[png-mng-misc] CFD eXIf 2017-05-31 [was DRAFT: eXIF 2017-05-28]

[Glenn Randers-Pehrson]

eXIf chunk is designed to be simple toencode. As willem said, KISS (keep
it simple, stupid).

Ok. I agree with eXIf chunk written as direct copy of EXIF data. However I
disagree that anyone could rely on such 'raw' data.
I think encoder should be able to parse EXIF if it want to keep
unsafe-to-copy metadata, but it may not parse by cost of loosing
unsafe-to-copy data(as any decoder should discard this data).
I don't think it is possible to keep unsafe-to-copy data within safe-to-copy
chunk and rely on them.

I think this paragraph in the current eXIf CFD provides sufficient
warning against relying on the 'raw' data:

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. 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.

Glenn

[Pavel Zlatovratskii]

It is beyond the scope of this specification to resolve potential conflicts between
data in the eXIf chunk and in other PNG 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.

And all these 'maybe' is looks like extra complexity for me, definitely
not KISS.
As a developer of png software I read "beyond the scope of this
specification " like "There is no guarantee for EXIF, but someone may
prefer EXIF over PNG chunks, so try to guess what data is actual". Guess
is not simple.

Same for

there is no expectation that any thumbnails present in the Exif

profile have (or have not) been updated if the main image was changed.
and

encoders should be aware that, if the Exif profile is converted to

JPEG, the total length of the Exif data (the sum of the lengths of the
Exif attributes IFD, the GPS IFD, the thumbnail IFD, and the TIFF
header) must not exceed 64 kbytes, because it must fit into a JPEG APP1
marker.