.,
     a cOMp chunk containing an eXIf chunk must appear not
     more than once); PNG editors that do not decode the cOMp chunk
     are not expected to enforce any special ordering rules but may
     handle the chunk as any other safe-to-copy public chunk.  Decoders
     that actually use the chunk can enforce any ordering rules.

I might change the order a bit (putting compression method
before original length):

<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>

With this order, the original_length+data+compressed_chunk_data
are an exact copy of the original chunk, minus its CRC.


On Sat, Dec 31, 2016 at 10:08 PM, John Bowler <
[email protected]> wrote:

> Ok, here is a formal proposal, contained verbatim below and attached:
>
> =================================================
> PNG CHUNK PROPOSAL: eXIf
>
> SPECIFICATION
> =============
>
> The four-byte chunk type field contains the decimal values
>
> 101 88 73 102
>
> The eXIf chunk contains EXIF metadata with a length given by the chunk
> length.  Such data shall be compatible with specifications issued by CIPA,
> "Exchangeable image file format for digital still cameras" [CIPA
> DC-008-2016] or its successors.
> =================================================
>
>
>

------------------------------------------------------------------------------
Check out the vibrant tech community on one of the world's most 
engaging tech sites, SlashDot.org! http://sdm.link/slashdot

_______________________________________________
png-mng-misc mailing list
[email protected]
https://lists.sourceforge.net/lists/listinfo/png-mng-misc