DRAFT: eXIf 2017-0210

DRAFT
eXIf 2017-0210
http://www.simplesystems.org/png-group/proposals/eXIf/png-proposed-eXIf-chunk-2017-0210.html
Glenn Randers-Pehrson

In hopes of obtaining consensus on a proposed eXIf chunk, I present this DRAFT
for consideration and discussion.  If and when it seems appropriate
I'll issue an
official CFD to start the two-week countdown to the call for vote.

The guts of this proposal are:

The data segment of the eXIf chunk contains an Exif profile in the
format specified in "4.7.2 Interoperability Structure of APP1 in
Compressed Data" of [CIPA DC-008-2016] except that the JPEG APP1
marker, length, and the "Exif ID code" described in 4.7.2(C), i.e.,
"Exif", NULL, padding byte, are not included.  It begins
with either "II" or "MM", depending upon the byte order used.

There are no ordering constraints upon the position of the eXIf beyond those
imposed by the PNG specification, i.e., if present, the eXIf chunk may
appear anywhere between the IHDR and IEND chunks except between IDAT
chunks.

The eXIf chunk contains metadata concerning the original image data.
If the image has been edited subsequent to creation of the Exif profile,
This data might no longer apply to the PNG image data.  It is beyond
the scope of this specification to resolve potential conflicts between
data in the eXIf chunk and in other PNG chunks, to include other eXIf
chunks.  It is recommended that unless a decoder has independent knowledge
of the validity of the Exif data, the data should be considered to be of
historical value only.

There is no expectation that any thumbnails present in the Exif profile
have been updated (or have not been updated) if the main image was changed.

Image editing applications should consider Paragraph E.3 of the Exif
Specification (CIPA DC-008, Exchangeable image file format for digital
still cameras), which discusses requirements for updating Exif data when
the image is changed.  Encoders should follow those requirements, but
decoders should not assume that it has been accomplished.

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