Re: Endless gamma in eXIf 2017-0119

On Tue, Feb 7, 2017 at 10:38 AM, Glenn Randers-Pehrson
<[email protected]> wrote:
> It's not sufficient because our copy-safe mechanism does
> not adequatly handle potential conflicts among ancillary
> chunks, such a gAMA and eXIf in this situation.

Yes it does!   Read the rules for PNG editors carefully.  They don't
say it is legal to write a PNG with the wrong metadata, even if you
include the right metadata as well and it would be contrary to common
sense to suggest that that was the intention.  We had to made an
explicit exception to common sense to allow eXIf to introduce
incorrect metadata and the result is intractable.

A editor that conforms to those rules with eXIF (unsafe) *must* either
update the relevant tags in EXIF, delete the relevant tags or delete
the whole chunk.  There is no other possible interpretation; it's
exactly the same as if an editor edits a PNG with an iCCP chunk.

The only argument against unsafe-to-copy was Cosmin's assertion that
the chunk has to be safe-to-copy because the textual metadata is too
valuable.  Everyone else who expressed an opinion effectively argued
strongly against the safe-to-copy semantics.  (Remember, that is why
you put the line in the "security" section; the thumbnail argument.)

I never regarded Cosmin's argument as compelling but none of the
anti-thumbnailers argued against it.

I accept Adam's reasons but I don't think adding more words that no
one will take any notice of helps.  The default has to be to delete
the chunk; something that understands it can easily maintain it; exiv2
is a complete, adequate, library implementation and so is exiftool.

John Bowler

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